THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM.

BY

CHARLES DARWIN, M.A., F.R.S., ETC.




CONTENTS.


CHAPTER I.

INTRODUCTORY REMARKS.

Various means which favour or determine the cross-fertilisation of
plants.--Benefits derived from cross-fertilisation.--Self-fertilisation
favourable to the propagation of the species.--Brief history of the
subject.--Object of the experiments, and the manner in which they were
tried.--Statistical value of the measurements.--The experiments carried
on during several successive generations.--Nature of the relationship of
the plants in the later generations.--Uniformity of the conditions to
which the plants were subjected.--Some apparent and some real causes of
error.--Amount of pollen employed.--Arrangement of the work.--Importance
of the conclusions.


CHAPTER II.

CONVOLVULACEAE.

Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.--Greater
constitutional vigour of the crossed plants.--The effects on the
offspring of crossing different flowers on the same plant, instead of
crossing distinct individuals.--The effects of a cross with a fresh
stock.--The descendants of the self-fertilised plant named
Hero.--Summary on the growth, vigour, and fertility of the successive
crossed and self-fertilised generations.--Small amount of pollen in the
anthers of the self-fertilised plants of the later generations, and the
sterility of their first-produced flowers.--Uniform colour of the
flowers produced by the self-fertilised plants.--The advantage from a
cross between two distinct plants depends on their differing in
constitution.


CHAPTER III.

SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.

Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.--Appearance of a
new, tall, and highly self-fertile variety.--Offspring from a cross
between self-fertilised plants.--Effects of a cross with a fresh
stock.--Effects of crossing flowers on the same plant.--Summary on
Mimulus luteus.--Digitalis purpurea, superiority of the crossed
plants.--Effects of crossing flowers on the same
plant.--Calceolaria.--Linaria vulgaris.--Verbascum thapsus.--Vandellia
nummularifolia.--Cleistogene flowers.--Gesneria pendulina.--Salvia
coccinea.--Origanum vulgare, great increase of the crossed plants by
stolons.--Thunbergia alata.


CHAPTER IV.

CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.

Brassica oleracea, crossed and self-fertilised plants.--Great effect of
a cross with a fresh stock on the weight of the offspring.--Iberis
umbellata.--Papaver vagum.--Eschscholtzia californica, seedlings from a
cross with a fresh stock not more vigorous, but more fertile than the
self-fertilised seedlings.--Reseda lutea and odorata, many individuals
sterile with their own pollen.--Viola tricolor, wonderful effects of a
cross.--Adonis aestivalis.--Delphinium consolida.--Viscaria oculata,
crossed plants hardly taller, but more fertile than the
self-fertilised.--Dianthus caryophyllus, crossed and self-fertilised
plants compared for four generations.--Great effects of a cross with a
fresh stock.--Uniform colour of the flowers on the self-fertilised
plants.--Hibiscus africanus.


CHAPTER V.

GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.

Pelargonium zonale, a cross between plants propagated by cuttings does
no good.--Tropaeolum minus.--Limnanthes douglasii.--Lupinus luteus and
pilosus.--Phaseolus multiflorus and vulgaris.--Lathyrus odoratus,
varieties of, never naturally intercross in England.--Pisum sativum,
varieties of, rarely intercross, but a cross between them highly
beneficial.--Sarothamnus scoparius, wonderful effects of a
cross.--Ononis minutissima, cleistogene flowers of.--Summary on the
Leguminosae.--Clarkia elegans.--Bartonia aurea.--Passiflora
gracilis.--Apium petroselinum.--Scabiosa atropurpurea.--Lactuca
sativa.--Specularia speculum.--Lobelia ramosa, advantages of a cross
during two generations.--Lobelia fulgens.--Nemophila insignis, great
advantages of a cross.--Borago officinalis.--Nolana prostrata.


CHAPTER VI.

SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.

Petunia violacea, crossed and self-fertilised plants compared for four
generations.--Effects of a cross with a fresh stock.--Uniform colour of
the flowers on the self-fertilised plants of the fourth
generation.--Nicotiana tabacum, crossed and self-fertilised plants of
equal height.--Great effects of a cross with a distinct sub-variety on
the height, but not on the fertility, of the offspring.--Cyclamen
persicum, crossed seedlings greatly superior to the
self-fertilised.--Anagallis collina.--Primula veris.--Equal-styled
variety of Primula veris, fertility of, greatly increased by a cross
with a fresh stock.--Fagopyrum esculentum.--Beta vulgaris.--Canna
warscewiczi, crossed and self-fertilised plants of equal height.--Zea
mays.--Phalaris canariensis.


CHAPTER VII.

SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED
PLANTS.

Number of species and plants measured.--Tables given.--Preliminary
remarks on the offspring of plants crossed by a fresh stock.--Thirteen
cases specially considered.--The effects of crossing a self-fertilised
plant either by another self-fertilised plant or by an intercrossed
plant of the old stock.--Summary of the results.--Preliminary remarks on
the crossed and self-fertilised plants of the same stock.--The
twenty-six exceptional cases considered, in which the crossed plants did
not exceed greatly in height the self-fertilised.--Most of these cases
shown not to be real exceptions to the rule that cross-fertilisation is
beneficial.--Summary of results.--Relative weights of the crossed and
self-fertilised plants.


CHAPTER VIII.

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL
VIGOUR AND IN OTHER RESPECTS.

Greater constitutional vigour of crossed plants.--The effects of great
crowding.--Competition with other kinds of plants.--Self-fertilised
plants more liable to premature death.--Crossed plants generally flower
before the self-fertilised.--Negative effects of intercrossing flowers
on the same plant.--Cases described.--Transmission of the good effects
of a cross to later generations.--Effects of crossing plants of closely
related parentage.--Uniform colour of the flowers on plants
self-fertilised during several generations and cultivated under similar
conditions.


CHAPTER IX.

THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE
PRODUCTION OF SEEDS.

Fertility of plants of crossed and self-fertilised parentage, both lots
being fertilised in the same manner.--Fertility of the parent-plants
when first crossed and self-fertilised, and of their crossed and
self-fertilised offspring when again crossed and
self-fertilised.--Comparison of the fertility of flowers fertilised with
their own pollen and with that from other flowers on the same
plant.--Self-sterile plants.--Causes of self-sterility.--The appearance
of highly self-fertile varieties.--Self-fertilisation apparently in some
respects beneficial, independently of the assured production of
seeds.--Relative weights and rates of germination of seeds from crossed
and self-fertilised flowers.


CHAPTER X.

MEANS OF FERTILISATION.

Sterility and fertility of plants when insects are excluded.--The means
by which flowers are cross-fertilised.--Structures favourable to
self-fertilisation.--Relation between the structure and conspicuousness
of flowers, the visits of insects, and the advantages of
cross-fertilisation.--The means by which flowers are fertilised with
pollen from a distinct plant.--Greater fertilising power of such
pollen.--Anemophilous species.--Conversion of anemophilous species into
entomophilous.--Origin of nectar.--Anemophilous plants generally have
their sexes separated.--Conversion of diclinous into hermaphrodite
flowers.--Trees often have their sexes separated.


CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they
can.--Cause of this habit.--Means by which bees recognise the flowers of
the same species.--Sudden secretion of nectar.--Nectar of certain
flowers unattractive to certain insects.--Industry of bees, and the
number of flowers visited within a short time.--Perforation of the
corolla by bees.--Skill shown in the operation.--Hive-bees profit by the
holes made by humble-bees.--Effects of habit.--The motive for
perforating flowers to save time.--Flowers growing in crowded masses
chiefly perforated.


CHAPTER XII.

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.--Allied species differ greatly in the means by which
cross-fertilisation is favoured and self-fertilisation avoided.--The
benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.--The evil effects not due to the
combination of morbid tendencies in the parents.--Nature of the
conditions to which plants are subjected when growing near together in a
state of nature or under culture, and the effects of such
conditions.--Theoretical considerations with respect to the interaction
of differentiated sexual elements.--Practical lessons.--Genesis of the
two sexes.--Close correspondence between the effects of
cross-fertilisation and self-fertilisation, and of the legitimate and
illegitimate unions of heterostyled plants, in comparison with hybrid
unions.


INDEX.


...


THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.


CHAPTER I.

INTRODUCTORY REMARKS.

Various means which favour or determine the cross-fertilisation of plants.
Benefits derived from cross-fertilisation.
Self-fertilisation favourable to the propagation of the species.
Brief history of the subject.
Object of the experiments, and the manner in which they were tried.
Statistical value of the measurements.
The experiments carried on during several successive generations.
Nature of the relationship of the plants in the later generations.
Uniformity of the conditions to which the plants were subjected.
Some apparent and some real causes of error.
Amount of pollen employed.
Arrangement of the work.
Importance of the conclusions.

There is weighty and abundant evidence that the flowers of most kinds of
plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe,
by a distinct plant. Cross-fertilisation is sometimes ensured by the
sexes being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised
by pollen from the same plant, but can be fertilised by pollen from any
other individual of the same species. There are also very many species
which are partially sterile with their own pollen. Lastly, there is a
large class in which the flowers present no apparent obstacle of any
kind to self-fertilisation, nevertheless these plants are frequently
intercrossed, owing to the prepotency of pollen from another individual
or variety over the plant’s own pollen.

As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone
that they derived some great advantage from the process; and it is the
object of the present work to show the nature and importance of the
benefits thus derived. There are, however, some exceptions to the rule
of plants being constructed so as to allow of or to favour
cross-fertilisation, for some few plants seem to be invariably
self-fertilised; yet even these retain traces of having been formerly
adapted for cross-fertilisation. These exceptions need not make us doubt
the truth of the above rule, any more than the existence of some few
plants which produce flowers, and yet never set seed, should make us
doubt that flowers are adapted for the production of seed and the
propagation of the species.

We should always keep in mind the obvious fact that the production of
seed is the chief end of the act of fertilisation; and that this end can
be gained by hermaphrodite plants with incomparably greater certainty by
self-fertilisation, than by the union of the sexual elements belonging
to two distinct flowers or plants. Yet it is as unmistakably plain that
innumerable flowers are adapted for cross-fertilisation, as that the
teeth and talons of a carnivorous animal are adapted for catching prey;
or that the plumes, wings, and hooks of a seed are adapted for its
dissemination. Flowers, therefore, are constructed so as to gain two
objects which are, to a certain extent, antagonistic, and this explains
many apparent anomalies in their structure. The close proximity of the
anthers to the stigma in a multitude of species favours, and often
leads, to self-fertilisation; but this end could have been gained far
more safely if the flowers had been completely closed, for then the
pollen would not have been injured by the rain or devoured by insects,
as often happens. Moreover, in this case, a very small quantity of
pollen would have been sufficient for fertilisation, instead of millions
of grains being produced. But the openness of the flower and the
production of a great and apparently wasteful amount of pollen are
necessary for cross-fertilisation. These remarks are well illustrated by
the plants called cleistogene, which bear on the same stock two kinds of
flowers. The flowers of the one kind are minute and completely closed,
so that they cannot possibly be crossed; but they are abundantly
fertile, although producing an extremely small quantity of pollen. The
flowers of the other kind produce much pollen and are open; and these
can be, and often are, cross-fertilised. Hermann Muller has also made
the remarkable discovery that there are some plants which exist under
two forms; that is, produce on distinct stocks two kinds of
hermaphrodite flowers. The one form bears small flowers constructed for
self-fertilisation; whilst the other bears larger and much more
conspicuous flowers plainly constructed for cross-fertilisation by the
aid of insects; and without their aid these produce no seed.

The adaptation of flowers for cross-fertilisation is a subject which has
interested me for the last thirty-seven years, and I have collected a
large mass of observations, but these are now rendered superfluous by
the many excellent works which have been lately published. In the year
1857 I wrote a short paper on the fertilisation of the kidney bean (1/1.
‘Gardeners’ Chronicle’ 1857 page 725 and 1858 pages 824 and 844. ‘Annals
and Magazine of Natural History’ 3rd series volume 2 1858 page 462.);
and in 1862 my work ‘On the Contrivances by which British and Foreign
Orchids are Fertilised by Insects’ appeared. It seemed to me a better
plan to work out one group of plants as carefully as I could, rather
than to publish many miscellaneous and imperfect observations. My
present work is the complement of that on Orchids, in which it was shown
how admirably these plants are constructed so as to permit of, or to
favour, or to necessitate cross-fertilisation. The adaptations for
cross-fertilisation are perhaps more obvious in the Orchideae than in
any other group of plants, but it is an error to speak of them, as some
authors have done, as an exceptional case. The lever-like action of the
stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by
which the anthers are depressed and rubbed on the backs of bees, shows
as perfect a structure as can be found in any orchid. Papilionaceous
flowers, as described by various authors--for instance, by Mr. T.H.
Farrer--offer innumerable curious adaptations for cross-fertilisation.
The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful
as that of the most wonderful orchid. The stamens, according to Fritz
Muller, are irritable, so that as soon as a moth visits a flower, the
anthers explode and cover the insect with pollen; one of the filaments
which is broader than the others then moves and closes the flower for
about twelve hours, after which time it resumes its original position.
(1/2. ‘Botanische Zeitung’ 1866 page 129.) Thus the stigma cannot be
fertilised by pollen from the same flower, but only by that brought by a
moth from some other flower. Endless other beautiful contrivances for
this same purpose could be specified.

Long before I had attended to the fertilisation of flowers, a remarkable
book appeared in 1793 in Germany, ‘Das Entdeckte Geheimniss der Natur,’
by C.K. Sprengel, in which he clearly proved by innumerable
observations, how essential a part insects play in the fertilisation of
many plants. But he was in advance of his age, and his discoveries were
for a long time neglected. Since the appearance of my book on Orchids,
many excellent works on the fertilisation of flowers, such as those by
Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter
papers, have been published. (1/3. Sir John Lubbock has given an
interesting summary of the whole subject in his ‘British Wild Flowers
considered in relation to Insects’ 1875. Hermann Muller’s work ‘Die
Befruchtung der Blumen durch Insekten’ 1873, contains an immense number
of original observations and generalisations. It is, moreover,
invaluable as a repertory with references to almost everything which has
been published on the subject. His work differs from that of all others
in specifying what kinds of insects, as far as known, visit the flowers
of each species. He likewise enters on new ground, by showing not only
that flowers are adapted for their own good to the visits of certain
insects; but that the insects themselves are excellently adapted for
procuring nectar or pollen from certain flowers. The value of H.
Muller’s work can hardly be over-estimated, and it is much to be desired
that it should be translated into English. Severin Axell’s work is
written in Swedish, so that I have not been able to read it.) A list
would occupy several pages, and this is not the proper place to give
their titles, as we are not here concerned with the means, but with the
results of cross-fertilisation. No one who feels interest in the
mechanism by which nature effects her ends, can read these books and
memoirs without the most lively interest.

From my own observations on plants, guided to a certain extent by the
experience of the breeders of animals, I became convinced many years ago
that it is a general law of nature that flowers are adapted to be
crossed, at least occasionally, by pollen from a distinct plant.
Sprengel at times foresaw this law, but only partially, for it does not
appear that he was aware that there was any difference in power between
pollen from the same plant and from a distinct plant. In the
introduction to his book (page 4) he says, as the sexes are separated in
so many flowers, and as so many other flowers are dichogamous, “it
appears that nature has not willed that any one flower should be
fertilised by its own pollen.” Nevertheless, he was far from keeping
this conclusion always before his mind, or he did not see its full
importance, as may be perceived by anyone who will read his observations
carefully; and he consequently mistook the meaning of various
structures. But his discoveries are so numerous and his work so
excellent, that he can well afford to bear a small amount of blame. A
most capable judge, H. Muller, likewise says: “It is remarkable in how
very many cases Sprengel rightly perceived that pollen is necessarily
transported to the stigmas of other flowers of the same species by the
insects which visit them, and yet did not imagine that this
transportation was of any service to the plants themselves.” (1/4. ‘Die
Befruchtung der Blumen’ 1873 page 4. His words are: “Es ist merkwurdig,
in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die
Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben
anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung
zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur
die Pflanzen selbst gesucht werden musse.”)

Andrew Knight saw the truth much more clearly, for he remarks, “Nature
intended that a sexual intercourse should take place between
neighbouring plants of the same species.” (1/5. ‘Philosophical
Transactions’ 1799 page 202.) After alluding to the various means by
which pollen is transported from flower to flower, as far as was then
imperfectly known, he adds, “Nature has something more in view than that
its own proper males would fecundate each blossom.” In 1811 Kolreuter
plainly hinted at the same law, as did afterwards another famous
hybridiser of plants, Herbert. (1/6. Kolreuter ‘Mem. de l’Acad. de St.
Petersbourg’ tome 3 1809 published 1811 page 197. After showing how well
the Malvaceae are adapted for cross-fertilisation, he asks, “An id
aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo
pulvere, sed semper eo aliarum suae speciei impregnentur, merito
quaeritur? Certe natura nil facit frustra.” Herbert ‘Amaryllidaceae,
with a Treatise on Cross-bred Vegetables’ 1837.) But none of these
distinguished observers appear to have been sufficiently impressed with
the truth and generality of the law, so as to insist on it and impress
their beliefs on others.

In 1862 I summed up my observations on Orchids by saying that nature
“abhors perpetual self-fertilisation.” If the word perpetual had been
omitted, the aphorism would have been false. As it stands, I believe
that it is true, though perhaps rather too strongly expressed; and I
should have added the self-evident proposition that the propagation of
the species, whether by self-fertilisation or by cross-fertilisation, or
asexually by buds, stolons, etc. is of paramount importance. Hermann
Muller has done excellent service by insisting repeatedly on this latter
point.

It often occurred to me that it would be advisable to try whether
seedlings from cross-fertilised flowers were in any way superior to
those from self-fertilised flowers. But as no instance was known with
animals of any evil appearing in a single generation from the closest
possible interbreeding, that is between brothers and sisters, I thought
that the same rule would hold good with plants; and that it would be
necessary at the sacrifice of too much time to self-fertilise and
intercross plants during several successive generations, in order to
arrive at any result. I ought to have reflected that such elaborate
provisions favouring cross-fertilisation, as we see in innumerable
plants, would not have been acquired for the sake of gaining a distant
and slight advantage, or of avoiding a distant and slight evil.
Moreover, the fertilisation of a flower by its own pollen corresponds to
a closer form of interbreeding than is possible with ordinary bi-sexual
animals; so that an earlier result might have been expected.

I was at last led to make the experiments recorded in the present volume
from the following circumstance. For the sake of determining certain
points with respect to inheritance, and without any thought of the
effects of close interbreeding, I raised close together two large beds
of self-fertilised and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully grown were
plainly taller and more vigorous than the self-fertilised ones. Bees
incessantly visit the flowers of this Linaria and carry pollen from one
to the other; and if insects are excluded, the flowers produce extremely
few seeds; so that the wild plants from which my seedlings were raised
must have been intercrossed during all previous generations. It seemed
therefore quite incredible that the difference between the two beds of
seedlings could have been due to a single act of self-fertilisation; and
I attributed the result to the self-fertilised seeds not having been
well ripened, improbable as it was that all should have been in this
state, or to some other accidental and inexplicable cause. During the
next year, I raised for the same purpose as before two large beds close
together of self-fertilised and crossed seedlings from the carnation,
Dianthus caryophyllus. This plant, like the Linaria, is almost sterile
if insects are excluded; and we may draw the same inference as before,
namely, that the parent-plants must have been intercrossed during every
or almost every previous generation. Nevertheless, the self-fertilised
seedlings were plainly inferior in height and vigour to the crossed.

My attention was now thoroughly aroused, for I could hardly doubt that
the difference between the two beds was due to the one set being the
offspring of crossed, and the other of self-fertilised flowers.
Accordingly I selected almost by hazard two other plants, which happened
to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea
purpurea, both of which, unlike the Linaria and Dianthus, are highly
self-fertile if insects are excluded. Some flowers on a single plant of
both species were fertilised with their own pollen, and others were
crossed with pollen from a distinct individual; both plants being
protected by a net from insects. The crossed and self-fertilised seeds
thus produced were sown on opposite sides of the same pots, and treated
in all respects alike; and the plants when fully grown were measured and
compared. With both species, as in the cases of the Linaria and
Dianthus, the crossed seedlings were conspicuously superior in height
and in other ways to the self-fertilised. I therefore determined to
begin a long series of experiments with various plants, and these were
continued for the following eleven years; and we shall see that in a
large majority of cases the crossed beat the self-fertilised plants.
Several of the exceptional cases, moreover, in which the crossed plants
were not victorious, can be explained.

It should be observed that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds,
seedlings, or plants; these terms implying that they are the product of
crossed or self-fertilised flowers. Cross-fertilisation always means a
cross between distinct plants which were raised from seeds and not from
cuttings or buds. Self-fertilisation always implies that the flowers in
question were impregnated with their own pollen.

My experiments were tried in the following manner. A single plant, if it
produced a sufficiency of flowers, or two or three plants were placed
under a net stretched on a frame, and large enough to cover the plant
(together with the pot, when one was used) without touching it. This
latter point is important, for if the flowers touch the net they may be
cross-fertilised by bees, as I have known to happen; and when the net is
wet the pollen may be injured. I used at first “white cotton net,” with
very fine meshes, but afterwards a kind of net with meshes one-tenth of
an inch in diameter; and this I found by experience effectually excluded
all insects excepting Thrips, which no net will exclude. On the plants
thus protected several flowers were marked, and were fertilised with
their own pollen; and an equal number on the same plants, marked in a
different manner, were at the same time crossed with pollen from a
distinct plant. The crossed flowers were never castrated, in order to
make the experiments as like as possible to what occurs under nature
with plants fertilised by the aid of insects. Therefore, some of the
flowers which were crossed may have failed to be thus fertilised, and
afterwards have been self-fertilised. But this and some other sources of
error will presently be discussed. In some few cases of spontaneously
self-fertile species, the flowers were allowed to fertilise themselves
under the net; and in still fewer cases uncovered plants were allowed to
be freely crossed by the insects which incessantly visited them. There
are some great advantages and some disadvantages in my having
occasionally varied my method of proceeding; but when there was any
difference in the treatment, it is always so stated under the head of
each species.

Care was taken that the seeds were thoroughly ripened before being
gathered. Afterwards the crossed and self-fertilised seeds were in most
cases placed on damp sand on opposite sides of a glass tumbler covered
by a glass plate, with a partition between the two lots; and the glass
was placed on the chimney-piece in a warm room. I could thus observe the
germination of the seeds. Sometimes a few would germinate on one side
before any on the other, and these were thrown away. But as often as a
pair germinated at the same time, they were planted on opposite sides of
a pot, with a superficial partition between the two; and I thus
proceeded until from half-a-dozen to a score or more seedlings of
exactly the same age were planted on the opposite sides of several pots.
If one of the young seedlings became sickly or was in any way injured,
it was pulled up and thrown away, as well as its antagonist on the
opposite side of the same pot.

As a large number of seeds were placed on the sand to germinate, many
remained after the pairs had been selected, some of which were in a
state of germination and others not so; and these were sown crowded
together on the opposite sides of one or two rather larger pots, or
sometimes in two long rows out of doors. In these cases there was the
most severe struggle for life among the crossed seedlings on one side of
the pot, and the self-fertilised seedlings on the other side, and
between the two lots which grew in competition in the same pot. A vast
number soon perished, and the tallest of the survivors on both sides
when fully grown were measured. Plants treated in this manner, were
subjected to nearly the same conditions as those growing in a state of
nature, which have to struggle to maturity in the midst of a host of
competitors.

On other occasions, from the want of time, the seeds, instead of being
allowed to germinate on damp sand, were sown on the opposite sides of
pots, and the fully grown plants measured. But this plan is less
accurate, as the seeds sometimes germinated more quickly on one side
than on the other. It was however necessary to act in this manner with
some few species, as certain kinds of seeds would not germinate well
when exposed to the light; though the glasses containing them were kept
on the chimney-piece on one side of a room, and some way from the two
windows which faced the north-east. (1/7. This occurred in the plainest
manner with the seeds of Papaver vagum and Delphinium consolida, and
less plainly with those of Adonis aestivalis and Ononis minutissima.
Rarely more than one or two of the seeds of these four species
germinated on the bare sand, though left there for some weeks; but when
these same seeds were placed on earth in pots, and covered with a thin
layer of sand, they germinated immediately in large numbers.)

The soil in the pots in which the seedlings were planted, or the seeds
sown, was well mixed, so as to be uniform in composition. The plants on
the two sides were always watered at the same time and as equally as
possible; and even if this had not been done, the water would have
spread almost equally to both sides, as the pots were not large. The
crossed and self-fertilised plants were separated by a superficial
partition, which was always kept directed towards the chief source of
the light, so that the plants on both sides were equally illuminated. I
do not believe it possible that two sets of plants could have been
subjected to more closely similar conditions, than were my crossed and
self-fertilised seedlings, as grown in the above described manner.

In comparing the two sets, the eye alone was never trusted. Generally
the height of every plant on both sides was carefully measured, often
more than once, namely, whilst young, sometimes again when older, and
finally when fully or almost fully grown. But in some cases, which are
always specified, owing to the want of time, only one or two of the
tallest plants on each side were measured. This plan, which is not a
good one, was never followed (except with the crowded plants raised from
the seeds remaining after the pairs had been planted) unless the tallest
plants on each side seemed fairly to represent the average difference
between those on both sides. It has, however, some great advantages, as
sickly or accidentally injured plants, or the offspring of ill-ripened
seeds, are thus eliminated. When the tallest plants alone on each side
were measured, their average height of course exceeds that of all the
plants on the same side taken together. But in the case of the much
crowded plants raised from the remaining seeds, the average height of
the tallest plants was less than that of the plants in pairs, owing to
the unfavourable conditions to which they were subjected from being
greatly crowded. For our purpose, however, of the comparison of the
crossed and self-fertilised plants, their absolute height signifies
little.

As the plants were measured by an ordinary English standard divided into
inches and eighths of an inch, I have not thought it worth while to
change the fractions into decimals. The average or mean heights were
calculated in the ordinary rough method by adding up the measurements of
all, and dividing the product by the number of plants measured; the
result being here given in inches and decimals. As the different species
grow to various heights, I have always for the sake of easy comparison
given in addition the average height of the crossed plants of each
species taken as 100, and have calculated the average height of the
self-fertilised plant in relation to this standard. With respect to the
crowded plants raised from the seeds remaining after the pairs had been
planted, and of which only some of the tallest on each side were
measured, I have not thought it worth while to complicate the results by
giving separate averages for them and for the pairs, but have added up
all their heights, and thus obtained a single average.

I long doubted whether it was worth while to give the measurements of
each separate plant, but have decided to do so, in order that it may be
seen that the superiority of the crossed plants over the
self-fertilised, does not commonly depend on the presence of two or
three extra fine plants on the one side, or of a few very poor plants on
the other side. Although several observers have insisted in general
terms on the offspring from intercrossed varieties being superior to
either parent-form, no precise measurements have been given (1/8. A
summary of these statements, with references, may be found in my
‘Variation of Animals and Plants under Domestication’ chapter 17 2nd
edition 1875 volume 2 page 109.); and I have met with no observations on
the effects of crossing and self-fertilising the individuals of the same
variety. Moreover, experiments of this kind require so much time--mine
having been continued during eleven years--that they are not likely soon
to be repeated.

As only a moderate number of crossed and self-fertilised plants were
measured, it was of great importance to me to learn how far the averages
were trustworthy. I therefore asked Mr. Galton, who has had much
experience in statistical researches, to examine some of my tables of
measurements, seven in number, namely, those of Ipomoea, Digitalis,
Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if
we took by chance a dozen or score of men belonging to two nations and
measured them, it would I presume be very rash to form any judgment from
such small numbers on their average heights. But the case is somewhat
different with my crossed and self-fertilised plants, as they were of
exactly the same age, were subjected from first to last to the same
conditions, and were descended from the same parents. When only from two
to six pairs of plants were measured, the results are manifestly of
little or no value, except in so far as they confirm and are confirmed
by experiments made on a larger scale with other species. I will now
give the report on the seven tables of measurements, which Mr. Galton
has had the great kindness to draw up for me.

[“I have examined the measurements of the plants with care, and by many
statistical methods, to find out how far the means of the several sets
represent constant realities, such as would come out the same so long as
the general conditions of growth remained unaltered. The principal
methods that were adopted are easily explained by selecting one of the
shorter series of plants, say of Zea mays, for an example.”

TABLE 1/1. Zea mays (young plants). (Mr. Galton.)

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed, as recorded by Mr. Darwin.

Column 3: Self-fertilised, as recorded by Mr. Darwin.

Column 4: Crossed, in Separate Pots, arranged in order of magnitude.

Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.

Column 6: Crossed, in a Single Series, arranged in order of magnitude.

Column 7: Self-fertilised, in a Single Series, arranged in order of
magnitude.

Column 8: Difference, in a Single Series, arranged in order of magnitude.

Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8.
Pot 1 : 12     : 20 3/8 :: 21     : 20     :: 23 2/8 : 20     : -3 2/8.
Pot 1 : 21     : 20     :: 12     : 17 3/8 :: 23     : 20     : -3.
Pot 1 : -      : -      :: -      : -      :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 1 : 22     : 20     :: 22     : 20     :: 22 1/8 : 18 5/8 : -3 4/8.

Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22     : 18 3/8 : -3 5/8.
Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18     : -3 5/8.
Pot 2 : -      : -      :: -      : -      :: 21 4/8 : 18     : -3 4/8.
Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21     : 18     : -3.
Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18     :: 21     : 17 3/8 : -3 5/8.

Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8.
Pot 3 : 21 5/8 : 18     :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8.
Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8.
Pot 3 : -      : -      :: -      : -      :: 12     : 15 2/8 : +3 2/8.
Pot 3 : 21     : 18     :: 23     : 18     :: 12     : 12 6/8 : +0 6/8.

Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18.
Pot 4 : 23     : 15 4/8 :: 21     : 15 4/8.
Pot 4 : 12     : 18     :: 12     : 12 6/8.

“The observations as I received them are shown in Table 1/1, Columns 2
and 3, where they certainly have no prima facie appearance of
regularity. But as soon as we arrange them the in order of their
magnitudes, as in columns 4 and 5, the case is materially altered. We
now see, with few exceptions, that the largest plant on the crossed side
in each pot exceeds the largest plant on the self-fertilised side, that
the second exceeds the second, the third the third, and so on. Out of
the fifteen cases in the table, there are only two exceptions to this
rule. We may therefore confidently affirm that a crossed series will
always be found to exceed a self-fertilised series, within the range of
the conditions under which the present experiment has been made.”

TABLE 1/2.

Column 1: Number (Name) of Pot.

Column 2: Crossed.

Column 3: Self-fertilised.

Column 4: Difference.

Pot 1 : 18 7/8 : 19 2/8 : +0 3/8.
Pot 2 : 20 7/8 : 19     : -1 7/8.
Pot 3 : 21 1/8 : 16 7/8 : -4 2/8.
Pot 4 : 19 6/8 : 16     : -3 6/8.

“Next as regards the numerical estimate of this excess. The mean values
of the several groups are so discordant, as is shown in Table 1/2, that
a fairly precise numerical estimate seems impossible. But the
consideration arises, whether the difference between pot and pot may not
be of much the same order of importance as that of the other conditions
upon which the growth of the plants has been modified. If so, and only
on that condition, it would follow that when all the measurements,
either of the crossed or the self-fertilised plants, were combined into
a single series, that series would be statistically regular. The
experiment is tried in Table 1/1, columns 7 and 8, where the regularity
is abundantly clear, and justifies us in considering its mean as
perfectly reliable. I have protracted these measurements, and revised
them in the usual way, by drawing a curve through them with a free hand,
but the revision barely modifies the means derived from the original
observations. In the present, and in nearly all the other cases, the
difference between the original and revised means is under 2 per cent of
their value. It is a very remarkable coincidence that in the seven kinds
of plants, whose measurements I have examined, the ratio between the
heights of the crossed and of the self-fertilised ranges in five cases
within very narrow limits. In Zea mays it is as 100 to 84, and in the
others it ranges between 100 to 76 and 100 to 86.”

“The determination of the variability (measured by what is technically
called the ‘probable error’) is a problem of more delicacy than that of
determining the means, and I doubt, after making many trials, whether it
is possible to derive useful conclusions from these few observations. We
ought to have measurements of at least fifty plants in each case, in
order to be in a position to deduce fair results. One fact, however,
bearing on variability, is very evident in most cases, though not in Zea
mays, namely, that the self-fertilised plants include the larger number
of exceptionally small specimens, while the crossed are more generally
full grown.”

“Those groups of cases in which measurements have been made of a few of
the tallest plants that grew in rows, each of which contained a
multitude of plants, show very clearly that the crossed plants exceed
the self-fertilised in height, but they do not tell by inference
anything about their respective mean values. If it should happen that a
series is known to follow the law of error or any other law, and if the
number of individuals in the series is known, it would be always
possible to reconstruct the whole series when a fragment of it has been
given. But I find no such method to be applicable in the present case.
The doubt as to the number of plants in each row is of minor importance;
the real difficulty lies in our ignorance of the precise law followed by
the series. The experience of the plants in pots does not help us to
determine that law, because the observations of such plants are too few
to enable us to lay down more than the middle terms of the series to
which they belong with any sort of accuracy, whereas the cases we are
now considering refer to one of its extremities. There are other special
difficulties which need not be gone into, as the one already mentioned
is a complete bar.”]

Mr. Galton sent me at the same time graphical representations which he
had made of the measurements, and they evidently form fairly regular
curves. He appends the words “very good” to those of Zea and Limnanthes.
He also calculated the average height of the crossed and self-fertilised
plants in the seven tables by a more correct method than that followed
by me, namely, by including the heights, as estimated in accordance with
statistical rules, of a few plants which died before they were measured;
whereas I merely added up the heights of the survivors, and divided the
sum by their number. The difference in our results is in one way highly
satisfactory, for the average heights of the self-fertilised plants, as
deduced by Mr. Galton, is less than mine in all the cases excepting one,
in which our averages are the same; and this shows that I have by no
means exaggerated the superiority of the crossed over the
self-fertilised plants.

After the heights of the crossed and self-fertilised plants had been
taken, they were sometimes cut down close to the ground, and an equal
number of both weighed. This method of comparison gives very striking
results, and I wish that it had been oftener followed. Finally a record
was often kept of any marked difference in the rate of germination of
the crossed and self-fertilised seeds,--of the relative periods of
flowering of the plants raised from them,--and of their productiveness,
that is, of the number of seed-capsules which they produced and of the
average number of seeds which each capsule contained.

When I began my experiments I did not intend to raise crossed and
self-fertilised plants for more than a single generation; but as soon as
the plants of the first generation were in flower I thought that I would
raise one more generation, and acted in the following manner. Several
flowers on one or more of the self-fertilised plants were again
self-fertilised; and several flowers on one or more of the crossed
plants were fertilised with pollen from another crossed plant of the
same lot. Having thus once begun, the same method was followed for as
many as ten successive generations with some of the species. The seeds
and seedlings were always treated in exactly the same manner as already
described. The self-fertilised plants, whether originally descended from
one or two mother-plants, were thus in each generation as closely
interbred as was possible; and I could not have improved on my plan. But
instead of crossing one of the crossed plants with another crossed
plant, I ought to have crossed the self-fertilised plants of each
generation with pollen taken from a non-related plant--that is, one
belonging to a distinct family or stock of the same species and variety.
This was done in several cases as an additional experiment, and gave
very striking results. But the plan usually followed was to put into
competition and compare intercrossed plants, which were almost always
the offspring of more or less closely related plants, with the
self-fertilised plants of each succeeding generation;--all having been
grown under closely similar conditions. I have, however, learnt more by
this method of proceeding, which was begun by an oversight and then
necessarily followed, than if I had always crossed the self-fertilised
plants of each succeeding generation with pollen from a fresh stock.

I have said that the crossed plants of the successive generations were
almost always inter-related. When the flowers on an hermaphrodite plant
are crossed with pollen taken from a distinct plant, the seedlings thus
raised may be considered as hermaphrodite brothers or sisters; those
raised from the same capsule being as close as twins or animals of the
same litter. But in one sense the flowers on the same plant are distinct
individuals, and as several flowers on the mother-plant were crossed by
pollen taken from several flowers on the father-plant, such seedlings
would be in one sense half-brothers or sisters, but more closely related
than are the half-brothers and sisters of ordinary animals. The flowers
on the mother-plant were, however, commonly crossed by pollen taken from
two or more distinct plants; and in these cases the seedlings might be
called with more truth half-brothers or sisters. When two or three
mother-plants were crossed, as often happened, by pollen taken from two
or three father-plants (the seeds being all intermingled), some of the
seedlings of the first generation would be in no way related, whilst
many others would be whole or half-brothers and sisters. In the second
generation a large number of the seedlings would be what may be called
whole or half first-cousins, mingled with whole and half-brothers and
sisters, and with some plants not at all related. So it would be in the
succeeding generations, but there would also be many cousins of the
second and more remote degrees. The relationship will thus have become
more and more inextricably complex in the later generations; with most
of the plants in some degree and many of them closely related.

I have only one other point to notice, but this is one of the highest
importance; namely, that the crossed and self-fertilised plants were
subjected in the same generation to as nearly similar and uniform
conditions as was possible. In the successive generations they were
exposed to slightly different conditions as the seasons varied, and they
were raised at different periods. But in other respects all were treated
alike, being grown in pots in the same artificially prepared soil, being
watered at the same time, and kept close together in the same greenhouse
or hothouse. They were therefore not exposed during successive years to
such great vicissitudes of climate as are plants growing out of doors.

ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS.

It has been objected to such experiments as mine, that covering plants
with a net, although only for a short time whilst in flower, may affect
their health and fertility. I have seen no such effect except in one
instance with a Myosotis, and the covering may not then have been the
real cause of injury. But even if the net were slightly injurious, and
certainly it was not so in any high degree, as I could judge by the
appearance of the plants and by comparing their fertility with that of
neighbouring uncovered plants, it would not have vitiated my
experiments; for in all the more important cases the flowers were
crossed as well as self-fertilised under a net, so that they were
treated in this respect exactly alike.

As it is impossible to exclude such minute pollen-carrying insects as
Thrips, flowers which it was intended to fertilise with their own pollen
may sometimes have been afterwards crossed with pollen brought by these
insects from another flower on the same plant; but as we shall hereafter
see, a cross of this kind does not produce any effect, or at most only a
slight one. When two or more plants were placed near one another under
the same net, as was often done, there is some real though not great
danger of the flowers which were believed to be self-fertilised being
afterwards crossed with pollen brought by Thrips from a distinct plant.
I have said that the danger is not great because I have often found that
plants which are self-sterile, unless aided by insects, remained sterile
when several plants of the same species were placed under the same net.
If, however, the flowers which had been presumably self-fertilised by me
were in any case afterwards crossed by Thrips with pollen brought from a
distinct plant, crossed seedlings would have been included amongst the
self-fertilised; but it should be especially observed that this
occurrence would tend to diminish and not to increase any superiority in
average height, fertility, etc., of the crossed over the self-fertilised
plants.

As the flowers which were crossed were never castrated, it is probable
or even almost certain that I sometimes failed to cross-fertilise them
effectually, and that they were afterwards spontaneously
self-fertilised. This would have been most likely to occur with
dichogamous species, for without much care it is not easy to perceive
whether their stigmas are ready to be fertilised when the anthers open.
But in all cases, as the flowers were protected from wind, rain, and the
access of insects, any pollen placed by me on the stigmatic surface
whilst it was immature, would generally have remained there until the
stigma was mature; and the flowers would then have been crossed as was
intended. Nevertheless, it is highly probable that self-fertilised
seedlings have sometimes by this means got included amongst the crossed
seedlings. The effect would be, as in the former case, not to exaggerate
but to diminish any average superiority of the crossed over the
self-fertilised plants.

Errors arising from the two causes just named, and from others,--such as
some of the seeds not having been thoroughly ripened, though care was
taken to avoid this error--the sickness or unperceived injury of any of
the plants,--will have been to a large extent eliminated, in those cases
in which many crossed and self-fertilised plants were measured and an
average struck. Some of these causes of error will also have been
eliminated by the seeds having been allowed to germinate on bare damp
sand, and being planted in pairs; for it is not likely that ill-matured
and well-matured, or diseased and healthy seeds, would germinate at
exactly the same time. The same result will have been gained in the
several cases in which only a few of the tallest, finest, and healthiest
plants on each side of the pots were measured.

Kolreuter and Gartner have proved that with some plants several, even as
many as from fifty to sixty, pollen-grains are necessary for the
fertilisation of all the ovules in the ovarium. (1/9. ‘Kentniss der
Befruchtung’ 1844 page 345. Naudin ‘Nouvelles Archives du Museum’ tome 1
page 27.) Naudin also found in the case of Mirabilis that if only one or
two of its very large pollen-grains were placed on the stigma, the
plants raised from such seeds were dwarfed. I was therefore careful to
give an amply sufficient supply of pollen, and generally covered the
stigma with it; but I did not take any special pains to place exactly
the same amount on the stigmas of the self-fertilised and crossed
flowers. After having acted in this manner during two seasons, I
remembered that Gartner thought, though without any direct evidence,
that an excess of pollen was perhaps injurious; and it has been proved
by Spallanzani, Quatrefages, and Newport, that with various animals an
excess of the seminal fluid entirely prevents fertilisation. (1/10.
‘Transactions of the Philosophical Society’ 1853 pages 253-258.) It was
therefore necessary to ascertain whether the fertility of the flowers
was affected by applying a rather small and an extremely large quantity
of pollen to the stigma. Accordingly a very small mass of pollen-grains
was placed on one side of the large stigma in sixty-four flowers of
Ipomoea purpurea, and a great mass of pollen over the whole surface of
the stigma in sixty-four other flowers. In order to vary the experiment,
half the flowers of both lots were on plants produced from
self-fertilised seeds, and the other half on plants from crossed seeds.
The sixty-four flowers with an excess of pollen yielded sixty-one
capsules; and excluding four capsules, each of which contained only a
single poor seed, the remainder contained on an average 5.07 seeds per
capsule. The sixty-four flowers with only a little pollen placed on one
side of the stigma yielded sixty-three capsules, and excluding one from
the same cause as before, the remainder contained on an average 5.129
seeds. So that the flowers fertilised with little pollen yielded rather
more capsules and seeds than did those fertilised with an excess; but
the difference is too slight to be of any significance. On the other
hand, the seeds produced by the flowers with an excess of pollen were a
little heavier of the two; for 170 of them weighed 79.67 grains, whilst
170 seeds from the flowers with very little pollen weighed 79.20 grains.
Both lots of seeds having been placed on damp sand presented no
difference in their rate of germination. We may therefore conclude that
my experiments were not affected by any slight difference in the amount
of pollen used; a sufficiency having been employed in all cases.

The order in which our subject will be treated in the present volume is
as follows. A long series of experiments will first be given in Chapters
2 to 6. Tables will afterwards be appended, showing in a condensed form
the relative heights, weights, and fertility of the offspring of the
various crossed and self-fertilised species. Another table exhibits the
striking results from fertilising plants, which during several
generations had either been self-fertilised or had been crossed with
plants kept all the time under closely similar conditions, with pollen
taken from plants of a distinct stock and which had been exposed to
different conditions. In the concluding chapters various related points
and questions of general interest will be discussed.

Anyone not specially interested in the subject need not attempt to read
all the details (marked []); though they possess, I think, some value,
and cannot be all summarised. But I would suggest to the reader to take
as an example the experiments on Ipomoea in Chapter 2; to which may be
added those on Digitalis, Origanum, Viola, or the common cabbage, as in
all these cases the crossed plants are superior to the self-fertilised
in a marked degree, but not in quite the same manner. As instances of
self-fertilised plants being equal or superior to the crossed, the
experiments on Bartonia, Canna, and the common pea ought to be read; but
in the last case, and probably in that of Canna, the want of any
superiority in the crossed plants can be explained.

Species were selected for experiment belonging to widely distinct
families, inhabiting various countries. In some few cases several genera
belonging to the same family were tried, and these are grouped together;
but the families themselves have been arranged not in any natural order,
but in that which was the most convenient for my purpose. The
experiments have been fully given, as the results appear to me of
sufficient value to justify the details. Plants bearing hermaphrodite
flowers can be interbred more closely than is possible with bisexual
animals, and are therefore well-fitted to throw light on the nature and
extent of the good effects of crossing, and on the evil effects of close
interbreeding or self-fertilisation. The most important conclusion at
which I have arrived is that the mere act of crossing by itself does no
good. The good depends on the individuals which are crossed differing
slightly in constitution, owing to their progenitors having been
subjected during several generations to slightly different conditions,
or to what we call in our ignorance spontaneous variation. This
conclusion, as we shall hereafter see, is closely connected with various
important physiological problems, such as the benefit derived from
slight changes in the conditions of life, and this stands in the closest
connection with life itself. It throws light on the origin of the two
sexes and on their separation or union in the same individual, and
lastly on the whole subject of hybridism, which is one of the greatest
obstacles to the general acceptance and progress of the great principle
of evolution.

In order to avoid misapprehension, I beg leave to repeat that throughout
this volume a crossed plant, seedling, or seed, means one of crossed
PARENTAGE, that is, one derived from a flower fertilised with pollen
from a distinct plant of the same species. And that a self-fertilised
plant, seedling, or seed, means one of self-fertilised PARENTAGE, that
is, one derived from a flower fertilised with pollen from the same
flower, or sometimes, when thus stated, from another flower on the same
plant.



CHAPTER II.

CONVOLVULACEAE.

Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.
Greater constitutional vigour of the crossed plants.
The effects on the offspring of crossing different flowers on the same
plant, instead of crossing distinct individuals.
The effects of a cross with a fresh stock.
The descendants of the self-fertilised plant named Hero.
Summary on the growth, vigour, and fertility of the successive crossed
and self-fertilised generations.
Small amount of pollen in the anthers of the self-fertilised plants of
the later generations, and the sterility of their first-produced
flowers.
Uniform colour of the flowers produced by the self-fertilised plants.
The advantage from a cross between two distinct plants depends on their
differing in constitution.

A plant of Ipomoea purpurea, or as it is often called in England the
convolvulus major, a native of South America, grew in my greenhouse. Ten
flowers on this plant were fertilised with pollen from the same flower;
and ten other flowers on the same plant were crossed with pollen from a
distinct plant. The fertilisation of the flowers with their own pollen
was superfluous, as this convolvulus is highly self-fertile; but I acted
in this manner to make the experiments correspond in all respects.
Whilst the flowers are young the stigma projects beyond the anthers; and
it might have been thought that it could not be fertilised without the
aid of humble-bees, which often visit the flowers; but as the flower
grows older the stamens increase in length, and their anthers brush
against the stigma, which thus receives some pollen. The number of seeds
produced by the crossed and self-fertilised flowers differed very
little.

[Crossed and self-fertilised seeds obtained in the above manner were
allowed to germinate on damp sand, and as often as pairs germinated at
the same time they were planted in the manner described in the
Introduction (Chapter 1), on the opposite sides of two pots. Five pairs
were thus planted; and all the remaining seeds, whether or not in a
state of germination, were planted on the opposite sides of a third pot,
so that the young plants on both sides were here greatly crowded and
exposed to very severe competition. Rods of iron or wood of equal
diameter were given to all the plants to twine up; and as soon as one of
each pair reached the summit both were measured. A single rod was placed
on each side of the crowded pot, Number 3, and only the tallest plant on
each side was measured.

TABLE 2/1. Ipomoea purpurea (First Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Seedlings from Crossed Plants.

Column 3: Seedlings from Self-fertilised Plants.

Pot 1 :  87 4/8 :  69.
Pot 1 :  87 4/8 :  66.
Pot 1 :  89     :  73.

Pot 2 :  88     :  68 4/8.
Pot 2 :  87     :  60 4/8.

Pot 3 :  77     :  57.
Plants crowded; the tallest one measured on each side.

Total : 516     : 394.

The average height of the six crossed plants is here 86 inches, whilst
that of the six self-fertilised plants is only 65.66 inches, so that the
crossed plants are to the self-fertilised in height as 100 to 76. It
should be observed that this difference is not due to a few of the
crossed plants being extremely tall, or to a few of the self-fertilised
being extremely short, but to all the crossed plants attaining a greater
height than their antagonists. The three pairs in Pot 1 were measured at
two earlier periods, and the difference was sometimes greater and
sometimes less than that at the final measuring. But it is an
interesting fact, of which I have seen several other instances, that one
of the self-fertilised plants, when nearly a foot in height, was half an
inch taller than the crossed plant; and again, when two feet high, it
was 1 3/8 of an inch taller, but during the ten subsequent days the
crossed plant began to gain on its antagonist, and ever afterward
asserted its supremacy, until it exceeded its self-fertilised opponent
by 16 inches.

The five crossed plants in Pots 1 and 2 were covered with a net, and
produced 121 capsules; the five self-fertilised plants produced
eighty-four capsules, so that the numbers of capsules were as 100 to 69.
Of the 121 capsules on the crossed plants sixty-five were the product of
flowers crossed with pollen from a distinct plant, and these contained
on an average 5.23 seeds per capsule; the remaining fifty-six capsules
were spontaneously self-fertilised. Of the eighty-four capsules on the
self-fertilised plants, all the product of renewed self-fertilisation,
fifty-five (which were alone examined) contained on an average 4.85
seeds per capsule. Therefore the cross-fertilised capsules, compared
with the self-fertilised capsules, yielded seeds in the proportion of
100 to 93. The crossed seeds were relatively heavier than the
self-fertilised seeds. Combining the above data (i.e., number of
capsules and average number of contained seeds), the crossed plants,
compared with the self-fertilised, yielded seeds in the ratio of 100 to
64.

These crossed plants produced, as already stated, fifty-six
spontaneously self-fertilised capsules, and the self-fertilised plants
produced twenty-nine such capsules. The former contained on an average,
in comparison with the latter, seeds in the proportion of 100 to 99.

In Pot 3, on the opposite sides of which a large number of crossed and
self-fertilised seeds had been sown and the seedlings allowed to
struggle together, the crossed plants had at first no great advantage.
At one time the tallest crossed was 25 1/8 inches high, and the tallest
self-fertilised plants 21 3/8. But the difference afterwards became much
greater. The plants on both sides, from being so crowded, were poor
specimens. The flowers were allowed to fertilise themselves
spontaneously under a net; the crossed plants produced thirty-seven
capsules, the self-fertilised plants only eighteen, or as 100 to 47. The
former contained on an average 3.62 seeds per capsule; and the latter
3.38 seeds, or as 100 to 93. Combining these data (i.e., number of
capsules and average number of seeds), the crowded crossed plants
produced seeds compared with the self-fertilised as 100 to 45. These
latter seeds, however, were decidedly heavier, a hundred weighing 41.64
grains, than those from the capsules on the crossed plants, of which a
hundred weighed 36.79 grains; and this probably was due to the fewer
capsules borne by the self-fertilised plants having been better
nourished. We thus see that the crossed plants in this the first
generation, when grown under favourable conditions, and when grown under
unfavourable conditions from being much crowded, greatly exceeded in
height, and in the number of capsules produced, and slightly in the
number of seeds per capsule, the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Flowers on the crossed plants of the last generation (Table 2/1) were
crossed by pollen from distinct plants of the same generation; and
flowers on the self-fertilised plants were fertilised by pollen from the
same flower. The seeds thus produced were treated in every respect as
before, and we have in Table 2/2 the result.

TABLE 2/2. Ipomoea purpurea (Second Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  87     :  67 4/8.
Pot 1 :  83     :  68 4/8.
Pot 1 :  83     :  80 4/8.

Pot 2 :  85 4/8 :  61 4/8.
Pot 2 :  89     :  79.
Pot 2 :  77 4/8 :  41.

Total : 505     : 398.

Here again every single crossed plant is taller than its antagonist. The
self-fertilised plant in Pot 1, which ultimately reached the unusual
height of 80 4/8 inches, was for a long time taller than the opposed
crossed plant, though at last beaten by it. The average height of the
six crossed plants is 84.16 inches, whilst that of the six
self-fertilised plants is 66.33 inches, or as 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Seeds from the crossed plants of the last generation (Table 2/2) again
crossed, and from the self-fertilised plants again self-fertilised, were
treated in all respects exactly as before, with the following result:--

TABLE 2/3. Ipomoea purpurea (Third Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  74     :  56 4/8.
Pot 1 :  72     :  51 4/8.
Pot 1 :  73 4/8 :  54.

Pot 2 :  82     :  59.
Pot 2 :  81     :  30.
Pot 2 :  82     :  66.

Total : 464.5   : 317.

Again all the crossed plants are higher than their antagonists: their
average height is 77.41 inches, whereas that of the self-fertilised is
52.83 inches, or as 100 to 68.

I attended closely to the fertility of the plants of this third
generation. Thirty flowers on the crossed plants were crossed with
pollen from other crossed plants of the same generation, and the
twenty-six capsules thus produced contained, on an average, 4.73 seeds;
whilst thirty flowers on the self-fertilised plants, fertilised with the
pollen from the same flower, produced twenty-three capsules, each
containing 4.43 seeds. Thus the average number of seeds in the crossed
capsules was to that in the self-fertilised capsules as 100 to 94. A
hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of
the self-fertilised seeds weighed only 37.63 grains. Many of these
lighter self-fertilised seeds placed on damp sand germinated before the
crossed; thus thirty-six of the former germinated whilst only thirteen
of the latter or crossed seeds germinated. In Pot 1 the three crossed
plants produced spontaneously under the net (besides the twenty-six
artificially cross-fertilised capsules) seventy-seven self-fertilised
capsules containing on an average 4.41 seeds; whilst the three
self-fertilised plants produced spontaneously (besides the twenty-three
artificially self-fertilised capsules) only twenty-nine self-fertilised
capsules, containing on an average 4.14 seeds. Therefore the average
number of seeds in the two lots of spontaneously self-fertilised
capsules was as 100 to 94. Taking into consideration the number of
capsules together with the average number of seeds, the crossed plants
(spontaneously self-fertilised) produced seeds in comparison with the
self-fertilised plants (spontaneously self-fertilised) in the proportion
of 100 to 35. By whatever method the fertility of these plants is
compared, the crossed are more fertile than the self-fertilised plants.

I tried in several ways the comparative vigour and powers of growth of
the crossed and self-fertilised plants of this third generation. Thus,
four self-fertilised seeds which had just germinated were planted on one
side of a pot, and after an interval of forty-eight hours, four crossed
seeds in the same state of germination were planted on the opposite
side; and the pot was kept in the hothouse. I thought that the advantage
thus given to the self-fertilised seedlings would have been so great
that they would never have been beaten by the crossed ones. They were
not beaten until all had grown to a height of 18 inches; and the degree
to which they were finally beaten is shown in Table 2/4. We here see
that the average height of the four crossed plants is 76.62, and of the
four self-fertilised plants 65.87 inches, or as 100 to 86; therefore
less than when both sides started fair.

TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised
plants having had a start of forty-eight hours).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 3 :  78 4/8 :  73 4/8.
Pot 3 :  77 4/8 :  53.
Pot 3 :  73     :  61 4/8.
Pot 3 :  77 4/8 :  75 4/8.

Total : 306.5   : 263.5.

Crossed and self-fertilised seeds of the third generation were also sown
out of doors late in the summer, and therefore under unfavourable
conditions, and a single stick was given to each lot of plants to twine
up. The two lots were sufficiently separate so as not to interfere with
each other’s growth, and the ground was clear of weeds. As soon as they
were killed by the first frost (and there was no difference in their
hardiness), the two tallest crossed plants were found to be 24.5 and
22.5 inches, whilst the two tallest self-fertilised plants were only 15
and 12.5 inches in height, or as 100 to 59.

I likewise sowed at the same time two lots of the same seeds in a part
of the garden which was shady and covered with weeds. The crossed
seedlings from the first looked the most healthy, but they twined up a
stick only to a height of 7 1/4 inches; whilst the self-fertilised were
not able to twine at all; and the tallest of them was only 3 1/2 inches
in height.

Lastly, two lots of the same seeds were sown in the midst of a bed of
candy-tuft (Iberis) growing vigorously. The seedlings came up, but all
the self-fertilised ones soon died excepting one, which never twined and
grew to a height of only 4 inches. Many of the crossed seedlings, on the
other hand, survived; and some twined up the stems of the Iberis to the
height of 11 inches. These cases prove that the crossed seedlings have
an immense advantage over the self-fertilised, both when growing
isolated under very unfavourable conditions, and when put into
competition with each other or with other plants, as would happen in a
state of nature.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seedlings raised as before from the crossed and self-fertilised plants
of the third generation in Table 2/3, gave results as follows:--

TABLE 2/5. Ipomoea purpurea (Fourth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  84       :  80.
Pot 1 :  47       :  44 1/2.

Pot 2 :  83       :  73 1/2.
Pot 2 :  59       :  51 1/2.

Pot 3 :  82       :  56 1/2.
Pot 3 :  65 1/2   :  63.
Pot 3 :  68       :  52.

Total : 488.5     : 421.0.

Here the average height of the seven crossed plants is 69.78 inches, and
that of the seven self-fertilised plants 60.14; or as 100 to 86. This
smaller difference relatively to that in the former generations, may be
attributed to the plants having been raised during the depth of winter,
and consequently to their not having grown vigorously, as was shown by
their general appearance and from several of them never reaching the
summits of the rods. In Pot 2, one of the self-fertilised plants was for
a long time taller by two inches than its opponent, but was ultimately
beaten by it, so that all the crossed plants exceeded their opponents in
height. Of twenty-eight capsules produced by the crossed plants
fertilised by pollen from a distinct plant, each contained on an average
4.75 seeds; of twenty-seven self-fertilised capsules on the
self-fertilised plants, each contained on an average 4.47 seeds; so that
the proportion of seeds in the crossed and self-fertilised capsules was
as 100 to 94.

Some of the same seeds, from which the plants in Table 2/5 had been
raised, were planted, after they had germinated on damp sand, in a
square tub, in which a large Brugmansia had long been growing. The soil
was extremely poor and full of roots; six crossed seeds were planted in
one corner, and six self-fertilised seeds in the opposite corner. All
the seedlings from the latter soon died excepting one, and this grew to
the height of only 1 1/2 inches. Of the crossed plants three survived,
and they grew to the height of 2 1/2 inches, but were not able to twine
round a stick; nevertheless, to my surprise, they produced some small
miserable flowers. The crossed plants thus had a decided advantage over
the self-fertilised plants under this extremity of bad conditions.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

These were raised in the same manner as before, and when measured gave
the following results:--

TABLE 2/6. Ipomoea purpurea (Fifth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  96        :  73.
Pot 1 :  86        :  78.
Pot 1 :  69        :  29.

Pot 2 :  84        :  51.
Pot 2 :  84        :  84.
Pot 2 :  76 1/4    :  59.

Total : 495.25     : 374.00.

The average height of the six crossed plants is 82.54 inches, and that
of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every
crossed plant exceeded its antagonist in height. In Pot 1 the middle
plant on the crossed side was slightly injured whilst young by a blow,
and was for a time beaten by its opponent, but ultimately recovered the
usual superiority. The crossed plants produced spontaneously a vast
number more capsules than did the self-fertilised plants; and the
capsules of the former contained on an average 3.37 seeds, whilst those
of the latter contained only 3.0 per capsule, or as 100 to 89. But
looking only to the artificially fertilised capsules, those on the
crossed plants again crossed contained on an average 4.46 seeds, whilst
those on the self-fertilised plants again self-fertilised contained 4.77
seeds; so that the self-fertilised capsules were the more fertile of the
two, and of this unusual fact I can offer no explanation.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

These were raised in the usual manner, with the following result. I
should state that there were originally eight plants on each side; but
as two of the self-fertilised became extremely unhealthy and never grew
to near their full height, these as well as their opponents have been
struck out of the list. If they had been retained, they would have made
the average height of the crossed plants unfairly greater than that of
the self-fertilised. I have acted in the same manner in a few other
instances, when one of a pair plainly became very unhealthy.

TABLE 2/7.  Ipomoea purpurea (Sixth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  93     :  50 1/2.
Pot 1 :  91     :  65.

Pot 2 :  79     :  50.
Pot 2 :  86 1/2 :  87.
Pot 2 :  88     :  62.

Pot 3 :  87 1/2 :  64 1/2.

Total : 525     : 379.

The average height of the six crossed plants is here 87.5, and of the
six self-fertilised plants 63.16, or as 100 to 72. This large difference
was chiefly due to most of the plants, especially the self-fertilised
ones, having become unhealthy towards the close of their growth, and
they were severely attacked by aphides. From this cause nothing can be
inferred with respect to their relative fertility. In this generation we
have the first instance of a self-fertilised plant in Pot 2 exceeding
(though only by half an inch) its crossed opponent. This victory was
fairly won after a long struggle. At first the self-fertilised plant was
several inches taller than its opponent, but when the latter was 4 1/2
feet high it had grown equal; it then grew a little taller than the
self-fertilised plant, but was ultimately beaten by it to the extent of
half an inch, as shown in Table 2/7. I was so much surprised at this
case that I saved the self-fertilised seeds of this plant, which I will
call the “Hero,” and experimented on its descendants, as will hereafter
be described.

Besides the plants included in Table 2/7, nine crossed and nine
self-fertilised plants of the same lot were raised in two other pots, 4
and 5. These pots had been kept in the hothouse, but from want of room
were, whilst the plants were young, suddenly moved during very cold
weather into the coldest part of the greenhouse. They all suffered
greatly, and never quite recovered. After a fortnight only two of the
nine self-fertilised seedlings were alive, whilst seven of the crossed
survived. The tallest of these latter plants when measured was 47 inches
in height, whilst the tallest of the two surviving self-fertilised
plants was only 32 inches. Here again we see how much more vigorous the
crossed plants are than the self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.

These were raised as heretofore with the following result:--

TABLE 2/8. Ipomoea purpurea (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  84 4/8    :  74 6/8.
Pot 1 :  84 6/8    :  84.
Pot 1 :  76 2/8    :  55 4/8.

Pot 2 :  84 4/8    :  65.
Pot 2 :  90        :  51 2/8.
Pot 2 :  82 2/8    :  80 4/8.

Pot 3 :  83        :  67 6/8.
Pot 3 :  86        :  60 2/8.

Pot 4 :  84 2/8    :  75 2/8.

Total : 755.50     : 614.25.

Each of these nine crossed plants is higher than its opponent, though in
one case only by three-quarters of an inch. Their average height is
83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to
81. These plants, after growing to their full height, became very
unhealthy and infested with aphides, just when the seeds were setting,
so that many of the capsules failed, and nothing can be said on their
relative fertility.

CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.

As just stated, the plants of the last generation, from which the
present ones were raised, were very unhealthy and their seeds of
unusually small size; and this probably accounts for the two lots
behaving differently to what they did in any of the previous or
succeeding generations. Many of the self-fertilised seeds germinated
before the crossed ones, and these were of course rejected. When the
crossed seedlings in Table 2/9 had grown to a height of between 1 and 2
feet, they were all, or almost all, shorter than their self-fertilised
opponents, but were not then measured. When they had acquired an average
height of 32.28 inches, that of the self-fertilised plants was 40.68, or
as 100 to 122. Moreover, every one of the self-fertilised plants, with a
single exception, exceeded its crossed opponent. When, however, the
crossed plants had grown to an average height of 77.56 inches, they just
exceeded (namely, by .7 of an inch) the average height of the
self-fertilised plants; but two of the latter were still taller than
their crossed opponents. I was so much astonished at this whole case,
that I tied string to the summits of the rods; the plants being thus
allowed to continue climbing upwards. When their growth was complete
they were untwined, stretched straight, and measured. The crossed plants
had now almost regained their accustomed superiority, as may be seen in
Table 2/9.

The average height of the eight crossed plants is here 113.25 inches,
and that of the self-fertilised plants 96.65, or as 100 to 85.
Nevertheless two of the self-fertilised plants, as may be seen in Table
2/9, were still higher than their crossed opponents. The latter
manifestly had much thicker stems and many more lateral branches, and
looked altogether more vigorous than the self-fertilised plants, and
generally flowered before them. The earlier flowers produced by these
self-fertilised plants did not set any capsules, and their anthers
contained only a small amount of pollen; but to this subject I shall
return. Nevertheless capsules produced by two other self-fertilised
plants of the same lot, not included in Table 2/9, which had been highly
favoured by being grown in separate pots, contained the large average
number of 5.1 seeds per capsule.

TABLE 2/9. Ipomoea purpurea (Eighth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 111 6/8 :  96.
Pot 1 : 127     :  54.
Pot 1 : 130 6/8 :  93 4/8.

Pot 2 :  97 2/8 :  94.
Pot 2 :  89 4/8 : 125 6/8.

Pot 3 : 103 6/8 : 115 4/8.
Pot 3 : 100 6/8 :   84 6/8.
Pot 3 : 147 4/8 : 109 6/8.

Total : 908.25  : 773.25.

CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.

The plants of this generation were raised in the same manner as before,
with the result shown in Table 2/10.

The fourteen crossed plants average in height 81.39 inches and the
fourteen self-fertilised plants 64.07, or as 100 to 79. One
self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in
height, its opponent. The self-fertilised plants showed no sign of
inheriting the precocious growth of their parents; this having been due,
as it would appear, to the abnormal state of the seeds from the
unhealthiness of their parents. The fourteen self-fertilised plants
yielded only forty spontaneously self-fertilised capsules, to which must
be added seven, the product of ten flowers artificially self-fertilised.
On the other hand, the fourteen crossed plants yielded 152 spontaneously
self-fertilised capsules; but thirty-six flowers on these plants were
crossed (yielding thirty-three capsules), and these flowers would
probably have produced about thirty spontaneously self-fertilised
capsules. Therefore an equal number of the crossed and self-fertilised
plants would have produced capsules in the proportion of about 182 to
47, or as 100 to 26. Another phenomenon was well pronounced in this
generation, but I believe had occurred previously to a slight extent;
namely, that most of the flowers on the self-fertilised plants were
somewhat monstrous. The monstrosity consisted in the corolla being
irregularly split so that it did not open properly, with one or two of
the stamens slightly foliaceous, coloured, and firmly coherent to the
corolla. I observed this monstrosity in only one flower on the crossed
plants. The self-fertilised plants, if well nourished, would almost
certainly, in a few more generations, have produced double flowers, for
they had already become in some degree sterile. (2/1. See on this
subject ‘Variation of Animals and Plants under Domestication’ chapter 18
2nd edition volume 2 page 152.)

TABLE 2/10. Ipomoea purpurea (Ninth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  83 4/8    :  57.
Pot 1 :  85 4/8    :  71.
Pot 1 :  83 4/8    :  48 3/8.

Pot 2 :  83 2/8    :  45.
Pot 2 :  64 2/8    :  43 6/8.
Pot 2 :  64 3/8    :  38 4/8.

Pot 3 :  79        :  63.
Pot 3 :  88 1/8    :  71.
Pot 3 :  61        :  89 4/8.

Pot 4 :  82 4/8    :  82 4/8.
Pot 4 :  90        :  76 1/8.

Pot 5 :  89 4/8    :  67.
Pot 5 :  92 4/8    :  74 2/8.
Pot 5 :  92 4/8    :  70.
Crowded plants.

Total : 1139.5     : 897.0.

CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.

Six plants were raised in the usual manner from the crossed plants of
the last generation (Table 2/10) again intercrossed, and from the
self-fertilised again self-fertilised. As one of the crossed plants in
Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and
producing hardly any capsules, it and its opponent have been struck out
of the table.

TABLE 2/11. Ipomoea purpurea (Tenth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 :  92 3/8   :  47 2/8.
Pot 1 :  94 4/8   :  34 6/8.

Pot 2 :  87       :  54 4/8.
Pot 2 :  89 5/8   :  49 2/8.
Pot 2 : 105       :  66 2/8.

Total : 468.5     : 252.0.

The five crossed plants average 93.7 inches, and the five
self-fertilised only 50.4, or as 100 to 54. This difference, however, is
so great that it must be looked at as in part accidental. The six
crossed plants (the diseased one here included) yielded spontaneously
101 capsules, and the six self-fertilised plants 88, the latter being
chiefly produced by one of the plants. But as the diseased plant, which
yielded hardly any seed, is here included, the ratio of 101 to 88 does
not fairly give the relative fertility of the two lots. The stems of the
six crossed plants looked so much finer than those of the six
self-fertilised plants, that after the capsules had been gathered and
most of the leaves had fallen off, they were weighed. Those of the
crossed plants weighed 2,693 grains, whilst those of the self-fertilised
plants weighed only 1,173 grains, or as 100 to 44; but as the diseased
and dwarfed crossed plant is here included, the superiority of the
former in weight was really greater.]

THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME
PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

In all the foregoing experiments, seedlings from flowers crossed by
pollen from a distinct plant (though in the later generations more or
less closely related) were put into competition with, and almost
invariably proved markedly superior in height to the offspring from
self-fertilised flowers. I wished, therefore, to ascertain whether a
cross between two flowers on the same plant would give to the offspring
any superiority over the offspring from flowers fertilised with their
own pollen. I procured some fresh seed and raised two plants, which were
covered with a net; and several of their flowers were crossed with
pollen from a distinct flower on the same plant. Twenty-nine capsules
thus produced contained on an average 4.86 seeds per capsule; and 100 of
these seeds weighed 36.77 grains. Several other flowers were fertilised
with their own pollen, and twenty-six capsules thus produced contained
on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains.
So that a cross of this kind appears to have increased slightly the
number of seeds per capsule, in the ratio of 100 to 91; but these
crossed seeds were lighter than the self-fertilised in the ratio of 86
to 100. I doubt, however, from other observations, whether these results
are fully trustworthy. The two lots of seeds, after germinating on sand,
were planted in pairs on the opposite sides of nine pots, and were
treated in every respect like the plants in the previous experiments.
The remaining seeds, some in a state of germination and some not so,
were sown on the opposite sides of a large pot (Number 10); and the four
tallest plants on each side of this pot were measured. The result is
shown in Table 2/12.

TABLE 2/12. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   82     :   77 4/8.
Pot 1  :   75     :   87.
Pot 1  :   65     :   64.
Pot 1  :   76     :   87 2/8.

Pot 2  :   78 4/8 :   84.
Pot 2  :   43     :   86 4/8.
Pot 2  :   65 4/8 :   90 4/8.

Pot 3  :   61 2/8 :   86.
Pot 3  :   85     :   69 4/8.
Pot 3  :   89     :   87 4/8.

Pot 4  :   83     :   80 4/8.
Pot 4  :   73 4/8 :   88 4/8.
Pot 4  :   67     :   84 4/8.

Pot 5  :   78     :   66 4/8.
Pot 5  :   76 6/8 :   77 4/8.
Pot 5  :   57     :   81 4/8.

Pot 6  :   70 4/8 :   80.
Pot 6  :   79     :   82 4/8.
Pot 6  :   79 6/8 :   55 4/8.

Pot 7  :   76     :   77.
Pot 7  :   84 4/8 :   83 4/8.
Pot 7  :   79     :   73 4/8.

Pot 8  :   73     :   76 4/8.
Pot 8  :   67     :   82.
Pot 8  :   83     :   80 4/8.

Pot 9  :   73 2/8 :   78 4/8.
Pot 9  :   78     :   67 4/8.

Pot 10 :   34     :   82 4/8.
Pot 10 :   82     :   36 6/8.
Pot 10 :   84 6/8 :   69 4/8.
Pot 10 :   71     :   75 2/8.
Crowded plants.

Total  : 2270.25  : 2399.75.

The average height of the thirty-one crossed plants is 73.23 inches, and
that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to
106. Looking to each pair, it may be seen that only thirteen of the
crossed plants, whilst eighteen of the self-fertilised plants exceed
their opponents. A record was kept with respect to the plant which
flowered first in each pot; and only two of the crossed flowered before
one of the self-fertilised in the same pot; whilst eight of the
self-fertilised flowered first. It thus appears that the crossed plants
are slightly inferior in height and in earliness of flowering to the
self-fertilised. But the inferiority in height is so small, namely as
100 to 106, that I should have felt very doubtful on this head, had I
not cut down all the plants (except those in the crowded pot Number 10)
close to the ground and weighed them. The twenty-seven crossed plants
weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20
1/2 ounces; and this gives a ratio of 100 to 124.

A self-fertilised plant of the same parentage as those in Table 2/12 had
been raised in a separate pot for a distinct purpose; and it proved
partially sterile, the anthers containing very little pollen. Several
flowers on this plant were crossed with the little pollen which could be
obtained from the other flowers on the same plant; and other flowers
were self-fertilised. From the seeds thus produced four crossed and four
self-fertilised plants were raised, which were planted in the usual
manner on the opposite sides of two pots. All these four crossed plants
were inferior in height to their opponents; they averaged 78.18 inches,
whilst the four self-fertilised plants averaged 84.8 inches; or as 100
to 108. (2/2. From one of these self-fertilised plants, spontaneously
self-fertilised, I gathered twenty-four capsules, and they contained on
an average only 3.2 seeds per capsule; so that this plant had apparently
inherited some of the sterility of its parent.) This case, therefore,
confirms the last. Taking all the evidence together, we must conclude
that these strictly self-fertilised plants grew a little taller, were
heavier, and generally flowered before those derived from a cross
between two flowers on the same plant. These latter plants thus present
a wonderful contrast with those derived from a cross between two
distinct individuals.

THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK
BELONGING TO THE SAME VARIETY.

From the two foregoing series of experiments we see, firstly, the good
effects during several successive generations of a cross between
distinct plants, although these were in some degree inter-related and
had been grown under nearly the same conditions; and, secondly, the
absence of all such good effects from a cross between flowers on the
same plant; the comparison in both cases being made with the offspring
of flowers fertilised with their own pollen. The experiments now to be
given show how powerfully and beneficially plants, which have been
intercrossed during many successive generations, having been kept all
the time under nearly uniform conditions, are affected by a cross with
another plant belonging to the same variety, but to a distinct family or
stock, which had grown under different conditions.

[Several flowers on the crossed plants of the ninth generation in Table
2/10, were crossed with pollen from another crossed plant of the same
lot. The seedlings thus raised formed the tenth intercrossed generation,
and I will call them the “INTERCROSSED PLANTS.” Several other flowers on
the same crossed plants of the ninth generation were fertilised (not
having been castrated) with pollen taken from plants of the same
variety, but belonging to a distinct family, which had been grown in a
distant garden at Colchester, and therefore under somewhat different
conditions. The capsules produced by this cross contained, to my
surprise, fewer and lighter seeds than did the capsules of the
intercrossed plants; but this, I think, must have been accidental. The
seedlings raised from them I will call the “COLCHESTER-CROSSED.” The two
lots of seeds, after germinating on sand, were planted in the usual
manner on the opposite sides of five pots, and the remaining seeds,
whether or not in a state of germination, were thickly sown on the
opposite sides of a very large pot, Number 6 in Table 2/13. In three of
the six pots, after the young plants had twined a short way up their
sticks, one of the Colchester-crossed plants was much taller than any
one of the intercrossed plants on the opposite side of the same pot; and
in the three other pots somewhat taller. I should state that two of the
Colchester-crossed plants in Pot 4, when about two-thirds grown, became
much diseased, and were, together with their intercrossed opponents,
rejected. The remaining nineteen plants, when almost fully grown, were
measured, with the following result:

TABLE 2/13. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Colchester-Crossed Plants.

Column 3: Intercrossed Plants of the Tenth Generation.

Pot 1 :   87      :   78.
Pot 1 :   87 4/8  :   68 4/8.
Pot 1 :   85 1/8  :   94 4/8.

Pot 2 :   93 6/8  :   60.
Pot 2 :   85 4/8  :   87 2/8.
Pot 2 :   90 5/8  :   45 4/8.

Pot 3 :   84 2/8  :   70 1/8.
Pot 3 :   92 4/8  :   81 6/8.
Pot 3 :   85      :   86 2/8.

Pot 4 :   95 6/8  :   65 1/8.

Pot 5 :   90 4/8  :   85 6/8.
Pot 5 :   86 6/8  :   63.
Pot 5 :   84      :   62 6/8.

Pot 6 :   90 4/8  :   43 4/8.
Pot 6 :   75      :   39 6/8.
Pot 6 :   71      :   30 2/8.
Pot 6 :   83 6/8  :   86.
Pot 6 :   63      :   53.
Pot 6 :   65      :   48 6/8.
Crowded plants in a very large pot.

Total : 1596.50   : 1249.75.

In sixteen out of these nineteen pairs, the Colchester-crossed plant
exceeded in height its intercrossed opponent. The average height of the
Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78
inches; or as 100 to 78. With respect to the fertility of the two lots,
it was too troublesome to collect and count the capsules on all the
plants; so I selected two of the best pots, 5 and 6, and in these the
Colchester-crossed produced 269 mature and half-mature capsules, whilst
an equal number of the intercrossed plants produced only 154 capsules;
or as 100 to 57. By weight the capsules from the Colchester-crossed
plants were to those from the intercrossed plants as 100 to 51; so that
the former probably contained a somewhat larger average number of
seeds.]

We learn from this important experiment that plants in some degree
related, which had been intercrossed during the nine previous
generations, when they were fertilised with pollen from a fresh stock,
yielded seedlings as superior to the seedlings of the tenth intercrossed
generation, as these latter were to the self-fertilised plants of the
corresponding generation. For if we look to the plants of the ninth
generation in Table 2/10 (and these offer in most respects the fairest
standard of comparison) we find that the intercrossed plants were in
height to the self-fertilised as 100 to 79, and in fertility as 100 to
26; whilst the Colchester-crossed plants are in height to the
intercrossed as 100 to 78, and in fertility as 100 to 51.

[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH
APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.

In the five generations before the sixth, the crossed plant of each pair
was taller than its self-fertilised opponent; but in the sixth
generation (Table 2/7, Pot 2) the Hero appeared, which after a long and
dubious struggle conquered its crossed opponent, though by only half an
inch. I was so much surprised at this fact, that I resolved to ascertain
whether this plant would transmit its powers of growth to its seedlings.
Several flowers on Hero were therefore fertilised with their own pollen,
and the seedlings thus raised were put into competition with
self-fertilised and intercrossed plants of the corresponding generation.
The three lots of seedlings thus all belong to the seventh generation.
Their relative heights are shown in Tables 2/14 and 2/15.

TABLE 2/14. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.

Column 3: Self-fertilised Plants of the Seventh Generation.

Pot 1 :  74     :  89 4/8.
Pot 1 :  60     :  61.
Pot 1 :  55 2/8 :  49.

Pot 2 :  92     :  82.
Pot 2 :  91 6/8 :  56.
Pot 2 :  74 2/8 :  38.

Total : 447.25  : 375.50.

The average height of the six self-fertilised children of Hero is 74.54
inches, whilst that of the ordinary self-fertilised plants of the
corresponding generation is only 62.58 inches, or as 100 to 84.

TABLE 2/15. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.

Column 3: Intercrossed Plants of the Seventh Generation.

Pot 3 :  92     :  76 6/8.

Pot 4 :  87     :  89.
Pot 4 :  87 6/8 :  86 6/8.

Total : 266.75  : 252.50.

Here the average height of the three self-fertilised children of Hero is
88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100
to 95. We thus see that the self-fertilised children of Hero certainly
inherit the powers of growth of their parents; for they greatly exceed
in height the self-fertilised offspring of the other self-fertilised
plants, and even exceed by a trifle the intercrossed plants,--all of the
corresponding generation.

Several flowers on the self-fertilised children of Hero in Table 2/14
were fertilised with pollen from the same flower; and from the seeds
thus produced, self-fertilised plants of the eighth generation
(grandchildren of Hero) were raised. Several other flowers on the same
plants were crossed with pollen from the other children of Hero. The
seedlings raised from this cross may be considered as the offspring of
the union of brothers and sisters. The result of the competition between
these two sets of seedlings (namely self-fertilised and the offspring of
brothers and sisters) is given in Table 2/16.

TABLE 2/16. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Grandchildren of Hero, from the
Self-fertilised Children. Eighth Generation.

Column 3:  Grandchildren from a cross between the self-fertilised
children of Hero. Eighth Generation.

Pot 1 :   86 6/8  :  95 6/8.
Pot 1 :   90 3/8  :  95 3/8.

Pot 2 :   96      :  85.
Pot 2 :   77 2/8  :  93.

Pot 3 :   73      :  86 2/8.
Pot 3 :   66      :  82 2/8.
Pot 3 :   84 4/8  :  70 6/8.

Pot 4 :   88 1/8  :  66 3/8.
Pot 4 :   84      :  15 4/8.
Pot 4 :   36 2/8  :  38.
Pot 4 :   74      :  78 3/8.

Pot 5 :   90 1/8  :  82 6/8.
Pot 5 :   90 5/8  :  83 6/8.

Total : 1037.00   : 973.16.

The average height of the thirteen self-fertilised grandchildren of Hero
is 79.76 inches, and that of the grandchildren from a cross between the
self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of
the crossed plants grew only to a height of 15 1/2 inches; and if this
plant and its opponent are struck out, as would be the fairest plan, the
average height of the crossed plants exceeds only by a fraction of an
inch that of the self-fertilised plants. It is therefore clear that a
cross between the self-fertilised children of Hero did not produce any
beneficial effect worth notice; and it is very doubtful whether this
negative result can be attributed merely to the fact of brothers and
sisters having been united, for the ordinary intercrossed plants of the
several successive generations must often have been derived from the
union of brothers and sisters (as shown in Chapter 1), and yet all of
them were greatly superior to the self-fertilised plants. We are
therefore driven to the suspicion, which we shall soon see strengthened,
that Hero transmitted to its offspring a peculiar constitution adapted
for self-fertilisation.

It would appear that the self-fertilised descendants of Hero have not
only inherited from Hero a power of growth equal to that of the ordinary
intercrossed plants, but have become more fertile when self-fertilised
than is usual with the plants of the present species. The flowers on the
self-fertilised grandchildren of Hero in Table 2.16 (the eighth
generation of self-fertilised plants) were fertilised with their own
pollen and produced plenty of capsules, ten of which (though this is too
few a number for a safe average) contained 5.2 seeds per capsule,--a
higher average than was observed in any other case with the
self-fertilised plants. The anthers produced by these self-fertilised
grandchildren were also as well developed and contained as much pollen
as those on the intercrossed plants of the corresponding generation;
whereas this was not the case with the ordinary self-fertilised plants
of the later generations. Nevertheless some few of the flowers produced
by the grandchildren of Hero were slightly monstrous, like those of the
ordinary self-fertilised plants of the later generations. In order not
to recur to the subject of fertility, I may add that twenty-one
self-fertilised capsules, spontaneously produced by the
great-grandchildren of Hero (forming the ninth generation of
self-fertilised plants), contained on an average 4.47 seeds; and this is
as high an average as the self-fertilised flowers of any generation
usually yielded.

Several flowers on the self-fertilised grandchildren of Hero in Table
2/16 were fertilised with pollen from the same flower; and the seedlings
raised from them (great-grandchildren of Hero) formed the ninth
self-fertilised generation. Several other flowers were crossed with
pollen from another grandchild, so that they may be considered as the
offspring of brothers and sisters, and the seedlings thus raised may be
called the INTERCROSSED great-grandchildren. And lastly, other flowers
were fertilised with pollen from a distinct stock, and the seedlings
thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In
my anxiety to see what the result would be, I unfortunately planted the
three lots of seeds (after they had germinated on sand) in the hothouse
in the middle of winter, and in consequence of this the seedlings
(twenty in number of each kind) became very unhealthy, some growing only
a few inches in height, and very few to their proper height. The result,
therefore, cannot be fully trusted; and it would be useless to give the
measurements in detail. In order to strike as fair an average as
possible, I first excluded all the plants under 50 inches in height,
thus rejecting all the most unhealthy plants. The six self-fertilised
thus left were on an average 66.86 inches high; the eight intercrossed
plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that
there was not much difference between the three sets, the
self-fertilised plants having a slight advantage. Nor was there any
great difference when only the plants under 36 inches in height were
excluded. Nor again when all the plants, however much dwarfed and
unhealthy, were included. In this latter case the Colchester-crossed
gave the lowest average of all; and if these plants had been in any
marked manner superior to the other two lots, as from my former
experience I fully expected they would have been, I cannot but think
that some vestige of such superiority would have been evident,
notwithstanding the very unhealthy condition of most of the plants. No
advantage, as far as we can judge, was derived from intercrossing two of
the grandchildren of Hero, any more than when two of the children were
crossed. It appears therefore that Hero and its descendants have varied
from the common type, not only in acquiring great power of growth, and
increased fertility when subjected to self-fertilisation, but in not
profiting from a cross with a distinct stock; and this latter fact, if
trustworthy, is a unique case, as far as I have observed in all my
experiments.]

SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE
GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea
purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.

In Table 2/17, we see the average or mean heights of the ten successive
generations of the intercrossed and self-fertilised plants, grown in
competition with each other; and in the right hand column we have the
ratios of the one to the other, the height of the intercrossed plants
being taken at 100. In the bottom line the mean height of the
seventy-three intercrossed plants is shown to be 85.84 inches, and that
of the seventy-three self-fertilised plants 66.02 inches, or as 100 to
77.

TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten
generations.

Heights of Plants in inches:

Column 1: Name of Generation.

Column 2: Number of Crossed Plants.

Column 3: Average Height of Crossed Plants.

Column 4: Number of Self-fertilised Plants.

Column 5: Average Height of Self-fertilised Plants.

Column 6: n in Ratio between Average Heights of Crossed and
Self-fertilised Plants, expressed as 100 to n.

First generation Table 2/1   :  6 :  86.00 :  6 : 65.66 : 76.

Second generation Table 2/2  :  6 :  84.16 :  6 : 66.33 : 79.

Third generation Table 2/3   :  6 :  77.41 :  6 : 52.83 : 68.

Fourth generation Table 2/5  :  7 :  69.78 :  7 : 60.14 : 86.

Fifth generation Table 2/6   :  6 :  82.54 :  6 : 62.33 : 75.

Sixth generation Table 2/7   :  6 :  87.50 :  6 : 63.16 : 72.

Seventh generation Table 2/8 :  9 :  83.94 :  9 : 68.25 : 81.

Eighth generation Table 2/9  :  8 : 113.25 :  8 : 96.65 : 85.

Ninth generation Table 2/10  : 14 :  81.39 : 14 : 64.07 : 79.

Tenth generation Table 2/11  :  5 :  93.70 :  5 : 50.40 : 54.

All ten generations together : 73 :  85.84 : 73 : 66.02 : 77.

(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and
self-fertilised plants of Ipomoea purpurea in the ten generations; the
mean height of the crossed plants being taken as 100. On the right hand,
the mean heights of the crossed and self-fertilised plants of all the
generations taken together are shown (as eleven pairs of unequal
vertical lines.))

The mean height of the self-fertilised plants in each of the ten
generations is also shown in the diagram 2/1, that of the intercrossed
plants being taken at 100, and on the right side we see the relative
heights of the seventy-three intercrossed plants, and of the
seventy-three self-fertilised plants. The difference in height between
the crossed and self-fertilised plants will perhaps be best appreciated
by an illustration: If all the men in a country were on an average 6
feet high, and there were some families which had been long and closely
interbred, these would be almost dwarfs, their average height during ten
generations being only 4 feet 8 1/4 inches.

It should be especially observed that the average difference between the
crossed and self-fertilised plants is not due to a few of the former
having grown to an extraordinary height, or to a few of the
self-fertilised being extremely short, but to all the crossed plants
having surpassed their self-fertilised opponents, with the few following
exceptions. The first occurred in the sixth generation, in which the
plant named “Hero” appeared; two in the eighth generation, but the
self-fertilised plants in this generation were in an anomalous
condition, as they grew at first at an unusual rate and conquered for a
time the opposed crossed plants; and two exceptions in the ninth
generation, though one of these plants only equalled its crossed
opponent. Therefore, of the seventy-three crossed plants, sixty-eight
grew to a greater height than the self-fertilised plants, to which they
were opposed.

In the right-hand column of figures, the difference in height between
the crossed and self-fertilised plants in the successive generations is
seen to fluctuate much, as might indeed have been expected from the
small number of plants measured in each generation being insufficient to
give a fair average. It should be remembered that the absolute height of
the plants goes for nothing, as each pair was measured as soon as one of
them had twined up to the summit of its rod. The great difference in the
tenth generation, namely, 100 to 54, no doubt was partly accidental,
though, when these plants were weighed, the difference was even greater,
namely, 100 to 44. The smallest amount of difference occurred in the
fourth and the eighth generations, and this was apparently due to both
the crossed and self-fertilised plants having become unhealthy, which
prevented the former attaining their usual degree of superiority. This
was an unfortunate circumstance, but my experiments were not thus
vitiated, as both lots of plants were exposed to the same conditions,
whether favourable or unfavourable.

There is reason to believe that the flowers of this Ipomoea, when
growing out of doors, are habitually crossed by insects, so that the
first seedlings which I raised from purchased seeds were probably the
offspring of a cross. I infer that this is the case, firstly from
humble-bees often visiting the flowers, and from the quantity of pollen
left by them on the stigmas of such flowers; and, secondly, from the
plants raised from the same lot of seed varying greatly in the colour of
their flowers, for as we shall hereafter see, this indicates much
intercrossing. (2/3. Verlot says ‘Sur la Production des Variétés’ 1865
page 66, that certain varieties of a closely allied plant, the
Convolvulus tricolor, cannot be kept pure unless grown at a distance
from all other varieties.) It is, therefore, remarkable that the plants
raised by me from flowers which were, in all probability,
self-fertilised for the first time after many generations of crossing,
should have been so markedly inferior in height to the intercrossed
plants as they were, namely, as 76 to 100. As the plants which were
self-fertilised in each succeeding generation necessarily became much
more closely interbred in the later than in the earlier generations, it
might have been expected that the difference in height between them and
the crossed plants would have gone on increasing; but, so far is this
from being the case, that the difference between the two sets of plants
in the seventh, eighth, and ninth generations taken together is less
than in the first and second generations together. When, however, we
remember that the self-fertilised and crossed plants are all descended
from the same mother-plant, that many of the crossed plants in each
generation were related, often closely related, and that all were
exposed to the same conditions, which, as we shall hereafter find, is a
very important circumstance, it is not at all surprising that the
difference between them should have somewhat decreased in the later
generations. It is, on the contrary, an astonishing fact, that the
crossed plants should have been victorious, even to a slight degree,
over the self-fertilised plants of the later generations.

The much greater constitutional vigour of the crossed than of the
self-fertilised plants, was proved on five occasions in various ways;
namely, by exposing them, while young, to a low temperature or to a
sudden change of temperature, or by growing them, under very
unfavourable conditions, in competition with full-grown plants of other
kinds.

With respect to the productiveness of the crossed and self-fertilised
plants of the successive generations, my observations unfortunately were
not made on any uniform plan, partly from the want of time, and partly
from not having at first intended to observe more than a single
generation. A summary of the results is here given in a tabulated form,
the fertility of the crossed plants being taken as 100.

TABLE 2/18. Ratio of productiveness of crossed and self-fertilised
plants. Ipomoea purpurea.

FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN
COMPETITION WITH ONE ANOTHER.

Sixty-five capsules produced from flowers on five crossed plants
fertilised by pollen from a distinct plant, and fifty-five capsules
produced from flowers on five self-fertilised plants fertilised by their
own pollen, contained seeds in the proportion of : 100 to 93.

Fifty-six spontaneously self-fertilised capsules on the above five
crossed plants, and twenty-five spontaneously self-fertilised capsules
on the above five self-fertilised plants, yielded seeds in the
proportion of : 100 to 99.

Combining the total number of capsules produced by these plants, and the
average number of seeds in each, the above crossed and self-fertilised
plants yielded seeds in the proportion of : 100 to 64.

Other plants of this first generation grown under unfavourable
conditions and spontaneously self-fertilised, yielded seeds in the
proportion of : 100 to 45.

THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Crossed capsules compared with self-fertilised capsules contained seeds
in the ratio of : 100 to 94.

An equal number of crossed and self-fertilised plants, both
spontaneously self-fertilised, produced capsules in the ratio of : 100
to 38.

And these capsules contained seeds in the ratio of : 100 to 94.

Combining these data, the productiveness of the crossed to the
self-fertilised plants, both spontaneously self-fertilised, was as : 100
to 35.

FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Capsules from flowers on the crossed plants fertilised by pollen from
another plant, and capsules from flowers on the self-fertilised plants
fertilised with their own pollen, contained seeds in the proportion of :
100 to 94.

FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

The crossed plants produced spontaneously a vast number more pods (not
actually counted) than the self-fertilised, and these contained seeds in
the proportion of : 100 to 89.

NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Fourteen crossed plants, spontaneously self-fertilised, and fourteen
self-fertilised plants spontaneously self-fertilised, yielded capsules
(the average number of seeds per capsule not having been ascertained) in
the proportion of : 100 to 26.

PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH
INTERCROSSED PLANTS.

The offspring of intercrossed plants of the ninth generation, crossed by
a fresh stock, compared with plants of the same stock intercrossed
during ten generations, both sets of plants left uncovered and naturally
fertilised, produced capsules by weight as : 100 to 51.

We see in this table that the crossed plants are always in some degree
more productive than the self-fertilised plants, by whatever standard
they are compared. The degree differs greatly; but this depends chiefly
on whether an average was taken of the seeds alone, or of the capsules
alone, or of both combined. The relative superiority of the crossed
plants is chiefly due to their producing a much greater number of
capsules, and not to each capsule containing a larger average number of
seeds. For instance, in the third generation the crossed and
self-fertilised plants produced capsules in the ratio of 100 to 38,
whilst the seeds in the capsules on the crossed plants were to those on
the self-fertilised plants only as 100 to 94. In the eighth generation
the capsules on two self-fertilised plants (not included in table 2/18),
grown in separate pots and thus not subjected to any competition,
yielded the large average of 5.1 seeds. The smaller number of capsules
produced by the self-fertilised plants may be in part, but not
altogether, attributed to their lessened size or height; this being
chiefly due to their lessened constitutional vigour, so that they were
not able to compete with the crossed plants growing in the same pots.
The seeds produced by the crossed flowers on the crossed plants were not
always heavier than the self-fertilised seeds on the self-fertilised
plants. The lighter seeds, whether produced from crossed or
self-fertilised flowers, generally germinated before the heavier seeds.
I may add that the crossed plants, with very few exceptions, flowered
before their self-fertilised opponents, as might have been expected from
their greater height and vigour.

The impaired fertility of the self-fertilised plants was shown in
another way, namely, by their anthers being smaller than those in the
flowers on the crossed plants. This was first observed in the seventh
generation, but may have occurred earlier. Several anthers from flowers
on the crossed and self-fertilised plants of the eighth generation were
compared under the microscope; and those from the former were generally
longer and plainly broader than the anthers of the self-fertilised
plants. The quantity of pollen contained in one of the latter was, as
far as could be judged by the eye, about half of that contained in one
from a crossed plant. The impaired fertility of the self-fertilised
plants of the eighth generation was also shown in another manner, which
may often be observed in hybrids--namely, by the first-formed flowers
being sterile. For instance, the fifteen first flowers on a
self-fertilised plant of one of the later generations were carefully
fertilised with their own pollen, and eight of them dropped off; at the
same time fifteen flowers on a crossed plant growing in the same pot
were self-fertilised, and only one dropped off. On two other crossed
plants of the same generation, several of the earliest flowers were
observed to fertilise themselves and to produce capsules. In the plants
of the ninth, and I believe of some previous generations, very many of
the flowers, as already stated, were slightly monstrous; and this
probably was connected with their lessened fertility.

All the self-fertilised plants of the seventh generation, and I believe
of one or two previous generations, produced flowers of exactly the same
tint, namely, of a rich dark purple. So did all the plants, without any
exception, in the three succeeding generations of self-fertilised
plants; and very many were raised on account of other experiments in
progress not here recorded. My attention was first called to this fact
by my gardener remarking that there was no occasion to label the
self-fertilised plants, as they could always be known by their colour.
The flowers were as uniform in tint as those of a wild species growing
in a state of nature; whether the same tint occurred, as is probable, in
the earlier generations, neither my gardener nor self could recollect.
The flowers on the plants which were first raised from purchased seed,
as well as during the first few generations, varied much in the depth of
the purple tint; many were more or less pink, and occasionally a white
variety appeared. The crossed plants continued to the tenth generation
to vary in the same manner as before, but to a much less degree, owing,
probably, to their having become more or less closely inter-related. We
must therefore attribute the extraordinary uniformity of colour in the
flowers on the plants of the seventh and succeeding self-fertilised
generations, to inheritance not having been interfered with by crosses
during several preceding generations, in combination with the conditions
of life having been very uniform.

A plant appeared in the sixth self-fertilised generation, named the
Hero, which exceeded by a little in height its crossed antagonist, and
which transmitted its powers of growth and increased self-fertility to
its children and grandchildren. A cross between the children of Hero did
not give to the grandchildren any advantage over the self-fertilised
grandchildren raised from the self-fertilised children. And as far as my
observations can be trusted, which were made on very unhealthy plants,
the great-grandchildren raised from intercrossing the grandchildren had
no advantage over the seedlings from the grandchildren the product of
continued self-fertilisation; and what is far more remarkable, the
great-grandchildren raised by crossing the grandchildren with a fresh
stock, had no advantage over either the intercrossed or self-fertilised
great-grandchildren. It thus appears that Hero and its descendants
differed in constitution in an extraordinary manner from ordinary plants
of the present species.

Although the plants raised during ten successive generations from
crosses between distinct yet inter-related plants almost invariably
exceeded in height, constitutional vigour, and fertility their
self-fertilised opponents, it has been proved that seedlings raised by
intercrossing flowers on the same plant are by no means superior, on the
contrary are somewhat inferior in height and weight, to seedlings raised
from flowers fertilised with their own pollen. This is a remarkable
fact, which seems to indicate that self-fertilisation is in some manner
more advantageous than crossing, unless the cross brings with it, as is
generally the case, some decided and preponderant advantage; but to this
subject I shall recur in a future chapter.

The benefits which so generally follow from a cross between two plants
apparently depend on the two differing somewhat in constitution or
character. This is shown by the seedlings from the intercrossed plants
of the ninth generation, when crossed with pollen from a fresh stock,
being as superior in height and almost as superior in fertility to the
again intercrossed plants, as these latter were to seedlings from
self-fertilised plants of the corresponding generation. We thus learn
the important fact that the mere act of crossing two distinct plants,
which are in some degree inter-related and which have been long
subjected to nearly the same conditions, does little good as compared
with that from a cross between plants belonging to different stocks or
families, and which have been subjected to somewhat different
conditions. We may attribute the good derived from the crossing of the
intercrossed plants during the ten successive generations to their still
differing somewhat in constitution or character, as was indeed proved by
their flowers still differing somewhat in colour. But the several
conclusions which may be deduced from the experiments on Ipomoea will be
more fully considered in the final chapters, after all my other
observations have been given.



CHAPTER III.

SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.

Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.
Appearance of a new,  tall, and highly self-fertile variety.
Offspring from a cross between self-fertilised plants.
Effects of a cross with a fresh stock.
Effects of crossing flowers on the same plant.
Summary on Mimulus luteus.
Digitalis purpurea, superiority of the crossed plants.
Effects of crossing flowers on the same plant.
Calceolaria.
Linaria vulgaris.
Verbascum thapsus.
Vandellia nummularifolia.
Cleistogene flowers.
Gesneria pendulina.
Salvia coccinea.
Origanum vulgare, great increase of the crossed plants by stolons.
Thunbergia alata.

In the family of the Scrophulariaceae I experimented on species in the
six following genera: Mimulus, Digitalis, Calceolaria, Linaria,
Verbascum, and Vandellia.

[3/2. SCROPHULARIACEAE.--Mimulus luteus.

The plants which I raised from purchased seed varied greatly in the
colour of their flowers, so that hardly two individuals were quite
alike; the corolla being of all shades of yellow, with the most
diversified blotches of purple, crimson, orange, and coppery brown. But
these plants differed in no other respect. (3/1. I sent several
specimens with variously coloured flowers to Kew, and Dr. Hooker informs
me that they all consisted of Mimulus luteus. The flowers with much red
have been named by horticulturists as var. Youngiana.) The flowers are
evidently well adapted for fertilisation by the agency of insects; and
in the case of a closely allied species, Mimulus rosea, I have watched
bees entering the flowers, thus getting their backs well dusted with
pollen; and when they entered another flower the pollen was licked off
their backs by the two-lipped stigma, the lips of which are irritable
and close like a forceps on the pollen-grains. If no pollen is enclosed
between the lips, these open again after a time. Mr. Kitchener has
ingeniously explained the use of these movements, namely, to prevent the
self-fertilisation of the flower. (3/2. ‘A Year’s Botany’ 1874 page
118.) If a bee with no pollen on its back enters a flower it touches the
stigma, which quickly closes, and when the bee retires dusted with
pollen, it can leave none on the stigma of the same flower. But as soon
as it enters any other flower, plenty of pollen is left on the stigma,
which will be thus cross-fertilised. Nevertheless, if insects are
excluded, the flowers fertilise themselves perfectly and produce plenty
of seed; but I did not ascertain whether this is effected by the stamens
increasing in length with advancing age, or by the bending down of the
pistil. The chief interest in my experiments on the present species,
lies in the appearance in the fourth self-fertilised generation of a
variety which bore large peculiarly-coloured flowers, and grew to a
greater height than the other varieties; it likewise became more highly
self-fertile, so that this variety resembles the plant named Hero, which
appeared in the sixth self-fertilised generation of Ipomoea.

Some flowers on one of the plants raised from the purchased seeds were
fertilised with their own pollen; and others on the same plant were
crossed with pollen from a distinct plant. The seeds from twelve
capsules thus produced were placed in separate watch-glasses for
comparison; and those from the six crossed capsules appeared to the eye
hardly more numerous than those from the six self-fertilised capsules.
But when the seeds were weighed, those from the crossed capsules
amounted to 1.02 grain, whilst those from the self-fertilised capsules
were only .81 grain; so that the former were either heavier or more
numerous than the latter, in the ratio of 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.

Having ascertained, by leaving crossed and self-fertilised seed on damp
sand, that they germinated simultaneously, both kinds were thickly sown
on opposite sides of a broad and rather shallow pan; so that the two
sets of seedlings, which came up at the same time, were subjected to the
same unfavourable conditions. This was a bad method of treatment, but
this species was one of the first on which I experimented. When the
crossed seedlings were on an average half an inch high, the
self-fertilised ones were only a quarter of an inch high. When grown to
their full height under the above unfavourable conditions, the four
tallest crossed plants averaged 7.62, and the four tallest
self-fertilised 5.87 inches in height; or as 100 to 77. Ten flowers on
the crossed plants were fully expanded before one on the self-fertilised
plants. A few of these plants of both lots were transplanted into a
large pot with plenty of good earth, and the self-fertilised plants, not
now being subjected to severe competition, grew during the following
year as tall as the crossed plants; but from a case which follows it is
doubtful whether they would have long continued equal. Some flowers on
the crossed plants were crossed with pollen from another plant, and the
capsules thus produced contained a rather greater weight of seed than
those on the self-fertilised plants again self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Seeds from the foregoing plants, fertilised in the manner just stated,
were sown on the opposite sides of a small pot (1) and came up crowded.
The four tallest crossed seedlings, at the time of flowering, averaged 8
inches in height, whilst the four tallest self-fertilised plants
averaged only 4 inches. Crossed seeds were sown by themselves in a
second small pot, and self-fertilised seeds were sown by themselves in a
third small pot so that there was no competition whatever between these
two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher
on an average than the self-fertilised. Both lots looked equally
vigorous, but the crossed plants flowered earlier and more profusely
than the self-fertilised. In Pot 1, in which the two lots competed with
each other, the crossed plants flowered first and produced a large
number of capsules, whilst the self-fertilised produced only nineteen.
The contents of twelve capsules from the crossed flowers on the crossed
plants, and of twelve capsules from self-fertilised flowers on the
self-fertilised plants, were placed in separate watch-glasses for
comparison; and the crossed seeds seemed more numerous by half than the
self-fertilised.

The plants on both sides of Pot 1, after they had seeded, were cut down
and transplanted into a large pot with plenty of good earth, and on the
following spring, when they had grown to a height of between 5 and 6
inches, the two lots were equal, as occurred in a similar experiment in
the last generation. But after some weeks the crossed plants exceeded
the self-fertilised ones on the opposite side of the same pot, though
not nearly to so great a degree as before, when they were subjected to
very severe competition.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Crossed seeds from the crossed plants, and self-fertilised seeds from
the self-fertilised plants of the last generation, were sown thickly on
opposite sides of a small pot, Number 1. The two tallest plants on each
side were measured after they had flowered, and the two crossed ones
were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2
inches in height; that is, in the ratio of 100 to 69. Twenty flowers on
the crossed plants were again crossed and produced twenty capsules; ten
of which contained 1.33 grain weight of seeds. Thirty flowers on the
self-fertilised plants were again self-fertilised and produced
twenty-six capsules; ten of the best of which (many being very poor)
contained only .87 grain weight of seeds; that is, in the ratio of 100
to 65 by weight.

The superiority of the crossed over the self-fertilised plants was
proved in various ways. Self-fertilised seeds were sown on one side of a
pot, and two days afterwards crossed seeds on the opposite side. The two
lots of seedlings were equal until they were above half an inch high;
but when fully grown the two tallest crossed plants attained a height of
12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants
were only 8 and 5 1/2 inches high.

In a third pot, crossed seeds were sown four days after the
self-fertilised, and the seedlings from the latter had at first, as
might have been expected, an advantage; but when the two lots were
between 5 and 6 inches in height, they were equal, and ultimately the
three tallest crossed plants were 11, 10, and 8 inches, whilst the three
tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So
that there was not much difference between them, the crossed plants
having an average advantage of only the third of an inch. The plants
were cut down, and without being disturbed were transplanted into a
larger pot. Thus the two lots started fair on the following spring, and
now the crossed plants showed their inherent superiority, for the two
tallest were 13 inches, whilst the two tallest self-fertilised plants
were only 11 and 8 1/2 inches in height; or as 100 to 75. The two lots
were allowed to fertilise themselves spontaneously: the crossed plants
produced a large number of capsules, whilst the self-fertilised produced
very few and poor ones. The seeds from eight of the capsules on the
crossed plants weighed .65 grain, whilst those from eight of the
capsules on the self-fertilised plants weighed only .22 grain; or as 100
to 34.

The crossed plants in the above three pots, as in almost all the
previous experiments, flowered before the self-fertilised. This occurred
even in the third pot in which the crossed seeds were sown four days
after the self-fertilised seeds.

Lastly, seeds of both lots were sown on opposite sides of a large pot in
which a Fuchsia had long been growing, so that the earth was full of
roots. Both lots grew miserably; but the crossed seedlings had an
advantage at all times, and ultimately attained to a height of 3 1/2
inches, whilst the self-fertilised seedlings never exceeded 1 inch. The
several foregoing experiments prove in a decisive manner the superiority
in constitutional vigour of the crossed over the self-fertilised plants.

In the three generations now described and taken together, the average
height of the ten tallest crossed plants was 8.19 inches, and that of
the ten tallest self-fertilised plants 5.29 inches (the plants having
been grown in small pots), or as 100 to 65.

In the next or fourth self-fertilised generation, several plants of a
new and tall variety appeared, which increased in the later
self-fertilised generations, owing to its great self-fertility, to the
complete exclusion of the original kinds. The same variety also appeared
amongst the crossed plants, but as it was not at first regarded with any
particular attention, I know not how far it was used for raising the
intercrossed plants; and in the later crossed generations it was rarely
present. Owing to the appearance of this tall variety, the comparison of
the crossed and self-fertilised plants of the fifth and succeeding
generations was rendered unfair, as all the self-fertilised and only a
few or none of the crossed plants consisted of it. Nevertheless, the
results of the later experiments are in some respects well worth giving.


CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seeds of the two kinds, produced in the usual way from the two sets of
plants of the third generation, were sown on opposite sides of two pots
(1 and 2); but the seedlings were not thinned enough and did not grow
well. Many of the self-fertilised plants, especially in one of the pots,
consisted of the new and tall variety above referred to, which bore
large and almost white flowers marked with crimson blotches. I will call
it the WHITE VARIETY. I believe that it first appeared amongst both the
crossed and self-fertilised plants of the last generation; but neither
my gardener nor myself could remember any such variety in the seedlings
raised from the purchased seed. It must therefore have arisen either
through ordinary variation, or, judging from its appearance amongst both
the crossed and self-fertilised plants, more probably through reversion
to a formerly existing variety.

In Pot 1 the tallest crossed plant was 8 1/2 inches, and the tallest
self-fertilised 5 inches in height. In Pot 2, the tallest crossed plant
was 6 1/2 inches, and the tallest self-fertilised plant, which consisted
of the white variety, 7 inches in height; and this was the first
instance in my experiments on Mimulus in which the tallest
self-fertilised plant exceeded the tallest crossed. Nevertheless, the
two tallest crossed plants taken together were to the two tallest
self-fertilised plants in height as 100 to 80. As yet the crossed plants
were superior to the self-fertilised in fertility; for twelve flowers on
the crossed plants were crossed and yielded ten capsules, the seeds of
which weighed 1.71 grain. Twenty flowers on the self-fertilised plants
were self-fertilised, and produced fifteen capsules, all appearing poor;
and the seeds from ten of them weighed only .68 grain, so that from an
equal number of capsules the crossed seeds were to the self-fertilised
in weight as 100 to 40.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

Seeds from both lots of the fourth generation, fertilised in the usual
manner, were sown on opposite sides of three pots. When the seedlings
flowered, most of the self-fertilised plants were found to consist of
the tall white variety. Several of the crossed plants in Pot 1 likewise
belonged to this variety, as did a very few in Pots 2 and 3. The tallest
crossed plant in Pot 1 was 7 inches, and the tallest self-fertilised
plant on the opposite side 8 inches; in Pots 2 and 3 the tallest crossed
were 4 1/2 and 5 1/2, and the tallest self-fertilised 7 and 6 1/2 inches
in height; so that the average height of the tallest plants in the two
lots was as 100 for the crossed to 126 for the self-fertilised; and thus
we have a complete reversal of what occurred in the four previous
generations. Nevertheless, in all three pots the crossed plants retained
their habit of flowering before the self-fertilised. The plants were
unhealthy from being crowded and from the extreme heat of the season,
and were in consequence more or less sterile; but the crossed plants
were somewhat less sterile than the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

Seeds from plants of the fifth generation crossed and self-fertilised in
the usual manner were sown on opposite sides of several pots. On the
self-fertilised side every single plant belonged to the tall white
variety. On the crossed side some plants belonged to this variety, but
the greater number approached in character to the old and shorter kinds
with smaller yellowish flowers blotched with coppery brown. When the
plants on both sides were from 2 to 3 inches in height they were equal,
but when fully grown the self-fertilised were decidedly the tallest and
finest plants, but, from want of time, they were not actually measured.
In half the pots the first plant which flowered was a self-fertilised
one, and in the other half a crossed one. And now another remarkable
change was clearly perceived, namely, that the self-fertilised plants
had become more self-fertile than the crossed. The pots were all put
under a net to exclude insects, and the crossed plants produced
spontaneously only fifty-five capsules, whilst the self-fertilised
plants produced eighty-one capsules, or as 100 to 147. The seeds from
nine capsules of both lots were placed in separate watch-glasses for
comparison, and the self-fertilised appeared rather the more numerous.
Besides these spontaneously self-fertilised capsules, twenty flowers on
the crossed plants again crossed yielded sixteen capsules; twenty-five
flowers on the self-fertilised plants again self-fertilised yielded
seventeen capsules, and this is a larger proportional number of capsules
than was produced by the self-fertilised flowers on the self-fertilised
plants in the previous generations. The contents of ten capsules of both
these lots were compared in separate watch-glasses, and the seeds from
the self-fertilised appeared decidedly more numerous than those from the
crossed plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.

Crossed and self-fertilised seeds from the crossed and self-fertilised
plants of the sixth generation were sown in the usual manner on opposite
sides of three pots, and the seedlings were well and equally thinned.
Every one of the self-fertilised plants (and many were raised) in this,
as well as in the eighth and ninth generations, belonged to the tall
white variety. Their uniformity of character, in comparison with the
seedlings first raised from the purchased seed, was quite remarkable. On
the other hand, the crossed plants differed much in the tints of their
flowers, but not, I think, to so great a degree as those first raised. I
determined this time to measure the plants on both sides carefully. The
self-fertilised seedlings came up rather before the crossed, but both
lots were for a time of equal height. When first measured, the average
height of the six tallest crossed plants in the three pots was 7.02, and
that of the six tallest self-fertilised plants 8.97 inches, or as 100 to
128. When fully grown the same plants were again measured, with the
result shown in Table 3/18.

TABLE 3/18. Mimulus luteus (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 11 2/8 : 19 1/8.
Pot 1 : 11 7/8 : 18.

Pot 2 : 12 6/8 : 18 2/8.
Pot 2 : 11 2/8 : 14 6/8.

Pot 3 :  9 6/8 : 12 6/8.
Pot 3 : 11 6/8 : 11.

Total : 68.63  : 93.88.

The average height of the six crossed is here 11.43, and that of the six
self-fertilised 15.64, or as 100 to 137.

As it is now evident that the tall white variety transmitted its
characters faithfully, and as the self-fertilised plants consisted
exclusively of this variety, it was manifest that they would always
exceed in height the crossed plants which belonged chiefly to the
original shorter varieties. This line of experiment was therefore
discontinued, and I tried whether intercrossing two self-fertilised
plants of the sixth generation, growing in distinct pots, would give
their offspring any advantage over the offspring of flowers on one of
the same plants fertilised with their own pollen. These latter seedlings
formed the seventh generation of self-fertilised plants, like those in
the right hand column in Table 3/18; the crossed plants were the product
of six previous self-fertilised generations with an intercross in the
last generation. The seeds were allowed to germinate on sand, and were
planted in pairs on opposite sides of four pots, all the remaining seeds
being sown crowded on opposite sides of Pot 5 in Table 3/19; the three
tallest on each side in this latter pot being alone measured. All the
plants were twice measured--the first time whilst young, and the average
height of the crossed plants to that of the self-fertilised was then as
100 to 122. When fully grown they were again measured, as in Table 3/19.

TABLE 3/19. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth
Generation.

Column 3: Self-fertilised Plants of the Seventh Generation.

Pot 1 :  12 6/8 :  15 2/8.
Pot 1 :  10 4/8 :  11 5/8.
Pot 1 :  10     :  11.
Pot 1 :  14 5/8 :  11.

Pot 2 :  10 2/8 :  11 3/8.
Pot 2 :   7 6/8 :  11 4/8.
Pot 2 :  12 1/8 :   8 5/8.
Pot 2 :   7     :  14 3/8.

Pot 3 :  13 5/8 :  10 3/8.
Pot 3 :  12 2/8 :  11 6/8.

Pot 4 :   7 1/8 :  14 6/8.
Pot 4 :   8 2/8 :   7.
Pot 4 :   7 2/8 :   8.

Pot 5 :   8 5/8 :  10 2/8
Pot 5 :   9     :   9 3/8.
Pot 5 :   8 2/8 :   9 2/8.
Crowded.

Total : 159.38  : 175.50.

The average height of the sixteen intercrossed plants is here 9.96
inches, and that of the sixteen self-fertilised plants 10.96, or as 100
to 110; so that the intercrossed plants, the progenitors of which had
been self-fertilised for the six previous generations, and had been
exposed during the whole time to remarkably uniform conditions, were
somewhat inferior in height to the plants of the seventh self-fertilised
generation. But as we shall presently see that a similar experiment made
after two additional generations of self-fertilisation gave a different
result, I know not how far to trust the present one. In three of the
five pots in Table 3/19 a self-fertilised plant flowered first, and in
the other two a crossed plant. These self-fertilised plants were
remarkably fertile, for twenty flowers fertilised with their own pollen
produced no less than nineteen very fine capsules!

THE EFFECTS OF A CROSS WITH A DISTINCT STOCK.

Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were
fertilised with their own pollen, and plants of the eighth
self-fertilised generation were thus raised, merely to serve as parents
in the following experiment. Several flowers on these plants were
allowed to fertilise themselves spontaneously (insects being of course
excluded), and the plants raised from these seeds formed the ninth
self-fertilised generation; they consisted wholly of the tall white
variety with crimson blotches. Other flowers on the same plants of the
eighth self-fertilised generation were crossed with pollen taken from
another plant of the same lot; so that the seedlings thus raised were
the offspring of eight previous generations of self-fertilisation with
an intercross in the last generation; these I will call the INTERCROSSED
PLANTS. Lastly, other flowers on the same plants of the eighth
self-fertilised generation were crossed with pollen taken from plants
which had been raised from seed procured from a garden at Chelsea. The
Chelsea plants bore yellow flowers blotched with red, but differed in no
other respect. They had been grown out of doors, whilst mine had been
cultivated in pots in the greenhouse for the last eight generations, and
in a different kind of soil. The seedlings raised from this cross with a
wholly different stock may be called the CHELSEA-CROSSED. The three lots
of seeds thus obtained were allowed to germinate on bare sand; and
whenever a seed in all three lots, or in only two, germinated at the
same time, they were planted in pots superficially divided into three or
two compartments. The remaining seeds, whether or not in a state of
germination, were thickly sown in three divisions in a large pot, 10, in
Table 3/20. When the plants had grown to their full height they were
measured, as shown in Table 3/20; but only the three tallest plants in
each of the three divisions in Pot 10 were measured.

TABLE 3/20. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Plants from Self-fertilised Plants of the Eighth Generation
crossed by Chelsea Plants.

Column 3: Plants from an intercross between the Plants of the Eighth
Self-fertilised Generation.

Column 4: Self-fertilised Plants of the Ninth Generation from Plants of
the Eighth Self-fertilised Generation.

Pot 1  :  30 7/8 :  14     :   9 4/8.
Pot 1  :  28 3/8 :  13 6/8 :  10 5/8.
Pot 1  :  --     :  13 7/8 :  10.

Pot 2  :  20 6/8 :  11 4/8 :  11 6/8.
Pot 2  :  22 2/8 :  12     :  12 3/8.
Pot 2  :  --     :   9 1/8 :  --.

Pot 3  :  23 6/8 :  12 2/8 :   8 5/8.
Pot 3  :  24 1/8 :  --     :  11 4/8.
Pot 3  :  25 6/8 :  --     :   6 7/8.

Pot 4  :  22 5/8 :   9 2/8 :   4.
Pot 4  :  22     :   8 1/8 :  13 3/8.
Pot 4  :  17     :  --     :  11.

Pot 5  :  22 3/8 :   9     :   4 4/8.
Pot 5  :  19 5/8 :  11     :  13.
Pot 5  :  23 4/8 :  --     :  13 4/8.

Pot 6  :  28 2/8 :  18 6/8 :  12.
Pot 6  :  22     :   7     :  16 1/8.
Pot 6  :  --     :  12 4/8 :  --.

Pot 7  :  12 4/8 :  15     :  --.
Pot 7  :  24 3/8 :  12 3/8 :  --.
Pot 7  :  20 4/8 :  11 2/8 :  --.
Pot 7  :  26 4/8 :  15 2/8 :  --.

Pot 8  :  17 2/8 :  13 3/8 :  --.
Pot 8  :  22 6/8 :  14 5/8 :  --.
Pot 8  :  27     :  14 3/8 :  --.

Pot 9  :  22 6/8 :  11 6/8 :  --.
Pot 9  :   6     :  17     :  --.
Pot 9  :  20 2/8 :  14 7/8 :  --.

Pot 10 :  18 1/8 :   9 2/8 :  10 3/8.
Pot 10 :  16 5/8 :   8 2/8 :   8 1/8.
Pot 10 :  17 4/8 :  10     :  11 2/8.
Crowded plants.

Total  : 605.38  : 329.50  : 198.50.

In this table the average height of the twenty-eight Chelsea-crossed
plants is 21.62 inches; that of the twenty-seven intercrossed plants
12.2; and that of the nineteen self-fertilised 10.44. But with respect
to the latter it will be the fairest plan to strike out two dwarfed ones
(only 4 inches in height), so as not to exaggerate the inferiority of
the self-fertilised plants; and this will raise the average height of
the seventeen remaining self-fertilised plants to 11.2 inches. Therefore
the Chelsea-crossed are to the intercrossed in height as 100 to 56; the
Chelsea-crossed to the self-fertilised as 100 to 52; and the
intercrossed to the self-fertilised as 100 to 92. We thus see how
immensely superior in height the Chelsea-crossed are to the intercrossed
and to the self-fertilised plants. They began to show their superiority
when only one inch high. They were also, when fully grown, much more
branched with larger leaves and somewhat larger flowers than the plants
of the other two lots, so that if they had been weighed, the ratio would
certainly have been much higher than that of 100 to 56 and 52.

The intercrossed plants are here to the self-fertilised in height as 100
to 92; whereas in the analogous experiment given in Table 3/19 the
intercrossed plants from the self-fertilised plants of the sixth
generation were inferior in height to the self-fertilised plants in the
ratio of 100 to 110. I doubt whether this discordance in the results of
the two experiments can be explained by the self-fertilised plants in
the present case having been raised from spontaneously self-fertilised
seeds, whereas in the former case they were raised from artificially
self-fertilised seeds; nor by the present plants having been
self-fertilised during two additional generations, though this is a more
probable explanation.

With respect to fertility, the twenty-eight Chelsea-crossed plants
produced 272 capsules; the twenty-seven intercrossed plants produced 24;
and the seventeen self-fertilised plants 17 capsules. All the plants
were left uncovered so as to be naturally fertilised, and empty capsules
were rejected.

Therefore 20 Chelsea-crossed plants would have produced 194.29 capsules.

Therefore 20 Intercrossed plants would have produced 17.77 capsules.

Therefore 20 Self-fertilised plants would have produced 20.00 capsules.

The seeds contained in 8 capsules from the Chelsea-crossed plants
weighed 1.1 grains.

The seeds contained in 8 capsules from the Intercrossed plants weighed
0.51 grains.

The seeds contained in 8 capsules from the Self-fertilised plants
weighed 0.33 grains.

If we combine the number of capsules produced together with the average
weight of contained seeds, we get the following extraordinary ratios:

Weight of seed produced by the same number of Chelsea-crossed and
intercrossed plants as 100 to 4.

Weight of seed produced by the same number of Chelsea-crossed and
self-fertilised plants as 100 to 3.

Weight of seeds produced by the same number of intercrossed and
self-fertilised plants as 100 to 73.

It is also a remarkable fact that the Chelsea-crossed plants exceeded
the two other lots in hardiness, as greatly as they did in height,
luxuriance, and fertility. In the early autumn most of the pots were
bedded out in the open ground; and this always injures plants which have
been long kept in a warm greenhouse. All three lots consequently
suffered greatly, but the Chelsea-crossed plants much less than the
other two lots. On the 3rd of October the Chelsea-crossed plants began
to flower again, and continued to do so for some time; whilst not a
single flower was produced by the plants of the other two lots, the
stems of which were cut almost down to the ground and seemed half dead.
Early in December there was a sharp frost, and the stems of
Chelsea-crossed were now cut down; but on the 23rd of December they
began to shoot up again from the roots, whilst all the plants of the
other two lots were quite dead.

Although several of the self-fertilised seeds, from which the plants in
the right hand column in Table 3/20 were raised, germinated (and were of
course rejected) before any of those of the other two lots, yet in only
one of the ten pots did a self-fertilised plant flower before the
Chelsea-crossed or the intercrossed plants growing in the same pots. The
plants of these two latter lots flowered at the same time, though the
Chelsea-crossed grew so much taller and more vigorously than the
intercrossed.

As already stated, the flowers of the plants originally raised from the
Chelsea seeds were yellow; and it deserves notice that every one of the
twenty-eight seedlings raised from the tall white variety fertilised,
without being castrated, with pollen from the Chelsea plants, produced
yellow flowers; and this shows how prepotent this colour, which is the
natural one of the species, is over the white colour.

THE EFFECTS ON THE OFFSPRING OF INTERCROSSING FLOWERS ON THE SAME PLANT,
INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

In all the foregoing experiments the crossed plants were the product of
a cross between distinct plants. I now selected a very vigorous plant in
Table 3/20, raised by fertilising a plant of the eighth self-fertilised
generation with pollen from the Chelsea stock. Several flowers on this
plant were crossed with pollen from other flowers on the same plant, and
several other flowers were fertilised with their own pollen. The seed
thus produced was allowed to germinate on bare sand; and the seedlings
were planted in the usual manner on the opposite sides of six pots. All
the remaining seeds, whether or not in a state of germination, were sown
thickly in Pot 7; the three tallest plants on each side of this latter
pot being alone measured. As I was in a hurry to learn the result, some
of these seeds were sown late in the autumn, but the plants grew so
irregularly during the winter, that one crossed plant was 28 1/2 inches,
and two others only 4, or less than 4 inches in height, as may be seen
in Table 3/21. Under such circumstances, as I have observed in many
other cases, the result is not in the least trustworthy; nevertheless I
feel bound to give the measurements.

TABLE 3/21. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Plants raised from a Cross between different Flowers on the
same Plant.

Column 3: Plants raised from Flowers fertilised with their own Pollen.

Pot 1  :  17     :  17.
Pot 1  :   9     :   3 1/8.

Pot 2  :  28 2/8 :   19 1/8.
Pot 2  :  16 4/8 :   6.
Pot 2  :  13 5/8 :   2.

Pot 3  :   4     :  15 6/8.
Pot 3  :   2 2/8 :  10.

Pot 4  :  23 4/8 :   6 2/8.
Pot 4  :  15 4/8 :   7 1/8.

Pot 5  :   7     :  13 4/8.

Pot 6  :  18 3/8 :   1 4/8.
Pot 6  :  11     :   2.

Pot 7  :  21     :  15 1/8.
Pot 7  :  11 6/8 :  11.
Pot 7  :  12 1/8 :  11 2/8.
Crowded.

Total  : 210.88  : 140.75.

The fifteen crossed plants here average 14.05, and the fifteen
self-fertilised plants 9.38 in height, or as 100 to 67. But if all the
plants under ten inches in height are struck out, the ratio of the
eleven crossed plants to the eight self-fertilised plants is as 100 to
82.

On the following spring, some remaining seeds of the two lots were
treated in exactly the same manner; and the measurements of the
seedlings are given in Table 3/22.

TABLE 3/22. Mimulus luteus.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Plants raised from a Cross between different Flowers on the
same Plant.

Column 3: Plants raised from Flowers fertilised with their own Pollen.

Pot 1  :  15 1/8 :  19 1/8.
Pot 1  :  12     :  20 5/8.
Pot 1  :  10 1/8 :  12 6/8.

Pot 2  :  16 2/8 :  11 2/8.
Pot 2  :  13 5/8 :  19 3/8.
Pot 2  :  20 1/8 :  17 4/8.

Pot 3  :  18 7/8 :  12 6/8.
Pot 3  :  15     :  15 6/8.
Pot 3  :  13 7/8 :  17.

Pot 4  :  19 2/8 :  16 2/8.
Pot 4  :  19 6/8 :  21 5/8.

Pot 5  :  25 3/8 :  22 5/8.

Pot 6  :  15     :  19 5/8.
Pot 6  :  20 2/8 :  16 2/8.
Pot 6  :  27 2/8 :  19 5/8.

Pot 7  :   7 6/8 :   7 6/8.
Pot 7  :  14     :   8.
Pot 7  :  13 4/8 :   7.

Pot 8  :  18 2/8 :  20 3/8.
Pot 8  :  18 6/8 :  17 6/8.
Pot 8  :  18 3/8 :  15 4/8.
Pot 8  :  18 3/8 :  15 1/8.
Crowded.

Total  : 370.88  : 353.63.

Here the average height of the twenty-two crossed plants is 16.85, and
that of the twenty-two self-fertilised plants 16.07; or as 100 to 95.
But if four of the plants in Pot 7, which are much shorter than any of
the others, are struck out (and this would be the fairest plan), the
twenty-one crossed are to the nineteen self-fertilised plants in height
as 100 to 100.6--that is, are equal. All the plants, except the crowded
ones in Pot 8, after being measured were cut down, and the eighteen
crossed plants weighed 10 ounces, whilst the same number of
self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if
the dwarfed plants in Pot 7 had been excluded, the self-fertilised would
have exceeded the crossed in weight in a higher ratio. In all the
previous experiments in which seedlings were raised from a cross between
distinct plants, and were put into competition with self-fertilised
plants, the former generally flowered first; but in the present case, in
seven out of the eight pots a self-fertilised plant flowered before a
crossed one on the opposite side. Considering all the evidence with
respect to the plants in Table3/ 22, a cross between two flowers on the
same plant seems to give no advantage to the offspring thus produced,
the self-fertilised plants being in weight superior. But this conclusion
cannot be absolutely trusted, owing to the measurements given in Table
3/21, though these latter, from the cause already assigned, are very
much less trustworthy than the present ones.]

SUMMARY OF OBSERVATIONS ON Mimulus luteus.

In the three first generations of crossed and self-fertilised plants,
the tallest plants alone on each side of the several pots were measured;
and the average height of the ten crossed to that of the ten
self-fertilised plants was as 100 to 64. The crossed were also much more
fertile than the self-fertilised, and so much more vigorous that they
exceeded them in height, even when sown on the opposite side of the same
pot after an interval of four days. The same superiority was likewise
shown in a remarkable manner when both kinds of seeds were sown on the
opposite sides of a pot with very poor earth full of the roots of
another plant. In one instance crossed and self-fertilised seedlings,
grown in rich soil and not put into competition with each other,
attained to an equal height. When we come to the fourth generation the
two tallest crossed plants taken together exceeded by only a little the
two tallest self-fertilised plants, and one of the latter beat its
crossed opponent,--a circumstance which had not occurred in the previous
generations. This victorious self-fertilised plant consisted of a new
white-flowered variety, which grew taller than the old yellowish
varieties. From the first it seemed to be rather more fertile, when
self-fertilised, than the old varieties, and in the succeeding
self-fertilised generations became more and more self-fertile. In the
sixth generation the self-fertilised plants of this variety compared
with the crossed plants produced capsules in the proportion of 147 to
100, both lots being allowed to fertilise themselves spontaneously. In
the seventh generation twenty flowers on one of these plants
artificially self-fertilised yielded no less than nineteen very fine
capsules!

This variety transmitted its characters so faithfully to all the
succeeding self-fertilised generations, up to the last or ninth, that
all the many plants which were raised presented a complete uniformity of
character; thus offering a remarkable contrast with the seedlings raised
from the purchased seeds. Yet this variety retained to the last a latent
tendency to produce yellow flowers; for when a plant of the eighth
self-fertilised generation was crossed with pollen from a
yellow-flowered plant of the Chelsea stock, every single seedling bore
yellow flowers. A similar variety, at least in the colour of its
flowers, also appeared amongst the crossed plants of the third
generation. No attention was at first paid to it, and I know not how far
it was at first used either for crossing or self-fertilisation. In the
fifth generation most of the self-fertilised plants, and in the sixth
and all the succeeding generations every single plant consisted of this
variety; and this no doubt was partly due to its great and increasing
self-fertility. On the other hand, it disappeared from amongst the
crossed plants in the later generations; and this was probably due to
the continued intercrossing of the several plants. From the tallness of
this variety, the self-fertilised plants exceeded the crossed plants in
height in all the generations from the fifth to the seventh inclusive;
and no doubt would have done so in the later generations, had they been
grown in competition with one another. In the fifth generation the
crossed plants were in height to the self-fertilised, as 100 to 126; in
the sixth, as 100 to 147; and in the seventh generation, as 100 to 137.
This excess of height may be attributed not only to this variety
naturally growing taller than the other plants, but to its possessing a
peculiar constitution, so that it did not suffer from continued
self-fertilisation.

This variety presents a strikingly analogous case to that of the plant
called the Hero, which appeared in the sixth self-fertilised generation
of Ipomoea. If the seeds produced by Hero had been as greatly in excess
of those produced by the other plants, as was the case with Mimulus, and
if all the seeds had been mingled together, the offspring of Hero would
have increased to the entire exclusion of the ordinary plants in the
later self-fertilised generations, and from naturally growing taller
would have exceeded the crossed plants in height in each succeeding
generation.

Some of the self-fertilised plants of the sixth generation were
intercrossed, as were some in the eighth generation; and the seedlings
from these crosses were grown in competition with self-fertilised plants
of the two corresponding generations. In the first trial the
intercrossed plants were less fertile than the self-fertilised, and less
tall in the ratio of 100 to 110. In the second trial, the intercrossed
plants were more fertile than the self-fertilised in the ratio of 100 to
73, and taller in the ratio of 100 to 92. Notwithstanding that the
self-fertilised plants in the second trial were the product of two
additional generations of self-fertilisation, I cannot understand this
discordance in the results of the two analogous experiments.

The most important of all the experiments on Mimulus are those in which
flowers on plants of the eighth self-fertilised generation were again
self-fertilised; other flowers on distinct plants of the same lot were
intercrossed; and others were crossed with a new stock of plants from
Chelsea. The Chelsea-crossed seedlings were to the intercrossed in
height as 100 to 56, and in fertility as 100 to 4; and they were to the
self-fertilised plants, in height as 100 to 52, and in fertility as 100
to 3. These Chelsea-crossed plants were also much more hardy than the
plants of the other two lots; so that altogether the gain from the cross
with a fresh stock was wonderfully great.

Lastly, seedlings raised from a cross between flowers on the same plant
were not superior to those from flowers fertilised with their own
pollen; but this result cannot be absolutely trusted, owing to some
previous observations, which, however, were made under very unfavourable
circumstances.

[Digitalis purpurea.

The flowers of the common Foxglove are proterandrous; that is, the
pollen is mature and mostly shed before the stigma of the same flower is
ready for fertilisation. This is effected by the larger humble-bees,
which, whilst in search of nectar, carry pollen from flower to flower.
The two upper and longer stamens shed their pollen before the two lower
and shorter ones. The meaning of this fact probably is, as Dr. Ogle
remarks, that the anthers of the longer stamens stand near to the
stigma, so that they would be the most likely to fertilise it (3/3.
‘Popular Science Review’ January 1870 page 50.); and as it is an
advantage to avoid self-fertilisation, they shed their pollen first,
thus lessening the chance. There is, however, but little danger of
self-fertilisation until the bifid stigma opens; for Hildebrand found
that pollen placed on the stigma before it had opened produced no
effect. (3/4. ‘Geschlechter-Vertheilung bei den Pflanzen’ 1867 page 20.)
The anthers, which are large, stand at first transversely with respect
to the tubular corolla, and if they were to dehisce in this position
they would, as Dr. Ogle also remarks, smear with pollen the whole back
and sides of an entering humble-bee in a useless manner; but the anthers
twist round and place themselves longitudinally before they dehisce. The
lower and inner side of the mouth of the corolla is thickly clothed with
hairs, and these collect so much of the fallen pollen that I have seen
the under surface of a humble-bee thickly dusted with it; but this can
never be applied to the stigma, as the bees in retreating do not turn
their under surfaces upwards. I was therefore puzzled whether these
hairs were of any use; but Mr. Belt has, I think, explained their use:
the smaller kinds of bees are not fitted to fertilise the flowers, and
if they were allowed to enter easily they would steal much nectar, and
fewer large bees would haunt the flowers. Humble-bees can crawl into the
dependent flowers with the greatest ease, using the “hairs as footholds
while sucking the honey; but the smaller bees are impeded by them, and
when, having at length struggled through them, they reach the slippery
precipice above, they are completely baffled.” Mr. Belt says that he
watched many flowers during a whole season in North Wales, and “only
once saw a small bee reach the nectary, though many were seen trying in
vain to do so.” (3/5. ‘The Naturalist in Nicaragua’ 1874 page 132. But
it appears from H. Muller ‘Die Befruchtung der Blumen’ 1873 page 285,
that small insects sometimes succeed in entering the flowers.)

I covered a plant growing in its native soil in North Wales with a net,
and fertilised six flowers each with its own pollen, and six others with
pollen from a distinct plant growing within the distance of a few feet.
The covered plant was occasionally shaken with violence, so as to
imitate the effects of a gale of wind, and thus to facilitate as far as
possible self-fertilisation. It bore ninety-two flowers (besides the
dozen artificially fertilised), and of these only twenty-four produced
capsules; whereas almost all the flowers on the surrounding uncovered
plants were fruitful. Of the twenty-four spontaneously self-fertilised
capsules, only two contained their full complement of seed; six
contained a moderate supply; and the remaining sixteen extremely few
seeds. A little pollen adhering to the anthers after they had dehisced,
and accidentally falling on the stigma when mature, must have been the
means by which the above twenty-four flowers were partially
self-fertilised; for the margins of the corolla in withering do not curl
inwards, nor do the flowers in dropping off turn round on their axes, so
as to bring the pollen-covered hairs, with which the lower surface is
clothed, into contact with the stigma--by either of which means
self-fertilisation might be effected.

Seeds from the above crossed and self-fertilised capsules, after
germinating on bare sand, were planted in pairs on the opposite sides of
five moderately-sized pots, which were kept in the greenhouse. The
plants after a time appeared starved, and were therefore, without being
disturbed, turned out of their pots, and planted in the open ground in
two close parallel rows. They were thus subjected to tolerably severe
competition with one another; but not nearly so severe as if they had
been left in the pots. At the time when they were turned out, their
leaves were between 5 and 8 inches in length, and the longest leaf on
the finest plant on each side of each pot was measured, with the result
that the leaves of the crossed plants exceeded, on an average, those of
the self-fertilised plants by .4 of an inch.

In the following summer the tallest flower-stem on each plant, when
fully grown, was measured. There were seventeen crossed plants; but one
did not produce a flower-stem. There were also, originally, seventeen
self-fertilised plants, but these had such poor constitutions that no
less than nine died in the course of the winter and spring, leaving only
eight to be measured, as in Table 3/23.

TABLE 3/23. Digitalis purpurea.

The tallest Flower-stem on each Plant measured in inches: 0 means that
the Plant died before a Flower-stem was produced.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3:  Self-fertilised Plants.

Pot 1  :  53 6/8 :  27 4/8.
Pot 1  :  57 4/8 :  55 6/8.
Pot 1  :  57 6/8 :   0.
Pot 1  :  65     :   0.

Pot 2  :  34 4/8 :  39.
Pot 2  :  52 4/8 :  32.
Pot 2  :  63 6/8 :  21.

Pot 3  :  57 4/8 :  53 4/8.
Pot 3  :  53 4/8 :   0.
Pot 3  :  50 6/8 :   0.
Pot 3  :  37 2/8 :   0.

Pot 4  :  64 4/8 :  34 4/8.
Pot 4  :  37 4/8 :  23 6/8.
Pot 4  :  --     :   0.

Pot 5  :  53     :   0.
Pot 5  :  47 6/8 :   0.
Pot 5  :  34 6/8 :   0.

Total  : 821.25  : 287.00.

The average height of the flower-stems of the sixteen crossed plants is
here 51.33 inches; and that of the eight self-fertilised plants, 35.87;
or as 100 to 70. But this difference in height does not give at all a
fair idea of the vast superiority of the crossed plants. These latter
produced altogether sixty-four flower-stems, each plant producing, on an
average, exactly four flower-stems, whereas the eight self-fertilised
plants produced only fifteen flower-stems, each producing an average
only of 1.87 stems, and these had a less luxuriant appearance. We may
put the result in another way: the number of flower-stems on the crossed
plants was to those on an equal number of self-fertilised plants as 100
to 48.

Three crossed seeds in a state of germination were also planted in three
separate pots; and three self-fertilised seeds in the same state in
three other pots. These plants were therefore at first exposed to no
competition with one another, and when turned out of their pots into the
open ground they were planted at a moderate distance apart, so that they
were exposed to much less severe competition than in the last case. The
longest leaves on the three crossed plants, when turned out, exceeded
those on the self-fertilised plants by a mere trifle, namely, on an
average by .17 of an inch. When fully grown the three crossed plants
produced twenty-six flower-stems; the two tallest of which on each plant
were on an average 54.04 inches in height. The three self-fertilised
plants produced twenty-three flower-stems, the two tallest of which on
each plant had an average height of 46.18 inches. So that the difference
between these two lots, which hardly competed together, is much less
than in the last case when there was moderately severe competition,
namely, as 100 to 85, instead of as 100 to 70.

THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE
SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

A fine plant growing in my garden (one of the foregoing seedlings) was
covered with a net, and six flowers were crossed with pollen from
another flower on the same plant, and six others were fertilised with
their own pollen. All produced good capsules. The seeds from each were
placed in separate watch-glasses, and no difference could be perceived
by the eye between the two lots of seeds; and when they were weighed
there was no difference of any significance, as the seeds from the
self-fertilised capsules weighed 7.65 grains, whilst those from the
crossed capsules weighed 7.7 grains. Therefore the sterility of the
present species, when insects are excluded, is not due to the impotence
of pollen on the stigma of the same flower. Both lots of seeds and
seedlings were treated in exactly the same manner as in Table 3/23,
excepting that after the pairs of germinating seeds had been planted on
the opposite sides of eight pots, all the remaining seeds were thickly
sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young
plants during the following spring were turned out of their pots,
without being disturbed, and planted in the open ground in two rows, not
very close together, so that they were subjected to only moderately
severe competition with one another. Very differently to what occurred
in the first experiment, when the plants were subjected to somewhat
severe mutual competition, an equal number on each side either died or
did not produce flower-stems. The tallest flower-stems on the surviving
plants were measured, as shown in Table 3/24.

TABLE 3/24. Digitalis purpurea.

The tallest Flower-stem on each Plant measured in inches: 0 signifies
that the Plant died, or did not produce a Flower-stem.

Column 1: Number (Name) of Pot.

Column 2: Plants raised from a Cross between different Flowers on the
same Plant.

Column 3:  Plants raised from Flowers fertilised with their own Pollen.

Pot 1   :   49 4/8 :  45 5/8.
Pot 1   :   46 7/8 :  52.
Pot 1   :   43 6/8 :   0.

Pot 2   :   38 4/8 :  54 4/8.
Pot 2   :   47 4/8 :  47 4/8.
Pot 2   :    0     :  32 5/8.

Pot 3   :   54 7/8 :  46 5/8.

Pot 4   :   32 1/8 :  41 3/8.
Pot 4   :    0     :  29 7/8.
Pot 4   :   43 7/8 :  37 1/8.

Pot 5   :   46 6/8 :  42 1/8.
Pot 5   :   40 4/8 :  42 1/8.
Pot 5   :   43     :   0.

Pot 6   :   48 2/8 :  47 7/8.
Pot 6   :   46 2/8 :  48 3/8.

Pot 7   :   48 5/8 :  25.
Pot 7   :   42     :  40 5/8.

Pot 8   :   46 7/8 :  39 1/8.

Pot 9   :   49     :  30 3/8.
Pot 9   :   50 3/8 :  15.
Pot 9   :   46 3/8 :  36 7/8.
Pot 9   :   47 6/8 :  44 1/8.
Pot 9   :    0     :  31 6/8.
Crowded Plants.

Pot 10  :   46 4/8 :  47 7/8.
Pot 10  :   35 2/8 :   0.
Pot 10  :   24 5/8 :  34 7/8.
Pot 10  :   41 4/8 :  40 7/8.
Pot 10  :   17 3/8 :  41 1/8.
Crowded Plants.

Total   : 1078.00  : 995.38.

The average height of the flower-stems on the twenty-five crossed plants
in all the pots taken together is 43.12 inches, and that of the
twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to
test this result, the plants planted in pairs in Pots 1 and 8 were
considered by themselves, and the average height of the sixteen crossed
plants is here 44.9, and that of the sixteen self-fertilised plants
42.03, or as 100 to 94. Again, the plants raised from the thickly sown
seed in Pots 9 and 10, which were subjected to very severe mutual
competition, were taken by themselves, and the average height of the
nine crossed plants is 39.86, and that of the nine self-fertilised
plants 35.88, or as 100 to 90. The plants in these two latter pots (9
and 10), after being measured, were cut down close to the ground and
weighed: the nine crossed plants weighed 57.66 ounces, and the nine
self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we
may conclude, especially from the evidence of weight, that seedlings
from a cross between flowers on the same plant have a decided, though
not great, advantage over those from flowers fertilised with their own
pollen, more especially in the case of the plants subjected to severe
mutual competition. But the advantage is much less than that exhibited
by the crossed offspring of distinct plants, for these exceeded the
self-fertilised plants in height as 100 to 70, and in the number of
flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and
almost certainly from Mimulus, as with these two species a cross between
flowers on the same plant did no good.

CALCEOLARIA.

A BUSHY GREENHOUSE VARIETY, WITH YELLOW FLOWERS BLOTCHED WITH PURPLE.

The flowers in this genus are constructed so as to favour or almost
ensure cross-fertilisation (3/6. Hildebrand as quoted by H. Muller ‘Die
Befruchtung der Blumen’ 1873 page 277.); and Mr. Anderson remarks that
extreme care is necessary to exclude insects in order to preserve any
kind true. (3/7. ‘Gardeners’ Chronicle’ 1853 page 534.) He adds the
interesting statement, that when the corolla is cut quite away, insects,
as far as he has seen, never discover or visit the flowers. This plant
is, however, self-fertile if insects are excluded. So few experiments
were made by me, that they are hardly worth giving. Crossed and
self-fertilised seeds were sown on opposite sides of a pot, and after a
time the crossed seedlings slightly exceeded the self-fertilised in
height. When a little further grown, the longest leaves on the former
were very nearly 3 inches in length, whilst those on the self-fertilised
plants were only 2 inches. Owing to an accident, and to the pot being
too small, only one plant on each side grew up and flowered; the crossed
plant was 19 1/2 inches in height, and the self-fertilised one 15
inches; or as 100 to 77.

Linaria vulgaris.

It has been mentioned in the introductory chapter that two large beds of
this plant were raised by me many years ago from crossed and
self-fertilised seeds, and that there was a conspicuous difference in
height and general appearance between the two lots. The trial was
afterwards repeated with more care; but as this was one of the first
plants experimented on, my usual method was not followed. Seeds were
taken from wild plants growing in this neighbourhood and sown in poor
soil in my garden. Five plants were covered with a net, the others being
left exposed to the bees, which incessantly visit the flowers of this
species, and which, according to H. Muller, are the exclusive
fertilisers. This excellent observer remarks that, as the stigma lies
between the anthers and is mature at the same time with them,
self-fertilisation is possible. (3/8. ‘Die Befruchtung’ etc. page 279.)
But so few seeds are produced by protected plants, that the pollen and
stigma of the same flower seem to have little power of mutual
interaction. The exposed plants bore numerous capsules forming solid
spikes. Five of these capsules were examined and appeared to contain an
equal number of seeds; and these being counted in one capsule, were
found to be 166. The five protected plants produced altogether only
twenty-five capsules, of which five were much finer than all the others,
and these contained an average of 23.6 seeds, with a maximum in one
capsule of fifty-five. So that the number of seeds in the capsules on
the exposed plants to the average number in the finest capsules on the
protected plants was as 100 to 14.

Some of the spontaneously self-fertilised seeds from under the net, and
some seeds from the uncovered plants naturally fertilised and almost
certainly intercrossed by the bees, were sown separately in two large
pots of the same size; so that the two lots of seedlings were not
subjected to any mutual competition. Three of the crossed plants when in
full flower were measured, but no care was taken to select the tallest
plants; their heights were 7 4/8, 7 2/8, and 6 4/8 inches; averaging
7.08 in height. The three tallest of all the self-fertilised plants were
then carefully selected, and their heights were 6 3/8, 5 5/8, and 5 2/8,
averaging 5.75 in height. So that the naturally crossed plants were to
the spontaneously self-fertilised plants in height, at least as much as
100 to 81.

Verbascum thapsus.

The flowers of this plant are frequented by various insects, chiefly by
bees, for the sake of the pollen. Hermann Muller, however, has shown
(‘Die Befruchtung’ etc. page 277) that V. nigrum secretes minute drops
of nectar. The arrangement of the reproductive organs, though not at all
complex, favours cross-fertilisation; and even distinct species are
often crossed, for a greater number of naturally produced hybrids have
been observed in this genus than in almost any other. (3/9. I have given
a striking case of a large number of such hybrids between Verbascum
thapsus and lychnitis found growing wild: ‘Journal of Linnean Society
Botany’ volume 10 page 451.) Nevertheless the present species is
perfectly self-fertile, if insects are excluded; for a plant protected
by a net was as thickly loaded with fine capsules as the surrounding
uncovered plants. Verbascum lychnitis is rather less self-fertile, for
some protected plants did not yield quite so many capsules as the
adjoining uncovered plants.

Plants of Verbascum thapsus had been raised for a distinct purpose from
self-fertilised seeds; and some flowers on these plants were again
self-fertilised, yielding seed of the second self-fertilised generation;
and other flowers were crossed with pollen from a distinct plant. The
seeds thus produced were sown on the opposite sides of four large pots.
They germinated, however, so irregularly (the crossed seedlings
generally coming up first) that I was able to save only six pairs of
equal age. These when in full flower were measured, as in Table 3/25.

TABLE 3/25. Verbascum thapsus.

Heights of Plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3:  Self-fertilised Plants of the Second Generation.

Pot 1  :  76 :  53 4/8.

Pot 2  :  54 :  66.

Pot 3  :  62 :  75.
Pot 3  :  60 5/8 :   30 4/8.

Pot 4  :  73 :  62.
Pot 4  :  66 4/8 :  52.

Total  : 392.13  : 339.00.

We here see that two of the self-fertilised plants exceed in height
their crossed opponents. Nevertheless the average height of the six
crossed plants is 65.34 inches, and that of the six self-fertilised
plants 56.5 inches; or as 100 to 86.

Vandellia nummularifolia.

Seeds were sent to me by Mr. J. Scott from Calcutta of this small Indian
weed, which bears perfect and cleistogene flowers. (3/10. The convenient
term of CLEISTOGENE was proposed by Kuhn in an article on the present
genus in ‘Bot. Zeitung’ 1867 page 65.) The latter are extremely small,
imperfectly developed, and never expand, yet yield plenty of seeds. The
perfect and open flowers are also small, of a white colour with purple
marks; they generally produce seed, although the contrary has been
asserted; and they do so even if protected from insects. They have a
rather complicated structure, and appear to be adapted for
cross-fertilisation, but were not carefully examined by me. They are not
easy to fertilise artificially, and it is possible that some of the
flowers which I thought that I had succeeded in crossing were afterwards
spontaneously self-fertilised under the net. Sixteen capsules from the
crossed perfect flowers contained on an average ninety-three seeds (with
a maximum in one capsule of 137), and thirteen capsules from the
self-fertilised perfect flowers contained sixty-two seeds (with a
maximum in one capsule of 135); or as 100 to 67. But I suspect that this
considerable excess was accidental, as on one occasion nine crossed
capsules were compared with seven self-fertilised capsules (both
included in the above number), and they contained almost exactly the
same average number of seed. I may add that fifteen capsules from
self-fertilised cleistogene flowers contained on an average sixty-four
seeds, with a maximum in one of eighty-seven.

Crossed and self-fertilised seeds from the perfect flowers, and other
seeds from the self-fertilised cleistogene flowers, were sown in five
pots, each divided superficially into three compartments. The seedlings
were thinned at an early age, so that twenty plants were left in each of
the three divisions. The crossed plants when in full flower averaged 4.3
inches, and the self-fertilised plants from the perfect flowers 4.27
inches in height; or as 100 to 99. The self-fertilised plants from the
cleistogene flowers averaged 4.06 inches in height; so that the crossed
were in height to these latter plants as 100 to 94.

I determined to compare again the growth of plants raised from crossed
and self-fertilised perfect flowers, and obtained two fresh lots of
seeds. These were sown on opposite sides of five pots, but they were not
sufficiently thinned, so that they grew rather crowded. When fully
grown, all those above 2 inches in height were selected, all below this
standard being rejected; the former consisted of forty-seven crossed and
forty-one self-fertilised plants; thus a greater number of the crossed
than of the self-fertilised plants grew to a height of above 2 inches.
Of the crossed plants, the twenty-four tallest were on an average 3.6
inches in height; whilst the twenty-four tallest self-fertilised plants
were 3.38 inches in average height; or as 100 to 94. All these plants
were then cut down close to the ground, and the forty-seven crossed
plants weighed 1090.3 grains, and the forty-one self-fertilised plants
weighed 887.4 grains. Therefore an equal number of crossed and
self-fertilised would have been to each other in weight as 100 to 97.
From these several facts we may conclude that the crossed plants had
some real, though very slight, advantage in height and weight over the
self-fertilised plants, when grown in competition with one another.

The crossed plants were, however, inferior in fertility to the
self-fertilised. Six of the finest plants were selected out of the
forty-seven crossed plants, and six out of the forty-one self-fertilised
plants; and the former produced 598 capsules, whilst the latter or
self-fertilised plants produced 752 capsules. All these capsules were
the product of cleistogene flowers, for the plants did not bear during
the whole of this season any perfect flowers. The seeds were counted in
ten cleistogene capsules produced by crossed plants, and their average
number was 46.4 per capsule; whilst the number in ten cleistogene
capsules produced by the self-fertilised plants was 49.4; or as 100 to
106.

3. GESNERIACEAE.--Gesneria pendulina.

In Gesneria the several parts of the flower are arranged on nearly the
same plan as in Digitalis, and most or all of the species are
dichogamous. (3/11. Dr. Ogle ‘Popular Science Review’ January 1870 page
51.) Plants were raised from seed sent me by Fritz Muller from South
Brazil. Seven flowers were crossed with pollen from a distinct plant,
and produced seven capsules containing by weight 3.01 grains of seeds.
Seven flowers on the same plants were fertilised with their own pollen,
and their seven capsules contained exactly the same weight of seeds.
Germinating seeds were planted on opposite sides of four pots, and when
fully grown measured to the tips of their leaves.

TABLE 3/26. Gesneria pendulina.

Heights of Plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3:  Self-fertilised Plants.

Pot 1  :  42 2/8 :  39.
Pot 1  :  24 4/8 :  27 3/8.

Pot 2  :  33 :  30 6/8.
Pot 2  :  27 :  19 2/8.

Pot 3  :  33 4/8 :  31 7/8.
Pot 3  :  29 4/8 :  28 6/8.

Pot 4  :  30 6/8 :  29 6/8.
Pot 4  :  36     :  26 3/8.

Total  : 256.50  : 233.13.

The average height of the eight crossed plants is 32.06 inches, and that
of the eight self-fertilised plants 29.14; or as 100 to 90.

4. LABIATAE.--Salvia coccinea. (3/12. The admirable mechanical
adaptations in this genus for favouring or ensuring cross-fertilisation,
have been fully described by Sprengel, Hildebrand, Delpino, H. Muller,
Ogle, and others, in their several works.)

This species, unlike most of the others in the same genus, yields a good
many seeds when insects are excluded. I gathered ninety-eight capsules
produced by flowers spontaneously self-fertilised under a net, and they
contained on an average 1.45 seeds, whilst flowers artificially
fertilised with their own pollen, in which case the stigma will have
received plenty of pollen, yielded on an average 3.3 seeds, or more than
twice as many. Twenty flowers were crossed with pollen from a distinct
plant, and twenty-six were self-fertilised. There was no great
difference in the proportional number of flowers which produced capsules
by these two processes, or in the number of the contained seeds, or in
the weight of an equal number of seeds.

Seeds of both kinds were sown rather thickly on opposite sides of three
pots. When the seedlings were about 3 inches in height, the crossed
showed a slight advantage over the self-fertilised. When two-thirds
grown, the two tallest plants on each side of each pot were measured;
the crossed averaged 16.37 inches, and the self-fertilised 11.75 in
height; or as 100 to 71. When the plants were fully grown and had done
flowering, the two tallest plants on each side were again measured, with
the results shown in Table 3/27.

TABLE 3/27. Salvia coccinea.

Heights of Plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  32 6/8 :  25.
Pot 1  :  20     :  18 6/8.

Pot 2  :  32 3/8 :  20 6/8.
Pot 2  :  24 4/8 :  19 4/8.

Pot 3  :  29 4/8 :  25.
Pot 3  :  28     :  18.

Total  : 167.13  : 127.00.

It may be here seen that each of the six tallest crossed plants exceeds
in height its self-fertilised opponent; the former averaged 27.85
inches, whilst the six tallest self-fertilised plants averaged 21.16
inches; or as 100 to 76. In all three pots the first plant which
flowered was a crossed one. All the crossed plants together produced 409
flowers, whilst all the self-fertilised together produced only 232
flowers; or as 100 to 57. So that the crossed plants in this respect
were far more productive than the self-fertilised.

Origanum vulgare.

This plant exists, according to H. Muller, under two forms; one
hermaphrodite and strongly proterandrous, so that it is almost certain
to be fertilised by pollen from another flower; the other form is
exclusively female, has a smaller corolla, and must of course be
fertilised by pollen from a distinct plant in order to yield any seeds.
The plants on which I experimented were hermaphrodites; they had been
cultivated for a long period as a pot-herb in my kitchen garden, and
were, like so many long-cultivated plants, extremely sterile. As I felt
doubtful about the specific name I sent specimens to Kew, and was
assured that the species was Origanum vulgare. My plants formed one
great clump, and had evidently spread from a single root by stolons. In
a strict sense, therefore, they all belonged to the same individual. My
object in experimenting on them was, firstly, to ascertain whether
crossing flowers borne by plants having distinct roots, but all derived
asexually from the same individual, would be in any respect more
advantageous than self-fertilisation; and, secondly, to raise for future
trial seedlings which would constitute really distinct individuals.
Several plants in the above clump were covered by a net, and about two
dozen seeds (many of which, however, were small and withered) were
obtained from the flowers thus spontaneously self-fertilised. The
remainder of the plants were left uncovered and were incessantly visited
by bees, so that they were doubtless crossed by them. These exposed
plants yielded rather more and finer seed (but still very few) than did
the covered plants. The two lots of seeds thus obtained were sown on
opposite sides of two pots; the seedlings were carefully observed from
their first growth to maturity, but they did not differ at any period in
height or in vigour, the importance of which latter observation we shall
presently see. When fully grown, the tallest crossed plant in one pot
was a very little taller than the tallest self-fertilised plant on the
opposite side, and in the other pot exactly the reverse occurred. So
that the two lots were in fact equal; and a cross of this kind did no
more good than crossing two flowers on the same plant of Ipomoea or
Mimulus.

The plants were turned out of the two pots without being disturbed and
planted in the open ground, in order that they might grow more
vigorously. In the following summer all the self-fertilised and some of
the quasi-crossed plants were covered by a net. Many flowers on the
latter were crossed by me with pollen from a distinct plant, and others
were left to be crossed by the bees. These quasi-crossed plants produced
rather more seed than did the original ones in the great clump when left
to the action of the bees. Many flowers on the self-fertilised plants
were artificially self-fertilised, and others were allowed to fertilise
themselves spontaneously under the net, but they yielded altogether very
few seeds. These two lots of seeds--the product of a cross between
distinct seedlings, instead of as in the last case between plants
multiplied by stolons, and the product of self-fertilised flowers--were
allowed to germinate on bare sand, and several equal pairs were planted
on opposite sides of two LARGE pots. At a very early age the crossed
plants showed some superiority over the self-fertilised, which was ever
afterwards retained. When the plants were fully grown, the two tallest
crossed and the two tallest self-fertilised plants in each pot were
measured, as shown in Table 3/28. I regret that from want of time I did
not measure all the pairs; but the tallest on each side seemed fairly to
represent the average difference between the two lots.

TABLE 3/28. Origanum vulgare.

Heights of Plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants (two tallest in each pot).

Column 3: Self-fertilised Plants (two tallest in each pot).

Pot 1  : 26   : 24.
Pot 1  : 21   : 21.

Pot 2  : 17   : 12.
Pot 2  : 16   : 11 4/8.

Total  : 80.0 : 68.5.

The average height of the crossed plants is here 20 inches, and that of
the self-fertilised 17.12; or as 100 to 86. But this excess of height by
no means gives a fair idea of the vast superiority in vigour of the
crossed over the self-fertilised plants. The crossed flowered first and
produced thirty flower-stems, whilst the self-fertilised produced only
fifteen, or half the number. The pots were then bedded out, and the
roots probably came out of the holes at the bottom and thus aided their
growth. Early in the following summer the superiority of the crossed
plants, owing to their increase by stolons, over the self-fertilised
plants was truly wonderful. In Pot 1, and it should be remembered that
very large pots had been used, the oval clump of crossed plants was 10
by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2
inches in height; whilst the clump of self-fertilised plants, on the
opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across,
with the tallest young stem 4 inches in height. In Pot 2, the clump of
crossed plants was 18 by 9 inches across, with the tallest young stem 8
1/2 inches in height; whilst the clump of self-fertilised plants on the
opposite side of the same pot was 12 by 4 1/2 inches across, with the
tallest young stem 6 inches in height. The crossed plants during this
season, as during the last, flowered first. Both the crossed and
self-fertilised plants being left freely exposed to the visits of bees,
manifestly produced much more seed than their grand-parents,--the plants
of the original clump still growing close by in the same garden, and
equally left to the action of the bees.

5. ACANTHACEAE.--Thunbergia alata.

It appears from Hildebrand’s description (‘Botanische Zeitung’ 1867 page
285) that the conspicuous flowers of this plant are adapted for
cross-fertilisation. Seedlings were twice raised from purchased seed;
but during the early summer, when first experimented on, they were
extremely sterile, many of the anthers containing hardly any pollen.
Nevertheless, during the autumn these same plants spontaneously produced
a good many seeds. Twenty-six flowers during the two years were crossed
with pollen from a distinct plant, but they yielded only eleven
capsules; and these contained very few seeds! Twenty-eight flowers were
fertilised with pollen from the same flower, and these yielded only ten
capsules, which, however, contained rather more seed than the crossed
capsules. Eight pairs of germinating seeds were planted on opposite
sides of five pots; and exactly half the crossed and half the
self-fertilised plants exceeded their opponents in height. Two of the
self-fertilised plants died young, before they were measured, and their
crossed opponents were thrown away. The six remaining pairs of these
grew very unequally, some, both of the crossed and self-fertilised
plants, being more than twice as tall as the others. The average height
of the crossed plants was 60 inches, and that of the self-fertilised
plants 65 inches, or as 100 to 108. A cross, therefore, between distinct
individuals here appears to do no good; but this result deduced from so
few plants in a very sterile condition and growing very unequally,
obviously cannot be trusted.]



CHAPTER IV.

CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.

Brassica oleracea, crossed and self-fertilised plants.
Great effect of a cross with a fresh stock on the weight of the
offspring.
Iberis umbellata.
Papaver vagum.
Eschscholtzia californica, seedlings from a cross with a fresh stock not
more vigorous, but more fertile than the self-fertilised seedlings.
Reseda lutea and odorata, many individuals sterile with their own pollen.
Viola tricolor, wonderful effects of a cross.
Adonis aestivalis.
Delphinium consolida.
Viscaria oculata, crossed plants hardly taller, but more fertile than
the self-fertilised.
Dianthus caryophyllus, crossed and self-fertilised plants compared for
four generations.
Great effects of a cross with a fresh stock.
Uniform colour of the flowers on the self-fertilised plants.
Hibiscus africanus.

[6. CRUCIFERAE.--Brassica oleracea.

VAR. CATTELL’S EARLY BARNES CABBAGE.

The flowers of the common cabbage are adapted, as shown by H. Muller,
for cross-fertilisation, and should this fail, for self-fertilisation.
(4/1. ‘Die Befruchtung’ etc. page 139.) It is well known that the
varieties are crossed so largely by insects, that it is impossible to
raise pure kinds in the same garden, if more than one kind is in flower
at the same time. Cabbages, in one respect, were not well fitted for my
experiments, as, after they had formed heads, they were often difficult
to measure. The flower-stems also differ much in height; and a poor
plant will sometimes throw up a higher stem than that of a fine plant.
In the later experiments, the fully-grown plants were cut down and
weighed, and then the immense advantage from a cross became manifest.

A single plant of the above variety was covered with a net just before
flowering, and was crossed with pollen from another plant of the same
variety growing close by; and the seven capsules thus produced contained
on an average 16.3 seeds, with a maximum of twenty in one capsule. Some
flowers were artificially self-fertilised, but their capsules did not
contain so many seeds as those from flowers spontaneously
self-fertilised under the net, of which a considerable number were
produced. Fourteen of these latter capsules contained on an average 4.1
seeds, with a maximum in one of ten seeds; so that the seeds in the
crossed capsules were in number to those in the self-fertilised capsules
as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed
3.88 grains, were, however, a little finer than those from the crossed
capsules, fifty-eight of which weighed 3.76 grains. When few seeds are
produced, these seem often to be better nourished and to be heavier than
when many are produced.

The two lots of seeds in an equal state of germination were planted,
some on opposite sides of a single pot, and some in the open ground. The
young crossed plants in the pot at first exceeded by a little in height
the self-fertilised; then equalled them; were then beaten; and lastly
were again victorious. The plants, without being disturbed, were turned
out of the pot, and planted in the open ground; and after growing for
some time, the crossed plants, which were all of nearly the same height,
exceeded the self-fertilised ones by 2 inches. When they flowered, the
flower-stems of the tallest crossed plant exceeded that of the tallest
self-fertilised plant by 6 inches. The other seedlings which were
planted in the open ground stood separate, so that they did not compete
with one another; nevertheless the crossed plants certainly grew to a
rather greater height than the self-fertilised; but no measurements were
made. The crossed plants which had been raised in the pot, and those
planted in the open ground, all flowered a little before the
self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Some flowers on the crossed plants of the last generation were again
crossed with pollen from another crossed plant, and produced fine
capsules. The flowers on the self-fertilised plants of the last
generation were allowed to fertilise themselves spontaneously under a
net, and they produced some remarkably fine capsules. The two lots of
seeds thus produced germinated on sand, and eight pairs were planted on
opposite sides of four pots. These plants were measured to the tips of
their leaves on the 20th of October of the same year, and the eight
crossed plants averaged in height 8.4 inches, whilst the self-fertilised
averaged 8.53 inches, so that the crossed were a little inferior in
height, as 100 to 101.5. By the 5th of June of the following year these
plants had grown much bulkier, and had begun to form heads. The crossed
had now acquired a marked superiority in general appearance, and
averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31
inches; or as 100 to 91. The plants were then turned out of their pots
and planted undisturbed in the open ground. By the 5th of August their
heads were fully formed, but several had grown so crooked that their
heights could hardly be measured with accuracy. The crossed plants,
however, were on the whole considerably taller than the self-fertilised.
In the following year they flowered; the crossed plants flowering before
the self-fertilised in three of the pots, and at the same time in Pot 2.
The flower-stems were now measured, as shown in Table 4/29.

TABLE 3/29. Brassica oleracea.

Measured in inches to tops of flower-stems: 0 signifies that a
Flower-stem was not formed.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  49 2/8 :  44.
Pot 1  :  39 4/8 :  41.

Pot 2  :  37 4/8 :  38.
Pot 2  :  33 4/8 :  35 4/8.

Pot 3  :  47     :  51 1/8.
Pot 3  :  40     :  41 2/8.
Pot 3  :  42     :  46 4/8.

Pot 4  :  43 6/8 :  20 2/8.
Pot 4  :  37 2/8 :  33 3/8.
Pot 4  :   0     :   0.

Total  : 369.75  : 351.00.

The nine flower-stems on the crossed plants here average 41.08 inches,
and the nine on the self-fertilised plants 39 inches in height, or as
100 to 95. But this small difference, which, moreover, depended almost
wholly on one of the self-fertilised plants being only 20 inches high,
does not in the least show the vast superiority of the crossed over the
self-fertilised plants. Both lots, including the two plants in Pot 4,
which did not flower, were now cut down close to the ground and weighed,
but those in Pot 2 were excluded, for they had been accidentally injured
by a fall during transplantation, and one was almost killed. The eight
crossed plants weighed 219 ounces, whilst the eight self-fertilised
plants weighed only 82 ounces, or as 100 to 37; so that the superiority
of the former over the latter in weight was great.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

Some flowers on a crossed plant of the last or second generation were
fertilised, without being castrated, by pollen taken from a plant of the
same variety, but not related to my plants, and brought from a nursery
garden (whence my seeds originally came) having a different soil and
aspect. The flowers on the self-fertilised plants of the last or second
generation (Table 4/29) were allowed to fertilise themselves
spontaneously under a net, and yielded plenty of seeds. These latter and
the crossed seeds, after germinating on sand, were planted in pairs on
the opposite sides of six large pots, which were kept at first in a cool
greenhouse. Early in January their heights were measured to the tips of
their leaves. The thirteen crossed plants averaged 13.16 inches in
height, and the twelve (for one had died) self-fertilised plants
averaged 13.7 inches, or as 100 to 104; so that the self-fertilised
plants exceeded by a little the crossed plants.

TABLE 3/30. Brassica oleracea.

Weights in ounces of plants after they had formed heads.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants from Pollen of fresh Stock.

Column 3: Self-fertilised Plants of the Third Generation.

Pot 1  : 130     :  18 2/4.

Pot 2  :  74     :  34 3/4.

Pot 3  : 121     :  17 2/4.

Pot 4  : 127 2/4 :  14.

Pot 5  :  90     :  11 2/4.

Pot 6  : 106 2/4 :  46.

Total  : 649.00  : 142.25.

Early in the spring the plants were gradually hardened, and turned out
of their pots into the open ground without being disturbed. By the end
of August the greater number had formed fine heads, but several grew
extremely crooked, from having been drawn up to the light whilst in the
greenhouse. As it was scarcely possible to measure their heights, the
finest plant on each side of each pot was cut down close to the ground
and weighed. In Table 4/30 we have the result.

The six finest crossed plants average 108.16 ounces, whilst the six
finest self-fertilised plants average only 23.7 ounces, or as 100 to 22.
This difference shows in the clearest manner the enormous benefit which
these plants derived from a cross with another plant belonging to the
same sub-variety, but to a fresh stock, and grown during at least the
three previous generations under somewhat different conditions.

THE OFFSPRING FROM A CUT-LEAVED, CURLED, AND VARIEGATED WHITE-GREEN
CABBAGE CROSSED WITH A CUT-LEAVED, CURLED, AND VARIEGATED CRIMSON-GREEN
CABBAGE, COMPARED WITH THE SELF-FERTILISED OFFSPRING FROM THE TWO
VARIETIES.

These trials were made, not for the sake of comparing the growth of the
crossed and self-fertilised seedlings, but because I had seen it stated
that these varieties would not naturally intercross when growing
uncovered and near one another. This statement proved quite erroneous;
but the white-green variety was in some degree sterile in my garden,
producing little pollen and few seeds. It was therefore no wonder that
seedlings raised from the self-fertilised flowers of this variety were
greatly exceeded in height by seedlings from a cross between it and the
more vigorous crimson-green variety; and nothing more need be said about
this experiment.

The seedlings from the reciprocal cross, that is, from the crimson-green
variety fertilised with pollen from the white-green variety, offer a
somewhat more curious case. A few of these crossed seedlings reverted to
a pure green variety with their leaves less cut and curled, so that they
were altogether in a much more natural state, and these plants grew more
vigorously and taller than any of the others. Now it is a strange fact
that a much larger number of the self-fertilised seedlings from the
crimson-green variety than of the crossed seedlings thus reverted; and
as a consequence the self-fertilised seedlings grew taller by 2 1/2
inches on an average than the crossed seedlings, with which they were
put into competition. At first, however, the crossed seedlings exceeded
the self-fertilised by an average of a quarter of an inch. We thus see
that reversion to a more natural condition acted more powerfully in
favouring the ultimate growth of these plants than did a cross; but it
should be remembered that the cross was with a semi-sterile variety
having a feeble constitution.

Iberis umbellata.

VAR. KERMESIANA.

This variety produced plenty of spontaneously self-fertilised seed under
a net. Other plants in pots in the greenhouse were left uncovered, and
as I saw small flies visiting the flowers, it seemed probable that they
would be intercrossed. Consequently seeds supposed to have been thus
crossed and spontaneously self-fertilised seeds were sown on opposite
sides of a pot. The self-fertilised seedlings grew from the first
quicker than the supposed crossed seedlings, and when both lots were in
full flower the former were from 5 to 6 inches higher than the crossed!
I record in my notes that the self-fertilised seeds from which these
self-fertilised plants were raised were not so well ripened as the
crossed; and this may possibly have caused the great difference in their
growth, in a somewhat analogous manner as occurred with the
self-fertilised plants of the eighth generation of Ipomoea raised from
unhealthy parents. It is a curious circumstance, that two other lots of
the above seeds were sown in pure sand mixed with burnt earth, and
therefore without any organic matter; and here the supposed crossed
seedlings grew to double the height of the self-fertilised, before both
lots died, as necessarily occurred at an early period. We shall
hereafter meet with another case apparently analogous to this of Iberis
in the third generation of Petunia.

The above self-fertilised plants were allowed to fertilise themselves
again under a net, yielding self-fertilised plants of the second
generation, and the supposed crossed plants were crossed by pollen of a
distinct plant; but from want of time this was done in a careless
manner, namely, by smearing one head of expanded flowers over another. I
should have thought that this would have succeeded, and perhaps it did
so; but the fact of 108 of the self-fertilised seeds weighing 4.87
grains, whilst the same number of the supposed crossed seeds weighed
only 3.57 grains, does not look like it. Five seedlings from each lot of
seeds were raised, and the self-fertilised plants, when fully grown,
exceeded in average height by a trifle (namely .4 of an inch) the five
probably crossed plants. I have thought it right to give this case and
the last, because had the supposed crossed plants proved superior to the
self-fertilised in height, I should have assumed without doubt that the
former had really been crossed. As it is, I do not know what to
conclude.

Being much surprised at the two foregoing trials, I determined to make
another, in which there should be no doubt about the crossing. I
therefore fertilised with great care (but as usual without castration)
twenty-four flowers on the supposed crossed plants of the last
generation with pollen from distinct plants, and thus obtained
twenty-one capsules. The self-fertilised plants of the last generation
were allowed to fertilise themselves again under a net, and the
seedlings reared from these seeds formed the third self-fertilised
generation. Both lots of seeds, after germinating on bare sand, were
planted in pairs on the opposite sides of two pots. All the remaining
seeds were sown crowded on opposite sides of a third pot; but as all the
self-fertilised seedlings in this latter pot died before they grew to
any considerable height, they were not measured. The plants in Pots 1
and 2 were measured when between 7 and 8 inches in height, and the
crossed exceeded the self-fertilised in average height by 1.57 inches.
When fully grown they were again measured to the summits of their
flower-heads, with the following result:--

TABLE 4/31. Iberis umbellata.

Heights of plants to the summits of their flower-heads, in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3:  Self-fertilised Plants of the Third Generation.

Pot 1  :  18     :  19.
Pot 1  :  21     :  21.
Pot 1  :  18 2/8 :  19 4/8.

Pot 2  :  19     :  16 6/8.
Pot 2  :  18 4/8 :   7 4/8.
Pot 2  :  17 6/8 :  14 4/8.
Pot 2  :  21 3/8 :  16 4/8.

Total  : 133.88  : 114.75.

The average height of the seven crossed plants is here 19.12 inches, and
that of the seven self-fertilised plants 16.39, or as 100 to 86. But as
the plants on the self-fertilised side grew very unequally, this ratio
cannot be fully trusted, and is probably too high. In both pots a
crossed plant flowered before any one of the self-fertilised. These
plants were left uncovered in the greenhouse; but from being too much
crowded they were not very productive. The seeds from all seven plants
of both lots were counted; the crossed produced 206, and the
self-fertilised 154; or as 100 to 75.

CROSS BY A FRESH STOCK.

From the doubts caused by the two first trials, in which it was not
known with certainty that the plants had been crossed; and from the
crossed plants in the last experiment having been put into competition
with plants self-fertilised for three generations, which moreover grew
very unequally, I resolved to repeat the trial on a larger scale, and in
a rather different manner. I obtained seeds of the same crimson variety
of Iberis umbellata from another nursery garden, and raised plants from
them. Some of these plants were allowed to fertilise themselves
spontaneously under a net; others were crossed by pollen taken from
plants raised from seed sent me by Dr. Durando from Algiers, where the
parent-plants had been cultivated for some generations. These latter
plants differed in having pale pink instead of crimson flowers, but in
no other respect. That the cross had been effective (though the flowers
on the crimson mother-plant had NOT been castrated) was well shown when
the thirty crossed seedlings flowered, for twenty-four of them produced
pale pink flowers, exactly like those of their father; the six others
having crimson flowers exactly like those of their mother and like those
of all the self-fertilised seedlings. This case offers a good instance
of a result which not rarely follows from crossing varieties having
differently coloured flowers; namely, that the colours do not blend, but
resemble perfectly those either of the father or mother plant. The seeds
of both lots, after germinating on sand, were planted on opposite sides
of eight pots. When fully grown, the plants were measured to the summits
of the flower-heads, as shown in Table 4/32.

TABLE 4/32. Iberis umbellata.

Height of Plants to the summits of the flower-heads, measured in inches:
0 signifies that the Plant died.

Column 1: Number (Name) of Pot.

Column 2: Plants from a Cross with a fresh Stock.

Column 3: Plants from Spontaneously Self-fertilised Seeds.

Pot 1  :  18 6/8 :  17 3/8.
Pot 1  :  17 5/8 :  16 7/8.
Pot 1  :  17 6/8 :  13 1/8.
Pot 1  :  20 1/8 :  15 3/8.

Pot 2  :  20 2/8 :   0.
Pot 2  :  15 7/8 :  16 6/8.
Pot 2  :  17     :  15 2/8.

Pot 3  :  19 2/8 :  13 6/8.
Pot 3  :  18 1/8 :  14 2/8.
Pot 3  :  15 2/8 :  13 4/8.

Pot 4  :  17 1/8 :  16 4/8.
Pot 4  :  18 7/8 :  14 4/8.
Pot 4  :  17 5/8 :  16.
Pot 4  :  15 6/8 :  15 3/8.
Pot 4  :  14 4/8 :  14 7/8.

Pot 5  :  18 1/8 :  16 4/8.
Pot 5  :  14 7/8 :  16 2/8.
Pot 5  :  16 2/8 :  14 2/8.
Pot 5  :  15 5/8 :  14 2/8.
Pot 5  :  12 4/8 :  16 1/8.

Pot 6  :  18 6/8 :  16 1/8.
Pot 6  :  18 6/8 :  15.
Pot 6  :  17 3/8 :  15 2/8.

Pot 7  :  18     :  16 3/8.
Pot 7  :  16 4/8 :  14 4/8.
Pot 7  :  18 2/8 :  13 5/8.

Pot 8  :  20 6/8 :  15 6/8.
Pot 8  :  17 7/8 :  16 3/8.
Pot 8  :  13 5/8 :  20 2/8.
Pot 8  :  19 2/8 :  15 6/8.

Total  : 520.38  :  449.88.

The average height of the thirty crossed plants is here 17.34, and that
of the twenty-nine self-fertilised plants (one having died) 15.51, or as
100 to 89. I am surprised that the difference did not prove somewhat
greater, considering that in the last experiment it was as 100 to 86;
but this latter ratio, as before explained, was probably too great. It
should, however, be observed that in the last experiment (Table 4/31),
the crossed plants competed with plants of the third self-fertilised
generation; whilst in the present case, plants derived from a cross with
a fresh stock competed with self-fertilised plants of the first
generation.

The crossed plants in the present case, as in the last, were more
fertile than the self-fertilised, both lots being left uncovered in the
greenhouse. The thirty crossed plants produced 103 seed-bearing
flowers-heads, as well as some heads which yielded no seeds; whereas the
twenty-nine self-fertilised plants produced only 81 seed-bearing heads;
therefore thirty such plants would have produced 83.7 heads. We thus get
the ratio of 100 to 81, for the number of seed-bearing flower-heads
produced by the crossed and self-fertilised plants. Moreover, a number
of seed-bearing heads from the crossed plants, compared with the same
number from the self-fertilised, yielded seeds by weight, in the ratio
of 100 to 92. Combining these two elements, namely, the number of
seed-bearing heads and the weight of seeds in each head, the
productiveness of the crossed to the self-fertilised plants was as 100
to 75.

The crossed and self-fertilised seeds, which remained after the above
pairs had been planted, (some in a state of germination and some not
so), were sown early in the year out of doors in two rows. Many of the
self-fertilised seedlings suffered greatly, and a much larger number of
them perished than of the crossed. In the autumn the surviving
self-fertilised plants were plainly less well-grown than the crossed
plants.

7. PAPAVERACEAE.--Papaver vagum.

A SUB-SPECIES OF Papaver dubium, FROM THE SOUTH OF FRANCE.

The poppy does not secrete nectar, but the flowers are highly
conspicuous and are visited by many pollen-collecting bees, flies and
beetles. The anthers shed their pollen very early, and in the case of
Papaver rhoeas, it falls on the circumference of the radiating stigmas,
so that this species must often be self-fertilised; but with Papaver
dubium the same result does not follow (according to H. Muller ‘Die
Befruchtung’ page 128), owing to the shortness of the stamens, unless
the flower happens to stand inclined. The present species, therefore,
does not seem so well fitted for self-fertilisation as most of the
others. Nevertheless Papaver vagum produced plenty of capsules in my
garden when insects were excluded, but only late in the season. I may
here add that Papaver somniferum produces an abundance of spontaneously
self-fertilised capsules, as Professor H. Hoffmann likewise found to be
the case. (4/2. ‘Zur Speciesfrage’ 1875 page 53.) Some species of
Papaver cross freely when growing in the same garden, as I have known to
be the case with Papaver bracteatum and orientale.

Plants of Papaver vagum were raised from seeds sent me from Antibes
through the kindness of Dr. Bornet. Some little time after the flowers
had expanded, several were fertilised with their own pollen, and others
(not castrated) with pollen from a distinct individual; but I have
reason to believe, from observations subsequently made, that these
flowers had been already fertilised by their own pollen, as this process
seems to take place soon after their expansion. (4/3. Mr. J. Scott found
‘Report on the Experimental Culture of the Opium Poppy’ Calcutta 1874
page 47, in the case of Papaver somniferum, that if he cut away the
stigmatic surface before the flower had expanded, no seeds were
produced; but if this was done “on the second day, or even a few hours
after the expansion of the flower on the first day, a partial
fertilisation had already been effected, and a few good seeds were
almost invariably produced.” This proves at how early a period
fertilisation takes place.) I raised, however, a few seedlings of both
lots, and the self-fertilised rather exceeded the crossed plants in
height.

Early in the following year I acted differently, and fertilised seven
flowers, very soon after their expansion, with pollen from another
plant, and obtained six capsules. From counting the seeds in a
medium-sized one, I estimated that the average number in each was at
least 120. Four out of twelve capsules, spontaneously self-fertilised at
the same time, were found to contain no good seeds; and the remaining
eight contained on an average 6.6 seeds per capsule. But it should be
observed that later in the season the same plants produced under a net
plenty of very fine spontaneously self-fertilised capsules.

The above two lots of seeds, after germinating on sand, were planted in
pairs on opposite sides of five pots. The two lots of seedlings, when
half an inch in height, and again when 6 inches high, were measured to
the tips of their leaves, but presented no difference. When fully grown,
the flower-stalks were measured to the summits of the seed capsules,
with the following result:--

TABLE 4/33. Papaver vagum.

Heights of flower-stalks to the summits of the seed capsules measured in
inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  24 2/8 :  21.
Pot 1  :  30     :  26 5/8.
Pot 1  :  18 4/8 :  16.

Pot 2  :  14 4/8 :  15 3/8.
Pot 2  :  22     :  20 1/8.
Pot 2  :  19 5/8 :  14 1/8.
Pot 2  :  21 5/8 :  16 4/8.

Pot 3  :  20 6/8 :  19 2/8.
Pot 3  :  20 2/8 :  13 2/8.
Pot 3  :  20 6/8 :  18.

Pot 4  :  25 3/8 :  23 2/8.
Pot 4  :  24 2/8 :  23.

Pot 5  :  20     :  18 3/8.
Pot 5  :  27 7/8 :  27.
Pot 5  :  19     :  21 2/8.

Total  : 328.75  : 293.13.

The fifteen crossed plants here average 21.91 inches, and the fifteen
self-fertilised plants 19.54 inches in height, or as 100 to 89. These
plants did not differ in fertility, as far as could be judged by the
number of capsules produced, for there were seventy-five on the crossed
side and seventy-four on the self-fertilised side.

Eschscholtzia californica.

This plant is remarkable from the crossed seedlings not exceeding in
height or vigour the self-fertilised. On the other hand, a cross greatly
increases the productiveness of the flowers on the parent-plant, and is
indeed sometimes necessary in order that they should produce any seed;
moreover, plants thus derived are themselves much more fertile than
those raised from self-fertilised flowers; so that the whole advantage
of a cross is confined to the reproductive system. It will be necessary
for me to give this singular case in considerable detail.

Twelve flowers on some plants in my flower-garden were fertilised with
pollen from distinct plants, and produced twelve capsules; but one of
these contained no good seed. The seeds of the eleven good capsules
weighed 17.4 grains. Eighteen flowers on the same plants were fertilised
with their own pollen and produced twelve good capsules, which contained
13.61 grains weight of seed. Therefore an equal number of crossed and
self-fertilised capsules would have yielded seed by weight as 100 to 71.
(4/4. Professor Hildebrand experimented on plants in Germany on a larger
scale than I did, and found them much more self-fertile. Eighteen
capsules, produced by cross-fertilisation, contained on an average
eighty-five seeds, whilst fourteen capsules from self-fertilised flowers
contained on an average only nine seeds; that is, as 100 to 11: ‘Jahrb.
fur Wissen Botanik.’ B. 7 page 467.) If we take into account of the fact
that a much greater proportion of flowers produced capsules when crossed
than when self-fertilised, the relative fertility of the crossed to the
self-fertilised flowers was as 100 to 52. Nevertheless these plants,
whilst still protected by the net, spontaneously produced a considerable
number of self-fertilised capsules.

The seeds of the two lots after germinating on sand were planted in
pairs on the opposite sides of four large pots. At first there was no
difference in their growth, but ultimately the crossed seedlings
exceeded the self-fertilised considerably in height, as shown in Table
4/34. But I believe from the cases which follow that this result was
accidental, owing to only a few plants having been measured, and to one
of the self-fertilised plants having grown only to a height of 15
inches. The plants had been kept in the greenhouse, and from being drawn
up to the light had to be tied to sticks in this and the following
trials. They were measured to the summits of their flower-stems.

TABLE 4/34. Eschscholtzia californica.

Heights of Plants to the summits of their flower-stems measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  33 4/8 :  25.

Pot 2  :  34 2/8 :  35.

Pot 3  :  29     :  27 2/8.

Pot 4  :  22     :  15.

Total  : 118.75  : 102.25.

The four crossed plants here average 29.68 inches, and the four
self-fertilised 25.56 in height; or as 100 to 86. The remaining seeds
were sown in a large pot in which a Cineraria had long been growing; and
in this case again the two crossed plants on the one side greatly
exceeded in height the two self-fertilised plants on the opposite side.
The plants in the above four pots from having been kept in the
greenhouse did not produce on this or any other similar occasion many
capsules; but the flowers on the crossed plants when again crossed were
much more productive than the flowers on the self-fertilised plants when
again self-fertilised. These plants after seeding were cut down and kept
in the greenhouse; and in the following year, when grown again, their
relative heights were reversed, as the self-fertilised plants in three
out of the four pots were now taller than and flowered before the
crossed plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The fact just given with respect to the growth of the cut-down plants
made me doubtful about my first trial, so I determined to make another
on a larger scale with crossed and self-fertilised seedlings raised from
the crossed and self-fertilised plants of the last generation. Eleven
pairs were raised and grown in competition in the usual manner; and now
the result was different, for the two lots were nearly equal during
their whole growth. It would therefore be superfluous to give a table of
their heights. When fully grown and measured, the crossed averaged
32.47, and the self-fertilised 32.81 inches in height; or as 100 to 101.
There was no great difference in the number of flowers and capsules
produced by the two lots when both were left freely exposed to the
visits of insects.

PLANTS RAISED FROM BRAZILIAN SEED.

Fritz Muller sent me from South Brazil seeds of plants which were there
absolutely sterile when fertilised with pollen from the same plant, but
were perfectly fertile when fertilised with pollen from any other plant.
The plants raised by me in England from these seeds were examined by
Professor Asa Gray, and pronounced to belong to E. Californica, with
which they were identical in general appearance. Two of these plants
were covered by a net, and were found not to be so completely
self-sterile as in Brazil. But I shall recur to this subject in another
part of this work. Here it will suffice to state that eight flowers on
these two plants, fertilised with pollen from another plant under the
net, produced eight fine capsules, each containing on an average about
eighty seeds. Eight flowers on these same plants, fertilised with their
own pollen, produced seven capsules, which contained on an average only
twelve seeds, with a maximum in one of sixteen seeds. Therefore the
cross-fertilised capsules, compared with the self-fertilised, yielded
seeds in the ratio of about 100 to 15. These plants of Brazilian
parentage differed also in a marked manner from the English plants in
producing extremely few spontaneously self-fertilised capsules under a
net.

Crossed and self-fertilised seeds from the above plants, after
germinating on bare sand, were planted in pairs on the opposite sides of
five large pots. The seedlings thus raised were the grandchildren of the
plants which grew in Brazil; the parents having been grown in England.
As the grandparents in Brazil absolutely require cross-fertilisation in
order to yield any seeds, I expected that self-fertilisation would have
proved very injurious to these seedlings, and that the crossed ones
would have been greatly superior in height and vigour to those raised
from self-fertilised flowers. But the result showed that my anticipation
was erroneous; for as in the last experiment with plants of the English
stock, so in the present one, the self-fertilised plants exceeded the
crossed by a little in height. It will be sufficient to state that the
fourteen crossed plants averaged 44.64, and the fourteen self-fertilised
45.12 inches in height; or as 100 to 101.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I now tried a different experiment. Eight flowers on the self-fertilised
plants of the last experiment (i.e., grandchildren of the plants which
grew in Brazil) were again fertilised with pollen from the same plant,
and produced five capsules, containing on an average 27.4 seeds, with a
maximum in one of forty-two seeds. The seedlings raised from these seeds
formed the second SELF-FERTILISED generation of the Brazilian stock.

Eight flowers on one of the crossed plants of the last experiment were
crossed with pollen from another grandchild, and produced five capsules.
These contained on an average 31.6 seeds, with a maximum in one of
forty-nine seeds. The seedlings raised from these seeds may be called
the INTERCROSSED.

Lastly, eight other flowers on the crossed plants of the last experiment
were fertilised with pollen from a plant of the English stock, growing
in my garden, and which must have been exposed during many previous
generations to very different conditions from those to which the
Brazilian progenitors of the mother-plant had been subjected. These
eight flowers produced only four capsules, containing on an average 63.2
seeds, with a maximum in one of ninety. The plants raised from these
seeds may be called the ENGLISH-CROSSED. As far as the above averages
can be trusted from so few capsules, the English-crossed capsules
contained twice as many seeds as the intercrossed, and rather more than
twice as many as the self-fertilised capsules. The plants which yielded
these capsules were grown in pots in the greenhouse, so that their
absolute productiveness must not be compared with that of plants growing
out of doors.

The above three lots of seeds, namely, the self-fertilised,
intercrossed, and English-crossed, were planted in an equal state of
germination (having been as usual sown on bare sand) in nine large pots,
each divided into three parts by superficial partitions. Many of the
self-fertilised seeds germinated before those of the two crossed lots,
and these were of course rejected. The seedlings thus raised are the
great-grandchildren of the plants which grew in Brazil. When they were
from 2 to 4 inches in height, the three lots were equal. They were
measured when four-fifths grown, and again when fully grown, and as
their relative heights were almost exactly the same at these two ages, I
will give only the last measurements. The average height of the nineteen
English-crossed plants was 45.92 inches; that of the eighteen
intercrossed plants (for one died), 43.38; and that of the nineteen
self-fertilised plants, 50.3 inches. So that we have the following
ratios in height:--

The English-crossed to the self-fertilised plants, as 100 to 109.

The English-crossed to the intercrossed plants, as 100 to 94.

The intercrossed to the self-fertilised plants, as 100 to 116.

After the seed-capsules had been gathered, all these plants were cut
down close to the ground and weighed. The nineteen English crossed
plants weighed 18.25 ounces; the intercrossed plants (with their weight
calculated as if there had been nineteen) weighed 18.2 ounces; and the
nineteen self-fertilised plants, 21.5 ounces. We have therefore for the
weights of the three lots of plants the following ratios:--

The English-crossed to the self-fertilised plants, as 100 to 118.

The English-crossed to the intercrossed plants, as 100 to 100.

The intercrossed to the self-fertilised plants, as 100 to 118.

We thus see that in weight, as in height, the self-fertilised plants had
a decided advantage over the English-crossed and intercrossed plants.

The remaining seeds of the three kinds, whether or not in a state of
germination, were sown in three long parallel rows in the open ground;
and here again the self-fertilised seedlings exceeded in height by
between 2 and 3 inches the seedlings in the two other rows, which were
of nearly equal heights. The three rows were left unprotected throughout
the winter, and all the plants were killed, with the exception of two of
the self-fertilised; so that as far as this little bit of evidence goes,
some of the self-fertilised plants were more hardy than any of the
crossed plants of either lot.

We thus see that the self-fertilised plants which were grown in the nine
pots were superior in height (as 116 to 100), and in weight (as 118 to
100), and apparently in hardiness, to the intercrossed plants derived
from a cross between the grandchildren of the Brazilian stock. The
superiority is here much more strongly marked than in the second trial
with the plants of the English stock, in which the self-fertilised were
to the crossed in height as 101 to 100. It is a far more remarkable
fact--if we bear in mind the effects of crossing plants with pollen from
a fresh stock in the cases of Ipomoea, Mimulus, Brassica, and
Iberis--that the self-fertilised plants exceeded in height (as 109 to
100), and in weight (as 118 to 100), the offspring of the Brazilian
stock crossed by the English stock; the two stocks having been long
subjected to widely different conditions.

If we now turn to the fertility of the three lots of plants we find a
very different result. I may premise that in five out of the nine pots
the first plant which flowered was one of the English-crossed; in four
of the pots it was a self-fertilised plant; and in not one did an
intercrossed plant flower first; so that these latter plants were beaten
in this respect, as in so many other ways. The three closely adjoining
rows of plants growing in the open ground flowered profusely, and the
flowers were incessantly visited by bees, and certainly thus
intercrossed. The manner in which several plants in the previous
experiments continued to be almost sterile as long as they were covered
by a net, but set a multitude of capsules immediately that they were
uncovered, proves how effectually the bees carry pollen from plant to
plant. My gardener gathered, at three successive times, an equal number
of ripe capsules from the plants of the three lots, until he had
collected forty-five from each lot. It is not possible to judge from
external appearance whether or not a capsule contains any good seeds; so
that I opened all the capsules. Of the forty-five from the
English-crossed plants, four were empty; of those from the intercrossed,
five were empty; and of those from the self-fertilised, nine were empty.
The seeds were counted in twenty-one capsules taken by chance out of
each lot, and the average number of seeds in the capsules from the
English-crossed plants was 67; from the intercrossed, 56; and from the
self-fertilised, 48.52. It therefore follows that:--

The forty-five capsules (the four empty ones included) from the
English-crossed plants contained 2747 seeds.

The forty-five capsules (the five empty ones included) from the
intercrossed plants contained 2240 seeds.

The forty-five capsules (the nine empty ones included) from the
self-fertilised plants contained 1746.7 seeds.

The reader should remember that these capsules are the product of
cross-fertilisation, effected by the bees; and that the difference in
the number of the contained seeds must depend on the constitution of the
plants;--that is, on whether they were derived from a cross with a
distinct stock, or from a cross between plants of the same stock, or
from self-fertilisation. From the above facts we obtain the following
ratios:--

Number of seeds contained in an equal number of naturally fertilised
capsules produced:--

By the English-crossed and self-fertilised plants, as 100 to 63.

By the English-crossed and intercrossed plants, as 100 to 81.

By the intercrossed and self-fertilised plants, as 100 to 78.

But to have ascertained the productiveness of the three lots of plants,
it would have been necessary to know how many capsules were produced by
the same number of plants. The three long rows, however, were not of
quite equal lengths, and the plants were much crowded, so that it would
have been extremely difficult to have ascertained how many capsules were
produced by them, even if I had been willing to undertake so laborious a
task as to collect and count all the capsules. But this was feasible
with the plants grown in pots in the greenhouse; and although these were
much less fertile than those growing out of doors, their relative
fertility appeared, after carefully observing them, to be the same. The
nineteen plants of the English-crossed stock in the pots produced
altogether 240 capsules; the intercrossed plants (calculated as
nineteen) produced 137.22 capsules; and the nineteen self-fertilised
plants, 152 capsules. Now, knowing the number of seeds contained in
forty-five capsules of each lot, it is easy to calculate the relative
numbers of seeds produced by an equal number of the plants of the three
lots.

Number of seeds produced by an equal number of naturally-fertilised
plants:--

Plants of English-crossed and self-fertilised parentage, as 100 to 40
seeds.

Plants of English-crossed and intercrossed parentage, as 100 to 45
seeds.

Plants of intercrossed and self-fertilised parentage, as 100 to 89
seeds.

The superiority in productiveness of the intercrossed plants (that is,
the product of a cross between the grandchildren of the plants which
grew in Brazil) over the self-fertilised, small as it is, is wholly due
to the larger average number of seeds contained in the capsules; for the
intercrossed plants produced fewer capsules in the greenhouse than did
the self-fertilised plants. The great superiority in productiveness of
the English-crossed over the self-fertilised plants is shown by the
larger number of capsules produced, the larger average number of
contained seeds, and the smaller number of empty capsules. As the
English-crossed and intercrossed plants were the offspring of crosses in
every previous generation (as must have been the case from the flowers
being sterile with their own pollen), we may conclude that the great
superiority in productiveness of the English-crossed over the
intercrossed plants is due to the two parents of the former having been
long subjected to different conditions.

The English-crossed plants, though so superior in productiveness, were,
as we have seen, decidedly inferior in height and weight to the
self-fertilised, and only equal to, or hardly superior to, the
intercrossed plants. Therefore, the whole advantage of a cross with a
distinct stock is here confined to productiveness, and I have met with
no similar case.

8. RESEDACEAE.--Reseda lutea.

Seeds collected from wild plants growing in this neighbourhood were sown
in the kitchen-garden; and several of the seedlings thus raised were
covered with a net. Of these, some were found (as will hereafter be more
fully described) to be absolutely sterile when left to fertilise
themselves spontaneously, although plenty of pollen fell on their
stigmas; and they were equally sterile when artificially and repeatedly
fertilised with their own pollen; whilst other plants produced a few
spontaneously self-fertilised capsules. The remaining plants were left
uncovered, and as pollen was carried from plant to plant by the hive and
humble-bees which incessantly visit the flowers, they produced an
abundance of capsules. Of the necessity of pollen being carried from one
plant to another, I had ample evidence in the case of this species and
of R. odorata; for those plants, which set no seeds or very few as long
as they were protected from insects, became loaded with capsules
immediately that they were uncovered.

Seeds from the flowers spontaneously self-fertilised under the net, and
from flowers naturally crossed by the bees, were sown on opposite sides
of five large pots. The seedlings were thinned as soon as they appeared
above ground, so that an equal number were left on the two sides. After
a time the pots were plunged into the open ground. The same number of
plants of crossed and self-fertilised parentage were measured up to the
summits of their flower-stems, with the result given in Table 4/35.
Those which did not produce flower-stems were not measured.

TABLE 4/35. Reseda lutea, in pots.

Heights of plants to the summits of the flower-stems measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  21     :  12 7/8.
Pot 1  :  14 2/8 :  16.
Pot 1  :  19 1/8 :  11 7/8.
Pot 1  :   7     :  15 2/8.
Pot 1  :  15 1/8 :  19 1/8.

Pot 2  :  20 4/8 :  12 4/8.
Pot 2  :  17 3/8 :  16 2/8.
Pot 2  :  23 7/8 :  16 2/8.
Pot 2  :  17 1/8 :  13 3/8.
Pot 2  :  20 6/8 :  13 5/8.

Pot 3  :  16 1/8 :  14 4/8.
Pot 3  :  17 6/8 :  19 4/8.
Pot 3  :  16 2/8 :  20 7/8.
Pot 3  :  10     :   7 7/8.
Pot 3  :  10     :  17 6/8.

Pot 4  :  22 1/8 :   9.
Pot 4  :  19     :  11 4/8.
Pot 4  :  18 7/8 :  11.
Pot 4  :  16 4/8 :  16.
Pot 4  :  19 2/8 :  16 3/8.

Pot 5  :  25 2/8 :  14 6/8.
Pot 5  :  22     :  16.
Pot 5  :   8 6/8 :  14 3/8.
Pot 5  :  14 2/8 :  14 2/8.

Total  : 412.25  : 350.86.

The average height of the twenty-four crossed plants is here 17.17
inches, and that of the same number of self-fertilised plants 14.61; or
as 100 to 85. Of the crossed plants all but five flowered, whilst
several of the self-fertilised did not do so. The above pairs, whilst
still in flower, but with some capsules already formed, were afterwards
cut down and weighed. The crossed weighed 90.5 ounces; and an equal
number of the self-fertilised only 19 ounces, or as 100 to 21; and this
is an astonishing difference.

Seeds of the same two lots were also sown in two adjoining rows in the
open ground. There were twenty crossed plants in the one row and
thirty-two self-fertilised plants in the other row, so that the
experiment was not quite fair; but not so unfair as it at first appears,
for the plants in the same row were not crowded so much as seriously to
interfere with each other’s growth, and the ground was bare on the
outside of both rows. These plants were better nourished than those in
the pots and grew to a greater height. The eight tallest plants in each
row were measured in the same manner as before, with the following
result:--

TABLE 4/36. Reseda lutea, growing in the open ground.

Heights of plants to the summits of the flower-stems measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

         28     :  33 2/8.
         27 3/8 :  23.
         27 5/8 :  21 5/8.
         28 6/8 :  20 4/8.
         29 7/8 :  21 5/8.
         26 6/8 :  22.
         26 2/8 :  21 2/8.
         30 1/8 :  21 7/8.

Total : 224.75  : 185.13

The average height of the crossed plants, whilst in full flower, was
here 28.09, and that of the self-fertilised 23.14 inches; or as 100 to
82. It is a singular fact that the tallest plant in the two rows, was
one of the self-fertilised. The self-fertilised plants had smaller and
paler green leaves than the crossed. All the plants in the two rows were
afterwards cut down and weighed. The twenty crossed plants weighed 65
ounces, and twenty self-fertilised (by calculation from the actual
weight of the thirty-two self-fertilised plants) weighed 26.25 ounces;
or as 100 to 40. Therefore the crossed plants did not exceed in weight
the self-fertilised plants in nearly so great a degree as those growing
in the pots, owing probably to the latter having been subjected to more
severe mutual competition. On the other hand, they exceeded the
self-fertilised in height in a slightly greater degree.

Reseda odorata.

Plants of the common mignonette were raised from purchased seed, and
several of them were placed under separate nets. Of these some became
loaded with spontaneously self-fertilised capsules; others produced a
few, and others not a single one. It must not be supposed that these
latter plants produced no seed because their stigmas did not receive any
pollen, for they were repeatedly fertilised with pollen from the same
plant with no effect; but they were perfectly fertile with pollen from
any other plant. Spontaneously self-fertilised seeds were saved from one
of the highly self-fertile plants, and other seeds were collected from
the plants growing outside the nets, which had been crossed by the bees.
These seeds after germinating on sand were planted in pairs on the
opposite sides of five pots. The plants were trained up sticks, and
measured to the summits of their leafy stems--the flower-stems not being
included. We here have the result:--

TABLE 4/37. Reseda odorata (seedlings from a highly self-fertile plant).

Heights of plants to the summits of the leafy stems, flower-stems not
included, measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  20 7/8 :  22 4/8.
Pot 1  :  34 7/8 :  28 5/8.
Pot 1  :  26 6/8 :  23 2/8.
Pot 1  :  32 6/8 :  30 4/8.

Pot 2  :  34 3/8 :  28 5/8.
Pot 2  :  34 5/8 :  30 5/8.
Pot 2  :  11 6/8 :  23.
Pot 2  :  33 3/8 :  30 1/8.

Pot 3  :  17 7/8 :   4 4/8.
Pot 3  :  27     :  25.
Pot 3  :  30 1/8 :  26 3/8.
Pot 3  :  30 2/8 :  25 1/8.

Pot 4  :  21 5/8 :  22 6/8.
Pot 4  :  28     :  25 4/8.
Pot 4  :  32 5/8 :  15 1/8.
Pot 4  :  32 3/8 :  24 6/8.

Pot 5  :  21     :  11 6/8.
Pot 5  :  25 2/8 :  19 7/8.
Pot 5  :  26 6/8 :  10 4/8.

Total  : 522.25  : 428.50.

The average height of the nineteen crossed plants is here 27.48, and
that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All
these plants were cut down in the early autumn and weighed: the crossed
weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to
67. These two lots having been left freely exposed to the visits of
insects, did not present any difference to the eye in the number of
seed-capsules which they produced.

The remainder of the same two lots of seeds were sown in two adjoining
rows in the open ground; so that the plants were exposed to only
moderate competition. The eight tallest on each side were measured, as
shown in Table 4/38.

TABLE 4/38. Reseda odorata, growing in the open ground.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

         24 4/8 :  26 5/8.
         27 2/8 :  25 7/8.
         24     :  25.
         26 6/8 :  28 3/8.
         25     :  29 7/8.
         26 2/8 :  25 7/8.
         27 2/8 :  26 7/8.
         25 1/8 :  28 2/8.

Total : 206.13  : 216.75

The average height of the eight crossed plants is 25.76, and that of the
eight self-fertilised 27.09; or as 100 to 105.

We here have the anomalous result of the self-fertilised plants being a
little taller than the crossed; of which fact I can offer no
explanation. It is of course possible, but not probable, that the labels
may have been interchanged by accident.

Another experiment was now tried: all the self-fertilised capsules,
though very few in number, were gathered from one of the
semi-self-sterile plants under a net; and as several flowers on this
same plant had been fertilised with pollen from a distinct individual,
crossed seeds were thus obtained. I expected that the seedlings from
this semi-self-sterile plant would have profited in a higher degree from
a cross, than did the seedlings from the fully self-fertile plants. But
my anticipation was quite wrong, for they profited in a less degree. An
analogous result followed in the case of Eschscholtzia, in which the
offspring of the plants of Brazilian parentage (which were partially
self-sterile) did not profit more from a cross, than did the plants of
the far more self-fertile English stock. The above two lots of crossed
and self-fertilised seeds from the same plant of Reseda odorata, after
germinating on sand, were planted on opposite sides of five pots, and
measured as in the last case, with the result in Table 4/39.

TABLE 4/39. Reseda odorata (seedlings from a semi-self-sterile plant).

Heights of plants to the summits of the leafy stems, flower-stems not
included, measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  33 4/8 :  31.
Pot 1  :  30 6/8 :  28.
Pot 1  :  29 6/8 :  13 2/8.
Pot 1  :  20     :  32.

Pot 2  :  22     :  21 6/8.
Pot 2  :  33 4/8 :  26 6/8.
Pot 2  :  31 2/8 :  25 2/8.
Pot 2  :  32 4/8 :  30 4/8.

Pot 3  :  30 1/8 :  17 2/8.
Pot 3  :  32 1/8 :  29 6/8.
Pot 3  :  31 4/8 :  24 6/8.
Pot 3  :  32 2/8 :  34 2/8.

Pot 4  :  19 1/8 :  20 6/8.
Pot 4  :  30 1/8 :  32 6/8.
Pot 4  :  24 3/8 :  31 4/8.
Pot 4  :  30 6/8 :  36 6/8.

Pot 5  :  34 6/8 :  24 5/8.
Pot 5  :  37 1/8 :  34.
Pot 5  :  31 2/8 :  22 2/8.
Pot 5  :  33     :  37 1/8.

Total  : 599.75  : 554.25.

The average height of the twenty crossed plants is here 29.98, and that
of the twenty self-fertilised 27.71 inches; or as 100 to 92. These
plants were then cut down and weighed; and the crossed in this case
exceeded the self-fertilised in weight by a mere trifle, namely, in the
ratio of 100 to 99. The two lots, left freely exposed to insects, seemed
to be equally fertile.

The remainder of the seed was sown in two adjoining rows in the open
ground; and the eight tallest plants in each row were measured, with the
result in Table 4/40.

TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant,
planted in the open ground).

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

         28 2/8 :  22 3/8.
         22 4/8 :  24 3/8.
         25 7/8 :  23 4/8.
         25 3/8 :  21 4/8.
         29 4/8 :  22 5/8.
         27 1/8 :  27 3/8.
         22 4/8 :  27 3/8.
         26 2/8 :  19 2/8.

Total : 207.38  : 188.38.

The average height of the eight crossed plants is here 25.92, and that
of the eight self-fertilised plants 23.54 inches; or as 100 to 90.

9. VIOLACEAE.--Viola tricolor.

Whilst the flowers of the common cultivated heartsease are young, the
anthers shed their pollen into a little semi-cylindrical passage, formed
by the basal portion of the lower petal, and surrounded by papillae. The
pollen thus collected lies close beneath the stigma, but can seldom gain
access into its cavity, except by the aid of insects, which pass their
proboscides down this passage into the nectary. (4/5. The flowers of
this plant have been fully described by Sprengel, Hildebrand, Delpino,
and H. Muller. The latter author sums up all the previous observations
in his ‘Befruchtung der Blumen’ and in ‘Nature’ November 20, 1873 page
44. See also Mr. A.W. Bennett in ‘Nature’ May 15, 1873 page 50 and some
remarks by Mr. Kitchener ibid page 143. The facts which follow on the
effects of covering up a plant of V. tricolor have been quoted by Sir J.
Lubbock in his ‘British Wild Flowers’ etc. page 62.) Consequently when I
covered up a large plant of a cultivated variety, it set only eighteen
capsules, and most of these contained very few good seeds--several from
only one to three; whereas an equally fine uncovered plant of the same
variety, growing close by, produced 105 fine capsules. The few flowers
which produce capsules when insects are excluded, are perhaps fertilised
by the curling inwards of the petals as their wither, for by this means
pollen-grains adhering to the papillae might be inserted into the cavity
of the stigma. But it is more probable that their fertilisation is
effected, as Mr. Bennett suggests, by Thrips and certain minute beetles
which haunt the flowers, and which cannot be excluded by any net.
Humble-bees are the usual fertilisers; but I have more than once seen
flies (Rhingia rostrata) at work, with the under sides of their bodies,
heads and legs dusted with pollen; and having marked the flowers which
they visited, I found them after a few days fertilised. (4/6. I should
add that this fly apparently did not suck the nectar, but was attracted
by the papillae which surround the stigma. Hermann Muller also saw a
small bee, an Andrena, which could not reach the nectar, repeatedly
inserting its proboscis beneath the stigma, where the papillae are
situated; so that these papillae must be in some way attractive to
insects. A writer asserts ‘Zoologist’ volume 3-4 page 1225, that a moth
(Plusia) frequently visits the flowers of the pansy. Hive-bees do not
ordinarily visit them, but a case has been recorded ‘Gardeners’
Chronicle’ 1844 page 374, of these bees doing so. Hermann Muller has
also seen the hive-bee at work, but only on the wild small-flowered
form. He gives a list ‘Nature’ 1873 page 45, of all the insects which he
has seen visiting both the large and small-flowered forms. From his
account, I suspect that the flowers of plants in a state of nature are
visited more frequently by insects than those of the cultivated
varieties. He has seen several butterflies sucking the flowers of wild
plants, and this I have never observed in gardens, though I have watched
the flowers during many years.) It is curious for how long a time the
flowers of the heartsease and of some other plants may be watched
without an insect being seen to visit them. During the summer of 1841, I
observed many times daily for more than a fortnight some large clumps of
heartsease growing in my garden, before I saw a single humble-bee at
work. During another summer I did the same, but at last saw some
dark-coloured humble-bees visiting on three successive days almost every
flower in several clumps; and almost all these flowers quickly withered
and produced fine capsules. I presume that a certain state of the
atmosphere is necessary for the secretion of nectar, and that as soon as
this occurs the insects discover the fact by the odour emitted, and
immediately frequent the flowers.

As the flowers require the aid of insects for their complete
fertilisation, and as they are not visited by insects nearly so often as
most other nectar-secreting flowers, we can understand the remarkable
fact discovered by H. Muller and described by him in ‘Nature,’ namely,
that this species exists under two forms. One of these bears conspicuous
flowers, which, as we have seen, require the aid of insects, and are
adapted to be cross-fertilised by them; whilst the other form has much
smaller and less conspicuously coloured flowers, which are constructed
on a slightly different plan, favouring self-fertilisation, and are thus
adapted to ensure the propagation of the species. The self-fertile form,
however, is occasionally visited, and may be crossed by insects, though
this is rather doubtful.

In my first experiments on Viola tricolor I was unsuccessful in raising
seedlings, and obtained only one full-grown crossed and self-fertilised
plant. The former was 12 1/2 inches and the latter 8 inches in height.
On the following year several flowers on a fresh plant were crossed with
pollen from another plant, which was known to be a distinct seedling;
and to this point it is important to attend. Several other flowers on
the same plant were fertilised with their own pollen. The average number
of seeds in the ten crossed capsules was 18.7, and in the twelve
self-fertilised capsules 12.83; or as 100 to 69. These seeds, after
germinating on bare sand, were planted in pairs on the opposite sides of
five pots. They were first measured when about a third of their full
size, and the crossed plants then averaged 3.87 inches, and the
self-fertilised only 2.00 inches in height; or as 100 to 52. They were
kept in the greenhouse, and did not grow vigorously. Whilst in flower
they were again measured to the summits of their stems (see Table 4/41),
with the following result:--

TABLE 4/41. Viola tricolor.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  8 2/8 :  0 2/8.
Pot 1  :  7 4/8 :  2 4/8.
Pot 1  :  5     :  1 2/8.

Pot 2  :  5     :  6.
Pot 2  :  4     :  4.
Pot 2  :  4 4/8 :  3 1/8.

Pot 3  :  9 4/8 :  3 1/8.
Pot 3  :  3 3/8 :  1 7/8.
Pot 3  :  8 4/8 :  0 5/8.

Pot 4  :  4 7/8 :  2 1/8.
Pot 4  :  4 2/8 :  1 6/8.
Pot 4  :  4     :  2 1/8.

Pot 5  :  6     :  3.
Pot 5  :  3 3/8 :  1 4/8.

Total  : 78.13  : 33.25.

The average height of the fourteen crossed plants is here 5.58 inches,
and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four
out of the five pots, a crossed plant flowered before any one of the
self-fertilised; as likewise occurred with the pair raised during the
previous year. These plants without being disturbed were now turned out
of their pots and planted in the open ground, so as to form five
separate clumps. Early in the following summer (1869) they flowered
profusely, and being visited by humble-bees set many capsules, which
were carefully collected from all the plants on both sides. The crossed
plants produced 167 capsules, and the self-fertilised only 17; or as 100
to 10. So that the crossed plants were more than twice the height of the
self-fertilised, generally flowered first, and produced ten times as
many naturally fertilised capsules.

By the early part of the summer of 1870 the crossed plants in all the
five clumps had grown and spread so much more than the self-fertilised,
that any comparison between them was superfluous. The crossed plants
were covered with a sheet of bloom, whilst only a single self-fertilised
plant, which was much finer than any of its brethren, flowered. The
crossed and self-fertilised plants had now grown all matted together on
the respective sides of the superficial partitions still separating
them; and in the clump which included the finest self-fertilised plant,
I estimated that the surface covered by the crossed plants was about
nine times as large as that covered by the self-fertilised plants. The
extraordinary superiority of the crossed over the self-fertilised plants
in all five clumps, was no doubt due to the crossed plants at first
having had a decided advantage over the self-fertilised, and then
robbing them more and more of their food during the succeeding seasons.
But we should remember that the same result would follow in a state of
nature even to a greater degree; for my plants grew in ground kept clear
of weeds, so that the self-fertilised had to compete only with the
crossed plants; whereas the whole surface of the ground is naturally
covered with various kinds of plants, all of which have to struggle
together for existence.

The ensuing winter was very severe, and in the following spring (1871)
the plants were again examined. All the self-fertilised were now dead,
with the exception of a single branch on one plant, which bore on its
summit a minute rosette of leaves about as large as a pea. On the other
hand, all the crossed plants without exception were growing vigorously.
So that the self-fertilised plants, besides their inferiority in other
respects, were more tender.

Another experiment was now tried for the sake of ascertaining how far
the superiority of the crossed plants, or to speak more correctly, the
inferiority of the self-fertilised plants, would be transmitted to their
offspring. The one crossed and one self-fertilised plant, which were
first raised, had been turned out of their pot and planted in the open
ground. Both produced an abundance of very fine capsules, from which
fact we may safely conclude that they had been cross-fertilised by
insects. Seeds from both, after germinating on sand, were planted in
pairs on the opposite sides of three pots. The naturally crossed
seedlings derived from the crossed plants flowered in all three pots
before the naturally crossed seedlings derived from the self-fertilised
plants. When both lots were in full flower, the two tallest plants on
each side of each pot were measured, and the result is shown in Table
4/42.

TABLE 4/42. Viola tricolor: seedlings from crossed and self-fertilised
plants, the parents of both sets having been left to be naturally
fertilised.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Naturally Crossed Plants from artificially crossed Plants.

Column 3: Naturally Crossed Plants from Self-fertilised Plants.

Pot 1  : 12 1/8 :  9 6/8.
Pot 1  : 11 6/8 :  8 3/8.

Pot 2  : 13 2/8 :  9 6/8.
Pot 2  : 10     : 11 4/8.

Pot 3  : 14 4/8 : 11 1/8.
Pot 3  : 13 6/8 : 11 3/8.

Total  : 75.38  : 61.88.

The average height of the six tallest plants derived from the crossed
plants is 12.56 inches; and that of the six tallest plants derived from
the self-fertilised plants is 10.31 inches; or as 100 to 82. We here see
a considerable difference in height between the two sets, though very
far from equalling that in the previous trials between the offspring
from crossed and self-fertilised flowers. This difference must be
attributed to the latter set of plants having inherited a weak
constitution from their parents, the offspring of self-fertilised
flowers; notwithstanding that the parents themselves had been freely
intercrossed with other plants by the aid of insects.

10. RANUNCULACEAE.--Adonis aestivalis.

The results of my experiments on this plant are hardly worth giving, as
I remark in my notes made at the time, “seedlings, from some unknown
cause, all miserably unhealthy.” Nor did they ever become healthy; yet I
feel bound to give the present case, as it is opposed to the general
results at which I have arrived. Fifteen flowers were crossed and all
produced fruit, containing on an average 32.5 seeds; nineteen flowers
were fertilised with their own pollen, and they likewise all yielded
fruit, containing a rather larger average of 34.5 seeds; or as 100 to
106. Seedlings were raised from these seeds. In one of the pots all the
self-fertilised plants died whilst quite young; in the two others, the
measurements were as follows:

TABLE 4/43. Adonis aestivalis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  : 14     : 13 4/8.
Pot 1  : 13 4/8 : 13 4/8.

Pot 2  : 16 2/8 : 15 2/8.
Pot 2  : 13 2/8 : 15.

Total  : 57.00  : 57.25.

The average height of the four crossed plants is 14.25, and that of the
four self-fertilised plants 14.31; or as 100 to 100.4; so that they were
in fact of equal height. According to Professor H. Hoffman, this plant
is proterandrous (4/7. ‘Zur Speciesfrage’ 1875 page 11.); nevertheless
it yields plenty of seeds when protected from insects.

Delphinium consolida.

It has been said in the case of this plant, as of so many others, that
the flowers are fertilised in the bud, and that distinct plants or
varieties can never naturally intercross. (4/8. Decaisne
‘Comptes-Rendus’ July 1863 page 5.) But this is an error, as we may
infer, firstly from the flowers being proterandrous,--the mature stamens
bending up, one after the other, into the passage which leads to the
nectary, and afterwards the mature pistils bending in the same
direction; secondly, from the number of humble-bees which visit the
flowers (4/9. Their structure is described by H. Muller ‘Befruchtung’
etc., page 122.); and thirdly, from the greater fertility of the flowers
when crossed with pollen from a distinct plant than when spontaneously
self-fertilised. In the year 1863 I enclosed a large branch in a net,
and crossed five flowers with pollen from a distinct plant; these
yielded capsules containing on an average 35.2 very fine seeds, with a
maximum of forty-two in one capsule. Thirty-two other flowers on the
same branch produced twenty-eight spontaneously self-fertilised
capsules, containing on an average 17.2 seeds, with a maximum in one of
thirty-six seeds. But six of these capsules were very poor, yielding
only from one to five seeds; if these are excluded, the remaining
twenty-two capsules give an average of 20.9 seeds, though many of these
seeds were small. The fairest ratio, therefore, for the number of seeds
produced by a cross and by spontaneous self-fertilisation is as 100 to
59. These seeds were not sown, as I had too many other experiments in
progress.

In the summer of 1867, which was a very unfavourable one, I again
crossed several flowers under a net with pollen from a distinct plant,
and fertilised other flowers on the same plant with their own pollen.
The former yielded a much larger proportion of capsules than the latter;
and many of the seeds in the self-fertilised capsules, though numerous,
were so poor that an equal number of seeds from the crossed and
self-fertilised capsules were in weight as 100 to 45. The two lots were
allowed to germinate on sand, and pairs were planted on the opposite
sides of four pots. When nearly two-thirds grown they were measured, as
shown in Table 4/44.

TABLE 4/44. Delphinium consolida.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  : 11     : 11.

Pot 2  : 19     : 16 2/8.
Pot 2  : 16 2/8 : 11 4/8.

Pot 3  : 26     : 22.

Pot 4  :  9 4/8 :  8 2/8.
Pot 4  :  8     :  6 4/8.

Total  : 89.75  : 75.50.

The six crossed plants here average 14.95, and the six self-fertilised
12.50 inches in height; or as 100 to 84. When fully grown they were
again measured, but from want of time only a single plant on each side
was measured; so that I have thought it best to give the earlier
measurements. At the later period the three tallest crossed plants still
exceeded considerably in height the three tallest self-fertilised, but
not in quite so great a degree as before. The pots were left uncovered
in the greenhouse, but whether the flowers were intercrossed by bees or
self-fertilised I do not know. The six crossed plants produced 282
mature and immature capsules, whilst the six self-fertilised plants
produced only 159; or as 100 to 56. So that the crossed plants were very
much more productive than the self-fertilised.

11. CARYOPHYLLACEAE.--Viscaria oculata.

Twelve flowers were crossed with pollen from another plant, and yielded
ten capsules, containing by weight 5.77 grains of seeds. Eighteen
flowers were fertilised with their own pollen and yielded twelve
capsules, containing by weight 2.63 grains. Therefore the seeds from an
equal number of crossed and self-fertilised flowers would have been in
weight as 100 to 38. I had previously selected a medium-sized capsule
from each lot, and counted the seeds in both; the crossed one contained
284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds
were sown on opposite sides of three pots, and several seedlings raised;
but only the tallest flower-stem of one plant on each side was measured.
The three on the crossed side averaged 32.5 inches, and the three on the
self-fertilised side 34 inches in height; or as 100 to 104. But this
trial was on much too small a scale to be trusted; the plants also grew
so unequally that one of the three flower-stems on the crossed plants
was very nearly twice as tall as that on one of the others; and one of
the three flower-stems on the self-fertilised plants exceeded in an
equal degree one of the others.

In the following year the experiment was repeated on a larger scale: ten
flowers were crossed on a new set of plants and yielded ten capsules
containing by weight 6.54 grains of seed. Eighteen spontaneously
self-fertilised capsules were gathered, of which two contained no seed;
the other sixteen contained by weight 6.07 grains of seed. Therefore the
weight of seed from an equal number of crossed and spontaneously
self-fertilised flowers (instead of artificially fertilised as in the
previous case) was as 100 to 58.

The seeds after germinating on sand were planted in pairs on the
opposite sides of four pots, with all the remaining seeds sown crowded
in the opposite sides of a fifth pot; in this latter pot only the
tallest plant on each side was measured. Until the seedlings had grown
about 5 inches in height no difference could be perceived in the two
lots. Both lots flowered at nearly the same time. When they had almost
done flowering, the tallest flower-stem on each plant was measured, as
shown in Table 4/45.

TABLE 4/45. Viscaria oculata.

Tallest flower-stem on each plant measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  19     :  32 3/8.
Pot 1  :  33     :  38.
Pot 1  :  41     :  38.
Pot 1  :  41     :  28 7/8.

Pot 2  :  37 4/8 :  36.
Pot 2  :  36 4/8 :  32 3/8.
Pot 2  :  38     :  35 6/8.

Pot 3  :  44 4/8 :  36.
Pot 3  :  39 4/8 :  20 7/8.
Pot 3  :  39     :  30 5/8.

Pot 4  :  30 2/8 :  36.
Pot 4  :  31     :  39.
Pot 4  :  33 1/8 :  29.
Pot 4  :  24     :  38 4/8.

Pot 5  :  30 2/8 :  32.
Crowded.

Total  : 517.63  : 503.36.

The fifteen crossed plants here average 34.5, and the fifteen
self-fertilised 33.55 inches in height; or as 100 to 97. So that the
excess of height of the crossed plants is quite insignificant. In
productiveness, however, the difference was much more plainly marked.
All the capsules were gathered from both lots of plants (except from the
crowded and unproductive ones in Pot 5), and at the close of the season
the few remaining flowers were added in. The fourteen crossed plants
produced 381, whilst the fourteen self-fertilised plants produced only
293 capsules and flowers; or as 100 to 77.

Dianthus caryophyllus.

The common carnation is strongly proterandrous, and therefore depends to
a large extent upon insects for fertilisation. I have seen only
humble-bees visiting the flowers, but I dare say other insects likewise
do so. It is notorious that if pure seed is desired, the greatest care
is necessary to prevent the varieties which grow in the same garden from
intercrossing. (4/10. ‘Gardeners’ Chronicle’ 1847 page 268.) The pollen
is generally shed and lost before the two stigmas in the same flower
diverge and are ready to be fertilised. I was therefore often forced to
use for self-fertilisation pollen from the same plant instead of from
the same flower. But on two occasions, when I attended to this point, I
was not able to detect any marked difference in the number of seeds
produced by these two forms of self-fertilisation.

Several single-flowered carnations were planted in good soil, and were
all covered with a net. Eight flowers were crossed with pollen from a
distinct plant and yielded six capsules, containing on an average 88.6
seeds, with a maximum in one of 112 seeds. Eight other flowers were
self-fertilised in the manner above described, and yielded seven
capsules containing on an average 82 seeds, with a maximum in one of 112
seeds. So that there was very little difference in the number of seeds
produced by cross-fertilisation and self-fertilisation, namely, as 100
to 92. As these plants were covered by a net, they produced
spontaneously only a few capsules containing any seeds, and these few
may perhaps be attributed to the action of Thrips and other minute
insects which haunt the flowers. A large majority of the spontaneously
self-fertilised capsules produced by several plants contained no seeds,
or only a single one. Excluding these latter capsules, I counted the
seeds in eighteen of the finest ones, and these contained on an average
18 seeds. One of the plants was spontaneously self-fertile in a higher
degree than any of the others. On another occasion a single covered-up
plant produced spontaneously eighteen capsules, but only two of these
contained any seed, namely 10 and 15.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.

The many seeds obtained from the above crossed and artificially
self-fertilised flowers were sown out of doors, and two large beds of
seedlings, closely adjoining one another, thus raised. This was the
first plant on which I experimented, and I had not then formed any
regular scheme of operation. When the two lots were in full flower, I
measured roughly a large number of plants but record only that the
crossed were on an average fully 4 inches taller than the
self-fertilised. Judging from subsequent measurements, we may assume
that the crossed plants were about 28 inches, and the self-fertilised
about 24 inches in height; and this will give us a ratio of 100 to 86.
Out of a large number of plants, four of the crossed ones flowered
before any one of the self-fertilised plants.

Thirty flowers on these crossed plants of the first generation were
again crossed with pollen from a distinct plant of the same lot, and
yielded twenty-nine capsules, containing on an average 55.62 seeds, with
a maximum in one of 110 seeds.

Thirty flowers on the self-fertilised plants were again self-fertilised;
eight of them with pollen from the same flower, and the remainder with
pollen from another flower on the same plant; and these produced
twenty-two capsules, containing on an average 35.95 seeds, with a
maximum in one of sixty-one seeds. We thus see, judging by the number of
seeds per capsule, that the crossed plants again crossed were more
productive than the self-fertilised again self-fertilised, in the ratio
of 100 to 65. Both the crossed and self-fertilised plants, from having
grown much crowded in the two beds, produced less fine capsules and
fewer seeds than did their parents.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The crossed and self-fertilised seeds from the crossed and
self-fertilised plants of the last generation were sown on opposite
sides of two pots; but the seedlings were not thinned enough, so that
both lots grew very irregularly, and most of the self-fertilised plants
after a time died from being smothered. My measurements were, therefore,
very incomplete. From the first the crossed seedlings appeared the
finest, and when they were on an average, by estimation, 5 inches high,
the self-fertilised plants were only 4 inches. In both pots the crossed
plants flowered first. The two tallest flower-stems on the crossed
plants in the two pots were 17 and 16 1/2 inches in height; and the two
tallest flower-stems on the self-fertilised plants 10 1/2 and 9 inches;
so that their heights were as 100 to 58. But this ratio, deduced from
only two pairs, obviously is not in the least trustworthy, and would not
have been given had it not been otherwise supported. I state in my notes
that the crossed plants were very much more luxuriant than their
opponents, and seemed to be twice as bulky. This latter estimate may be
believed from the ascertained weights of the two lots in the next
generation. Some flowers on these crossed plants were again crossed with
pollen from another plant of the same lot, and some flowers on the
self-fertilised plants again self-fertilised; and from the seeds thus
obtained the plants of the next generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

The seeds just alluded to were allowed to germinate on bare sand, and
were planted in pairs on the opposite sides of four pots. When the
seedlings were in full flower, the tallest stem on each plant was
measured to the base of the calyx. The measurements are given in Table
4/46. In Pot 1 the crossed and self-fertilised plants flowered at the
same time; but in the other three pots the crossed flowered first. These
latter plants also continued flowering much later in the autumn than the
self-fertilised.

TABLE 4/46. Dianthus caryophyllus (third generation).

Tallest flower-stem on each plant measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  28 6/8 :  30.
Pot 1  :  27 3/8 :  26.

Pot 2  :  29     :  30 7/8.
Pot 2  :  29 4/8 :  27 4/8.

Pot 3  :  28 4/8 :  31 6/8.
Pot 3  :  23 4/8 :  24 5/8.

Pot 4  :  27     :  30.
Pot 4  :  33 4/8 :  25.

Total  : 227.13  : 225.75.

The average height of the eight crossed plants is here 28.39 inches, and
of the eight self-fertilised 28.21; or as 100 to 99. So that there was
no difference in height worth speaking of; but in general vigour and
luxuriance there was an astonishing difference, as shown by their
weights. After the seed-capsules had been gathered, the eight crossed
and the eight self-fertilised plants were cut down and weighed; the
former weighed 43 ounces, and the latter only 21 ounces; or as 100 to
49.

These plants were all kept under a net, so that the capsules which they
produced must have been all spontaneously self-fertilised. The eight
crossed plants produced twenty-one such capsules, of which only twelve
contained any seed, averaging 8.5 per capsule. On the other hand, the
eight self-fertilised plants produced no less than thirty-six capsules,
of which I examined twenty-five, and, with the exception of three, all
contained seeds, averaging 10.63 seeds per capsule. Thus the
proportional number of seeds per capsule produced by the plants of
crossed origin to those produced by the plants of self-fertilised origin
(both lots being spontaneously self-fertilised) was as 100 to 125. This
anomalous result is probably due to some of the self-fertilised plants
having varied so as to mature their pollen and stigmas more nearly at
the same time than is proper to the species; and we have already seen
that some plants in the first experiment differed from the others in
being slightly more self-fertile.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

Twenty flowers on the self-fertilised plants of the last or third
generation, in Table 4/46, were fertilised with their own pollen, but
taken from other flowers on the same plants. These produced fifteen
capsules, which contained (omitting two with only three and six seeds)
on an average 47.23 seeds, with a maximum of seventy in one. The
self-fertilised capsules from the self-fertilised plants of the first
generation yielded the much lower average of 35.95 seeds; but as these
latter plants grew extremely crowded, nothing can be inferred with
respect to this difference in their self-fertility. The seedlings raised
from the above seeds constitute the plants of the fourth self-fertilised
generation in Table 4/47.

Twelve flowers on the same plants of the third self-fertilised
generation, in Table 4/46, were crossed with pollen from the crossed
plants in the same table. These crossed plants had been intercrossed for
the three previous generations; and many of them, no doubt, were more or
less closely inter-related, but not so closely as in some of the
experiments with other species; for several carnation plants had been
raised and crossed in the earlier generations. They were not related, or
only in a distant degree, to the self-fertilised plants. The parents of
both the self-fertilised and crossed plants had been subjected to as
nearly as possible the same conditions during the three previous
generations. The above twelve flowers produced ten capsules, containing
on an average 48.66 seeds, with a maximum in one of seventy-two seeds.
The plants raised from these seeds may be called the INTERCROSSED.

Lastly, twelve flowers on the same self-fertilised plants of the third
generation were crossed with pollen from plants which had been raised
from seeds purchased in London. It is almost certain that the plants
which produced these seeds had grown under very different conditions to
those to which my self-fertilised and crossed plants had been subjected;
and they were in no degree related. The above twelve flowers thus
crossed all produced capsules, but these contained the low average of
37.41 seeds per capsule, with a maximum in one of sixty-four seeds. It
is surprising that this cross with a fresh stock did not give a much
higher average number of seeds; for, as we shall immediately see, the
plants raised from these seeds, which may be called the LONDON-CROSSED,
benefited greatly by the cross, both in growth and fertility.

The above three lots of seeds were allowed to germinate on bare sand.
Many of the London-crossed germinated before the others, and were
rejected; and many of the intercrossed later than those of the other two
lots. The seeds after thus germinating were planted in ten pots, made
tripartite by superficial divisions; but when only two kinds of seeds
germinated at the same time, they were planted on the opposite sides of
other pots; and this is indicated by blank spaces in one of the three
columns in Table 4/47. A 0 in the table signifies that the seedling died
before it was measured; and a + signifies that the plant did not produce
a flower-stem, and therefore was not measured. It deserves notice that
no less than eight out of the eighteen self-fertilised plants either
died or did not flower; whereas only three out of the eighteen
intercrossed, and four out of the twenty London-crossed plants, were in
this predicament. The self-fertilised plants had a decidedly less
vigorous appearance than the plants of the other two lots, their leaves
being smaller and narrower. In only one pot did a self-fertilised plant
flower before one of the two kinds of crossed plants, between which
there was no marked difference in the period of flowering. The plants
were measured to the base of the calyx, after they had completed their
growth, late in the autumn.

TABLE 4/47. Dianthus caryophyllus.

Heights of plants to the base of the calyx, measured in inches.

Column 1: Number (Name) of Pot.

Column 2: London-Crossed Plants.

Column 3: Intercrossed Plants.

Column 4: Self-fertilised Plants.

Pot 1   :  39 5/8 :  25 1/8 :  29 2/8.
Pot 1   :  30 7/8 :  21 6/8 :   +.

Pot 2   :  36 2/8 :         :  22 3/8.
Pot 2   :   0     :         :   +.

Pot 3   :  28 5/8 :  30 2/8 :    .
Pot 3   :   +     :  23 1/8 :    .

Pot 4   :  33 4/8 :  35 5/8 :  30.
Pot 4   :  28 7/8 :  32     :  24 4/8.

Pot 5   :  28     :  34 4/8 :   +.
Pot 5   :   0     :  24 2/8 :   +.

Pot 6   :  32 5/8 :  24 7/8 :  30 3/8.
Pot 6   :  31     :  26     :  24 4/8.

Pot 7   :  41 7/8 :  29 7/8 :  27 7/8.
Pot 7   :  34 7/8 :  26 4/8 :  27.

Pot 8   :  34 5/8 :  29     :  26 6/8.
Pot 8   :  28 5/8 :   0     :   +.

Pot 9   :  25 5/8 :  28 5/8 :   +.
Pot 9   :   0     :   +     :   0.

Pot 10  :  38     :  28 4/8 :  22 7/8.
Pot 10  :  32 1/8 :   +     :   0.

Total   : 525.13  : 420.00  : 265.50.

The average height of the sixteen London-crossed plants in Table 4/47 is
32.82 inches; that of the fifteen intercrossed plants, 28 inches; and
that of the ten self-fertilised plants, 26.55.

So that in height we have the following ratios:--

The London-crossed to the self-fertilised as 100 to 81.

The London-crossed to the intercrossed as 100 to 85.

The intercrossed to the self-fertilised as 100 to 95.

These three lots of plants, which it should be remembered were all
derived on the mother-side from plants of the third self-fertilised
generation, fertilised in three different ways, were left exposed to the
visits of insects, and their flowers were freely crossed by them. As the
capsules of each lot became ripe they were gathered and kept separate,
the empty or bad ones being thrown away. But towards the middle of
October, when the capsules could no longer ripen, all were gathered and
were counted, whether good or bad. The capsules were then crushed, and
the seed cleaned by sieves and weighed. For the sake of uniformity the
results are given from calculation, as if there had been twenty plants
in each lot.

The sixteen London-crossed plants actually produced 286 capsules;
therefore twenty such plants would have produced 357.5 capsules; and
from the actual weight of the seeds, the twenty plants would have
yielded 462 grains weight of seeds.

The fifteen intercrossed plants actually produced 157 capsules;
therefore twenty of them would have produced 209.3 capsules and the
seeds would have weighed 208.48 grains.

The ten self-fertilised plants actually produced 70 capsules, therefore
twenty of them would have produced 140 capsules; and the seeds would
have weighed 153.2 grains.

From these data we get the following ratios:--

NUMBER OF CAPSULES PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE
LOTS.

NUMBER OF CAPSULES:

The London-crossed to the self-fertilised as 100 to 39.

The London-crossed to the intercrossed as 100 to 45.

The intercrossed to the self-fertilised as 100 to 67.

WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE LOTS.

WEIGHT OF SEED:

The London-crossed to the self-fertilised as 100 to 33.

The London-crossed to the intercrossed as 100 to 45.

The intercrossed to the self-fertilised as 100 to 73.

We thus see how greatly the offspring from the self-fertilised plants of
the third generation crossed by a fresh stock, had their fertility
increased, whether tested by the number of capsules produced or by the
weight of the contained seeds; this latter being the more trustworthy
method. Even the offspring from the self-fertilised plants crossed by
one of the crossed plants of the same stock, notwithstanding that both
lots had been long subjected to the same conditions, had their fertility
considerably increased, as tested by the same two methods.

In conclusion it may be well to repeat in reference to the fertility of
these three lots of plants, that their flowers were left freely exposed
to the visits of insects and were undoubtedly crossed by them, as may be
inferred from the large number of good capsules produced. These plants
were all the offspring of the same mother-plants, and the strongly
marked difference in their fertility must be attributed to the nature of
the pollen employed in fertilising their parents; and the difference in
the nature of the pollen must be attributed to the different treatment
to which the pollen-bearing parents had been subjected during several
previous generations.

COLOUR OF THE FLOWERS.

The flowers produced by the self-fertilised plants of the last or fourth
generation were as uniform in tint as those of a wild species, being of
a pale pink or rose colour. Analogous cases with Mimulus and Ipomoea,
after several generations of self-fertilisation, have been already
given. The flowers of the intercrossed plants of the fourth generation
were likewise nearly uniform in colour. On the other hand, the flowers
of the London-crossed plants, or those raised from a cross with the
fresh stock which bore dark crimson flowers, varied extremely in colour,
as might have been expected, and as is the general rule with seedling
carnations. It deserves notice that only two or three of the
London-crossed plants produced dark crimson flowers like those of their
fathers, and only a very few of a pale pink like those of their mothers.
The great majority had their petals longitudinally and variously striped
with the two colours,--the groundwork tint being, however, in some cases
darker than that of the mother-plants.

12. MALVACEAE.--Hibiscus africanus.

Many flowers on this Hibiscus were crossed with pollen from a distinct
plant, and many others were self-fertilised. A rather larger
proportional number of the crossed than of the self-fertilised flowers
yielded capsules, and the crossed capsules contained rather more seeds.
The self-fertilised seeds were a little heavier than an equal number of
the crossed seeds, but they germinated badly, and I raised only four
plants of each lot. In three out of the four pots, the crossed plants
flowered first.

TABLE 4/48. Hibiscus africanus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  13 4/8 :  16 2/8.

Pot 2  :  14     :  14.

Pot 3  :   8     :   7.

Pot 4  :  17 4/8 :  20 4/8.

Total  :  53.00  :  57.75.

The four crossed plants average 13.25, and the four self-fertilised
14.43 inches in height; or as 100 to 109. Here we have the unusual case
of self-fertilised plants exceeding the crossed in height; but only four
pairs were measured, and these did not grow well or equally. I did not
compare the fertility of the two lots.



CHAPTER V.

GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.

Pelargonium zonale, a cross between plants propagated by cuttings does
no good.
Tropaeolum minus.
Limnanthes douglasii.
Lupinus luteus and pilosus.
Phaseolus multiflorus and vulgaris.
Lathyrus odoratus, varieties of, never naturally intercross in England.
Pisum sativum, varieties of, rarely intercross, but a cross between them
highly beneficial.
Sarothamnus scoparius, wonderful effects of a cross.
Ononis minutissima, cleistogene flowers of.
Summary on the Leguminosae.
Clarkia elegans.
Bartonia aurea.
Passiflora gracilis.
Apium petroselinum.
Scabiosa atropurpurea.
Lactuca sativa.
Specularia speculum.
Lobelia ramosa, advantages of a cross during two generations.
Lobelia fulgens.
Nemophila insignis, great advantages of a cross.
Borago officinalis.
Nolana prostrata.

13. GERANIACEAE.--Pelargonium zonale.

This plant, as a general rule, is strongly proterandrous, and is
therefore adapted for cross-fertilisation by the aid of insects. (5/1.
Mr. J. Denny, a great raiser of new varieties of pelargoniums, after
stating that this species is proterandrous, adds ‘The Florist and
Pomologist’ January 1872 page 11, “there are some varieties, especially
those with petals of a pink colour, or which possess a weakly
constitution, where the pistil expands as soon as or even before the
pollen-bag bursts, and in which also the pistil is frequently short, so
when it expands it is smothered as it were by the bursting anthers;
these varieties are great seeders, each pip being fertilised by its own
pollen. I would instance Christine as an example of this fact.” We have
here an interesting case of variability in an important functional
point.) Some flowers on a common scarlet variety were self-fertilised,
and other flowers were crossed with pollen from another plant; but no
sooner had I done so, than I remembered that these plants had been
propagated by cuttings from the same stock, and were therefore parts in
a strict sense of the same individual. Nevertheless, having made the
cross I resolved to save the seeds, which, after germinating on sand,
were planted on the opposite sides of three pots. In one pot the
quasi-crossed plant was very soon and ever afterwards taller and finer
than the self-fertilised. In the two other pots the seedlings on both
sides were for a time exactly equal; but when the self-fertilised plants
were about 10 inches in height, they surpassed their antagonists by a
little, and ever afterwards showed a more decided and increasing
advantage; so that the self-fertilised plants, taken altogether, were
somewhat superior to the quasi-crossed plants. In this case, as in that
of the Origanum, if individuals which have been asexually propagated
from the same stock, and which have been long subjected to the same
conditions, are crossed, no advantage whatever is gained.

Several flowers on another plant of the same variety were fertilised
with pollen from the younger flowers on the same plant, so as to avoid
using the old and long-shed pollen from the same flower, as I thought
that this latter might be less efficient than fresh pollen. Other
flowers on the same plant were crossed with fresh pollen from a plant
which, although closely similar, was known to have arisen as a distinct
seedling. The self-fertilised seeds germinated rather before the others;
but as soon as I got equal pairs they were planted on the opposite sides
of four pots.

TABLE 5/49. Pelargonium zonale.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  22 3/8 :  25 5/8.
Pot 1  :  19 6/8 :  12 4/8.

Pot 2  :  15     :  19 6/8.
Pot 2  :  12 2/8 :  22 3/8.

Pot 3  :  30 5/8 :  19 4/8.
Pot 3  :  18 4/8 :   7 4/8.

Pot 4  :  38     :   9 1/8.

Total  : 156.50  : 116.38.

When the two lots of seedlings were between 4 and 5 inches in height
they were equal, excepting in Pot 4, in which the crossed plant was much
the tallest. When between 11 and 14 inches in height, they were measured
to the tips of their uppermost leaves; the crossed averaged 13.46, and
the self-fertilised 11.07 inches in height, or as 100 to 82. Five months
later they were again measured in the same manner, and the results are
given in Table 5/49.

The seven crossed plants now averaged 22.35, and the seven
self-fertilised 16.62 inches in height, or as 100 to 74. But from the
great inequality of the several plants, the result is less trustworthy
than in most other cases. In Pot 2 the two self-fertilised plants always
had an advantage, except whilst quite young over the two crossed plants.

As I wished to ascertain how these plants would behave during a second
growth, they were cut down close to the ground whilst growing freely.
The crossed plants now showed their superiority in another way, for only
one out of the seven was killed by the operation, whilst three of the
self-fertilised plants never recovered. There was, therefore, no use in
keeping any of the plants excepting those in Pots 1 and 3; and in the
following year the crossed plants in these two pots showed during their
second growth nearly the same relative superiority over the
self-fertilised plants as before.

Tropaeolum minus.

The flowers are proterandrous, and are manifestly adapted for
cross-fertilisation by insects, as shown by Sprengel and Delpino. Twelve
flowers on some plants growing out of doors were crossed with pollen
from a distinct plant and produced eleven capsules, containing
altogether twenty-four good seeds. Eighteen flowers were fertilised with
their own pollen and produced only eleven capsules, containing
twenty-two good seeds; so that a much larger proportion of the crossed
than of the self-fertilised flowers produced capsules, and the crossed
capsules contained rather more seed than the self-fertilised in the
ratio of 100 to 92. The seeds from the self-fertilised capsules were
however the heavier of the two, in the ratio of 100 to 87.

Seeds in an equal state of germination were planted on the opposite
sides of four pots, but only the two tallest plants on each side of each
pot were measured to the tops of their stems. The pots were placed in
the greenhouse, and the plants trained up sticks, so that they ascended
to an unusual height. In three of the pots the crossed plants flowered
first, but in the fourth at the same time with the self-fertilised. When
the seedlings were between 6 and 7 inches in height, the crossed began
to show a slight advantage over their opponents. When grown to a
considerable height the eight tallest crossed plants averaged 44.43, and
the eight tallest self-fertilised plants 37.34 inches, or as 100 to 84.
When their growth was completed they were again measured, as shown in
Table 5/50.

TABLE 5/50. Tropaeolum minus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  65     :  31.
Pot 1  :  50     :  45.

Pot 2  :  69     :  42.
Pot 2  :  35     :  45.

Pot 3  :  70     :  50 4/8.
Pot 3  :  59 4/8 :  55 4/8.

Pot 4  :  61 4/8 :  37 4/8.
Pot 4  :  57 4/8 :  61 4/8.

Total  : 467.5   : 368.0.

The eight tallest crossed plants now averaged 58.43, and the eight
tallest self-fertilised plants 46 inches in height, or as 100 to 79.

There was also a great difference in the fertility of the two lots which
were left uncovered in the greenhouse. On the 17th of September the
capsules from all the plants were gathered, and the seeds counted. The
crossed plants yielded 243, whilst the same number of self-fertilised
plants yielded only 155 seeds, or as 100 to 64.

Limnanthes douglasii.

Several flowers were crossed and self-fertilised in the usual manner,
but there was no marked difference in the number of seeds which they
yielded. A vast number of spontaneously self-fertilised capsules were
also produced under the net. Seedlings were raised in five pots from the
above seeds, and when the crossed were about 3 inches in height they
showed a slight advantage over the self-fertilised. When double this
height, the sixteen crossed and sixteen self-fertilised plants were
measured to the tips of their leaves; the former averaged 7.3 inches,
and the self-fertilised 6.07 inches in height, or as 100 to 83. In all
the pots, excepting 4, a crossed plant flowered before any one of the
self-fertilised plants. The plants, when fully grown, were again
measured to the summits of their ripe capsules, with the result in Table
5/51.

TABLE 5/51. Limnanthes douglasii.

Heights of plants to the summits of their ripe capsules, measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  17 7/8 :  15 1/8.
Pot 1  :  17 6/8 :  16 4/8.
Pot 1  :  13     :  11.

Pot 2  :  20     :  14 4/8.
Pot 2  :  22     :  15 6/8.
Pot 2  :  21     :  16 1/8.
Pot 2  :  18 4/8 :  17.

Pot 3  :  15 6/8 :  11 4/8.
Pot 3  :  17 2/8 :  10 4/8.
Pot 3  :  14     :   0.

Pot 4  :  20 4/8 :  13 4/8.
Pot 4  :  14     :  13.
Pot 4  :  18     :  12 2/8.

Pot 5  :  17     :  14 2/8.
Pot 5  :  18 5/8 :  14 1/8.
Pot 5  :  14 2/8 :  12 5/8.

Total  : 279.50  : 207.75.

The sixteen crossed plants now averaged 17.46, and the fifteen (for one
had died) self-fertilised plants 13.85 inches in height, or as 100 to
79. Mr. Galton considers that a higher ratio would be fairer, namely,
100 to 76. He made a graphical representation of the above measurements,
and adds the words “very good” to the curvature thus formed. Both lots
of plants produced an abundance of seed-capsules, and, as far as could
be judged by the eye, there was no difference in their fertility.]

14. LEGUMINOSAE.

In this family I experimented on the following six genera, Lupinus,
Phaseolus, Lathyrus, Pisum, Sarothamnus, and Ononis.

[Lupinus luteus. (5/2. The structure of the flowers of this plant, and
their manner of fertilisation, have been described by H. Muller
‘Befruchtung’ etc. page 243. The flowers do not secrete free nectar, and
bees generally visit them for their pollen. Mr. Farrer, however, remarks
‘Nature’ 1872 page 499, that “there is a cavity at the back and base of
the vexillum, in which I have not been able to find nectar. But the
bees, which constantly visit these flowers, certainly go to this cavity
for what they want, and not to the staminal tube.”)

A few flowers were crossed with pollen from a distinct plant, but owing
to the unfavourable season only two crossed seeds were produced. Nine
seeds were saved from flowers spontaneously self-fertilised under a net,
on the same plant which yielded the two crossed seeds. One of these
crossed seeds was sown in a pot with two self-fertilised seeds on the
opposite side; the latter came up between two and three days before the
crossed seed. The second crossed seed was sown in like manner with two
self-fertilised seeds on the opposite side; these latter also came up
about a day before the crossed one. In both pots, therefore, the crossed
seedlings from germinating later, were at first completely beaten by the
self-fertilised; nevertheless, this state of things was afterwards
completely reversed. The seeds were sown late in the autumn, and the
pots, which were much too small, were kept in the greenhouse. The plants
in consequence grew badly, and the self-fertilised suffered most in both
pots. The two crossed plants when in flower during the following spring
were 9 inches in height; one of the self-fertilised plants was 8, and
the three others only 3 inches in height, being thus mere dwarfs. The
two crossed plants produced thirteen pods, whilst the four
self-fertilised plants produced only a single one. Some other
self-fertilised plants which had been raised separately in larger pots
produced several spontaneously self-fertilised pods under a net, and
seeds from these were used in the following experiment.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The spontaneously self-fertilised seeds just mentioned, and crossed
seeds obtained by intercrossing the two crossed plants of the last
generation, after germinating on sand, were planted in pairs on the
opposite sides of three large pots. When the seedlings were only 4
inches in height, the crossed had a slight advantage over their
opponents. When grown to their full height, every one of the crossed
plants exceeded its opponent in height. Nevertheless the self-fertilised
plants in all three pots flowered before the crossed! The measurements
are given in Table 5/52.

TABLE 5/52. Lupinus luteus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  33 2/8 :  24 4/8.
Pot 1  :  30 4/8 :  18 4/8.
Pot 1  :  30     :  28.

Pot 2  :  29 4/8 :  26.
Pot 2  :  30     :  25.

Pot 3  :  30 4/8 :  28.
Pot 3  :  31     :  27 2/8.
Pot 3  :  31 4/8 :  24 4/8.

Total  : 246.25  : 201.75.

The eight crossed plants here average 30.78, and the eight
self-fertilised 25.21 inches in height; or as 100 to 82. These plants
were left uncovered in the greenhouse to set their pods, but they
produced very few good ones, perhaps in part owing to few bees visiting
them. The crossed plants produced nine pods, containing on an average
3.4 seeds, and the self-fertilised plants seven pods, containing on an
average 3 seeds, so that the seeds from an equal number of plants were
as 100 to 88.

Two other crossed seedlings, each with two self-fertilised seedlings on
the opposite sides of the same large pot, were turned out of their pots
early in the season, without being disturbed, into open ground of good
quality. They were thus subjected to but little competition with one
another, in comparison with the plants in the above three pots. In the
autumn the two crossed plants were about 3 inches taller than the four
self-fertilised plants; they looked also more vigorous and produced many
more pods.

Two other crossed and self-fertilised seeds of the same lot, after
germinating on sand, were planted on the opposite sides of a large pot,
in which a Calceolaria had long been growing, and were therefore exposed
to unfavourable conditions: the two crossed plants ultimately attained a
height of 20 1/2 and 20 inches, whilst the two self-fertilised were only
18 and 9 1/2 inches high.

Lupinus pilosus.

From a series of accidents I was again unfortunate in obtaining a
sufficient number of crossed seedlings; and the following results would
not be worth giving, did they not strictly accord with those just given
with respect to Lupinus luteus. I raised at first only a single crossed
seedling, which was placed in competition with two self-fertilised ones
on the opposite side of the same pot. These plants, without being
disturbed, were soon afterwards turned into the open ground. By the
autumn the crossed plant had grown to so large a size that it almost
smothered the two self-fertilised plants, which were mere dwarfs; and
the latter died without maturing a single pod. Several self-fertilised
seeds had been planted at the same time separately in the open ground;
and the two tallest of these were 33 and 32 inches, whereas the one
crossed plant was 38 inches in height. This latter plant also produced
many more pods than did any one of the self-fertilised plants, although
growing separately. A few flowers on the one crossed plant were crossed
with pollen from one of the self-fertilised plants, for I had no other
crossed plant from which to obtain pollen. One of the self-fertilised
plants having been covered by a net produced plenty of spontaneously
self-fertilised pods.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

From crossed and self-fertilised seeds obtained in the manner just
described, I succeeded in raising to maturity only a pair of plants,
which were kept in a pot in the greenhouse. The crossed plant grew to a
height of 33 inches, and the self-fertilised to that of 26 1/2 inches.
The former produced, whilst still kept in the greenhouse, eight pods,
containing on an average 2.77 seeds; and the latter only two pods,
containing on an average 2.5 seeds. The average height of the two
crossed plants of the two generations taken together was 35.5, and that
of the three self-fertilised plants of the same two generations 30.5; or
as 100 to 86. (5/3. We here see that both Lupinus luteus and pilosus
seed freely when insects are excluded; but Mr. Swale, of Christchurch,
in New Zealand, informs me ‘Gardeners’ Chronicle’ 1858 page 828, that
the garden varieties of the lupine are not there visited by any bees,
and that they seed less freely than any other introduced leguminous
plant, with the exception of red clover. He adds “I have, for amusement,
during the summer, released the stamens with a pin, and a pod of seed
has always rewarded me for my trouble, the adjoining flowers not so
served having all proved blind.” I do not know to what species this
statement refers.)

Phaseolus multiflorus.

This plant, the scarlet-runner of English gardeners and the Phaseolus
coccineus of Lamarck, originally came from Mexico, as I am informed by
Mr. Bentham. The flowers are so constructed that hive and humble-bees,
which visit them incessantly, almost always alight on the left
wing-petal, as they can best suck the nectar from this side. Their
weight and movements depress the petal, and this causes the stigma to
protrude from the spirally-wound keel, and a brush of hairs round the
stigma pushes out the pollen before it. The pollen adheres to the head
or proboscis of the bee which is at work, and is thus placed either on
the stigma of the same flower, or is carried to another flower. (5/4.
The flowers have been described by Delpino, and in an admirable manner
by Mr. Farrer in the ‘Annals and Magazine of Natural History’ volume 2
4th series October 1868 page 256. My son Francis has explained ‘Nature’
January 8, 1874 page 189, the use of one peculiarity in their structure,
namely, a little vertical projection on the single free stamen near its
base, which seems placed as if to guard the entrance into the two
nectar-holes in the staminal sheath. He shows that this projection
prevents the bees reaching the nectar, unless they go to the left side
of the flower, and it is absolutely necessary for cross-fertilisation
that they should alight on the left wing-petal.) Several years ago I
covered some plants under a large net, and these produced on one
occasion about one-third, and on another occasion about one-eighth, of
the number of pods which the same number of uncovered plants growing
close alongside produced. (5/5. ‘Gardeners’ Chronicle’ 1857 page 725 and
more especially ibid 1858 page 828. Also ‘Annals and Magazine of Natural
History’ 3rd series volume 2 1858 page 462.) This lessened fertility was
not caused by any injury from the net, as I moved the wing-petals of
several protected flowers, in the same manner as bees do, and these
produced remarkably fine pods. When the net was taken off, the flowers
were immediately visited by bees, and it was interesting to observe how
quickly the plants became covered with young pods. As the flowers are
much frequented by Thrips, the self-fertilisation of most of the flowers
under the net may have been due to the action of these minute insects.
Dr. Ogle likewise covered up a large portion of a plant, and “out of a
vast number of blossoms thus protected not a single one produced a pod,
while the unprotected blossoms were for the most part fruitful.” Mr.
Belt gives a more curious case; this plant grows well and flowers in
Nicaragua; but as none of the native bees visit the flowers, not a
single pod is ever produced. (5/6. Dr. Ogle ‘Popular Science Review’
1870 page 168. Mr. Belt ‘The Naturalist in Nicaragua’ 1874 page 70. The
latter author gives a case ‘Nature’ 1875 page 26, of a late crop of
Phaseolus multiflorus near London which “was rendered barren” by the
humble-bees cutting, as they frequently do, holes at the bases of the
flowers instead of entering them in the proper manner.)

From the facts now given we may feel nearly sure that individuals of the
same variety or of different varieties, if growing near each other and
in flower at the same time, would intercross; but I cannot myself
advance any direct evidence of such an occurrence, as only a single
variety is commonly cultivated in England. I have, however, received an
account from the Reverend W.A. Leighton, that plants raised by him from
ordinary seed produced seeds differing in an extraordinary manner in
colour and shape, leading to the belief that their parents must have
been crossed. In France M. Fermond more than once planted close together
varieties which ordinarily come true and which bear differently coloured
flowers and seeds; and the offspring thus raised varied so greatly that
there could hardly be a doubt that they had intercrossed. (5/7.
‘Fécondation chez les Végétaux’ 1859 pages 34-40. He adds that M.
Villiers has described a spontaneous hybrid, which he calls Phaseolus
coccineus hybridus, in the ‘Annales de la Soc. R. de Horticulture’ June
1844.) On the other hand, Professor H. Hoffman does not believe in the
natural crossing of the varieties; for although seedlings raised from
two varieties growing close together produced plants which yielded seeds
of a mixed character, he found that this likewise occurred with plants
separated by a space of from 40 to 150 paces from any other variety; he
therefore attributes the mixed character of the seed to spontaneous
variability. (5/8. ‘Bestimmung des Werthes von Species und Varietat’
1869 pages 47-72.) But the above distance would be very far from
sufficient to prevent intercrossing: cabbages have been known to cross
at several times this distance; and the careful Gartner gives many
instances of plants growing at from 600 to 800 yards apart fertilising
one another. (5/9. ‘Kenntnis der Befruchtung’ 1844 pages 573, 577.)
Professor Hoffman even maintains that the flowers of the kidney-bean are
specially adapted for self-fertilisation. He enclosed several flowers in
bags; and as the buds often dropped off, he attributes the partial
sterility of these flowers to the injurious effects of the bags, and not
to the exclusion of insects. But the only safe method of experimenting
is to cover up a whole plant, which then never suffers.

Self-fertilised seeds were obtained by moving up and down in the same
manner as bees do the wing-petals of flowers protected by a net; and
crossed seeds were obtained by crossing two of the plants under the same
net. The seeds after germinating on sand were planted on the opposite
sides of two large pots, and equal-sized sticks were given them to twine
up. When 8 inches in height, the plants on the two sides were equal. The
crossed plants flowered before the self-fertilised in both pots. As soon
as one of each pair had grown to the summit of its stick both were
measured.

TABLE 5/53. Phaseolus multiflorus.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  87     :  84 6/8.
Pot 1  :  88     :  87.
Pot 1  :  82 4/8 :  76.

Pot 2  :  90     :  76 4/8.
Pot 2  :  82 4/8 :  87 4/8.

Total  : 430.00  : 411.75.

The average height of the five crossed plants is 86 inches, and that of
the five self-fertilised plants 82.35; or as 100 to 96. The pots were
kept in the greenhouse, and there was little or no difference in the
fertility of the two lots. Therefore as far as these few observations
serve, the advantage gained by a cross is very small.

Phaseolus vulgaris.

With respect to this species, I merely ascertained that the flowers were
highly fertile when insects were excluded, as indeed must be the case,
for the plants are often forced during the winter when no insects are
present. Some plants of two varieties (namely Canterbury and Fulmer’s
Forcing Bean) were covered with a net, and they seemed to produce as
many pods, containing as many beans, as some uncovered plants growing
alongside; but neither the pods nor the beans were actually counted.
This difference in self-fertility between Phaseolus vulgaris and
multifloris is remarkable, as these two species are so closely related
that Linnaeus thought that they formed one. When the varieties of
Phaseolus vulgaris grow near one another in the open ground, they
sometimes cross largely, notwithstanding their capacity for
self-fertilisation. Mr. Coe has given me a remarkable instance of this
fact with respect to the negro and a white-seeded and a brown-seeded
variety, which were all grown together. The diversity of character in
the seedlings of the second generation raised by me from his plants was
wonderful. I could add other analogous cases, and the fact is well-known
to gardeners. (5/10. I have given Mr. Coe’s case in the ‘Gardeners’
Chronicle’ 1858 page 829. See also for another case ibid page 845.)

Lathyrus odoratus.

Almost everyone who has studied the structure of papilionaceous flowers
has been convinced that they are specially adapted for
cross-fertilisation, although many of the species are likewise capable
of self-fertilisation. The case therefore of Lathyrus odoratus or the
sweet-pea is curious, for in this country it seems invariably to
fertilise itself. I conclude that this is so, as five varieties,
differing greatly in the colour of their flowers but in no other
respect, are commonly sold and come true; yet on inquiry from two great
raisers of seed for sale, I find that they take no precautions to insure
purity--the five varieties being habitually grown close together. (5/11.
See Mr. W. Earley in ‘Nature’ 1872 page 242, to the same effect. He
once, however, saw bees visiting the flowers, and supposed that on this
occasion they would have been intercrossed.) I have myself purposely
made similar trials with the same result. Although the varieties always
come true, yet, as we shall presently see, one of the five well-known
varieties occasionally gives birth to another, which exhibits all its
usual characters. Owing to this curious fact, and to the darker-coloured
varieties being the most productive, these increase, to the exclusion of
the others, as I was informed by the late Mr. Masters, if there be no
selection.

In order to ascertain what would be the effect of crossing two
varieties, some flowers on the Purple sweet-pea, which has a dark
reddish-purple standard-petal with violet-coloured wing-petals and keel,
were castrated whilst very young, and were fertilised with pollen of the
Painted Lady. This latter variety has a pale cherry-coloured standard,
with almost white wings and keel. On two occasions I raised from a
flower thus crossed plants perfectly resembling both parent-forms; but
the greater number resembled the paternal variety. So perfect was the
resemblance, that I should have suspected some mistake in the label, had
not the plants, which were at first identical in appearance with the
father or Painted Lady, later in the season produced flowers blotched
and streaked with dark purple. This is an interesting example of partial
reversion in the same individual plant as it grows older. The
purple-flowered plants were thrown away, as they might possibly have
been the product of the accidental self-fertilisation of the
mother-plant, owing to the castration not having been effectual. But the
plants which resembled in the colour of their flowers the paternal
variety or Painted Lady were preserved, and their seeds saved. Next
summer many plants were raised from these seeds, and they generally
resembled their grandfather the Painted Lady, but most of them had their
wing-petals streaked and stained with dark pink; and a few had pale
purple wings with the standard of a darker crimson than is natural to
the Painted Lady, so that they formed a new sub-variety. Amongst these
plants a single one appeared having purple flowers like those of the
grandmother, but with the petals slightly streaked with a paler tint:
this was thrown away. Seeds were again saved from the foregoing plants,
and the seedlings thus raised still resembled the Painted Lady, or
great-grandfather; but they now varied much, the standard petal varying
from pale to dark red, in a few instances with blotches of white; and
the wing-petals varied from nearly white to purple, the keel being in
all nearly white.

As no variability of this kind can be detected in plants raised from
seeds, the parents of which have grown during many successive
generations in close proximity, we may infer that they cannot have
intercrossed. What does occasionally occur is that in a row of plants
raised from seeds of one variety, another variety true of its kind
appears; for instance, in a long row of Scarlets (the seeds of which had
been carefully gathered from Scarlets for the sake of this experiment)
two Purples and one Painted Lady appeared. Seeds from these three
aberrant plants were saved and sown in separate beds. The seedlings from
both the Purples were chiefly Purples, but with some Painted Ladies and
some Scarlets. The seedlings from the aberrant Painted Lady were chiefly
Painted Ladies with some Scarlets. Each variety, whatever its parentage
may have been, retained all its characters perfect, and there was no
streaking or blotching of the colours, as in the foregoing plants of
crossed origin. Another variety, however, is often sold, which is
striped and blotched with dark purple; and this is probably of crossed
origin, for I found, as well as Mr. Masters, that it did not transmit
its characters at all truly.

From the evidence now given, we may conclude that the varieties of the
sweet-pea rarely or never intercross in this country; and this is a
highly remarkable fact, considering, firstly, the general structure of
the flowers; secondly, the large quantity of pollen produced, far more
than is requisite for self-fertilisation; and thirdly, the occasional
visit of insects. That insects should sometimes fail to cross-fertilise
the flowers is intelligible, for I have thrice seen humble-bees of two
kinds, as well as hive-bees, sucking the nectar, and they did not
depress the keel-petals so as to expose the anthers and stigma; they
were therefore quite inefficient for fertilising the flowers. One of
these bees, namely, Bombus lapidarius, stood on one side at the base of
the standard and inserted its proboscis beneath the single separate
stamen, as I afterwards ascertained by opening the flower and finding
this stamen prised up. Bees are forced to act in this manner from the
slit in the staminal tube being closely covered by the broad membranous
margin of the single stamen, and from the tube not being perforated by
nectar-passages. On the other hand, in the three British species of
Lathyrus which I have examined, and in the allied genus Vicia, two
nectar-passages are present. Therefore British bees might well be
puzzled how to act in the case of the sweet-pea. I may add that the
staminal tube of another exotic species, Lathyrus grandiflorus, is not
perforated by nectar-passages, and this species has rarely set any pods
in my garden, unless the wing-petals were moved up and down, in the same
manner as bees ought to do; and then pods were generally formed, but
from some cause often dropped off afterwards. One of my sons caught an
elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but
this insect would not depress the wing-petals and keel. On the other
hand, I have seen on one occasion hive-bees, and two or three occasions
the Megachile willughbiella in the act of depressing the keel; and these
bees had the under sides of their bodies thickly covered with pollen,
and could not thus fail to carry pollen from one flower to the stigma of
another. Why then do not the varieties occasionally intercross, though
this would not often happen, as insects so rarely act in an efficient
manner? The fact cannot, as it appears, be explained by the flowers
being self-fertilised at a very early age; for although nectar is
sometimes secreted and pollen adheres to the viscid stigma before the
flowers are fully expanded, yet in five young flowers which were
examined by me the pollen-tubes were not exserted. Whatever the cause
may be, we may conclude, that in England the varieties never or very
rarely intercross. But it does not follow from this, that they would not
be cross by the aid of other and larger insects in their native country,
which in botanical works is said to be the south of Europe and the East
Indies. Accordingly I wrote to Professor Delpino, in Florence, and he
informs me “that it is the fixed opinion of gardeners there that the
varieties do intercross, and that they cannot be preserved pure unless
they are sown separately.”

It follows also from the foregoing facts that the several varieties of
the sweet-pea must have propagated themselves in England by
self-fertilisation for very many generations, since the time when each
new variety first appeared. From the analogy of the plants of Mimulus
and Ipomoea, which had been self-fertilised for several generations, and
from trials previously made with the common pea, which is in nearly the
same state as the sweet-pea, it appeared to me very improbable that a
cross between the individuals of the same variety would benefit the
offspring. A cross of this kind was therefore not tried, which I now
regret. But some flowers of the Painted Lady, castrated at an early age,
were fertilised with pollen from the Purple sweet-pea; and it should be
remembered that these varieties differ in nothing except in the colour
of their flowers. The cross was manifestly effectual (though only two
seeds were obtained), as was shown by the two seedlings, when they
flowered, closely resembling their father, the Purple pea, excepting
that they were a little lighter coloured, with their keels slightly
streaked with pale purple. Seeds from flowers spontaneously
self-fertilised under a net were at the same time saved from the same
mother-plant, the Painted Lady. These seeds unfortunately did not
germinate on sand at the same time with the crossed seeds, so that they
could not be planted simultaneously. One of the two crossed seeds in a
state of germination was planted in a pot (Number 1) in which a
self-fertilised seed in the same state had been planted four days
before, so that this latter seedling had a great advantage over the
crossed one. In Pot 2 the other crossed seed was planted two days before
a self-fertilised one; so that here the crossed seedling had a
considerable advantage over the self-fertilised one. But this crossed
seedling had its summit gnawed off by a slug, and was in consequence for
a time quite beaten by the self-fertilised plant. Nevertheless I allowed
it to remain, and so great was its constitutional vigour that it
ultimately beat its uninjured self-fertilised rival. When all four
plants were almost fully grown they were measured, as here shown:--

TABLE 5/54. Lathyrus odoratus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  80     :  64 4/8.

Pot 2  :  78 4/8 :  63.

Total  : 158.5   : 127.5.

The two crossed plants here average 79.25, and the two self-fertilised
63.75 inches in height, or as 100 to 80. Six flowers on these two
crossed plants were reciprocally crossed with pollen from the other
plant, and the six pods thus produced contained on an average six peas,
with a maximum in one of seven. Eighteen spontaneously self-fertilised
pods from the Painted Lady, which, as already stated, had no doubt been
self-fertilised for many previous generations, contained on an average
only 3.93 peas, with a maximum in one of five peas; so that the number
of peas in the crossed and self-fertilised pods was as 100 to 65. The
self-fertilised peas were, however, quite as heavy as those from the
crossed pods. From these two lots of seeds, the plants of the next
generation were raised.

PLANTS OF THE SECOND GENERATION.

Many of the self-fertilised peas just referred to germinated on sand
before any of the crossed ones, and were rejected. As soon as I got
equal pairs, they were planted on the opposite sides of two large pots,
which were kept in the greenhouse. The seedlings thus raised were the
grandchildren of the Painted Lady, which was first crossed by the Purple
variety. When the two lots were from 4 to 6 inches in height there was
no difference between them. Nor was there any marked difference in the
period of their flowering. When fully grown they were measured, as
follows:--

TABLE 5/55. Lathyrus odoratus (Second Generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Seedlings from Plants Crossed during the two previous
Generations.

Column 3: Seedlings from Plants Self-fertilised during many previous
Generations.

Pot 1  :  72 4/8 :  57 4/8.
Pot 1  :  71     :  67.
Pot 1  :  52 2/8 :  56 2/8.

Pot 2  :  81 4/8 :  66 2/8.
Pot 2  :  45 2/8 :  38 7/8.
Pot 2  :  55     :  46.

Total  : 377.50  : 331.86.

The average height of the six crossed plants is here 62.91, and that of
the six self-fertilised 55.31 inches; or as 100 to 88. There was not
much difference in the fertility of the two lots; the crossed plants
having produced in the greenhouse thirty-five pods, and the
self-fertilised thirty-two pods.

Seeds were saved from the self-fertilised flowers on these two lots of
plants, for the sake of ascertaining whether the seedlings thus raised
would inherit any difference in growth or vigour. It must therefore be
understood that both lots in the following trial are plants of
self-fertilised parentage; but that in the one lot the plants were the
children of plants which had been crossed during two previous
generations, having been before that self-fertilised for many
generations; and that in the other lot they were the children of plants
which had not been crossed for very many previous generations. The seeds
germinated on sand and were planted in pairs on the opposite sides of
four pots. They were measured, when fully grown, with the following
result:--

TABLE 5/56. Lathyrus odoratus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants from Crossed Plants.

Column 3: Self-fertilised Plants from Self-fertilised Plants.

Pot 1  :  72     :  65.
Pot 1  :  72     :  61 4/8.

Pot 2  :  58     :  64.
Pot 2  :  68     :  68 2/8.
Pot 2  :  72 4/8 :  56 4/8.

Pot 3  :  81     :  60 2/8.

Pot 4  :  77 4/8 :  76 4/8.

Total  : 501     : 452.

The average height of the seven self-fertilised plants, the offspring of
crossed plants, is 71.57, and that of the seven self-fertilised plants,
the offspring of self-fertilised plants, is 64.57; or as 100 to 90. The
self-fertilised plants from the self-fertilised produced rather more
pods--namely, thirty-six--than the self-fertilised plants from the
crossed, for these produced only thirty-one pods.

A few seeds of the same two lots were sown in the opposite corners of a
large box in which a Brugmansia had long been growing, and in which the
soil was so exhausted that seeds of Ipomoea purpurea would hardly
vegetate; yet the two plants of the sweet-pea which were raised
flourished well. For a long time the self-fertilised plant from the
self-fertilised beat the self-fertilised plant from the crossed plant;
the former flowered first, and was at one time 77 1/2 inches, whilst the
latter was only 68 1/2 in height; but ultimately the plant from the
previous cross showed its superiority and attained a height of 108 1/2
inches, whilst the other was only 95 inches. I also sowed some of the
same two lots of seeds in poor soil in a shady place in a shrubbery.
Here again the self-fertilised plants from the self-fertilised for a
long time exceeded considerably in height those from the previously
crossed plants; and this may probably be attributed, in the present as
in the last case, to these seeds having germinated rather sooner than
those from the crossed plants; but at the close of the season the
tallest of the self-fertilised plants from the crossed plants was 30
inches, whilst the tallest of the self-fertilised from the
self-fertilised was 29 3/8 inches in height.

From the various facts now given we see that plants derived from a cross
between two varieties of the sweet-pea, which differ in no respect
except in the colour of their flowers, exceed considerably in height the
offspring from self-fertilised plants, both in the first and second
generations. The crossed plants also transmit their superiority in
height and vigour to their self-fertilised offspring.

Pisum sativum.

The common pea is perfectly fertile when its flowers are protected from
the visits of insects; I ascertained this with two or three different
varieties, as did Dr. Ogle with another. But the flowers are likewise
adapted for cross-fertilisation; Mr. Farrer specifies the following
points, namely: “The open blossom displaying itself in the most
attractive and convenient position for insects; the conspicuous
vexillum; the wings forming an alighting place; the attachment of the
wings to the keel, by which any body pressing on the former must press
down the latter; the staminal tube enclosing nectar, and affording by
means of its partially free stamen with apertures on each side of its
base an open passage to an insect seeking the nectar; the moist and
sticky pollen placed just where it will be swept out of the apex of the
keel against the entering insect; the stiff elastic style so placed that
on a pressure being applied to the keel it will be pushed upwards out of
the keel; the hairs on the style placed on that side of the style only
on which there is space for the pollen, and in such a direction as to
sweep it out; and the stigma so placed as to meet an entering
insect,--all these become correlated parts of one elaborate mechanism,
if we suppose that the fertilisation of these flowers is effected by the
carriage of pollen from one to the other.” (5/12. ‘Nature’ October 10,
1872 page 479. Hermann Muller gives an elaborate description of the
flowers ‘Befruchtung’ etc. page 247.) Notwithstanding these manifest
provisions for cross-fertilisation, varieties which have been cultivated
for very many successive generations in close proximity, although
flowering at the same time, remain pure. I have elsewhere given evidence
on this head, and if required could give more. (5/13. ‘Variation of
Animals and Plants under Domestication’ chapter 9 2nd edition volume 1
page 348.) There can hardly be a doubt that some of Knight’s varieties,
which were originally produced by an artificial cross and were very
vigorous, lasted for at least sixty years, and during all these years
were self-fertilised; for had it been otherwise, they would not have
kept true, as the several varieties are generally grown near together.
Most of the varieties, however, endure for a shorter period; and this
may be in part due to their weakness of constitution from long-continued
self-fertilisation.

It is remarkable, considering that the flowers secrete much nectar and
afford much pollen, how seldom they are visited by insects either in
England, or, as H. Muller remarks, in North Germany. I have observed the
flowers for the last thirty years, and in all this time have only thrice
seen bees of the proper kind at work (one of them being Bombus
muscorum), such as were sufficiently powerful to depress the keel, so as
to get the undersides of their bodies dusted with pollen. These bees
visited several flowers, and could hardly have failed to cross-fertilise
them. Hive-bees and other small kinds sometimes collect pollen from old
and already fertilised flowers, but this is of no account. The rarity of
the visits of efficient bees to this exotic plant is, I believe, the
chief cause of the varieties so seldom intercrossing. That a cross does
occasionally take place, as might be expected from what has just been
stated, is certain, from the recorded cases of the direct action of the
pollen of one variety on the seed-coats of another. (5/14. ‘Variation of
Animals and Plants under Domestication’ chapter 11 2nd edition volume 1
page 428.) The late Mr. Masters, who particularly attended to the
raising of new varieties of peas, was convinced that some of them had
originated from accidental crosses. But as such crosses are rare, the
old varieties would not often be thus deteriorated, more especially as
plants departing from the proper type are generally rejected by those
who collect seed for sale. There is another cause which probably tends
to render cross-fertilisation rare, namely, the early age at which the
pollen-tubes are exserted; eight flowers not fully expanded were
examined, and in seven of these the pollen-tubes were in this state; but
they had not as yet penetrated the stigma. Although so few insects visit
the flowers of the pea in this country or in North Germany, and although
the anthers seem here to open abnormally soon, it does not follow that
the species in its native country would be thus circumstanced.

Owing to the varieties having been self-fertilised for many generations,
and to their having been subjected in each generation to nearly the same
conditions (as will be explained in a future chapter) I did not expect
that a cross between two such plants would benefit the offspring; and so
it proved on trial. In 1867 I covered up several plants of the Early
Emperor pea, which was not then a very new variety, so that it must
already have been propagated by self-fertilisation for at least a dozen
generations. Some flowers were crossed with pollen from a distinct plant
growing in the same row, and others were allowed to fertilise themselves
under a net. The two lots of seeds thus obtained were sown on opposite
sides of two large pots, but only four pairs came up at the same time.
The pots were kept in the greenhouse. The seedlings of both lots when
between 6 and 7 inches in height were equal. When nearly full-grown they
were measured, as in Table 5/57.

TABLE 5/57. Pisum sativum.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  35     :  29 6/8.

Pot 2  :  31 4/8 :  51.
Pot 2  :  35     :  45.
Pot 2  :  37     :  33.

Total  : 138.50  : 158.75.

The average height of the four crossed plants is here 34.62, and that of
the four self-fertilised plants 39.68, or as 100 to 115. So that the
crossed plants, far from beating the self-fertilised, were completely
beaten by them.

There can be no doubt that the result would have been widely different,
if any two varieties out of the numberless ones which exist had been
crossed. Notwithstanding that both had been self-fertilised for many
previous generations, each would almost certainly have possessed its own
peculiar constitution; and this degree of differentiation would have
been sufficient to make a cross highly beneficial. I have spoken thus
confidently of the benefit which would have been derived from crossing
any two varieties of the pea from the following facts: Andrew Knight in
speaking of the results of crossing reciprocally very tall and short
varieties, says, “I had in this experiment a striking instance of the
stimulative effects of crossing the breeds; for the smallest variety,
whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the
height of the large and luxuriant kind was very little diminished.”
(5/15. ‘Philosophical Transactions’ 1799 page 200.) Recently Mr. Laxton
has made numerous crosses, and everyone had been astonished at the
vigour and luxuriance of the new varieties which he has thus raised and
afterwards fixed by selection. He gave me seed-peas produced from
crosses between four distinct kinds; and the plants thus raised were
extraordinarily vigorous, being in each case from 1 to 2 or even 3 feet
taller than the parent-forms, which were raised at the same time close
alongside. But as I did not measure their actual height I cannot give
the exact ratio, but it must have been at least as 100 to 75. A similar
trial was subsequently made with two other peas from a different cross,
and the result was nearly the same. For instance, a crossed seedling
between the Maple and Purple-podded pea was planted in poor soil and
grew to the extraordinary height of 116 inches; whereas the tallest
plant of either parent variety, namely, a Purple-podded pea, was only 70
inches in height; or as 100 to 60.

Sarothamnus scoparius.

Bees incessantly visit the flowers of the common Broom, and these are
adapted by a curious mechanism for cross-fertilisation. When a bee
alights on the wing-petals of a young flower, the keel is slightly
opened and the short stamens spring out, which rub their pollen against
the abdomen of the bee. If a rather older flower is visited for the
first time (or if the bee exerts great force on a younger flower), the
keel opens along its whole length, and the longer as well as the shorter
stamens, together with the much elongated curved pistil, spring forth
with violence. The flattened, spoon-like extremity of the pistil rests
for a time on the back of the bee, and leaves on it the load of pollen
with which it is charged. As soon as the bee flies away, the pistil
instantly curls round, so that the stigmatic surface is now upturned and
occupies a position, in which it would be rubbed against the abdomen of
another bee visiting the same flower. Thus, when the pistil first
escapes from the keel, the stigma is rubbed against the back of the bee,
dusted with pollen from the longer stamens, either of the same or
another flower; and afterwards against the lower surface of the bee
dusted with pollen from the shorter stamens, which is often shed a day
or two before that from the longer stamens. (5/16. These observations
have been quoted in an abbreviated form by the Reverend G. Henslow, in
the ‘Journal of Linnean Society Botany’ volume 9 1866 page 358. Hermann
Muller has since published a full and excellent account of the flower in
his ‘Befruchtung’ etc. page 240.) By this mechanism cross-fertilisation
is rendered almost inevitable, and we shall immediately see that pollen
from a distinct plant is more effective than that from the same flower.
I need only add that, according to H. Muller, the flowers do not secrete
nectar, and he thinks that bees insert their proboscides only in the
hope of finding nectar; but they act in this manner so frequently and
for so long a time that I cannot avoid the belief that they obtain
something palatable within the flowers.

If the visits of bees are prevented, and if the flowers are not dashed
by the wind against any object, the keel never opens, so that the
stamens and pistil remain enclosed. Plants thus protected yield very few
pods in comparison with those produced by neighbouring uncovered bushes,
and sometimes none at all. I fertilised a few flowers on a plant growing
almost in a state of nature with pollen from another plant close
alongside, and the four crossed capsules contained on an average 9.2
seeds. This large number no doubt was due to the bush being covered up,
and thus not exhausted by producing many pods; for fifty pods gathered
from an adjoining plant, the flowers of which had been fertilised by the
bees, contained an average of only 7.14 seeds. Ninety-three pods
spontaneously self-fertilised on a large bush which had been covered up,
but had been much agitated by the wind, contained an average of 2.93
seeds. Ten of the finest of these ninety-three capsules yielded an
average of 4.30 seeds, that is less than half the average number in the
four artificially crossed capsules. The ratio of 7.14 to 2.93, or as 100
to 41, is probably the fairest for the number of seeds per pod, yielded
by naturally-crossed and spontaneously self-fertilised flowers. The
crossed seeds compared with an equal number of the spontaneously
self-fertilised seeds were heavier, in the ratio of 100 to 88. We thus
see that besides the mechanical adaptations for cross-fertilisation, the
flowers are much more productive with pollen from a distinct plant than
with their own pollen.

Eight pairs of the above crossed and self-fertilised seeds, after they
had germinated on sand, were planted (1867) on the opposite sides of two
large pots. When several of the seedlings were an inch and a half in
height, there was no marked difference between the two lots. But even at
this early age the leaves of the self-fertilised seedlings were smaller
and of not so bright a green as those of the crossed seedlings. The pots
were kept in the greenhouse, and as the plants on the following spring
(1868) looked unhealthy and had grown but little, they were plunged,
still in their pots, into the open ground. The plants all suffered much
from the sudden change, especially the self-fertilised, and two of the
latter died. The remainder were measured, and I give the measurements in
Table 5/58, because I have not seen in any other species so great a
difference between the crossed and self-fertilised seedlings at so early
an age.

TABLE 5/58. Sarothamnus scoparius (very young plants).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   4 4/8 :   2 4/8.
Pot 1  :   6     :   1 4/8.
Pot 1  :   2     :   1.

Pot 2  :   2     :   1 4/8.
Pot 2  :   2 4/8 :   1.
Pot 2  :   0 4/8 :   0 4/8.

Total  :  17.5   :   8.0.

The six crossed plants here average 2.91, and the six self-fertilised
1.33 inches in height; so that the former were more than twice as high
as the latter, or as 100 to 46.

In the spring of the succeeding year (1869) the three crossed plants in
Pot 1 had all grown to nearly a foot in height, and they had smothered
the three little self-fertilised plants so completely that two were
dead; and the third, only an inch and a half in height, was dying. It
should be remembered that these plants had been bedded out in their
pots, so that they were subjected to very severe competition. This pot
was now thrown away.

The six plants in Pot 2 were all alive. One of the self-fertilised was
an inch and a quarter taller than any one of the crossed plants; but the
other two self-fertilised plants were in a very poor condition. I
therefore resolved to leave these plants to struggle together for some
years. By the autumn of the same year (1869) the self-fertilised plant
which had been victorious was now beaten. The measurements are shown in
Table 5/59.

TABLE 5/59. Pot 2.--Sarothamnus scoparius.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

       :  15 6/8 :  13 1/8.
       :   9 6/8 :   3.
       :   8 2/8 :   2 4/8.

The same plants were again measured in the autumn of the following year,
1870.

TABLE 5/60. Pot 2.--Sarothamnus scoparius.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

       :  26 2/8 :  14 2/8.
       :  16 4/8 :  11 4/8.
       :  14     :   9 6/8.

Total  :  56.75  :  35.50.

The three crossed plants now averaged 18.91, and the three
self-fertilised 11.83 inches in height; or as 100 to 63. The three
crossed plants in Pot 1, as already shown, had beaten the three
self-fertilised plants so completely, that any comparison between them
was superfluous.

The winter of 1870-1871 was severe. In the spring the three crossed
plants in Pot 2 had not even the tips of their shoots in the least
injured, whereas all three self-fertilised plants were killed half-way
down to the ground; and this shows how much more tender they were. In
consequence not one of these latter plants bore a single flower during
the ensuing summer of 1871, whilst all three crossed plants flowered.

Ononis minutissima.

This plant, of which seeds were sent me from North Italy, produces,
besides the ordinary papilionaceous flowers, minute, imperfect, closed
or cleistogene flowers, which can never be cross-fertilised, but are
highly self-fertile. Some of the perfect flowers were crossed with
pollen from a distinct plant, and six capsules thus produced yielded on
an average 3.66 seeds, with a maximum of five in one. Twelve perfect
flowers were marked and allowed to fertilise themselves spontaneously
under a net, and they yielded eight capsules, containing on an average
2.38 seeds, with a maximum of three seeds in one. So that the crossed
and self-fertilised capsules from the perfect flowers yielded seeds in
the proportion of 100 to 65. Fifty-three capsules produced by the
cleistogene flowers contained on an average 4.1 seeds, so that these
were the most productive of all; and the seeds themselves looked finer
even than those from the crossed perfect flowers.

The seeds from the crossed perfect flowers and from the self-fertilised
cleistogene flowers were allowed to germinate on sand; but unfortunately
only two pairs germinated at the same time. These were planted on the
opposite sides of the same pot, which was kept in the greenhouse. In the
summer of the same year, when the seedlings were about 4 1/2 inches in
height, the two lots were equal. In the autumn of the following year
(1868) the two crossed plants were of exactly the same height, namely,
11 4/8 inches, and the two self-fertilised plants 12 6/8 and 7 2/8
inches; so that one of the self-fertilised exceeded considerably in
height all the others. By the autumn of 1869 the two crossed plants had
acquired the supremacy; their height being 16 4/8 and 15 1/8, whilst
that of the two self-fertilised plants was 14 5/8 and 11 4/8 inches.

By the autumn of 1870, the heights were as follows:--

TABLE 5/61. Ononis minutissima.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.

       :  20 3/8 :  17 4/8.
       :  19 2/8 :  17 2/8.

Total  :  39.63  :  34.75.

So that the mean height of the two crossed plants was 19.81, and that of
the two self-fertilised 17.37 inches; or as 100 to 88. It should be
remembered that the two lots were at first equal in height; that one of
the self-fertilised plants then had the advantage, the two crossed
plants being at last victorious.]

SUMMARY ON THE LEGUMINOSAE.

Six genera in this family were experimented on, and the results are in
some respects remarkable. The crossed plants of the two species of
Lupinus were conspicuously superior to the self-fertilised plants in
height and fertility; and when grown under very unfavourable conditions,
in vigour. The scarlet-runner (Phaseolus multiflorus) is partially
sterile if the visits of bees are prevented, and there is reason to
believe that varieties growing near one another intercross. The five
crossed plants, however, exceeded in height the five self-fertilised
only by a little. Phaseolus vulgaris is perfectly self-sterile;
nevertheless, varieties growing in the same garden sometimes intercross
largely. The varieties of Lathyrus odoratus, on the other hand, appear
never to intercross in this country; and though the flowers are not
often visited by efficient insects, I cannot account for this fact, more
especially as the varieties are believed to intercross in North Italy.
Plants raised from a cross between two varieties, differing only in the
colour of their flowers, grew much taller and were under unfavourable
conditions more vigorous than the self-fertilised plants; they also
transmitted, when self-fertilised, their superiority to their offspring.
The many varieties of the common Pea (Pisum sativum), though growing in
close proximity, very seldom intercross; and this seems due to the
rarity in this country of the visits of bees sufficiently powerful to
effect cross-fertilisation. A cross between the self-fertilised
individuals of the same variety does no good whatever to the offspring;
whilst a cross between distinct varieties, though closely allied, does
great good, of which we have excellent evidence. The flowers of the
Broom (Sarothamnus) are almost sterile if they are not disturbed and if
insects are excluded. The pollen from a distinct plant is more effective
than that from the same flower in producing seeds. The crossed seedlings
have an enormous advantage over the self-fertilised when grown together
in close competition. Lastly, only four plants of the Ononis minutissima
were raised; but as these were observed during their whole growth, the
advantage of the crossed over the self-fertilised plants may, I think,
be fully trusted.

[15. ONAGRACEAE.--Clarkia elegans.

Owing to the season being very unfavourable (1867), few of the flowers
which I fertilised formed capsules; twelve crossed flowers produced only
four, and eighteen self-fertilised flowers yielded only one capsule. The
seeds after germinating on sand were planted in three pots, but all the
self-fertilised plants died in one of them. When the two lots were
between 4 and 5 inches in height, the crossed began to show a slight
superiority over the self-fertilised. When in full flower they were
measured, with the following result:--

TABLE 5/62. Clarkia elegans.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  40 4/8 :  33.
Pot 1  :  35     :  24.
Pot 1  :  25     :  23.

Pot 2  :  33 4/8 :  30 4/8.

Total  : 134.0   : 110.5.

The average height of the four crossed plants is 33.5, and that of the
four self-fertilised plants 27.62 inches, or as 100 to 82. The crossed
plants altogether produced 105 and the self-fertilised plants 63
capsules; or as 100 to 60. In both pots a self-fertilised plant flowered
before any one of the crossed plants.

16. LOASACEAE.--Bartonia aurea.

Some flowers were crossed and self-fertilised in the usual manner during
two seasons; but as I reared on the first occasion only two pairs, the
results are given together. On both occasions the crossed capsules
contained slightly more seeds than the self-fertilised. During the first
year, when the plants were about 7 inches in height, the self-fertilised
were the tallest, and in the second year the crossed were the tallest.
When the two lots were in full flower they were measured, as in Table
5/63.

TABLE 5/63. Bartonia aurea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  31     :  37.

Pot 2  :  18 4/8 :  20 4/8.

Pot 3  :  19 4/8 :  40 4/8.

Pot 4  :  25     :  35.
Pot 4  :  36     :  15 4/8.

Pot 5  :  31     :  18.
Pot 5  :  16     :  11 4/8.

Pot 6  :  20     :  32 4/8.

Total  : 197.0   : 210.5.

The average height of the eight crossed plants is 24.62, and that of the
eight self-fertilised 26.31 inches; or as 100 to 107. So that the
self-fertilised had a decided advantage over the crossed. But the plants
from some cause never grew well, and finally became so unhealthy that
only three crossed and three self-fertilised plants survived to set any
capsules, and these were few in number. The two lots seemed to be about
equally unproductive.

17. PASSIFLORACEAE.--Passiflora gracilis.

This annual species produces spontaneously numerous fruits when insects
are excluded, and behaves in this respect very differently from most of
the other species in the genus, which are extremely sterile unless
fertilised with pollen from a distinct plant. (5/17. ‘Variation of
Animals and Plants under Domestication’ chapter 17 2nd edition volume 2
page 118.) Fourteen fruits from crossed flowers contained on an average
24.14 seeds. Fourteen fruits (two poor ones being rejected),
spontaneously self-fertilised under a net, contained on an average 20.58
seeds per fruit; or as 100 to 85. These seeds were sown on the opposite
sides of three pots, but only two pairs came up at the same time; and
therefore a fair judgment cannot be formed.

TABLE 5/64. Passiflora gracilis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  56     :  38.

Pot 2  :  42     :  64.

Total  :  98     : 102.

The mean of the two crossed is 49 inches, and that of the two
self-fertilised 51 inches; or as 100 to 104.

18. UMBELLIFERAE.--Apium petroselinum.

The Umbelliferae are proterandrous, and can hardly fail to be
cross-fertilised by the many flies and small Hymenoptera which visit the
flowers. (5/18. Hermann Muller ‘Befruchtung’ etc. page 96. According to
M. Mustel as stated by Godron ‘De l’espèce’ tome 2 page 58 1859,
varieties of the carrot growing near each other readily intercross.) A
plant of the common parsley was covered by a net, and it apparently
produced as many and as fine spontaneously self-fertilised fruits or
seeds as the adjoining uncovered plants. The flowers on the latter were
visited by so many insects that they must have received pollen from one
another. Some of these two lots of seeds were left on sand, but nearly
all the self-fertilised seeds germinated before the others, so that I
was forced to throw all away. The remaining seeds were then sown on the
opposite sides of four pots. At first the self-fertilised seedlings were
a little taller in most of the pots than the naturally crossed
seedlings, and this no doubt was due to the self-fertilised seeds having
germinated first. But in the autumn all the plants were so equal that it
did not seem worth while to measure them. In two of the pots they were
absolutely equal; in a third, if there was any difference, it was in
favour of the crossed plants, and in a somewhat plainer manner in the
fourth pot. But neither side had any substantial advantage over the
other; so that in height they may be said to be as 100 to 100.

19. DIPSACEAE.--Scabiosa atro-purpurea.

The flowers, which are proterandrous, were fertilised during the
unfavourable season of 1867, so that I got few seeds, especially from
the self-fertilised heads, which were extremely sterile. The crossed and
self-fertilised plants raised from these seeds were measured before they
were in full flower, as in Table 5/65.

TABLE 5/65. Scabiosa atro-purpurea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  14     :  20.

Pot 2  :  15     :  14 4/8.

Pot 3  :  21     :  14.
Pot 3  :  18 4/8 :  13.

Total  :  68.5   :  61.5.

The four crossed plants averaged 17.12, and the four self-fertilised
15.37 inches in height; or as 100 to 90. One of the self-fertilised
plants in Pot 3 was killed by an accident, and its fellow pulled up; so
that when they were again measured to the summits of their flowers,
there were only three on each side; the crossed now averaged in height
32.83, and the self-fertilised 30.16 inches; or as 100 to 92.

20. COMPOSITAE.--Lactuca sativa. (5/19. The Compositae are well-adapted
for cross-fertilisation, but a nurseryman on whom I can rely, told me
that he had been in the habit of sowing several kinds of lettuce near
together for the sake of seed, and had never observed that they became
crossed. It is very improbable that all the varieties which were thus
cultivated near together flowered at different times; but two which I
selected by hazard and sowed near each other did not flower at the same
time; and my trial failed.)

Three plants of Lettuce (Great London Cos var.) grew close together in
my garden; one was covered by a net, and produced self-fertilised seeds,
the other two were allowed to be naturally crossed by insects; but the
season (1867) was unfavourable, and I did not obtain many seeds. Only
one crossed and one self-fertilised plant were raised in Pot 1, and
their measurements are given in Table 5/66. The flowers on this one
self-fertilised plant were again self-fertilised under a net, not with
pollen from the same floret, but from other florets on the same head.
The flowers on the two crossed plants were left to be crossed by
insects, but the process was aided by some pollen being occasionally
transported by me from plant to plant. These two lots of seeds, after
germinating on sand, were planted in pairs on the opposite sides of Pots
2 and 3, which were at first kept in the greenhouse and then turned out
of doors. The plants were measured when in full flower. Table 5/66,
therefore, includes plants belonging to two generations. When the
seedlings of the two lots were only 5 or 6 inches in height they were
equal. In Pot 3 one of the self-fertilised plants died before flowering,
as has occurred in so many other cases.

TABLE 5/66. Lactuca sativa.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :  27     :  21 4/8.
Pot 1  :  25     :  20.
First generation, planted in open ground.

Pot 2  :  29 4/8 :  24.
Pot 2  :  17 4/8 :  10.
Pot 2  :  12 4/8 :  11.
Second generation, planted in open ground.

Pot 3  :  14     :   9 4/8.
Pot 3  :  10 4/8 :   0.
Second generation, kept in the pot.

Total  : 136     :  96.

The average height of the seven crossed plants is 19.43, and that of the
six self-fertilised plants 16 inches; or as 100 to 82.

21. CAMPANULACEAE.--Specularia speculum.

In the closely allied genus, Campanula, in which Specularia was formerly
included, the anthers shed at an early period their pollen, and this
adheres to the collecting hairs which surround the pistil beneath the
stigma; so that without some mechanical aid the flowers cannot be
fertilised. For instance, I covered up a plant of Campanula carpathica,
and it did not produce a single capsule, whilst the surrounding
uncovered plants seeded profusely. On the other hand, the present
species of Specularia appears to set almost as many capsules when
covered up, as when left to the visits of the Diptera, which, as far as
I have seen, are the only insects that frequent the flowers. (5/20. It
has long been known that another species of the genus, Specularia
perfoliata, produces cleistogene as well as perfect flowers, and the
former are of course self-fertile.) I did not ascertain whether the
naturally crossed and spontaneously self-fertilised capsules contained
an equal number of seeds, but a comparison of artificially crossed and
self-fertilised flowers, showed that the former were probably the most
productive. It appears that this plant is capable of producing a large
number of self-fertilised capsules owing to the petals closing at night,
as well as during cold weather. In the act of closing, the margins of
the petals become reflexed, and their inwardly projecting midribs then
pass between the clefts of the stigma, and in doing so push the pollen
from the outside of the pistil on to the stigmatic surfaces. (5/21. Mr.
Meehan has lately shown ‘Proceedings of the Academy of Natural Science
Philadelphia’ May 16, 1876 page 84, that the closing of the flowers of
Claytonia virginica and Ranunculus bulbosus during the night causes
their self-fertilisation.)

Twenty flowers were fertilised by me with their own pollen, but owing to
the bad season, only six capsules were produced; they contained on an
average 21.7 seeds, with a maximum of forty-eight in one. Fourteen
flowers were crossed with pollen from another plant, and these produced
twelve capsules, containing on an average 30 seeds, with a maximum in
one of fifty-seven seeds; so that the crossed seeds were to the
self-fertilised from an equal number of capsules as 100 to 72. The
former were also heavier than an equal number of self-fertilised seeds,
in the ratio of 100 to 86. Thus, whether we judge by the number of
capsules produced from an equal number of flowers, or by the average
number of the contained seeds, or the maximum number in any one capsule,
or by their weight, crossing does great good in comparison with
self-fertilisation. The two lots of seeds were sown on the opposite
sides of four pots; but the seedlings were not sufficiently thinned.
Only the tallest plant on each side was measured, when fully grown. The
measurements are given in Table 5/67. In all four pots the crossed
plants flowered first. When the seedlings were only about an inch and a
half in height both lots were equal.

TABLE 5/67. Specularia speculum.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Tallest Crossed Plant in each Pot.

Column 3: Tallest Self-fertilised Plant in each Pot.

Pot 1  :  18     :  15 6/8.

Pot 2  :  17     :  19.

Pot 3  :  22 1/8 :  18.

Pot 4  :  20     :  23.

Total  :  77.13  :  75.75.

The four tallest crossed plants averaged 19.28, and the four tallest
self-fertilised 18.93 inches in height; or as 100 to 98. So that there
was no difference worth speaking of between the two lots in height;
though other great advantages are derived, as we have seen, from
cross-fertilisation. From being grown in pots and kept in the
greenhouse, none of the plants produced any capsules.

Lobelia ramosa. (5/22. I have adopted the name given to this plant in
the ‘Gardeners’ Chronicle’ 1866. Professor T. Dyer, however, informs me
that it probably is a white variety of L. tenuior of R. Brown, from W.
Australia.)

VAR. SNOW-FLAKE.

The well-adapted means by which cross-fertilisation is ensured in this
genus have been described by several authors. (5/23. See the works of
Hildebrand and Delpino. Mr. Farrer also has given a remarkably clear
description of the mechanism by which cross-fertilisation is effected in
this genus, in the ‘Annals and Magazine of Natural History’ volume 2 4th
series 1868 page 260. In the allied genus Isotoma, the curious spike
which projects rectangularly from the anthers, and which when shaken
causes the pollen to fall on the back of an entering insect, seems to
have been developed from a bristle, like one of those which spring from
the anthers in some of or all the species of Lobelia, as described by
Mr. Farrer.) The pistil as it slowly increases in length pushes the
pollen out of the conjoined anthers, by the aid of a ring of bristles;
the two lobes of the stigma being at this time closed and incapable of
fertilisation. The extrusion of the pollen is also aided by insects,
which rub against the little bristles that project from the anthers. The
pollen thus pushed out is carried by insects to the older flowers, in
which the stigma of the now freely projecting pistil is open and ready
to be fertilised. I proved the importance of the gaily-coloured corolla,
by cutting off the large flowers of Lobelia erinus; and these flowers
were neglected by the hive-bees which were incessantly visiting the
other flowers.

A capsule was obtained by crossing a flower of L. ramosa with pollen
from another plant, and two other capsules from artificially
self-fertilised flowers. The contained seeds were sown on the opposite
sides of four pots. Some of the crossed seedlings which came up before
the others had to be pulled up and thrown away. Whilst the plants were
very small there was not much difference in height between the two lots;
but in Pot 3 the self-fertilised were for a time the tallest. When in
full flower the tallest plant on each side of each pot was measured, and
the result is shown in Table 5/68. In all four pots a crossed plant
flowered before any one of its opponents.

TABLE 5/68. Lobelia ramosa (First Generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Tallest Crossed Plant in each Pot.

Column 3: Tallest Self-fertilised Plant in each Pot.

Pot 1  :  22 4/8 :  17 4/8.

Pot 2  :  27 4/8 :  24.

Pot 3  :  16 4/8 :  15.

Pot 4  :  22 4/8 :  17.

Total  :  89.0   :  73.5.

The four tallest crossed plants averaged 22.25, and the four tallest
self-fertilised 18.37 inches in height; or as 100 to 82. I was surprised
to find that the anthers of a good many of these self-fertilised plants
did not cohere and did not contain any pollen; and the anthers even of a
very few of the crossed plants were in the same condition. Some flowers
on the crossed plants were again crossed, four capsules being thus
obtained; and some flowers on the self-fertilised plants were again
self-fertilised, seven capsules being thus obtained. The seeds from both
lots were weighed, and it was calculated that an equal number of
capsules would have yielded seed in the proportion by weight of 100 for
the crossed to 60 for the self-fertilised capsules. So that the flowers
on the crossed plants again crossed were much more fertile than those on
the self-fertilised plants again self-fertilised.

PLANTS OF THE SECOND GENERATION.

The above two lots of seeds were placed on damp sand, and many of the
crossed seeds germinated, as on the last occasion, before the
self-fertilised, and were rejected. Three or four pairs in the same
state of germination were planted on the opposite sides of two pots; a
single pair in a third pot; and all the remaining seeds were sown
crowded in a fourth pot. When the seedlings were about one and a half
inches in height, they were equal on both sides of the three first pots;
but in Pot 4, in which they grew crowded and were thus exposed to severe
competition, the crossed were about a third taller than the
self-fertilised. In this latter pot, when the crossed averaged 5 inches
in height, the self-fertilised were about 4 inches; nor did they look
nearly such fine plants. In all four pots the crossed plants flowered
some days before the self-fertilised. When in full flower the tallest
plant on each side was measured; but before this time the single crossed
plant in Pot 3, which was taller than its antagonist, had died and was
not measured. So that only the tallest plant on each side of three pots
was measured, as in Table 5/69.

TABLE 5/69. Lobelia ramosa (Second Generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Tallest Crossed Plant in each Pot.

Column 3: Tallest Self-fertilised Plant in each Pot.

Pot 1  :  27 4/8 :  18 4/8.

Pot 2  :  21     :  19 4/8.

Pot 3  :  21 4/8 :  19.
Crowded.

Total  :  70     :  57.

The average height of the three tallest crossed plants is here 23.33,
and that of the tallest self-fertilised 19 inches; or as 100 to 81.
Besides this difference in height, the crossed plants were much more
vigorous and more branched than the self-fertilised plants, and it is
unfortunate that they were not weighed.

Lobelia fulgens.

This species offers a somewhat perplexing case. In the first generation
the self-fertilised plants, though few in number, greatly exceeded the
crossed in height; whilst in the second generation, when the trial was
made on a much larger scale, the crossed beat the self-fertilised
plants. As this species is generally propagated by off-sets, some
seedlings were first raised, in order to have distinct plants. On one of
these plants several flowers were fertilised with their own pollen; and
as the pollen is mature and shed long before the stigma of the same
flower is ready for fertilisation, it was necessary to number each
flower and keep its pollen in paper with a corresponding number. By this
means well-matured pollen was used for self-fertilisation. Several
flowers on the same plant were crossed with pollen from a distinct
individual, and to obtain this the conjoined anthers of young flowers
were roughly squeezed, and as it is naturally protruded very slowly by
the growth of the pistil, it is probable that the pollen used by me was
hardly mature, certainly less mature than that employed for
self-fertilisation. I did not at the time think of this source of error,
but I now suspect that the growth of the crossed plants was thus
injured. Anyhow the trial was not perfectly fair. Opposed to the belief
that the pollen used in crossing was not in so good a state as that used
for self-fertilisation, is the fact that a greater proportional number
of the crossed than of the self-fertilised flowers produced capsules;
but there was no marked difference in the amount of seed contained in
the capsules of the two lots. (5/24. Gartner has shown that certain
plants of Lobelia fulgens are quite sterile with pollen from the same
plant, though this pollen is efficient on any other individual; but none
of the plants on which I experimented, which were kept in the
greenhouse, were in this peculiar condition.)

As the seeds obtained by the above two methods would not germinate when
left on bare sand, they were sown on the opposite sides of four pots;
but I succeeded in raising only a single pair of seedlings of the same
age in each pot. The self-fertilised seedlings, when only a few inches
in height, were in most of the pots taller than their opponents; and
they flowered so much earlier in all the pots, that the height of the
flower-stems could be fairly compared only in Pots 1 and 2.

TABLE 5/70. Lobelia fulgens (First Generation).

Heights of flower-stems measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Height of Flower-stems on the Crossed Plants.

Column 3: Height of Flower-stems on the Self-fertilised Plants.

Pot 1  :  33     :  50.

Pot 2  :  36 4/8 :  38 4/8.

Pot 3  :  21*    :  43.

Pot 4  :  12*    :  35 6/8.

*Not in full flower.

The mean height of the flower-stems of the two crossed plants in Pots 1
and 2 is here 34.75 inches, and that of the two self-fertilised plants
in the same pots 44.25 inches; or as 100 to 127. The self-fertilised
plants in Pots 3 and 4 were in every respect very much finer than the
crossed plants.

I was so much surprised at this great superiority of the self-fertilised
over the crossed plants, that I determined to try how they would behave
in one of the pots during a second growth. The two plants, therefore, in
Pot 1 were cut down, and repotted without being disturbed in a much
larger pot. In the following year the self-fertilised plant showed even
a greater superiority than before; for the two tallest flower-stems
produced by the one crossed plant were only 29 4/8 and 30 1/8 inches in
height, whereas the two tallest stems on the one self-fertilised plant
were 49 4/8 and 49 6/8 inches; and this gives a ratio of 100 to 167.
Considering all the evidence, there can be no doubt that these
self-fertilised plants had a great superiority over the crossed plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

TABLE 5/71. Lobelia fulgens (Second Generation).

Heights of flower-stems measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   27 3/8 :  32 3/8.
Pot 1  :   26     :  26 3/8.
Pot 1  :   24 3/8 :  25 1/8.
Pot 1  :   24 4/8 :  26 2/8.

Pot 2  :   34     :  36 2/8.
Pot 2  :   26 6/8 :  28 6/8.
Pot 2  :   25 1/8 :  30 1/8.
Pot 2  :   26     :  32 2/8.

Pot 3  :   40 4/8 :  30 4/8.
Pot 3  :   37 5/8 :  28 2/8.
Pot 3  :   32 1/8 :  23.

Pot 4  :   34 5/8 :  29 4/8.
Pot 4  :   32 2/8 :  28 3/8.
Pot 4  :   29 3/8 :  26.
Pot 4  :   27 1/8 :  25 2/8.

Pot 5  :   28 1/8 :  29.
Pot 5  :   27     :  24 6/8.
Pot 5  :   25 3/8 :  23 2/8.
Pot 5  :   24 3/8 :  24.

Pot 6  :   33 5/8 :  44 2/8.
Pot 6  :   32     :  37 6/8.
Pot 6  :   26 1/8 :  37.
Pot 6  :   25     :  35.

Pot 7  :   30 6/8 :  27 2/8.
Pot 7  :   30 3/8 :  19 2/8.
Pot 7  :   29 2/8 :  21.

Pot 8  :   39 3/8 :  23 1/8.
Pot 8  :   37 2/8 :  23 4/8.
Pot 8  :   36     :  25 4/8.
Pot 8  :   36     :  25 1/8.

Pot 9  :   33 3/8 :  19 3/8.
Pot 9  :   25     :  16 3/8.
Pot 9  :   25 3/8 :  19.
Pot 9  :   21 7/8 :  18 6/8.

Total  : 1014.00  : 921.63.

I determined on this occasion to avoid the error of using pollen of not
quite equal maturity for crossing and self-fertilisation; so that I
squeezed pollen out of the conjoined anthers of young flowers for both
operations. Several flowers on the crossed plant in Pot 1 in Table 5/70
were again crossed with pollen from a distinct plant. Several other
flowers on the self-fertilised plant in the same pot were again
self-fertilised with pollen from the anthers of other flowers on the
SAME PLANT. Therefore the degree of self-fertilisation was not quite so
close as in the last generation, in which pollen from the SAME FLOWER,
kept in paper, was used. These two lots of seeds were thinly sown on
opposite sides of nine pots; and the young seedlings were thinned, an
equal number of nearly as possible the same age being left on the two
sides. In the spring of the following year (1870), when the seedlings
had grown to a considerable size, they were measured to the tips of
their leaves; and the twenty-three crossed plants averaged 14.04 inches
in height, whilst the twenty-three self-fertilised seedlings were 13.54
inches; or as 100 to 96.

In the summer of the same year several of these plants flowered, the
crossed and self-fertilised plants flowering almost simultaneously, and
all the flower-stems were measured. Those produced by eleven of the
crossed plants averaged 30.71 inches, and those by nine of the
self-fertilised plants 29.43 inches in height; or as 100 to 96.

The plants in these nine pots, after they had flowered, were repotted
without being disturbed in much larger pots; and in the following year,
1871, all flowered freely; but they had grown into such an entangled
mass, that the separate plants on each side could no longer be
distinguished. Accordingly three or four of the tallest flower-stems on
each side of each pot were measured; and the measurements in Table 5/71
are, I think, more trustworthy than the previous ones, from being more
numerous, and from the plants being well established and growing
vigorously.

The average height of the thirty-four tallest flower-stems on the
twenty-three crossed plants is 29.82 inches, and that of the same number
of flower-stems on the same number of self-fertilised plants is 27.10
inches, or as 100 to 91. So that the crossed plants now showed a decided
advantage over their self-fertilised opponents.

22. POLEMONIACEAE.--Nemophila insignis.

Twelve flowers were crossed with pollen from a distinct plant, but
produced only six capsules, containing on an average 18.3 seeds.
Eighteen flowers were fertilised with their own pollen and produced ten
capsules, containing on an average 12.7 seeds, so that the seeds per
capsule were as 100 to 69. (5/25. Several species of Polemoniaceae are
known to be proterandrous, but I did not attend to this point in
Nemophila. Verlot says ‘Des Variétés’ 1865 page 66, that varieties
growing near one another spontaneously intercross.) The crossed seeds
weighed a little less than an equal number of self-fertilised seeds, in
the proportion of 100 to 105; but this was clearly due to some of the
self-fertilised capsules containing very few seeds, and these were much
bulkier than the others, from having been better nourished. A subsequent
comparison of the number of seeds in a few capsules did not show so
great a superiority on the side of the crossed capsules as in the
present case.

The seeds were placed on sand, and after germinating were planted in
pairs on the opposite sides of five pots, which were kept in the
greenhouse. When the seedlings were from 2 to 3 inches in height, most
of the crossed had a slight advantage over the self-fertilised. The
plants were trained up sticks, and thus grew to a considerable height.
In four out of the five pots a crossed plant flowered before any one of
the self-fertilised. The plants were first measured to the tips of their
leaves, before they had flowered and when the crossed were under a foot
in height. The twelve crossed plants averaged 11.1 inches in height,
whilst the twelve self-fertilised were less than half of this height,
namely, 5.45; or as 100 to 49. Before the plants had grown to their full
height, two of the self-fertilised died, and as I feared that this might
happen with others, they were again measured to the tops of their stems,
as shown in Table 5/72.

TABLE 5/72. Nemophila insignis; 0 means that the plant died.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   32 4/8 :  21 2/8.

Pot 2  :   34 4/8 :  23 5/8.

Pot 3  :   33 1/8 :  19.
Pot 3  :   22 2/8 :   7 2/8.
Pot 3  :   29     :  17 4/8.

Pot 4  :   35 4/8 :  10 4/8.
Pot 4  :   33 4/8 :  27.

Pot 5  :   35     :   0.
Pot 5  :   38     :  18 3/8.
Pot 5  :   36     :  20 4/8.
Pot 5  :   37 4/8 :  34.
Pot 5  :   32 4/8 :   0.

Total  :  399.38  : 199.00.

The twelve crossed plants now averaged 33.28, and the ten
self-fertilised 19.9 inches in height, or as 100 to 60; so that they
differed somewhat less than before.

The plants in Pots 3 and 5 were placed under a net in the greenhouse,
two of the crossed plants in the latter pot being pulled up on account
of the death of two of the self-fertilised; so that altogether six
crossed and six self-fertilised plants were left to fertilise themselves
spontaneously. The pots were rather small, and the plants did not
produce many capsules. The small size of the self-fertilised plants will
largely account for the fewness of the capsules which they produced. The
six crossed plants bore 105, and the six self-fertilised only 30
capsules; or as 100 to 29.

The self-fertilised seeds thus obtained from the crossed and
self-fertilised plants, after germinating on sand, were planted on the
opposite sides of four small pots, and treated as before. But many of
the plants were unhealthy, and their heights were so unequal--some on
both sides being five times as tall as the others--that the averages
deduced from the measurements in Table 5/73 are not in the least
trustworthy. Nevertheless I have felt bound to give them, as they are
opposed to my general conclusions.

The seven self-fertilised plants from the crossed plants here average
15.73, and the seven self-fertilised from the self-fertilised 21 inches
in height; or as 100 to 133. Strictly analogous experiments with Viola
tricolor and Lathyrus odoratus gave a very different result.

TABLE 5/73. Nemophila insignis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants from Crossed Plants.

Column 3: Self-fertilised Plants from Self-fertilised Plants.

Pot 1  :   27     :  27 4/8.
Pot 1  :   14     :  34 2/8.

Pot 2  :   17 6/8 :  23.
Pot 2  :   24 4/8 :  32.

Pot 3  :   16     :   7.

Pot 4  :    5 3/8 :   7 2/8.
Pot 4  :    5 4/8 :  16.

Total  :  110.13  : 147.00.

23. BORAGINACEAE.--Borago officinalis.

This plant is frequented by a greater number of bees than any other one
which I have observed. It is strongly proterandrous (H. Muller
‘Befruchtung’ etc. page 267), and the flowers can hardly fail to be
cross-fertilised; but should this not occur, they are capable of
self-fertilisation to a limited extent, as some pollen long remains
within the anthers, and is apt to fall on the mature stigma. In the year
1863 I covered up a plant, and examined thirty-five flowers, of which
only twelve yielded any seeds; whereas of thirty-five flowers on an
exposed plant growing close by, all with the exception of two yielded
seeds. The covered-up plant, however, produced altogether twenty-five
spontaneously self-fertilised seeds; the exposed plant producing
fifty-five seeds, the product, no doubt, of cross-fertilisation.

In the year 1868 eighteen flowers on a protected plant were crossed with
pollen from a distinct plant, but only seven of these produced fruit;
and I suspect that I applied pollen to many of the stigmas before they
were mature. These fruits contained on an average 2 seeds, with a
maximum in one of three seeds. Twenty-four spontaneously self-fertilised
fruits were produced by the same plant, and these contained on an
average 1.2 seeds, with a maximum of two in one fruit. So that the
fruits from the artificially crossed flowers yielded seeds compared with
those from the spontaneously self-fertilised flowers, in the ratio of
100 to 60. But the self-fertilised seeds, as often occurs when few are
produced, were heavier than the crossed seeds in the ratio of 100 to 90.

These two lots of seeds were sown on opposite sides of two large pots;
but I succeeded in raising only four pairs of equal age. When the
seedlings on both sides were about 8 inches in height they were equal.
When in full flower they were measured, as follows:--

TABLE 5/74. Borago officinalis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   19     :  13 4/8.
Pot 1  :   21     :  18 6/8.
Pot 1  :   16 4/8 :  20 2/8.

Pot 2  :   26 2/8 :  32 2/8.

Total  :   82.75  :  84.75.

The average height of the four crossed plants is here 20.68, and that of
the four self-fertilised 21.18 inches; or as 100 to 102. The
self-fertilised plants thus exceeded the crossed in height by a little;
but this was entirely due to the tallness of one of the self-fertilised.
The crossed plants in both pots flowered before the self-fertilised.
Therefore I believe if more plants had been raised, the result would
have been different. I regret that I did not attend to the fertility of
the two lots.

24. NOLANACEAE.--Nolana prostrata.

In some of the flowers the stamens are considerably shorter than the
pistil, in others equal to it in length. I suspected, therefore, but
erroneously as it proved, that this plant was dimorphic, like Primula,
Linum, etc., and in the year 1862 twelve plants, covered by a net in the
greenhouse, were subjected to trial. The spontaneously self-fertilised
flowers yielded 64 grains weight of seeds, but the product of fourteen
artificially crossed flowers is here included, which falsely increases
the weight of the self-fertilised seeds. Nine uncovered plants, the
flowers of which were eagerly visited by bees for their pollen and were
no doubt intercrossed by them, produced 79 grains weight of seeds:
therefore twelve plants thus treated would have yielded 105 grains. Thus
the seeds produced by the flowers on an equal number of plants, when
crossed by bees, and spontaneously self-fertilised (the product of
fourteen artificially crossed flowers being, however, included in the
latter) were in weight as 100 to 61.

In the summer of 1867 the trial was repeated; thirty flowers were
crossed with pollen from a distinct plant and produced twenty-seven
capsules, each containing five seeds. Thirty-two flowers were fertilised
with their own pollen, and produced only six capsules, each with five
seeds. So that the crossed and self-fertilised capsules contained the
same number of seeds, though many more capsules were produced by the
cross-fertilised than by the self-fertilised flowers, in the ratio of
100 to 21.

An equal number of seeds of both lots were weighed, and the crossed
seeds were to the self-fertilised in weight as 100 to 82. Therefore a
cross increases the number of capsules produced and the weight of the
seeds, but not the number of seeds in each capsule.

These two lots of seeds, after germinating on sand, were planted on the
opposite sides of three pots. The seedlings when from 6 to 7 inches in
height were equal. The plants were measured when fully grown, but their
heights were so unequal in the several pots, that the result cannot be
fully trusted.

TABLE 5/75. Nolana prostrata.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :    8 4/8 :   4 2/8.
Pot 1  :    6 4/8 :   7 4/8.

Pot 2  :   10 4/8 :  14 4/8.
Pot 2  :   18     :  18.

Pot 3  :   20 2/8 :  22 6/8.

Total  :   63.75  :  67.00.

The five crossed plants average 12.75, and the five self-fertilised 13.4
inches in height; or as 100 to 105.



CHAPTER VI.

SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.

Petunia violacea, crossed and self-fertilised plants compared for four
generations.
Effects of a cross with a fresh stock.
Uniform colour of the flowers on the self-fertilised plants of the
fourth generation.
Nicotiana tabacum, crossed and self-fertilised plants of equal height.
Great effects of a cross with a distinct sub-variety on the height, but
not on the fertility, of the offspring.
Cyclamen persicum, crossed seedlings greatly superior to the self-fertilised.
Anagallis collina.
Primula veris.
Equal-styled variety of Primula veris, fertility of, greatly increased
by a cross with a fresh stock.
Fagopyrum esculentum.
Beta vulgaris.
Canna warscewiczi, crossed and self-fertilised plants of equal height.
Zea mays.
Phalaris canariensis.

25. SOLANACEAE. Petunia violacea.

DINGY PURPLE VARIETY.

The flowers of this plant are so seldom visited during the day by
insects in this country, that I have never seen an instance; but my
gardener, on whom I can rely, once saw some humble-bees at work. Mr.
Meehan says, that in the United States bees bore through the corolla for
the nectar, and adds that their “fertilisation is carried on by
night-moths.” (6/1. ‘Proceedings of the Academy of Natural Science of
Philadelphia’ August 2, 1870 page 90.)

In France M. Naudin, after castrating a large number of flowers whilst
in bud, left them exposed to the visits of insects, and about a quarter
produced capsules (6/2. ‘Annales des Sc. Nat.’ 4th series Bot. Tome 9
cah. 5); but I am convinced that a much larger proportion of flowers in
my garden are cross-fertilised by insects, for protected flowers with
their own pollen placed on the stigma never yielded nearly a full
complement of seed; whilst those left uncovered produced fine capsules,
showing that pollen from other plants must have been brought to them,
probably by moths. Plants growing vigorously and flowering in pots in
the greenhouse, never yielded a single capsule; and this may be
attributed, at least in chief part, to the exclusion of moths.

Six flowers on a plant covered by a net were crossed with pollen from a
distinct plant and produced six capsules, containing by weight 4.44
grains of seed. Six other flowers were fertilised with their own pollen
and produced only three capsules, containing only 1.49 grains weight of
seed. From this it follows that an equal number of crossed and
self-fertilised capsules would have contained seeds by weight as 100 to
67. I should not have thought the proportional contents of so few
capsules worth giving, had not nearly the same result been confirmed by
several subsequent trials.

Seeds of the two lots were placed on sand, and many of the
self-fertilised seeds germinated before the crossed, and were rejected.
Several pairs in an equal state of germination were planted on the
opposite sides of Pots 1 and 2; but only the tallest plant on each side
was measured. Seeds were also sown thickly on the two sides of a large
pot (3), the seedlings being afterwards thinned, so that an equal number
was left on each side; the three tallest on each side being measured.
The pots were kept in the greenhouse, and the plants were trained up
sticks. For some time the young crossed plants had no advantage in
height over the self-fertilised; but their leaves were larger. When
fully grown and in flower the plants were measured, as follows:--

TABLE 6/76. Petunia violacea (first generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   30     :  20 4/8.

Pot 2  :   34 4/8 :  27 4/8.

Pot 3  :   34     :  28 4/8.
Pot 3  :   30 4/8 :  27 4/8.
Pot 3  :   25     :  26.

Total  :  154     : 130.

The five tallest crossed plants here average 30.8, and the five tallest
self-fertilised 26 inches in height, or as 100 to 84.

Three capsules were obtained by crossing flowers on the above crossed
plants, and three other capsules by again self-fertilising flowers on
the self-fertilised plants. One of the latter capsules appeared as fine
as any one of the crossed capsules; but the other two contained many
imperfect seeds. From these two lots of seeds the plants of the
following generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

As in the last generation, many of the self-fertilised seeds germinated
before the crossed.

Seeds in an equal state of germination were planted on the opposite
sides of three pots. The crossed seedlings soon greatly exceeded in
height the self-fertilised. In Pot 1, when the tallest crossed plant was
10 1/2 inches high, the tallest self-fertilised was only 3 1/2 inches;
in Pot 2 the excess in height of the crossed was not quite so great. The
plants were treated as in the last generation, and when fully grown
measured as before. In Pot 3 both the crossed plants were killed at an
early age by some animal, so that the self-fertilised had no
competitors. Nevertheless these two self-fertilised plants were
measured, and are included in Table 6/77. The crossed plants flowered
long before their self-fertilised opponents in Pots 1 and 2, and before
those growing separately in Pot 3.

TABLE 6/77. Petunia violacea (Second generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   57 2/8 :  13 4/8.
Pot 1  :   36 2/8 :   8.

Pot 2  :   44 4/8 :  33 2/8.
Pot 2  :   24     :  28.

Pot 3  :    0     :  46 2/8.
Pot 3  :    0     :  28 4/8.

Total  :  162.0   : 157.5.

The four crossed plants average 40.5, and the six self-fertilised 26.25
inches in height; or as 100 to 65. But this great inequality is in part
accidental, owing to some of the self-fertilised plants being very
short, and to one of the crossed being very tall.

Twelve flowers on these crossed plants were again crossed, and eleven
capsules were produced; of these, five were poor and six good; the
latter contained by weight 3.75 grains of seeds. Twelve flowers on the
self-fertilised plants were again fertilised with their own pollen and
produced no less than twelve capsules, and the six finest of these
contained by weight 2.57 grains of seeds. It should however be observed
that these latter capsules were produced by the plants in Pot 3, which
were not exposed to any competition. The seeds in the six fine crossed
capsules to those in the six finest self-fertilised capsules were in
weight as 100 to 68. From these seeds the plants of the next generation
were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

TABLE 6/78. Petunia violacea (third generation; plants very young).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :    1 4/8 :   5 6/8.
Pot 1  :    1     :   4 4/8.

Pot 2  :    5 7/8 :   8 3/8.
Pot 2  :    5 6/8 :   6 7/8.

Pot 3  :    4     :   5 5/8.

Pot 4  :    1 4/8 :   5 3/8.

Total  :   19.63  :  36.50.

The above seeds were placed on sand, and after germinating were planted
in pairs on the opposite sides of four pots; and all the remaining seeds
were thickly sown on the two sides of a fifth large pot. The result was
surprising, for the self-fertilised seedlings very early in life beat
the crossed, and at one time were nearly double their height. At first
the case appeared like that of Mimulus, in which after the third
generation a tall and highly self-fertile variety appeared. But as in
the two succeeding generations the crossed plants resumed their former
superiority over the self-fertilised, the case must be looked at as an
anomaly. The sole conjecture which I can form is that the crossed seeds
had not been sufficiently ripened, and thus produced weakly plants, as
occurred with Iberis. When the crossed plants were between 3 and 4
inches in height, the six finest in four of the pots were measured to
the summits of their stems, and at the same time the six finest of the
self-fertilised plants. The measurements are given in Table 6/78, and it
may be here seen that all the self-fertilised plants exceed their
opponents in height, whereas when subsequently measured the excess of
the self-fertilised depended chiefly on the unusual tallness of two of
the plants in Pot 2. The crossed plants here average 3.27, and the
self-fertilised 6.08 inches in height; or as 100 to 186.

When fully grown they were again measured, as follows:--

TABLE 6/79. Petunia violacea (third generation; plants fully grown).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   41 4/8 :  40 6/8.
Pot 1  :   48     :  39.
Pot 1  :   36     :  48.

Pot 2  :   36     :  47.
Pot 2  :   21     :  80 2/8.
Pot 2  :   36 2/8 :  86 2/8.

Pot 3  :   52     :  46.

Pot 4  :   57     :  43 6/8.

Total  :  327.75  : 431.00.

The eight crossed plants now averaged 40.96, and the eight
self-fertilised plants 53.87 inches in height, or as 100 to 131; and
this excess chiefly depended, as already stated, on the unusual tallness
of two of the self-fertilised plants in Pot 2. The self-fertilised had
therefore lost some of their former great superiority over the crossed
plants. In three of the pots the self-fertilised plants flowered first;
but in Pot 3 at the same time with the crossed.

The case is rendered the more strange, because the crossed plants in the
fifth pot (not included in the two last tables), in which all the
remaining seeds had been thickly sown, were from the first finer plants
than the self-fertilised, and had larger leaves. At the period when the
two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high,
the two tallest self-fertilised were only 4 inches. When the two crossed
plants were 12 and 10 inches high, the two self-fertilised were only 8
inches. These latter plants, as well as many others on the same side of
this pot never grew any higher, whereas several of the crossed plants
grew to the height of two feet! On account of this great superiority of
the crossed plants, the plants on neither side of this pot have been
included in the two last tables.

Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were
again crossed, and produced seventeen capsules. Thirty flowers on the
self-fertilised plants in the same two pots were again self-fertilised,
but produced only seven capsules. The contents of each capsule of both
lots were placed in separate watch-glasses, and the seeds from the
crossed appeared to the eye to be at least double the number of those
from the self-fertilised capsules.

In order to ascertain whether the fertility of the self-fertilised
plants had been lessened by the plants having been self-fertilised for
the three previous generations, thirty flowers on the crossed plants
were fertilised with their own pollen. These yielded only five capsules,
and their seeds being placed in separate watch-glasses did not seem more
numerous than those from the capsules on the self-fertilised plants
self-fertilised for the fourth time. So that as far as can be judged
from so few capsules, the self-fertility of the self-fertilised plants
had not decreased in comparison with that of the plants which had been
intercrossed during the three previous generations. It should, however,
be remembered that both lots of plants had been subjected in each
generation to almost exactly similar conditions.

Seeds from the crossed plants again crossed, and from the
self-fertilised again self-fertilised, produced by the plants in Pot 1
(Table 6/79), in which the three self-fertilised plants were on an
average only a little taller than the crossed, were used in the
following experiment. They were kept separate from two similar lots of
seeds produced by the two plants in Pot 4 in the same table, in which
the crossed plant was much taller than its self-fertilised opponent.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM
THE PLANTS IN POT 1, TABLE 6/79).

Crossed and self-fertilised seeds from plants of the last generation in
Pot 1 in Table 6/79, were placed on sand, and after germinating, were
planted in pairs on the opposite sides of four pots. The seedlings when
in full flower were measured to the base of the calyx. The remaining
seeds were sown crowded on the two sides of Pot 5; and the four tallest
plants on each side of this pot were measured in the same manner.

TABLE 6/80. Petunia violacea (fourth generation; raised from plants of
the third generation in Pot 1, table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   29 2/8 :  30 2/8.
Pot 1  :   36 2/8 :  34 6/8.
Pot 1  :   49     :  31 3/8.

Pot 2  :   33 3/8 :  31 5/8.
Pot 2  :   37 3/8 :  38 2/8.
Pot 2  :   56 4/8 :  38 4/8.

Pot 3  :   46     :  45 1/8.
Pot 3  :   67 2/8 :  45.
Pot 3  :   54 3/8 :  23 2/8.

Pot 4  :   51 6/8 :  34.
Pot 4  :   51 7/8 :   0.

Pot 5  :   49 4/8 :  22 3/8.
Pot 5  :   46 3/8 :  24 2/8.
Pot 5  :   40     :  24 6/8.
Pot 5  :   53     :  30.
Crowded plants.

Total  :  701.88  : 453.50.

The fifteen crossed plants average 46.79, and the fourteen (one having
died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So
that the crossed plants in this generation had recovered their wonted
superiority over the self-fertilised plants; though the parents of the
latter in Pot 1, Table 6/79, were a little taller than their crossed
opponents.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM
THE PLANTS IN POT 4, TABLE 6/79).

Two similar lots of seeds, obtained from the plants in Pot 4 in Table
6/79, in which the single crossed plant was at first shorter, but
ultimately much taller than its self-fertilised opponent, were treated
in every way like their brethren of the same generation in the last
experiment. We have in Table 6/81 the measurements of the present
plants. Although the crossed plants greatly exceeded in height the
self-fertilised; yet in three out of the five pots a self-fertilised
plant flowered before any one of the crossed; in a fourth pot
simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered
first.

TABLE 6/81. Petunia violacea (fourth generation; raised from plants of
the third generation in Pot 4, Table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   46     :  30 2/8.
Pot 1  :   46     :  28.

Pot 2  :   50 6/8 :  25.
Pot 2  :   40 2/8 :  31 3/8.
Pot 2  :   37 3/8 :  22 4/8.

Pot 3  :   54 2/8 :  22 5/8.
Pot 3  :   61 1/8 :  26 6/8.
Pot 3  :   45     :  32.

Pot 4  :   30     :  24 4/8.
Pot 4  :   29 1/8 :  26.

Pot 5  :   37 4/8 :  40 2/8.
Pot 5  :   63     :  18 5/8.
Pot 5  :   41 2/8 :  17 4/8.
Crowded plants.

Total  :  581.63  : 349.36.

The thirteen crossed plants here average 44.74, and the thirteen
self-fertilised plants 26.87 inches in height; or as 100 to 60. The
crossed parents of these were much taller, relatively to the
self-fertilised parents, than in the last case; and apparently they
transmitted some of this superiority to their crossed offspring. It is
unfortunate that I did not turn these plants out of doors, so as to
observe their relative fertility, for I compared the pollen from some of
the crossed and self-fertilised plants in Pot 1, Table 6/81, and there
was a marked difference in its state; that of the crossed plants
contained hardly any bad and empty grains, whilst such abounded in the
pollen of the self-fertilised plants.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I procured from a garden in Westerham, whence my plants originally came,
a fresh plant differing in no respect from mine except in the colour of
the flowers, which was a fine purple. But this plant must have been
exposed during at least four generations to very different conditions
from those to which my plants had been subjected, as these had been
grown in pots in the greenhouse. Eight flowers on the self-fertilised
plants in Table 6/81, of the last or fourth self-fertilised generation,
were fertilised with pollen from this fresh stock; all eight produced
capsules containing together by weight 5.01 grains of seeds. The plants
raised from these seeds may be called the Westerham-crossed.

Eight flowers on the crossed plants of the last or fourth generation in
Table 6/81 were again crossed with pollen from one of the other crossed
plants, and produced five capsules, containing by weight 2.07 grains of
seeds. The plants raised from these seeds may be called the
INTERCROSSED; and these form the fifth intercrossed generation.

Eight flowers on the self-fertilised plants of the same generation in
Table 6/81 were again self-fertilised, and produced seven capsules,
containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants
raised from these seeds form the fifth self-fertilised generation. These
latter plants and the intercrossed are comparable in all respects with
the crossed and self-fertilised plants of the four previous generations.

From the foregoing data it is easy to calculate that:

Ten Westerham-crossed capsules would have contained 6.26 grains weight
of seed.

Ten intercrossed capsules would have contained 4.14 grains weight of
seed.

Ten self-fertilised capsules would have contained 3.00 grains weight of
seed.

We thus get the following ratios:--

Seeds from the Westerham-crossed capsules to those from the capsules of
the fifth self-fertilised generation, in weight as 100 to 48.

Seeds from the Westerham-crossed capsules to those from the capsules of
the fifth intercrossed generation, in weight as 100 to 66.

Seeds from the intercrossed capsules to those from the self-fertilised
capsules, in weight as 100 to 72.

So that a cross with pollen from a fresh stock greatly increased the
productiveness of the flowers on plants which had been self-fertilised
for the four previous generations, in comparison not only with the
flowers on the same plants self-fertilised for the fifth time, but with
the flowers on the crossed plants crossed with pollen from another plant
of the same old stock for the fifth time.

These three lots of seeds were placed on sand, and were planted in an
equal state of germination in seven pots, each made tripartite by three
superficial partitions. Some of the remaining seeds, whether or not in a
state of germination, were thickly sown in an eighth pot. The pots were
kept in the greenhouse, and the plants trained up sticks. They were
first measured to the tops of their stems when coming into flower; and
the twenty-two Westerham-crossed plants then averaged 25.51 inches; the
twenty-three intercrossed plants 30.38; and the twenty-three
self-fertilised plants 23.40 inches in height. We thus get the following
ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to
91.

The Westerham-crossed plants in height to the intercrossed as 100 to
119.

The intercrossed plants in height to the self-fertilised as 100 to 77.

These plants were again measured when their growth appeared on a casual
inspection to be complete. But in this I was mistaken, for after cutting
them down, I found that the summits of the stems of the
Westerham-crossed plants were still growing vigorously; whilst the
intercrossed had almost, and the self-fertilised had quite completed
their growth. Therefore I do not doubt, if the three lots had been left
to grow for another month, that the ratios would have been somewhat
different from those deduced from the measurements in Table 6/82.

TABLE 6/82. Petunia violacea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Westerham-Crossed Plants (from self-fertilised Plants of
fourth generation crossed by a fresh stock).

Column 3: Intercrossed Plants (Plants of one and the same stock
intercrossed for five generations).

Column 4: Self-fertilised Plants (self-fertilised for five generations).

Pot 1  :   64 5/8 :   57 2/8 :   43 6/8.
Pot 1  :   24     :   64     :   56 3/8.
Pot 1  :   51 4/8 :   58 6/8 :   31 5/8.

Pot 2  :   48 7/8 :   59 7/8 :   41 5/8.
Pot 2  :   54 4/8 :   58 2/8 :   41 2/8.
Pot 2  :   58 1/8 :   53     :   18 2/8.

Pot 3  :   62     :   52 2/8 :   46 6/8.
Pot 3  :   53 2/8 :   54 6/8 :   45.
Pot 3  :   62 7/8 :   61 6/8 :   19 4/8.

Pot 4  :   44 4/8 :   58 7/8 :   37 5/8.
Pot 4  :   49 2/8 :   65 2/8 :   33 2/8.
Pot 4  :   ..     :   59 6/8 :   32 2/8.

Pot 5  :   43 1/8 :   35 6/8 :   41 6/8.
Pot 5  :   53 7/8 :   34 6/8 :   26 4/8.
Pot 5  :   53 2/8 :   54 6/8 :    0.

Pot 6  :   37 4/8 :   56     :   46 4/8.
Pot 6  :   61     :   63 5/8 :   29 6/8.
Pot 6  :    0     :   57 7/8 :   14 4/8.

Pot 7  :   59 6/8 :   51     :   43.
Pot 7  :   43 4/8 :   49 6/8 :   12 2/8.
Pot 7  :   50 5/8 :    0     :    0.

Pot 8  :   37 7/8 :   38 5/8 :   21 6/8.
Pot 8  :   37 2/8 :   44 5/8 :   14 5/8.

Total  : 1051.25  : 1190.50  :  697.88.

The twenty-one Westerham-crossed plants now averaged 50.05 inches; the
twenty-two intercrossed plants, 54.11 inches; and the twenty-one
self-fertilised plants, 33.23 inches in height. We thus get the
following ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to
66.

The Westerham-crossed plants in height to the intercrossed as 100 to
108.

The intercrossed plants in height to the self-fertilised as 100 to 61.

We here see that the Westerham-crossed (the offspring of plants
self-fertilised for four generations and then crossed with a fresh
stock) have gained greatly in height, since they were first measured,
relatively to the plants self-fertilised for five generations. They were
then as 100 to 91, and now as 100 to 66 in height. The intercrossed
plants (i.e., those which had been intercrossed for the last five
generations) likewise exceed in height the self-fertilised plants, as
occurred in all the previous generations with the exception of the
abnormal plants of the third generation. On the other hand, the
Westerham-crossed plants are exceeded in height by the intercrossed; and
this is a surprising fact, judging from most of the other strictly
analogous cases. But as the Westerham-crossed plants were still growing
vigorously, while the intercrossed had almost ceased to grow, there can
hardly be a doubt that if left to grow for another month they would have
beaten the intercrossed in height. That they were gaining on them is
clear, as when measured before they were as 100 to 119, and now as only
100 to 108 in height. The Westerham-crossed plants had also leaves of a
darker green, and looked altogether more vigorous than the intercrossed;
and what is much more important, they produced, as we shall presently
see, much heavier seed-capsules. So that in fact the offspring from the
self-fertilised plants of the fourth generation crossed by a fresh stock
were superior to the intercrossed, as well as to the self-fertilised
plants of the fifth generation--of which latter fact there could not be
the least doubt.

These three lots of plants were cut down close to the ground and
weighed. The twenty-one Westerham-crossed plants weighed 32 ounces; the
twenty-two intercrossed plants, 34 ounces, and the twenty-one
self-fertilised plants 7 1/4 ounces. The following ratios are calculated
for an equal number of plants of each kind. But as the self-fertilised
plants were just beginning to wither, their relative weight is here
slightly too small; and as the Westerham-crossed were still growing
vigorously, their relative weight with time allowed would no doubt have
greatly increased.

The Westerham-crossed plants in weight to the self-fertilised as 100 to
22.

The Westerham-crossed plants in weight to the intercrossed as 100 to
101.

The intercrossed plants in weight to the self-fertilised as 100 to 22.3.

We here see, judging by weight instead of as before by height, that the
Westerham-crossed and the intercrossed have an immense advantage over
the self-fertilised. The Westerham-crossed are inferior to the
intercrossed by a mere trifle; but it is almost certain that if they had
been allowed to go on growing for another month, the former would have
completely beaten the latter.

As I had an abundance of seeds of the same three lots, from which the
foregoing plants had been raised, these were sown in three long parallel
and adjoining rows in the open ground, so as to ascertain whether under
these circumstances the results would be nearly the same as before. Late
in the autumn (November 13) the ten tallest plants were carefully
selected out of each row, and their heights measured, with the following
result:--

TABLE 6/83. Petunia violacea (plants growing in the open ground).

Heights of plants measured in inches.

Column 1: Westerham-Crossed Plants (from self-fertilised Plants of the
fourth generation crossed by a fresh stock).

Column 2: intercrossed Plants (Plants of one and the same stock
intercrossed for five generations).

Column 3: self-fertilised Plants (self-fertilised for five generations).

   34 2/8 :   38     :   27 3/8.
   36 2/8 :   36 2/8 :   23.
   35 2/8 :   39 5/8 :   25.
   32 4/8 :   37     :   24 1/8.
   37     :   36     :   22 4/8.
   36 4/8 :   41 3/8 :   23 3/8.
   40 7/8 :   37 2/8 :   21 5/8.
   37 2/8 :   40     :   23 4/8.
   38 2/8 :   41 2/8 :   21 3/8.
   38 5/8 :   36     :   21 2/8.

 366.76   :  382.76  :  233.13.

The ten Westerham-crossed plants here average 36.67 inches in height;
the ten intercrossed plants, 38.27 inches; and the ten self-fertilised,
23.31 inches. These three lots of plants were also weighed; the
Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41
ounces; and the self-fertilised, 14.75 ounces. We thus get the following
ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to
63.

The Westerham-crossed plants in weight to the self-fertilised as 100 to
53.

The Westerham-crossed plants in height to the intercrossed as 100 to
104.

The Westerham-crossed plants in weight to the intercrossed as 100 to
146.

The intercrossed plants in height to the self-fertilised as 100 to 61.

The intercrossed plants in weight to the self-fertilised as 100 to 36.

Here the relative heights of the three lots are nearly the same (within
three or four per cent) as with the plants in the pots. In weight there
is a much greater difference: the Westerham-crossed exceed the
self-fertilised by much less than they did before; but the
self-fertilised plants in the pots had become slightly withered, as
before stated, and were in consequence unfairly light. The
Westerham-crossed plants are here inferior in weight to the intercrossed
plants in a much higher degree than in the pots; and this appeared due
to their being much less branched, owing to their having germinated in
greater numbers and consequently being much crowded. Their leaves were
of a brighter green than those of the intercrossed and self-fertilised
plants.

RELATIVE FERTILITY OF THE THREE LOTS OF PLANTS.

None of the plants in pots in the greenhouse ever produced a capsule;
and this may be attributed in chief part to the exclusion of moths.
Therefore the fertility of the three lots could be judged of only by
that of the plants growing out of doors, which from being left uncovered
were probably cross-fertilised. The plants in the three rows were
exactly of the same age and had been subjected to closely similar
conditions, so that any difference in their fertility must be attributed
to their different origin; namely, to the one lot being derived from
plants self-fertilised for four generations and then crossed with a
fresh stock; to the second lot being derived from plants of the same old
stock intercrossed for five generations; and to the third lot being
derived from plants self-fertilised for five generations. All the
capsules, some nearly mature and some only half-grown, were gathered,
counted, and weighed from the ten finest plants in each of the three
rows, of which the measurements and weights have already been given. The
intercrossed plants, as we have seen, were taller and considerably
heavier than the plants of the other two lots, and they produced a
greater number of capsules than did even the Westerham-crossed plants;
and this may be attributed to the latter having grown more crowded and
being in consequence less branched. Therefore the average weight of an
equal number of capsules from each lot of plants seems to be the fairest
standard of comparison, as their weights will have been determined
chiefly by the number of the included seeds. As the intercrossed plants
were taller and heavier than the plants of the other two lots, it might
have been expected that they would have produced the finest or heaviest
capsules; but this was very far from being the case.

The ten tallest Westerham-crossed plants produced 111 ripe and unripe
capsules, weighing 121.2 grains. Therefore 100 of such capsules would
have weighed 109.18 grains.

The ten tallest intercrossed plants produced 129 capsules, weighing
76.45 grains. Therefore 100 of these capsules would have weighed 59.26
grains.

The ten tallest self-fertilised plants produced only 44 capsules,
weighing 22.35 grains. Therefore 100 of these capsules would have
weighed 50.79 grains.

From these data we get the following ratios for the fertility of the
three lots, as deduced from the relative weights of an equal number of
capsules from the finest plants in each lot:--

Westerham-crossed plants to self-fertilised plants as 100 to 46.

Westerham-crossed plants to intercrossed plants as 100 to 54.

Intercrossed plants to self-fertilised plants as 100 to 86.

We here see how potent the influence of a cross with pollen from a fresh
stock has been on the fertility of plants self-fertilised for four
generations, in comparison with plants of the old stock when either
intercrossed or self-fertilised for five generations; the flowers on all
these plants having been left to be freely crossed by insects or to
fertilise themselves. The Westerham-crossed plants were also much taller
and heavier plants than the self-fertilised, both in the pots and open
ground; but they were less tall and heavy than the intercrossed plants.
This latter result, however, would almost certainly have been reversed,
if the plants had been allowed to grow for another month, as the
Westerham-crossed were still growing vigorously, whilst the intercrossed
had almost ceased to grow. This case reminds us of the somewhat
analogous one of Eschscholtzia, in which plants raised from a cross with
a fresh stock did not grow higher than the self-fertilised or
intercrossed plants, but produced a greater number of seed-capsules,
which contained a far larger average number of seeds.

COLOUR OF THE FLOWERS ON THE ABOVE THREE LOTS OF PLANTS.

The original mother-plant, from which the five successive
self-fertilised generations were raised, bore dingy purple flowers. At
no time was any selection practised, and the plants were subjected in
each generation to extremely uniform conditions. The result was, as in
some previous cases, that the flowers on all the self-fertilised plants,
both in the pots and open ground, were absolutely uniform in tint; this
being a dull, rather peculiar flesh colour. This uniformity was very
striking in the long row of plants growing in the open ground, and these
first attracted my attention. I did not notice in which generation the
original colour began to change and to become uniform, but I have every
reason to believe that the change was gradual. The flowers on the
intercrossed plants were mostly of the same tint, but not nearly so
uniform as those on the self-fertilised plants, and many of them were
pale, approaching almost to white. The flowers on the plants from the
cross with the purple-flowered Westerham stock were, as might have been
expected, much more purple and not nearly so uniform in tint. The
self-fertilised plants were also remarkably uniform in height, as judged
by the eye; the intercrossed less so, whilst the Westerham-crossed
plants varied much in height.

Nicotiana tabacum.

This plant offers a curious case. Out of six trials with crossed and
self-fertilised plants, belonging to three successive generations, in
one alone did the crossed show any marked superiority in height over the
self-fertilised; in four of the trials they were approximately equal;
and in one (i.e., in the first generation) the self-fertilised plants
were greatly superior to the crossed. In no case did the capsules from
flowers fertilised with pollen from a distinct plant yield many more,
and sometimes they yielded much fewer seeds than the capsules from
self-fertilised flowers. But when the flowers of one variety were
crossed with pollen from a slightly different variety, which had grown
under somewhat different conditions,--that is, by a fresh stock,--the
seedlings derived from this cross exceeded in height and weight those
from the self-fertilised flowers in an extraordinary degree.

Twelve flowers on some plants of the common tobacco, raised from
purchased seeds, were crossed with pollen from a distinct plant of the
same lot, and these produced ten capsules. Twelve flowers on the same
plants were fertilised with their own pollen, and produced eleven
capsules. The seeds in the ten crossed capsules weighed 31.7 grains,
whilst those in ten of the self-fertilised capsules weighed 47.67
grains; or as 100 to 150. The much greater productiveness of the
self-fertilised than of the crossed capsules can hardly be attributed to
chance, as all the capsules of both lots were very fine and healthy
ones.

The seeds were placed on sand, and several pairs in an equal state of
germination were planted on the opposite sides of three pots. The
remaining seeds were thickly sown on the two sides of Pot 4, so that the
plants in this pot were much crowded. The tallest plant on each side of
each pot was measured. Whilst the plants were quite young the four
tallest crossed plants averaged 7.87 inches, and the four tallest
self-fertilised 14.87 inches in height; or as 100 to 189. The heights at
this age are given in the two left columns of Table 6/84.

When in full flower the tallest plants on each side were again measured,
see the two right hand columns in Table 6/84. But I should state that
the pots were not large enough, and the plants never grew to their
proper height. The four tallest crossed plants now averaged 18.5, and
the four tallest self-fertilised plants 32.75 inches in height; or as
100 to 178. In all four pots a self-fertilised plant flowered before any
one of the crossed.

In Pot 4, in which the plants were extremely crowded, the two lots were
at first equal; and ultimately the tallest crossed plant exceeded by a
trifle the tallest self-fertilised plant. This recalled to my mind an
analogous case in the one generation of Petunia, in which the
self-fertilised plants were throughout their growth taller than the
crossed in all the pots except in the crowded one. Accordingly another
trial was made, and some of the same crossed and self-fertilised seeds
of tobacco were sown thickly on opposite sides of two additional pots;
the plants being left to grow up much crowded. When they were between 13
and 14 inches in height there was no difference between the two sides,
nor was there any marked difference when the plants had grown as tall as
they could; for in one pot the tallest crossed plant was 26 1/2 inches
in height, and exceeded by 2 inches the tallest self-fertilised plant,
whilst in the other pot, the tallest crossed plant was shorter by 3 1/2
inches than the tallest self-fertilised plant, which was 22 inches in
height.

TABLE 6/84. Nicotiana tabacum (first generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants, May 20, 1868.

Column 3: self-fertilised Plants, May 20, 1868.

Column 4: Crossed Plants, December 6, 1868.

Column 5: self-fertilised Plants, December 6, 1868.

Pot 1  :   15 4/8 :   26     :   40     :   44.

Pot 2  :    3     :   15     :    6 4/8 :   43.

Pot 3  :    8     :   13 4/8 :   16     :   33.

Pot 4  :    5     :    5     :   11 4/8 :   11.

Total  :   31.5   :   59.5   :   74.0   :  131.0.

As the plants did not grow to their proper height in the above small
pots in Table 6/84, four crossed and four self-fertilised plants were
raised from the same seed, and were planted in pairs on the opposite
sides of four very large pots containing rich soil; so that they were
not exposed to at all severe mutual competition. When these plants were
in flower I neglected to measure them, but record in my notes that all
four self-fertilised plants exceeded in height the four crossed plants
by 2 or 3 inches. We have seen that the flowers on the original or
parent-plants which were crossed with pollen from a distinct plant
yielded much fewer seeds than those fertilised with their own pollen;
and the trial just given, as well as that in Table 6/84, show us clearly
that the plants raised from the crossed seeds were inferior in height to
those from the self-fertilised seeds; but only when not greatly crowded.
When crowded and thus subjected to very severe competition, the crossed
and self-fertilised plants were nearly equal in height.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Twelve flowers on the crossed plants of the last generation growing in
the four large pots just mentioned, were crossed with pollen from a
crossed plant growing in one of the other pots; and twelve flowers on
the self-fertilised plants were fertilised with their own pollen. All
these flowers of both lots produced fine capsules. Ten of the crossed
capsules contained by weight 38.92 grains of seeds, and ten of the
self-fertilised capsules 37.74 grains; or as 100 to 97. Some of these
seeds in an equal state of germination were planted in pairs on the
opposite sides of five large pots. A good many of the crossed seeds
germinated before the self-fertilised, and were of course rejected. The
plants thus raised were measured when several of them were in full
flower.

TABLE 6/85. Nicotiana tabacum (second generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   14 4/8 :  27 6/8.
Pot 1  :   78 4/8 :   8 6/8.
Pot 1  :    9     :  56.

Pot 2  :   60 4/8 :  16 6/8.
Pot 2  :   44 6/8 :   7.
Pot 2  :   10     :  50 4/8.

Pot 3  :   57 1/8 :  87     (A).
Pot 3  :    1 2/8 :  81 2/8 (B).

Pot 4  :    6 6/8 :  19.
Pot 4  :   31     :  43 2/8.
Pot 4  :   69 4/8 :   4.

Pot 5  :   99 4/8 :   9 4/8.
Pot 5  :   29 2/8 :   3.

Total  :  511.63  : 413.75.

The thirteen crossed plants here average 39.35, and the thirteen
self-fertilised plants 31.82 inches in height; or as 100 to 81. But it
would be a very much fairer plan to exclude all the starved plants of
only 10 inches and under in height; and in this case the nine remaining
crossed plants average 53.84, and the seven remaining self-fertilised
plants 51.78 inches in height, or as 100 to 96; and this difference is
so small that the crossed and self-fertilised plants may be considered
as of equal heights.

In addition to these plants, three crossed plants were planted
separately in three large pots, and three self-fertilised plants in
three other large pots, so that they were not exposed to any
competition; and now the self-fertilised plants exceeded the crossed in
height by a little, for the three crossed averaged 55.91, and the three
self-fertilised 59.16 inches; or as 100 to 106.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

TABLE 6/86. Nicotiana tabacum (third generation). Seedlings from the
self-fertilised plant A in pot 3, Table 6/85, of the last or second
generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the
third Self-fertilised generation.

Pot 1  :  100 2/8 :  98.
Pot 1  :   91     :  79.

Pot 2  :  110 2/8 :  59 1/8.
Pot 2  :  100 4/8 :  66 6/8.

Pot 3  :  104     :  79 6/8.

Pot 4  :   84 2/8 : 110 4/8.
Pot 4  :   76 4/8 :  64 1/8.

Total  :  666.75  : 557.25.

As I wished to ascertain, firstly, whether those self-fertilised plants
of the last generation, which greatly exceeded in height their crossed
opponents, would transmit the same tendency to their offspring, and
secondly, whether they possessed the same sexual constitution, I
selected for experiment the two self-fertilised plants marked A and B in
Pot 3 in Table 6/85, as these two were of nearly equal height, and were
greatly superior to their crossed opponents. Four flowers on each plant
were fertilised with their own pollen, and four others on the same
plants were crossed with pollen from one of the crossed plants growing
in another pot. This plan differs from that before followed, in which
seedlings from crossed plants again crossed, have been compared with
seedlings from self-fertilised plants again self-fertilised. The seeds
from the crossed and self-fertilised capsules of the above two plants
were placed in separate watch-glasses and compared, but were not
weighed; and in both cases those from the crossed capsules seemed to be
rather less numerous than those from the self-fertilised capsules. These
seeds were planted in the usual manner, and the heights of the crossed
and self-fertilised seedlings, when fully grown, are given in Tables
6/86 and 6/87.

The seven crossed plants in the first of these two tables average 95.25,
and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In
half the pots a crossed plant, and in the other half a self-fertilised
plant flowered first.

We now come to the seedlings raised from the other parent-plant B.

TABLE 6/87. Nicotiana tabacum (third generation). Seedlings from the
self-fertilised plant B in pot 3, Table 6/85, of the last or second
generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the
third Self-fertilised generation.

Pot 1  :   87 2/8 :  72 4/8.
Pot 1  :   49     :  14 2/8.

Pot 2  :   98 4/8 :  73.
Pot 2  :    0     : 110 4/8.

Pot 3  :   99     : 106 4/8.
Pot 3  :   15 2/8 :  73 6/8.

Pot 4  :   97 6/8 :  48 6/8.

Pot 5  :   48 6/8 :  81 2/8.
Pot 5  :    0     :  61 2/8.

Total  :  495.50  : 641.75.

The seven crossed plants (for two of them died) here average 70.78
inches, and the nine self-fertilised plants 71.3 inches in height; or as
100 to barely 101. In four out of these five pots, a self-fertilised
plant flowered before any one of the crossed plants. So that,
differently from the last case, the self-fertilised plants are in some
respects slightly superior to the crossed.

If we now consider the crossed and self-fertilised plants of the three
generations, we find an extraordinary diversity in their relative
heights. In the first generation, the crossed plants were inferior to
the self-fertilised as 100 to 178; and the flowers on the original
parent-plants which were crossed with pollen from a distinct plant
yielded much fewer seeds than the self-fertilised flowers, in the
proportion of 100 to 150. But it is a strange fact that the
self-fertilised plants, which were subjected to very severe competition
with the crossed, had on two occasions no advantage over them. The
inferiority of the crossed plants of this first generation cannot be
attributed to the immaturity of the seeds, for I carefully examined
them; nor to the seeds being diseased or in any way injured in some one
capsule, for the contents of the ten crossed capsules were mingled
together and a few taken by chance for sowing. In the second generation
the crossed and self-fertilised plants were nearly equal in height. In
the third generation, crossed and self-fertilised seeds were obtained
from two plants of the previous generation, and the seedlings raised
from them differed remarkably in constitution; the crossed in the one
case exceeded the self-fertilised in height in the ratio of 100 to 83,
and in the other case were almost equal. This difference between the two
lots, raised at the same time from two plants growing in the same pot,
and treated in every respect alike, as well as the extraordinary
superiority of the self-fertilised over the crossed plants in the first
generation, considered together, make me believe that some individuals
of the present species differ to a certain extent from others in their
sexual affinities (to use the term employed by Gartner), like closely
allied species of the same genus. Consequently if two plants which thus
differ are crossed, the seedlings suffer and are beaten by those from
the self-fertilised flowers, in which the sexual elements are of the
same nature. It is known that with our domestic animals certain
individuals are sexually incompatible, and will not produce offspring,
although fertile with other individuals. (6/3. I have given evidence on
this head in my ‘Variation of Animals and Plants under Domestication’
chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a
case which bears more closely on our present one, as it shows that in
the genus Nicotiana the varieties differ in their sexual affinities.
(6/4. ‘Das Geschlecht der Pflanzen, Zweite Fortsetzung’ 1764 pages
55-60.) He experimented on five varieties of the common tobacco, and
proved that they were varieties by showing that they were perfectly
fertile when reciprocally crossed; but one of these varieties, if used
either as the father or the mother, was more fertile than any of the
others when crossed with a widely distinct species, N. glutinosa. As the
different varieties thus differ in their sexual affinities, there is
nothing surprising in the individuals of the same variety differing in a
like manner to a slight degree.

Taking the plants of the three generations altogether, the crossed show
no superiority over the self-fertilised, and I can account for this fact
only by supposing that with this species, which is perfectly
self-fertile without insect aid, most of the individuals are in the same
condition, as those of the same variety of the common pea and of a few
other exotic plants, which have been self-fertilised for many
generations. In such cases a cross between two individuals does no good;
nor does it in any case, unless the individuals differ in general
constitution, either from so-called spontaneous variation, or from their
progenitors having been subjected to different conditions. I believe
that this is the true explanation in the present instance, because, as
we shall immediately see, the offspring of plants, which did not profit
at all by being crossed with a plant of the same stock, profited to an
extraordinary degree by a cross with a slightly different sub-variety.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I procured some seed of N. tabacum from Kew and raised some plants,
which formed a slightly different sub-variety from my former plants; as
the flowers were a shade pinker, the leaves a little more pointed, and
the plants not quite so tall. Therefore the advantage in height which
the seedlings gained by this cross cannot be attributed to direct
inheritance. Two of the plants of the third self-fertilised generation,
growing in Pots 2 and 5 in Table 6/87, which exceeded in height their
crossed opponents (as did their parents in a still higher degree) were
fertilised with pollen from the Kew plants, that is, by a fresh stock.
The seedlings thus raised may be called the Kew-crossed. Some other
flowers on the same two plants were fertilised with their own pollen,
and the seedlings thus raised from the fourth self-fertilised
generation. The crossed capsules produced by the plant in Pot 2, Table
6/87, were plainly less fine than the self-fertilised capsules on the
same plant. In Pot 5 the one finest capsule was also a self-fertilised
one; but the seeds produced by the two crossed capsules together
exceeded in number those produced by the two self-fertilised capsules on
the same plant. Therefore as far as the flowers on the parent-plants are
concerned, a cross with pollen from a fresh stock did little or no good;
and I did not expect that the offspring would have received any benefit,
but in this I was completely mistaken.

The crossed and self-fertilised seeds from the two plants were placed on
bare sand, and very many of the crossed seeds of both sets germinated
before the self-fertilised seeds, and protruded their radicles at a
quicker rate. Hence many of the crossed seeds had to be rejected, before
pairs in an equal state of germination were obtained for planting on the
opposite sides of sixteen large pots. The two series of seedlings raised
from the parent-plants in the two Pots 2 and 5 were kept separate, and
when fully grown were measured to the tips of their highest leaves, as
shown in Table 6/88. But as there was no uniform difference in height
between the crossed and self-fertilised seedlings raised from the two
plants, their heights have been added together in calculating the
averages. I should state that by the accidental fall of a large bush in
the greenhouse, several plants in both the series were much injured.
These were at once measured together with their opponents and afterwards
thrown away. The others were left to grow to their full height, and were
measured when in flower. This accident accounts for the small height of
some of the pairs; but as all the pairs, whether only partly or fully
grown, were measured at the same time, the measurements are fair.

The average height of the twenty-six crossed plants in the sixteen pots
of the two series is 63.29, and that of the twenty-six self-fertilised
plants is 41.67 inches; or as 100 to 66. The superiority of the crossed
plants was shown in another way, for in every one of the sixteen pots a
crossed plant flowered before a self-fertilised one, with the exception
of Pot 6 of the second series, in which the plants on the two sides
flowered simultaneously.

TABLE 6/88. Nicotiana tabacum. Plants raised from two plants of the
third self-fertilised generation in Pots 2 and 5, in Table 6/87.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Kew-crossed Plants, pot 2, Table 6/87.

Column 3: Plants of the fourth Self-fertilised generation, pot 2, Table
6/87.

Column 4: Kew-crossed Plants, pot 5, Table 6/87.

Column 5: Plants of the fourth Self-fertilised generation, pot 5, Table
6/87.

Pot 1  :   84 6/8 :  68 4/8 :   77 6/8 :  56.
Pot 1  :   31     :   5     :    7 2/8 :   5 3/8.

Pot 2  :   78 4/8 :  51 4/8 :   55 4/8 :  27 6/8.
Pot 2  :   48     :  70     :   18     :   7.

Pot 3  :   77 3/8 :  12 6/8 :   76 2/8 :  60 6/8.
Pot 3  :   77 1/8 :   6 6/8.

Pot 4  :   49 2/8 :   29 4/8 :   90 4/8 :  11 6/8.
Pot 4  :   15 6/8 :   32     :   22 2/8 :   4 1/8.

Pot 5  :   89     :   85     :   94 2/8 :  28 4/8.
Pot 5  :   17     :    5 3/8.

Pot 6  :   90     :   80     :   78     :   78 6/8.

Pot 7  :   84 4/8 :   48 6/8 :   85 4/8 :   61 4/8.
Pot 7  :   76 4/8 :   56 4/8.

Pot 8  :   83 4/8 :   84 4/8 :   65 5/8 :   78 3/8.
Pot 8  :          :          :   72 2/8 :   27 4/8.

Total  :  902.63  :  636.13  :  743.13  :  447.38.

Some of the remaining seeds of both series, whether or not in a state of
germination, were thickly sown on the opposite sides of two very large
pots; and the six highest plants on each side of each pot were measured
after they had grown to nearly their full height. But their heights were
much less than in the former trials, owing to their extremely crowded
condition. Even whilst quite young, the crossed seedlings manifestly had
much broader and finer leaves than the self-fertilised seedlings.

TABLE 6/89. Nicotiana tabacum. Plants of the same parentage as those in
Table 6/88, but grown extremely crowded in two large pots.

Heights of plants measured in inches.

Column 1: Kew-crossed Plants, from pot 2, Table 6/87.

Column 2: Plants of the fourth Self-fertilised generation, from pot 2,
Table 6/87.

Column 3: Kew-crossed Plants, from pot 5, Table 6/87.

Column 4: Plants of the fourth Self-fertilised generation, from pot 5,
Table 6/87.

 42 4/8 :  22 4/8 :  44 6/8 :  22 4/8.
 34     :  19 2/8 :  42 4/8 :  21.
 30 4/8 :  14 2/8 :  27 4/8 :  18.
 23 4/8 :  16     :  31 2/8 :  15 2/8.
 26 6/8 :  13 4/8 :  32     :  13 5/8.
 18 3/8 :  16     :  24 6/8 :  14 6/8.

175.63  : 101.50  : 202.75  : 105.13.

The twelve tallest crossed plants in the two pots belonging to the two
series average here 31.53, and the twelve tallest self-fertilised plants
17.21 inches in height; or as 100 to 54. The plants on both sides, when
fully grown, some time after they had been measured, were cut down close
to the ground and weighed. The twelve crossed plants weighed 21.25
ounces; and the twelve self-fertilised plants only 7.83 ounces; or in
weight as 100 to 37.

The rest of the crossed and self-fertilised seeds from the two
parent-plants (the same as in the last experiment) was sown on the 1st
of July in four long parallel and separate rows in good soil in the open
ground; so that the seedlings were not subjected to any mutual
competition. The summer was wet and unfavourable for their growth.
Whilst the seedlings were very small the two crossed rows had a clear
advantage over the two self-fertilised rows. When fully grown the twenty
tallest crossed plants and the twenty tallest self-fertilised plants
were selected and measured on the 11th of November to the extremities of
their leaves, as shown in Table 6/90. Of the twenty crossed plants,
twelve had flowered; whilst of the twenty self-fertilised plants one
alone had flowered.

TABLE 6/90. Nicotiana tabacum. Plants raised from the same seeds as in
the last two experiments, but sown separately in the open ground, so as
not to compete together.

Heights of plants measured in inches.

Column 1: Kew-crossed Plants, from pot 2, Table 6/87.

Column 2: Plants of the fourth Self-fertilised generation, from pot 2,
Table 6/87.

Column 3: Kew-crossed Plants, from pot 5, Table 6/87.

Column 4: Plants of the fourth Self-fertilised generation, from pot 5,
Table 6/87.

 42 2/8 :  22 6/8 :  54 4/8 :  34 4/8.
 54 5/8 :  37 4/8 :  51 4/8 :  38 5/8.
 39 3/8 :  34 4/8 :  45     :  40 6/8.
 53 2/8 :  30     :  43     :  43 2/8.
 49 3/8 :  28 6/8 :  43     :  40.
 50 3/8 :  31 2/8 :  48 6/8 :  38 2/8.
 47 1/8 :  25 4/8 :  44     :  35 6/8.
 57 3/8 :  26 2/8 :  48 2/8 :  39 6/8.
 37     :  22 3/8 :  55 1/8 :  47 6/8.
 48     :  28     :  63     :  58 5/8.

478.75  : 286.86  : 496.13  : 417.25

The twenty tallest crossed plants here average 48.74, and the twenty
tallest self-fertilised 35.2 inches in height; or as 100 to 72. These
plants after being measured were cut down close to the ground, and the
twenty crossed plants weighed 195.75 ounces, and the twenty
self-fertilised plants 123.25 ounces; or as 100 to 63.

In Tables 6/88, 6/89 and 6/90, we have the measurements of fifty-six
plants derived from two plants of the third self-fertilised generation
crossed with pollen from a fresh stock, and of fifty-six plants of the
fourth self-fertilised generation derived from the same two plants.
These crossed and self-fertilised plants were treated in three different
ways, having been put, firstly, into moderately close competition with
one another in pots; secondly, having been subjected to unfavourable
conditions and to very severe competition from being greatly crowded in
two large pots; and thirdly, having been sown separately in open and
good ground, so as not to suffer from any mutual competition. In all
these cases the crossed plants in each lot were greatly superior to the
self-fertilised. This was shown in several ways,--by the earlier
germination of the crossed seeds, by the more rapid growth of the
seedlings whilst quite young, by the earlier flowering of the mature
plants, as well as by the greater height which they ultimately attained.
The superiority of the crossed plants was shown still more plainly when
the two lots were weighed; the weight of the crossed plants to that of
the self-fertilised in the two crowded pots being as 100 to 37. Better
evidence could hardly be desired of the immense advantage derived from a
cross with a fresh stock.

26. PRIMULACEAE.--Cyclamen persicum. (6/5. Cyclamen repandum according
to Lecoq ‘Geographie Botanique de l’Europe’ tome 8 1858 page 150, is
proterandrous, and this I believe to be the case with Cyclamen
persicum.)

Ten flowers crossed with pollen from plants known to be distinct
seedlings, yielded nine capsules, containing on an average 34.2 seeds,
with a maximum of seventy-seven in one. Ten flowers self-fertilised
yielded eight capsules, containing on an average only 13.1 seeds, with a
maximum of twenty-five in one. This gives a ratio of 100 to 38 for the
average number of seeds per capsule for the crossed and self-fertilised
flowers. The flowers hang downwards, and as the stigmas stand close
beneath the anthers, it might have been expected that pollen would have
fallen on them, and that they would have been spontaneously
self-fertilised; but these covered-up plants did not produce a single
capsule. On some other occasions uncovered plants in the same greenhouse
produced plenty of capsules, and I suppose that the flowers had been
visited by bees, which could hardly fail to carry pollen from plant to
plant.

The seeds obtained in the manner just described were placed on sand, and
after germinating were planted in pairs,--three crossed and three
self-fertilised plants on the opposite sides of four pots. When the
leaves were 2 or 3 inches in length, including the foot-stalks, the
seedlings on both sides were equal. In the course of a month or two the
crossed plants began to show a slight superiority over the
self-fertilised, which steadily increased; and the crossed flowered in
all four pots some weeks before, and much more profusely than the
self-fertilised. The two tallest flower-stems on the crossed plants in
each pot were now measured, and the average height of the eight stems
was 9.49 inches. After a considerable interval of time the
self-fertilised plants flowered, and several of their flower-stems (but
I forgot to record how many) were roughly measured, and their average
height was a little under 7.5 inches; so that the flower-stems on the
crossed plants to those on the self-fertilised were at least as 100 to
79. The reason why I did not make more careful measurements of the
self-fertilised plants was, that they looked such poor specimens that I
determined to there them re-potted in larger pots and in the following
year to measure them carefully; but we shall see that this was partly
frustrated by so few flower-stems being then produced.

These plants were left uncovered in the greenhouse; and the twelve
crossed plants produced forty capsules, whilst the twelve
self-fertilised plants produced only five; or as 100 to 12. But this
difference does not give a just idea of the relative fertility of the
two lots. I counted the seeds in one of the finest capsules on the
crossed plants, and it contained seventy-three; whilst the finest of the
five capsules produced by the self-fertilised plants contained only
thirty-five good seeds. In the other four capsules most of the seeds
were barely half as large as those in the crossed capsules.

TABLE 6/91. Cyclamen persicum: 0 implies that no flower-stem was
produced.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   10     :   0.
Pot 1  :    9 2/8 :   0.
Pot 1  :   10 2/8 :   0.

Pot 2  :    9 2/8 :   0.
Pot 2  :   10     :   0.
Pot 2  :   10 2/8 :   0.

Pot 3  :    9 1/8 :   8.
Pot 3  :    9 5/8 :   6 7/8.
Pot 3  :    9 5/8 :   6 6/8.

Pot 4  :   11 1/8 :   0.
Pot 4  :   10 5/8 :   7 7/8.
Pot 4  :   10 6/8 :   0.

Total  :  119.88  :  29.50.

In the following year the crossed plants again bore many flowers before
the self-fertilised bore a single one. The three tallest flower-stems on
the crossed plants in each of the pots were measured, as shown in Table
6/91. In Pots 1 and 2 the self-fertilised plants did not produce a
single flower-stem; in Pot 4 only one; and in Pot 3 six, of which the
three tallest were measured.

The average height of the twelve flower-stems on the crossed plants is
9.99, and that of the four flower-stems on the self-fertilised plants
7.37 inches; or as 100 to 74. The self-fertilised plants were miserable
specimens, whilst the crossed ones looked very vigorous.

ANAGALLIS.

Anagallis collina, var. grandiflora (pale red and blue-flowered
sub-varieties).

Firstly, twenty-five flowers on some plants of the red variety were
crossed with pollen from a distinct plant of the same variety, and
produced ten capsules; thirty-one flowers were fertilised with their own
pollen, and produced eighteen capsules. These plants, which were grown
in pots in the greenhouse, were evidently in a very sterile condition,
and the seeds in both sets of capsules, especially in the
self-fertilised, although numerous, were of so poor a quality that it
was very difficult to determine which were good and which bad. But as
far as I could judge, the crossed capsules contained on an average 6.3
good seeds, with a maximum in one of thirteen; whilst the
self-fertilised contained 6.05 such seeds, with a maximum in one of
fourteen.

Secondly, eleven flowers on the red variety were castrated whilst young
and fertilised with pollen from the blue variety, and this cross
evidently much increased their fertility; for the eleven flowers yielded
seven capsules, which contained on an average twice as many good seeds
as before, namely, 12.7; with a maximum in two of the capsules of
seventeen seeds. Therefore these crossed capsules yielded seeds compared
with those in the foregoing self-fertilised capsules, as 100 to 48.
These seeds were also conspicuously larger than those from the cross
between two individuals of the same red variety, and germinated much
more freely. The flowers on most of the plants produced by the cross
between the two-coloured varieties (of which several were raised), took
after their mother, and were red-coloured. But on two of the plants the
flowers were plainly stained with blue, and to such a degree in one case
as to be almost intermediate in tint.

The crossed seeds of the two foregoing kinds and the self-fertilised
were sown on the opposite sides of two large pots, and the seedlings
were measured when fully grown, as shown in Tables 6/92a and 6/92b.

TABLE 6/92a. Anagallis collina: Red variety crossed by a distinct plant
of the red variety, and red variety self-fertilised.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   23 4/8 :  15 4/8.
Pot 1  :   21     :  15 4/8.
Pot 1  :   17 2/8 :  14.

Total  :   61.75  :  45.00.

TABLE 6/92b. Anagallis collina: Red variety crossed by blue variety, and
red variety self-fertilised.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 2  :   30 4/8 :  24 4/8.
Pot 2  :   27 3/8 :  18 4/8.
Pot 2  :   25     :  11 6/8.

Total  :   82.88  :  54.75.

Total of both lots:
       :  144.63  :  99.75.

As the plants of the two lots are few in number, they may be run
together for the general average; but I may first state that the height
of the seedlings from the cross between two individuals of the red
variety is to that of the self-fertilised plants of the red variety as
100 to 73; whereas the height of the crossed offspring from the two
varieties to the self-fertilised plants of the red variety is as 100 to
66. So that the cross between the two varieties is here seen to be the
most advantageous. The average height of all six crossed plants in the
two lots taken together is 48.20, and that of the six self-fertilised
plants 33.25; or as 100 to 69.

These six crossed plants produced spontaneously twenty-six capsules,
whilst the six self-fertilised plants produced only two, or as 100 to 8.
There is therefore the same extraordinary difference in fertility
between the crossed and self-fertilised plants as in the last genus,
Cyclamen, which belongs to the same family of the Primulaceae.

Primula veris. British flora. (var. officinalis, Linn.).

THE COWSLIP.

Most of the species in this genus are heterostyled or dimorphic; that
is, they present two forms,--one long-styled with short stamens, and the
other short-styled with long stamens. (6/6. See my paper ‘On the Two
Forms or Dimorphic Condition in the Species of Primula’ in ‘Journal of
the Proceedings of the Linnean Society’ volume 6 1862 page 77. A second
paper, to which I presently refer ‘On the Hybrid-like Nature of the
Offspring from the Illegitimate Unions of Dimorphic and Trimorphic
Plants’ was published in volume 10 1867 page 393 of the same journal.)
For complete fertilisation it is necessary that pollen from the one form
should be applied to the stigma of the other form; and this is effected
under nature by insects. Such unions, and the seedlings raised from
them, I have called legitimate. If one form is fertilised with pollen
from the same form, the full complement of seed is not produced; and in
the case of some heterostyled genera no seed at all is produced. Such
unions, and the seedlings raised from them, I have called illegitimate.
These seedlings are often dwarfed and more or less sterile, like
hybrids. I possessed some long-styled plants of Primula veris, which
during four successive generations had been produced from illegitimate
unions between long-styled plants; they were, moreover, in some degree
inter-related, and had been subjected all the time to similar conditions
in pots in the greenhouse. As long as they were cultivated in this
manner, they grew well and were healthy and fertile. Their fertility
even increased in the later generations, as if they were becoming
habituated to illegitimate fertilisation. Plants of the first
illegitimate generation when taken from the greenhouse and planted in
moderately good soil out of doors grew well and were healthy; but when
those of the two last illegitimate generations were thus treated they
became excessively sterile and dwarfed, and remained so during the
following year, by which time they ought to have become accustomed to
growing out of doors, so that they must have possessed a weak
constitution.

Under these circumstances, it seemed advisable to ascertain what would
be the effect of legitimately crossing long-styled plants of the fourth
illegitimate generation with pollen taken from non-related short-styled
plants, growing under different conditions. Accordingly several flowers
on plants of the fourth illegitimate generation (i.e.,
great-great-grandchildren of plants which had been legitimately
fertilised), growing vigorously in pots in the greenhouse, were
legitimately fertilised with pollen from an almost wild short-styled
cowslip, and these flowers yielded some fine capsules. Thirty other
flowers on the same illegitimate plants were fertilised with their own
pollen, and these yielded seventeen capsules, containing on an average
thirty-two seeds. This is a high degree of fertility; higher, I believe,
than that which generally obtains with illegitimately fertilised
long-styled plants growing out of doors, and higher than that of the
previous illegitimate generations, although their flowers were
fertilised with pollen taken from a distinct plant of the same form.

These two lots of seeds were sown (for they will not germinate well when
placed on bare sand) on the opposite sides of four pots, and the
seedlings were thinned, so that an equal number were left on the two
sides. For some time there was no marked difference in height between
the two lots; and in Pot 3, Table 6/93, the self-fertilised plants were
rather the tallest. But by the time that they had thrown up young
flower-stems, the legitimately crossed plants revealed much the finest,
and had greener and larger leaves. The breadth of the largest leaf on
each plant was measured, and those on the crossed plants were on an
average a quarter of an inch (exactly .28 of an inch) broader than those
on the self-fertilised plants. The plants, from being too much crowded,
produced poor and short flower-stems. The two finest on each side were
measured; the eight on the legitimately crossed plants averaged 4.08,
and the eight on the illegitimately self-fertilised plants averaged 2.93
inches in height; or as 100 to 72.

These plants after they had flowered were turned out of their pots, and
planted in fairly good soil in the open ground. In the following year
(1870), when in full flower, the two tallest flower-stems on each side
were again measured, as shown in Table 6/93, which likewise gives the
number of flower-stems produced on both sides of all the pots.

TABLE 6/93. Primula veris.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Height: Legitimately crossed Plants.

Column 3: Number of Flower-stems produced: Legitimately crossed Plants.

Column 4: Height: Illegitimately crossed Plants.

Column 5: Number of Flower-stems produced: Illegitimately crossed
Plants.

Pot 1  :    9     :  16     :    2 1/8 :  3.
Pot 1  :    8     :         :    3 4/8.

Pot 2  :    7     :  16     :    6     :  3.
Pot 2  :    6 4/8 :         :    5 4/8.

Pot 3  :    6     :  16     :    3     :  4.
Pot 3  :    6 2/8 :         :    0 4/8.

Pot 4  :    7 3/8 :  14     :    2 5/8 :  5.
Pot 4  :    6 1/8 :         :    2 4/8.

Total  :   56.26  :  62     :   25.75  : 15.

The average height of the eight tallest flower-stems on the crossed
plants is here 7.03 inches, and that of the eight tallest flower-stems
on the self-fertilised plants 3.21 inches; or as 100 to 46. We see,
also, that the crossed plants bore sixty-two flower-stems; that is,
above four times as many as those (namely fifteen) borne by the
self-fertilised plants. The flowers were left exposed to the visits of
insects, and as many plants of both forms grew close by, they must have
been legitimately and naturally fertilised. Under these circumstances
the crossed plants produced 324 capsules, whilst the self-fertilised
produced only 16; and these were all produced by a single plant in Pot
2, which was much finer than any other self-fertilised plant. Judging by
the number of capsules produced, the fertility of an equal number of
crossed and self-fertilised plants was as 100 to 5.

In the succeeding year (1871) I did not count all the flower-stems on
these plants, but only those which produced capsules containing good
seeds. The season was unfavourable, and the crossed plants produced only
forty such flower-stems, bearing 168 good capsules, whilst the
self-fertilised plants produced only two such flower-stems, bearing only
6 capsules, half of which were very poor ones. So that the fertility of
the two lots, judging by the number of capsules, was as 100 to 3.5.

In considering the great difference in height and the wonderful
difference in fertility between the two sets of plants, we should bear
in mind that this is the result of two distinct agencies. The
self-fertilised plants were the product of illegitimate fertilisation
during five successive generations, in all of which, excepting the last,
the plants had been fertilised with pollen taken from a distinct
individual belonging to the same form, but which was more or less
closely related. The plants had also been subjected in each generation
to closely similar conditions. This treatment alone, as I know from
other observations, would have greatly reduced the size and fertility of
the offspring. On the other hand, the crossed plants were the offspring
of long-styled plants of the fourth illegitimate generation legitimately
crossed with pollen from a short-styled plant, which, as well as its
progenitors, had been exposed to very different conditions; and this
latter circumstance alone would have given great vigour to the
offspring, as we may infer from the several analogous cases already
given. How much proportional weight ought to be attributed to these two
agencies,--the one tending to injure the self-fertilised offspring, and
the other to benefit the crossed offspring,--cannot be determined. But
we shall immediately see that the greater part of the benefit, as far as
increased fertility is concerned, must be attributed to the cross having
been made with a fresh stock.

Primula veris.

EQUAL-STYLED AND RED-FLOWERED VAR.

I have described in my paper ‘On the Illegitimate Unions of Dimorphic
and Trimorphic Plants’ this remarkable variety, which was sent to me
from Edinburgh by Mr. J. Scott. It possessed a pistil proper to the
long-styled form, and stamens proper to the short-styled form; so that
it had lost the heterostyled or dimorphic character common to most of
the species of the genus, and may be compared with an hermaphrodite form
of a bisexual animal. Consequently the pollen and stigma of the same
flower are adapted for complete mutual fertilisation, instead of its
being necessary that pollen should be brought from one form to another,
as in the common cowslip. From the stigma and anthers standing nearly on
the same level, the flowers are perfectly self-fertile when insects are
excluded. Owing to the fortunate existence of this variety, it is
possible to fertilise its flowers in a legitimate manner with their own
pollen, and to cross other flowers in a legitimate manner with pollen
from another variety or fresh stock. Thus the offspring from both unions
can be compared quite fairly, free from any doubt from the injurious
effects of an illegitimate union.

The plants on which I experimented had been raised during two successive
generations from spontaneously self-fertilised seeds produced by plants
under a net; and as the variety is highly self-fertile, its progenitors
in Edinburgh may have been self-fertilised during some previous
generations. Several flowers on two of my plants were legitimately
crossed with pollen from a short-styled common cowslip growing almost
wild in my orchard; so that the cross was between plants which had been
subjected to considerably different conditions. Several other flowers on
the same two plants were allowed to fertilise themselves under a net;
and this union, as already explained, is a legitimate one.

The crossed and self-fertilised seeds thus obtained were sown thickly on
the opposite sides of three pots, and the seedlings thinned, so that an
equal number were left on the two sides. The seedlings during the first
year were nearly equal in height, excepting in Pot 3, Table 6/94, in
which the self-fertilised plants had a decided advantage. In the autumn
the plants were bedded out, in their pots; owing to this circumstance,
and to many plants growing in each pot, they did not flourish, and none
were very productive in seeds. But the conditions were perfectly equal
and fair for both sides. In the following spring I record in my notes
that in two of the pots the crossed plants are “incomparably the finest
in general appearance,” and in all three pots they flowered before the
self-fertilised. When in full flower the tallest flower-stem on each
side of each pot was measured, and the number of the flower-stems on
both sides counted, as shown in Table 6/94. The plants were left
uncovered, and as other plants were growing close by, the flowers no
doubt were crossed by insects. When the capsules were ripe they were
gathered and counted, and the result is likewise shown in Table 6/94.

TABLE 6/94. Primula veris (equal-styled, red-flowered variety).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Height of tallest flower-stem: crossed Plants.

Column 3: Number of Flower-stems: crossed Plants.

Column 4: Number of good capsules: crossed Plants.

Column 5: Height of tallest flower-stem: self-fertilised Plants.

Column 6: Number of Flower-stems: self-fertilised Plants.

Column 7: Number of good capsules: self-fertilised Plants.

Pot 1  : 10     : 14     : 163     :  6 4/8 :  6     :  6.

Pot 2  :  8 4/8 : 12     :   *     :  5     :  2     :  0.
                          *Several, not counted.

Pot 3  :  7 4/8 :  7     :  43     : 10 4/8 :  5     : 26.

Totals : 26.0   : 33     : 206     : 22.0   : 13     : 32.

The average height of the three tallest flower-stems on the crossed
plants is 8.66 inches, and that of the three on the self-fertilised
plants 7.33 inches; or as 100 to 85.

All the crossed plants together produced thirty-three flower-stems,
whilst the self-fertilised bore only thirteen. The number of the
capsules were counted only on the plants in Pots 1 and 3, for the
self-fertilised plants in Pot 2 produced none; therefore those on the
crossed plants on the opposite side were not counted. Capsules not
containing any good seeds were rejected. The crossed plants in the above
two pots produced 206, and the self-fertilised in the same pots only 32
capsules; or as 100 to 15. Judging from the previous generations, the
extreme unproductiveness of the self-fertilised plants in this
experiment was wholly due to their having been subjected to unfavourable
conditions, and to severe competition with the crossed plants; for had
they grown separately in good soil, it is almost certain that they would
have produced a large number of capsules. The seeds were counted in
twenty capsules from the crossed plants, and they averaged 24.75; whilst
in twenty capsules from the self-fertilised plants the average was
17.65; or as 100 to 71. Moreover, the seeds from the self-fertilised
plants were not nearly so fine as those from the crossed plants. If we
consider together the number of capsules produced and the average number
of contained seeds, the fertility of the crossed plants to the
self-fertilised plants was as 100 to 11. We thus see what a great
effect, as far as fertility is concerned, was produced by a cross
between the two varieties, which had been long exposed to different
conditions, in comparison with self-fertilisation; the fertilisation
having been in both cases of the legitimate order.

Primula sinensis.

As the Chinese primrose is a heterostyled or dimorphic plant, like the
common cowslip, it might have been expected that the flowers of both
forms when illegitimately fertilised with their own pollen or with that
from flowers on another plant of the same form, would have yielded less
seed than the legitimately crossed flowers; and that the seedlings
raised from illegitimately self-fertilised seeds would have been
somewhat dwarfed and less fertile, in comparison with the seedlings from
legitimately crossed seeds. This holds good in relation to the fertility
of the flowers; but to my surprise there was no difference in growth
between the offspring from a legitimate union between two distinct
plants, and from an illegitimate union whether between the flowers on
the same plant, or between distinct plants of the same form. But I have
shown, in the paper before referred to, that in England this plant is in
an abnormal condition, such as, judging from analogous cases, would tend
to render a cross between two individuals of no benefit to the
offspring. Our plants have been commonly raised from self-fertilised
seeds; and the seedlings have generally been subjected to nearly uniform
conditions in pots in greenhouses. Moreover, many of the plants are now
varying and changing their character, so as to become in a greater or
less degree equal-styled, and in consequence highly self-fertile. From
the analogy of Primula veris there can hardly be a doubt that if a plant
of Primula sinensis could have been procured direct from China, and if
it had been crossed with one of our English varieties, the offspring
would have shown wonderful superiority in height and fertility (though
probably not in the beauty of their flowers) over our ordinary plants.

My first experiment consisted in fertilising many flowers on long-styled
and short-styled plants with their own pollen, and other flowers on the
same plants with pollen taken from distinct plants belonging to the same
form; so that all the unions were illegitimate. There was no uniform and
marked difference in the number of seeds obtained from these two modes
of self-fertilisation, both of which were illegitimate. The two lots of
seeds from both forms were sown thickly on opposite sides of four pots,
and numerous plants thus raised. But there was no difference in their
growth, excepting in one pot, in which the offspring from the
illegitimate union of two long-styled plants exceeded in a decided
manner in height the offspring of flowers on the same plants fertilised
with their own pollen. But in all four pots the plants raised from the
union of distinct plants belonging to the same form, flowered before the
offspring from the self-fertilised flowers.

Some long-styled and short-styled plants were now raised from purchased
seeds, and flowers on both forms were legitimately crossed with pollen
from a distinct plant; and other flowers on both forms were
illegitimately fertilised with pollen from the flowers on the same
plant. The seeds were sown on opposite sides of Pots 1 to 4 in Table
6/95; a single plant being left on each side. Several flowers on the
illegitimate long-styled and short-styled plants described in the last
paragraph, were also legitimately and illegitimately fertilised in the
manner just described, and their seeds were sown in Pots 5 to 8 in the
same table. As the two sets of seedlings did not differ in any essential
manner, their measurements are given in a single table. I should add
that the legitimate unions in both cases yielded, as might have been
expected, many more seeds than the illegitimate unions. The seedlings
whilst half-grown presented no difference in height on the two sides of
the several pots. When fully grown they were measured to the tips of
their longest leaves, and the result is given in Table 6/95.

TABLE 6/95. Primula sinensis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Plants from legitimately Crossed seeds.

Column 3: Plants from illegitimately Self-fertilised seeds.

Pot 1  :    8 2/8 :   8.
From short-styled mother.

Pot 2  :    7 4/8 :   8 5/8.
From short-styled mother.

Pot 3  :    9 5/8 :   9 3/8.
From long-styled mother.

Pot 4  :    8 4/8 :   8 2/8.
From long-styled mother.

Pot 5  :    9 3/8 :   9.
From illegitimate short-styled mother.

Pot 6  :    9 7/8 :   9 4/8.
From illegitimate short-styled mother.

Pot 7  :    8 4/8 :   9 4/8.
From illegitimate long-styled mother.

Pot 8  :   10 4/8 :  10.
From illegitimate long-styled mother.

Total  :   72.13  :  72.25.

In six out of the eight pots the legitimately crossed plants exceeded in
height by a trifle the illegitimately self-fertilised plants; but the
latter exceeded the former in two of the pots in a more strongly marked
manner. The average height of the eight legitimately crossed plants is
9.01, and that of the eight illegitimately self-fertilised 9.03 inches,
or as 100 to 100.2. The plants on the opposite sides produced, as far as
could be judged by the eye, an equal number of flowers. I did not count
the capsules or the seeds produced by them; but undoubtedly, judging
from many previous observations, the plants derived from the
legitimately crossed seeds would have been considerably more fertile
than those from the illegitimately self-fertilised seeds. The crossed
plants, as in the previous case, flowered before the self-fertilised
plants in all the pots except in Pot 2, in which the two sides flowered
simultaneously; and this early flowering may, perhaps, be considered as
an advantage.

27. POLYGONEAE.--Fagopyrum esculentum.

This plant was discovered by Hildebrand to be heterostyled, that is, to
present, like the species of Primula, a long-styled and a short-styled
form, which are adapted for reciprocal fertilisation. Therefore the
following comparison of the growth of the crossed and self-fertilised
seedlings is not fair, for we do not know whether the difference in
their heights may not be wholly due to the illegitimate fertilisation of
the self-fertilised flowers.

I obtained seeds by legitimately crossing flowers on long-styled and
short-styled plants, and by fertilising other flowers on both forms with
pollen from the same plant. Rather more seeds were obtained by the
former than by the latter process; and the legitimately crossed seeds
were heavier than an equal number of the illegitimately self-fertilised
seeds, in the ratio of 100 to 82. Crossed and self-fertilised seeds from
the short-styled parents, after germinating on sand, were planted in
pairs on the opposite sides of a large pot; and two similar lots of
seeds from long-styled parents were planted in a like manner on the
opposite sides of two other pots. In all three pots the legitimately
crossed seedlings, when a few inches in height, were taller than the
self-fertilised; and in all three pots they flowered before them by one
or two days. When fully grown they were all cut down close to the
ground, and as I was pressed for time, they were placed in a long row,
the cut end of one plant touching the tip of another, and the total
length of the legitimately crossed plants was 47 feet 7 inches, and of
the illegitimately self-fertilised plants 32 feet 8 inches. Therefore
the average height of the fifteen crossed plants in all three pots was
38.06 inches, and that of the fifteen self-fertilised plants 26.13
inches; or as 100 to 69.

28. CHENOPODIACEAE.--Beta vulgaris.

A single plant, no others growing in the same garden, was left to
fertilise itself, and the self-fertilised seeds were collected. Seeds
were also collected from a plant growing in the midst of a large bed in
another garden; and as the incoherent pollen is abundant, the seeds of
this plant will almost certainly have been the product of a crossed
between distinct plants by means of the wind. Some of the two lots of
seeds were sown on the opposite sides of two very large pots; and the
young seedlings were thinned, so that an equal but considerable number
was left on the two sides. These plants were thus subjected to very
severe competition, as well as to poor conditions. The remaining seeds
were sown out of doors in good soil in two long and not closely
adjoining rows, so that these seedlings were placed under favourable
conditions, and were not subjected to any mutual competition. The
self-fertilised seeds in the open ground came up very badly; and on
removing the soil in two or three places, it was found that many had
sprouted under ground and had then died. No such case had been observed
before. Owing to the large number of seedlings which thus perished, the
surviving self-fertilised plants grew thinly in the row, and thus had an
advantage over the crossed plants, which grew very thickly in the other
row. The young plants in the two rows were protected by a little straw
during the winter, and those in the two large pots were placed in the
greenhouse.

There was no difference between the two lots in the pots until the
ensuing spring, when they had grown a little, and then some of the
crossed plants were finer and taller than any of the self-fertilised.
When in full flower their stems were measured, and the measurements are
given in Table 6/96.

TABLE 6/96. Beta vulgaris.

Heights of flower stems measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   34 6/8 :  36.
Pot 1  :   30     :  20 1/8.
Pot 1  :   33 6/8 :  32 2/8.
Pot 1  :   34 4/8 :  32.

Pot 2  :   42 3/8 :  42 1/8.
Pot 2  :   33 1/8 :  26 4/8.
Pot 2  :   31 2/8 :  29 2/8.
Pot 2  :   33     :  20 2/8.

Total  :  272.75  : 238.50.

The average height of the eight crossed plants is here 34.09, and that
of the eight self-fertilised plants 29.81; or as 100 to 87.

With respect to the plants in the open ground, each long row was divided
into half, so as to diminish the chance of any accidental advantage in
one part of either row; and the four tallest plants in the two halves of
the two rows were carefully selected and measured. The eight tallest
crossed plants averaged 30.92, and the eight tallest self-fertilised
30.7 inches in height, or as 100 to 99; so that they were practically
equal. But we should bear in mind that the trial was not quite fair, as
the self-fertilised plants had a great advantage over the crossed in
being much less crowded in their own row, owing to the large number of
seeds which had perished under ground after sprouting. Nor were the lots
in the two rows subjected to any mutual competition.

29. CANNACEAE.--Canna warscewiczi.

In most or all the species belonging to this genus, the pollen is shed
before the flower expands, and adheres in a mass to the foliaceous
pistil close beneath the stigmatic surface. As the edge of this mass
generally touches the edge of the stigma, and as it was ascertained by
trials purposely made that a very few pollen-grains suffice for
fertilisation, the present species and probably all the others of the
genus are highly self-fertile. Exceptions occasionally occur in which,
from the stamen being slightly shorter than usual, the pollen is
deposited a little beneath the stigmatic surface, and such flowers drop
off unimpregnated unless they are artificially fertilised. Sometimes,
though rarely, the stamen is a little longer than usual, and then the
whole stigmatic surface gets thickly covered with pollen. As some pollen
is generally deposited in contact with the edge of the stigma, certain
authors have concluded that the flowers are invariably self-fertilised.
This is an extraordinary conclusion, for it implies that a great amount
of pollen is produced for no purpose. On this view, also, the large size
of the stigmatic surface is an unintelligible feature in the structure
of the flower, as well as the relative position of all the parts, which
is such that when insects visit the flowers to suck the copious nectar,
they cannot fail to carry pollen from one flower to another. (6/7.
Delpino has described ‘Bot. Zeitung’ 1867 page 277 and ‘Scientific
Opinion’ 1870 page 135, the structure of the flowers in this genus, but
he was mistaken in thinking that self-fertilisation is impossible, at
least in the case of the present species. Dr. Dickie and Professor
Faivre state that the flowers are fertilised in the bud, and that
self-fertilisation is inevitable. I presume that they were misled by the
pollen being deposited at a very early period on the pistil: see
‘Journal of Linnean Society Botany’ volume 10 page 55 and ‘Variabilité
des Espèces’ 1868 page 158.)

According to Delpino, bees eagerly visit the flowers in North Italy, but
I have never seen any insect visiting the flowers of the present species
in my hothouse, although many plants grew there during several years.
Nevertheless these plants produced plenty of seed, as they likewise did
when covered by a net; they are therefore fully capable of
self-fertilisation, and have probably been self-fertilised in this
country for many generations. As they are cultivated in pots, and are
not exposed to competition with surrounding plants, they have also been
subjected for a considerable time to somewhat uniform conditions. This,
therefore, is a case exactly parallel with that of the common pea, in
which we have no right to expect much or any good from intercrossing
plants thus descended and thus treated; and no good did follow,
excepting that the cross-fertilised flowers yielded rather more seeds
than the self-fertilised. This species was one of the earlier ones on
which I experimented, and as I had not then raised any self-fertilised
plants for several successive generations under uniform conditions, I
did not know or even suspect that such treatment would interfere with
the advantages to be gained from a cross. I was therefore much surprised
at the crossed plants not growing more vigorously than the
self-fertilised, and a large number of plants were raised,
notwithstanding that the present species is an extremely troublesome one
to experiment on. The seeds, even those which have been long soaked in
water, will not germinate well on bare sand; and those that were sown in
pots (which plan I was forced to follow) germinated at very unequal
intervals of time; so that it was difficult to get pairs of the same
exact age, and many seedlings had to be pulled up and thrown away. My
experiments were continued during three successive generations; and in
each generation the self-fertilised plants were again self-fertilised,
their early progenitors in this country having probably been
self-fertilised for many previous generations. In each generation, also,
the crossed plants were fertilised with pollen from another crossed
plant.

Of the flowers which were crossed in the three generations, taken
together, a rather larger proportion yielded capsules than did those
which were self-fertilised. The seeds were counted in forty-seven
capsules from the crossed flowers, and they contained on an average 9.95
seeds; whereas forty-eight capsules from the self-fertilised flowers
contained on an average 8.45 seeds; or as 100 to 85. The seeds from the
crossed flowers were not heavier, on the contrary a little lighter, than
those from the self-fertilised flowers, as was thrice ascertained. On
one occasion I weighed 200 of the crossed and 106 of the self-fertilised
seeds, and the relative weight of an equal number was as 100 for the
crossed to 101.5 for the self-fertilised. With other plants, when the
seeds from the self-fertilised flowers were heavier than those from the
crossed flowers, this appeared to be due generally to fewer having been
produced by the self-fertilised flowers, and to their having been in
consequence better nourished. But in the present instance the seeds from
the crossed capsules were separated into two lots,--namely, those from
the capsules containing over fourteen seeds, and those from the capsules
containing under fourteen seeds, and the seeds from the more productive
capsules were the heavier of the two; so that the above explanation here
fails.

As pollen is deposited at a very early age on the pistil, generally in
contact with the stigma, some flowers whilst still in bud were castrated
for my first experiment, and were afterwards fertilised with pollen from
a distinct plant. Other flowers were fertilised with their own pollen.
From the seeds thus obtained, I succeeded in rearing only three pairs of
plants of equal age. The three crossed plants averaged 32.79 inches, and
the three self-fertilised 32.08 inches in height; so that they were
nearly equal, the crossed having a slight advantage. As the same result
followed in all three generations, it would be superfluous to give the
heights of all the plants, and I will give only the averages.

In order to raise crossed and self-fertilised plants of the second
generation, some flowers on the above crossed plants were crossed within
twenty-four hours after they had expanded with pollen from a distinct
plant; and this interval would probably not be too great to allow of
cross-fertilisation being effectual. Some flowers on the self-fertilised
plants of the last generation were also self-fertilised. From these two
lots of seeds, ten crossed and twelve self-fertilised plants of equal
ages were raised; and these were measured when fully grown. The crossed
averaged 36.98, and the self-fertilised averaged 37.42 inches in height;
so that here again the two lots were nearly equal; but the
self-fertilised had a slight advantage.

In order to raise plants of the third generation, a better plan was
followed, and flowers on the crossed plants of the second generation
were selected in which the stamens were too short to reach the stigmas,
so that they could not possibly have been self-fertilised. These flowers
were crossed with pollen from a distinct plant. Flowers on the
self-fertilised plants of the second generation were again
self-fertilised. From the two lots of seeds thus obtained, twenty-one
crossed and nineteen self-fertilised plants of equal age, and forming
the third generation, were raised in fourteen large pots. They were
measured when fully grown, and by an odd chance the average height of
the two lots was exactly the same, namely, 35.96 inches; so that neither
side had the least advantage over the other. To test this result, all
the plants on both sides in ten out of the above fourteen pots were cut
down after they had flowered, and in the ensuing year the stems were
again measured; and now the crossed plants exceeded by a little (namely,
1.7 inches) the self-fertilised. They were again cut down, and on their
flowering for the third time, the self-fertilised plants had a slight
advantage (namely, 1.54 inches) over the crossed. Hence the result
arrived at with these plants during the previous trials was confirmed,
namely, that neither lot had any decided advantage over the other. It
may, however, be worth mentioning that the self-fertilised plants showed
some tendency to flower before the crossed plants: this occurred with
all three pairs of the first generation; and with the cut down plants of
the third generation, a self-fertilised plant flowered first in nine out
of the twelve pots, whilst in the remaining three pots a crossed plant
flowered first.

If we consider all the plants of the three generations taken together,
the thirty-four crossed plants average 35.98, and the thirty-four
self-fertilised plants 36.39 inches in height; or as 100 to 101. We may
therefore conclude that the two lots possessed equal powers of growth;
and this I believe to be the result of long-continued
self-fertilisation, together with exposure to similar conditions in each
generation, so that all the individuals had acquired a closely similar
constitution.

30. GRAMINACEAE.--Zea mays.

This plant is monoecious, and was selected for trial on this account, no
other such plant having been experimented on. (6/8. Hildebrand remarks
that this species seems at first sight adapted to be fertilised by
pollen from the same plant, owing to the male flowers standing above the
female flowers; but practically it must generally be fertilised by
pollen from another plant, as the male flowers usually shed their pollen
before the female flowers are mature: ‘Monatsbericht der K. Akad.’
Berlin October 1872 page 743.) It is also anemophilous, or is fertilised
by the wind; and of such plants only the common beet had been tried.
Some plants were raised in the greenhouse, and were crossed with pollen
taken from a distinct plant; and a single plant, growing quite
separately in a different part of the house, was allowed to fertilise
itself spontaneously. The seeds thus obtained were placed on damp sand,
and as they germinated in pairs of equal age were planted on the
opposite sides of four very large pots; nevertheless they were
considerably crowded. The pots were kept in the hothouse. The plants
were first measured to the tips of their leaves when only between 1 and
2 feet in height, as shown in Table 6/97.

TABLE 6/97. Zea mays.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   23 4/8 :  17 3/8.
Pot 1  :   12     :  20 3/8.
Pot 1  :   21     :  20.

Pot 2  :   22     :  20.
Pot 2  :   19 1/8 :  18 3/8.
Pot 2  :   21 4/8 :  18 5/8.

Pot 3  :   22 1/8 :  18 5/8.
Pot 3  :   20 3/8 :  15 2/8.
Pot 3  :   18 2/8 :  16 4/8.
Pot 3  :   21 5/8 :  18.
Pot 3  :   23 2/8 :  16 2/8.

Pot 4  :   21     :  18.
Pot 4  :   22 1/8 :  12 6/8.
Pot 4  :   23     :  15 4/8.
Pot 4  :   12     :  18.

Total  :  302.88  : 263.63.

The fifteen crossed plants here average 20.19, and the fifteen
self-fertilised plants 17.57 inches in height; or as 100 to 87. Mr.
Galton made a graphical representation, in accordance with the method
described in the introductory chapter, of the above measurements, and
adds the words “very good” to the curves thus formed.

Shortly afterwards one of the crossed plants in Pot 1 died; another
became much diseased and stunted; and the third never grew to its full
height. They seemed to have been all injured, probably by some larva
gnawing their roots. Therefore all the plants on both sides of this pot
were rejected in the subsequent measurements. When the plants were fully
grown they were again measured to the tips of the highest leaves, and
the eleven crossed plants now averaged 68.1, and the eleven
self-fertilised plants 62.34 inches in height; or as 100 to 91. In all
four pots a crossed plant flowered before any one of the
self-fertilised; but three of the plants did not flower at all. Those
that flowered were also measured to the summits of the male flowers: the
ten crossed plants averaged 66.51, and the nine self-fertilised plants
61.59 inches in height; or as 100 to 93.

A large number of the same crossed and self-fertilised seeds were sown
in the middle of the summer in the open ground in two long rows. Very
much fewer of the self-fertilised than of the crossed plants produced
flowers; but those that did flower, flowered almost simultaneously. When
fully grown the ten tallest plants in each row were selected and
measured to the tips of their highest leaves, as well as to the summits
of their male flowers. The crossed averaged to the tips of their leaves
54 inches in height, and the self-fertilised 44.65, or as 100 to 83; and
to the summits of their male flowers, 53.96 and 43.45 inches; or as 100
to 80.

Phalaris canariensis.

Hildebrand has shown in the paper referred to under the last species,
that this hermaphrodite grass is better adapted for cross-fertilisation
than for self-fertilisation. Several plants were raised in the
greenhouse close together, and their flowers were mutually intercrossed.
Pollen from a single plant growing quite separately was collected and
placed on the stigmas of the same plant. The seeds thus produced were
self-fertilised, for they were fertilised with pollen from the same
plant, but it will have been a mere chance whether with pollen from the
same flowers. Both lots of seeds, after germinating on sand, were
planted in pairs on the opposite sides of four pots, which were kept in
the greenhouse. When the plants were a little over a foot in height they
were measured, and the crossed plants averaged 13.38, and the
self-fertilised 12.29 inches in height; or as 100 to 92.

When in full flower they were again measured to the extremities of their
culms, as shown in Table 6/98.

TABLE 6/98. Phalaris canariensis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1  :   42 2/8 :  41 2/8.
Pot 1  :   39 6/8 :  45 4/8.

Pot 2  :   37     :  31 6/8.
Pot 2  :   49 4/8 :  37 2/8.
Pot 4  :   29     :  42 3/8.
Pot 2  :   37     :  34 7/8.

Pot 3  :   37 6/8 :  28.
Pot 3  :   35 4/8 :  28.
Pot 3  :   43     :  34.

Pot 4  :   40 2/8 :  35 1/8.
Pot 4  :   37     :  34 4/8.

Total  :  428.00  : 392.63.

The eleven crossed plants now averaged 38.9, and the eleven
self-fertilised plants 35.69 inches in height; or as 100 to 92, which is
the same ratio as before. Differently to what occurred with the maize,
the crossed plants did not flower before the self-fertilised; and though
both lots flowered very poorly from having been kept in pots in the
greenhouse, yet the self-fertilised plants produced twenty-eight
flower-heads, whilst the crossed produced only twenty!

Two long rows of the same seeds were sown out of doors, and care was
taken that they were sown in nearly equal number; but a far greater
number of the crossed than of the self-fertilised seeds yielded plants.
The self-fertilised plants were in consequence not so much crowded as
the crossed, and thus had an advantage over them. When in full flower,
the twelve tallest plants were carefully selected from both rows and
measured, as shown in Table 6/99.

TABLE 6/99. Phalaris canariensis (growing in the open ground).

Heights of plants measured in inches.

Column 1: Crossed Plants, twelve tallest.

Column 2: Self-fertilised Plants, twelve tallest.

          34 1/8 :  35 2/8.
          35 7/8 :  31 1/8.
          36     :  33.
          35 5/8 :  32.
          35 5/8 :  31 5/8.
          36 1/8 :  36.
          36 6/8 :  33.
          38 6/8 :  32.
          36 2/8 :  35 1/8.
          35 5/8 :  33 5/8.
          34 1/8 :  34 2/8.
          34 5/8 :  35.

Total :  429.5   : 402.0.

The twelve crossed plants here average 35.78, and the twelve
self-fertilised 33.5 inches in height; or as 100 to 93. In this case the
crossed plants flowered rather before the self-fertilised, and thus
differed from those growing in the pots.]



CHAPTER VII.

SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED
PLANTS.

Number of species and plants measured.
Tables given.
Preliminary remarks on the offspring of plants crossed by a fresh stock.
Thirteen cases specially considered.
The effects of crossing a self-fertilised plant either by another
self-fertilised plant or by an intercrossed plant of the old stock.
Summary of the results.
Preliminary remarks on the crossed and self-fertilised plants of the
same stock.
The twenty-six exceptional cases considered, in which the crossed plants
did not exceed greatly in height the self-fertilised.
Most of these cases shown not to be real exceptions to the rule that
cross-fertilisation is beneficial.
Summary of results.
Relative weights of the crossed and self-fertilised plants.

The details which have been given under the head of each species are so
numerous and so intricate, that it is necessary to tabulate the results.
In Table 7/A, the number of plants of each kind which were raised from a
cross between two individuals of the same stock and from self-fertilised
seeds, together with their mean or average heights, are given. In the
right hand column, the mean height of the crossed to that of the
self-fertilised plants, the former being taken as 100, is shown. To make
this clear, it may be advisable to give an example. In the first
generation of Ipomoea, six plants derived from a cross between two
plants were measured, and their mean height is 86.00 inches; six plants
derived from flowers on the same parent-plant fertilised with their own
pollen were measured, and their mean height is 65.66 inches. From this
it follows, as shown in the right hand column, that if the mean height
of the crossed plants be taken as 100, that of the self-fertilised
plants is 76. The same plan is followed with all the other species.

The crossed and self-fertilised plants were generally grown in pots in
competition with one another, and always under as closely similar
conditions as could be attained. They were, however, sometimes grown in
separate rows in the open ground. With several of the species, the
crossed plants were again crossed, and the self-fertilised plants again
self-fertilised, and thus successive generations were raised and
measured, as may be seen in Table 7/A. Owing to this manner of
proceeding, the crossed plants became in the later generations more or
less closely inter-related.

In Table 7/B the relative weights of the crossed and self-fertilised
plants, after they had flowered and had been cut down, are given in the
few cases in which they were ascertained. The results are, I think, more
striking and of greater value as evidence of constitutional vigour than
those deduced from the relative heights of the plants.

The most important table is Table 7/C, as it includes the relative
heights, weights, and fertility of plants raised from parents crossed by
a fresh stock (that is, by non-related plants grown under different
conditions), or by a distinct sub-variety, in comparison with
self-fertilised plants, or in a few cases with plants of the same old
stock intercrossed during several generations. The relative fertility of
the plants in this and the other tables will be more fully considered in
a future chapter.

TABLE 7/A. Relative heights of plants from parents crossed with pollen
from other plants of the same stock, and self-fertilised.

Heights of plants measured in inches.

Column 1: Name of Plant.

Column 2: Number of Crossed Plants measured.

Column 3: Average Height of Crossed Plants.

Column 4: Number of Self-fertilised Plants measured.

Column 5: Average Height of Self-fertilised Plants.

Column 6: x, where the ratio of the Average Height of the Crossed to the
Self-fertilised Plants is expressed as 100 to x.

Ipomoea purpurea--first generation:
  6 :  86.00 :   6 :  65.66 :  76.

Ipomoea purpurea--second generation:
  6 :  84.16 :   6 :  66.33 :  79.

Ipomoea purpurea--third generation:
  6 :  77.41 :   6 :  52.83 :  68.

Ipomoea purpurea--fourth generation:
  7 :  69.78 :   7 :  60.14 :  86.

Ipomoea purpurea--fifth generation:
  6 :  82.54 :   6 :  62.33 :  75.

Ipomoea purpurea--sixth generation:
  6 :  87.50 :   6 :  63.16 :  72.

Ipomoea purpurea--seventh generation:
  9 :  83.94 :   9 :  68.25 :  81.

Ipomoea purpurea--eighth generation:
  8 : 113.25 :   8 :  96.65 :  85.

Ipomoea purpurea--ninth generation:
 14 :  81.39 :  14 :  64.07 :  79.

Ipomoea purpurea--tenth generation:
  5 :  93.70 :   5 :  50.40 :  54.

Ipomoea purpurea--Number and average height of all the plants of the ten
generations:
 73 :  85.84 :  73 :  66.02 :  77.

Mimulus luteus--three first generations, before the new and taller
self-fertilised variety appeared:
 10 :   8.19 :  10 :   5.29 :  65.

Digitalis purpurea:
 16 :  51.33 :   8 :  35.87 :  70.

Calceolaria--(common greenhouse variety):
  1 :  19.50 :   1 :  15.00 :  77.

Linaria vulgaris:
  3 :   7.08 :   3 :   5.75 :  81.

Verbascum thapsus:
  6 :  65.34 :   6 :  56.50 :  86.

Vandellia nummularifolia--crossed and self-fertilised plants, raised
from perfect flowers:
 20 :   4.30 :  20 :   4.27 :  99.

Vandellia nummularifolia--crossed and self-fertilised plants, raised
from perfect flowers: second trial, plants crowded:
 24 :   3.60 :  24 :   3.38 :  94.

Vandellia nummularifolia--crossed plants raised from perfect flowers,
and self-fertilised plants from cleistogene flowers:
 20 :   4.30 :  20 :   4.06 :  94.

Gesneria pendulina:
  8 :  32.06 :   8 :  29.14 :  90.

Salvia coccinea:
  6 :  27.85 :   6 :  21.16 :  76.

Origanum vulgare:
  4 :  20.00 :   4 :  17.12 :  86.

Thunbergia alata:
  6 :  60.00 :   6 :  65.00 : 108.

Brassica oleracea:
  9 :  41.08 :   9 :  39.00 :  95.

Iberis umbellata--the self-fertilised plants of the third generation:
  7 :  19.12 :   7 :  16.39 :  86.

Papaver vagum:
 15 :  21.91 :  15 :  19.54 :  89.

Eschscholtzia californica--English stock, first generation:
  4 :  29.68 :   4 :  25.56 :  86.

Eschscholtzia californica--English stock, second generation:
 11 :  32.47 :  11 :  32.81 : 101.

Eschscholtzia californica--Brazilian stock, first generation:
 14 :  44.64 :  14 :  45.12 : 101.

Eschscholtzia californica--Brazilian stock, second generation:
 18 :  43.38 :  19 :  50.30 : 116.

Eschscholtzia californica--average height and number of all the plants
of Eschscholtzia:
 47 :  40.03 :  48 :  42.72 : 107.

Reseda lutea--grown in pots:
 24 :  17.17 :  24 :  14.61 :  85.

Reseda lutea--grown in open ground :
  8 :  28.09 :   8 :  23.14 :  82.

Reseda odorata--self-fertilised seeds from a highly self-fertile plant,
grown in pots:
 19 :  27.48 :  19 :  22.55 :  82.

Reseda odorata--self-fertilised seeds from a highly self-fertile plant,
grown in open ground:
  8 :  25.76 :   8 :  27.09 : 105.

Reseda odorata--self-fertilised seeds from a semi-self-fertile plant,
grown in pots:
 20 :  29.98 :  20 :  27.71 :  92.

Reseda odorata--self-fertilised seeds from a semi-self-fertile plant,
grown in open ground:
  8 :  25.92 :   8 :  23.54 :  90.

Viola tricolor:
 14 :   5.58 :  14 :   2.37 :  42.

Adonis aestivalis:
  4 :  14.25 :   4 :  14.31 : 100.

Delphinium consolida:
  6 :  14.95 :   6 :  12.50 :  84.

Viscaria oculata:
 15 :  34.50 :  15 :  33.55 :  97.

Dianthus caryophyllus--open ground, about :
  6?:  28?   :   6?:  24?   :  86.

Dianthus caryophyllus--second generation, in pots, crowded:
  2 :  16.75 :   2 :   9.75 :  58.

Dianthus caryophyllus--third generation, in pots:
  8 :  28.39 :   8 :  28.21 :  99.

Dianthus caryophyllus--offspring from plants of the third
self-fertilised generation crossed by intercrossed plants of the third
generation, compared with plants of fourth self-fertilised generation:
 15 :  28.00 :  10 :  26.55 :  95.

Dianthus caryophyllus--number and average height of all the plants of
Dianthus:
 31 :  27.37 :  26 :  25.18 :  92.

Hibiscus africanus:
  4 :  13.25 :   4 :  14.43 : 109.

Pelargonium zonale:
  7 :  22.35 :   7 :  16.62 :  74.

Tropaeolum minus:
  8 :  58.43 :   8 :  46.00 :  79.

Limnanthes douglasii:
 16 :  17.46 :  16 :  13.85 :  79.

Lupinus luteus--second generation:
  8 :  30.78 :   8 :  25.21 :  82.

Lupinus pilosus--plants of two generations:
  2 :  35.50 :   3 :  30.50 :  86.

Phaseolus multiflorus:
  5 :  86.00 :   5 :  82.35 :  96.

Pisum sativum:
  4 :  34.62 :   4 :  39.68 : 115.

Sarothamnus scoparius--small seedlings:
  6 :   2.91 :   6 :   1.33 :  46.

Sarothamnus scoparius--the three survivors on each side after three
years’ growth:
    :  18.91 :     :  11.83 :  63.

Ononis minutissima:
  2 :  19.81 :   2 :  17.37 :  88.

Clarkia elegans:
  4 :  33.50 :   4 :  27.62 :  82.

Bartonia aurea:
  8 :  24.62 :   8 :  26.31 : 107.

Passiflora gracilis:
  2 :  49.00 :   2 :  51.00 : 104.

Apium petroselinum:
  * :        :   * :        : 100.
*not measured.

Scabiosa atro-purpurea:
  4 :  17.12 :   4 :  15.37 :  90.

Lactuca sativa--plants of two generations:
  7 :  19.43 :   6 :  16.00 :  82.

Specularia speculum:
  4 :  19.28 :   4 :  18.93 :  98.

Lobelia ramosa--first generation:
  4 :  22.25 :   4 :  18.37 :  82.

Lobelia ramosa--second generation:
  3 :  23.33 :   3 :  19.00 :  81.

Lobelia fulgens--first generation:
  2 :  34.75 :   2 :  44.25 : 127.

Lobelia fulgens--second generation:
 23 :  29.82 :  23 :  27.10 :  91.

Nemophila insignis--half-grown:
 12 :  11.10 :  12 :   5.45 :  49.

Nemophila insignis--the same fully-grown:
    :  33.28 :     :  19.90 :  60.

Borago officinalis:
  4 :  20.68 :   4 :  21.18 : 102.

Nolana prostrata:
  5 :  12.75 :   5 :  13.40 : 105.

Petunia violacea--first generation:
  5 :  30.80 :   5 :  26.00 :  84.

Petunia violacea--second generation:
  4 :  40.50 :   6 :  26.25 :  65.

Petunia violacea--third generation:
  8 :  40.96 :   8 :  53.87 : 131.

Petunia violacea--fourth generation:
 15 :  46.79 :  14 :  32.39 :  69.

Petunia violacea--fourth generation, from a distinct parent:
 13 :  44.74 :  13 :  26.87 :  60.

Petunia violacea--fifth generation:
 22 :  54.11 :  21 :  33.23 :  61.

Petunia violacea--fifth generation, in open ground:
 10 :  38.27 :  10 :  23.31 :  61.

Petunia violacea--Number and average height of all the plants in pots of
Petunia:
 67 :  46.53 :  67 :  33.12 :  71.

Nicotiana tabacum--first generation:
  4 :  18.50 :   4 :  32.75 : 178.

Nicotiana tabacum--second generation:
  9 :  53.84 :   7 :  51.78 :  96.

Nicotiana tabacum--third generation:
  7 :  95.25 :   7 :  79.60 :  83.

Nicotiana tabacum--third generation but raised from a distinct plant:
  7 :  70.78 :   9 :  71.30 : 101.

Nicotiana tabacum--Number and average height of all the plants of
Nicotiana:
 27 :  63.73 :  27 :  61.31 :  96.

Cyclamen persicum:
  8 :   9.49 :   8?:   7.50 :  79.

Anagallis collina:
  6 :  42.20 :   6 :  33.35 :  69.

Primula sinensis--a dimorphic species:
  8 :   9.01 :   8 :   9.03 : 100.

Fagopyrum esculentum--a dimorphic species:
 15 :  38.06 :  15 :  26.13 :  69.

Beta vulgaris--in pots:
  8 :  34.09 :   8 :  29.81 :  87.

Beta vulgaris--in open ground:
  8 :  30.92 :   8 :  30.70 :  99.

Canna warscewiczi--plants of three generations:
 34 :  35.98 :  34 :  36.39 : 101.

Zea mays--in pots, whilst young, measured to tips of leaves:
 15 :  20.19 :  15 :  17.57 :  87.

Zea mays--when full-grown, after the death of some, measured to tips of
leaves:
    :  68.10 :     :  62.34 :  91.

Zea mays--when full-grown, after the death of some, measured to tips of
flowers:
    :  66.51 :     :  61.59 :  93.

Zea mays--grown in open ground, measured to tips of leaves:
 10 :  54.00 :  10 :  44.55 :  83.

Zea mays--grown in open ground, measured to tips of flowers:
    :  53.96 :     :  43.45 :  80.

Phalaris canariensis--in pots.
 11 :  38.90 :  11 :  35.69 :  92.

Phalaris canariensis--in open ground:
 12 :  35.78 :  12 :  33.50 :  93.

TABLE 7/B.--Relative weights of plants from parents crossed with pollen
from distinct plants of the same stock, and self-fertilised.

Column 1: Names of plants.

Column 2: Number of crossed plants.

Column 3: Number of self-fertilised plants.

Column 4: x, where the ratio of the Weight of the Crossed to the
Self-fertilised Plants is expressed as 100 to x.

Ipomoea purpurea--plants of the tenth generation:
   6 :   6 :   44.

Vandellia nummularifolia--first generation:
  41 :  41 :   97.

Brassica oleracea--first generation:
   9 :   9 :   37.

Eschscholtzia californica--plants of the second generation:
  19 :  19 :  118.

Reseda lutea--first generation, grown in pots:
  24 :  24 :   21.

Reseda lutea--first generation, grown in open ground:
   8 :   8 :   40.

Reseda odorata--first generation, descended from a highly self-fertile
plant, grown in pots:
  19 :  19 :   67.

Reseda odorata--first generation, descended from a semi-self-fertile
plant, grown in pots:
  20 :  20 :   99.

Dianthus caryophyllus--plants of the third generation:
   8 :   8 :   49.

Petunia violacea--plants of the fifth generation, in pots:
  22 :  21 :   22.

Petunia violacea--plants of the fifth generation, in open ground:
  10 :  10 :   36.

TABLE 7/C.--Relative heights, weights, and fertility of plants from
parents crossed by a fresh stock, and from parents either
self-fertilised or intercrossed with plants of the same stock.

Column 1: Names of the plants and nature of the experiments.

Column 2: Number of plants from a cross with a fresh stock.

Column 3: Average height in inches and weight.

Column 4: Number of the plants from self-fertilised or intercrossed
parents of the same stock.

Column 5: Average height in inches and weight.

Column 4: x, where the ratio of the Height, Weight and Fertility of the
plants from the Cross with a fresh stock is expressed as 100 to x.

Ipomoea purpurea--offspring of plants intercrossed for nine generations
and then crossed by a fresh stock, compared with plants of the tenth
intercrossed generation:
  19 :  84.03 :   19 :  65.78 :  78.

Ipomoea purpurea--offspring of plants intercrossed for nine generations
and then crossed by a fresh stock, compared with plants of the tenth
intercrossed generation, in fertility:
  .. :     .. :   .. :     .. :  51.

Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with plants of
the ninth self-fertilised generation:
  28 :  21.62 :   19 :  10.44 :  52.

Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with plants of
the ninth self-fertilised generation, in fertility:
  .. :     .. :   .. :     .. :   3.

Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with the
offspring of a plant self-fertilised for eight generations, and then
intercrossed with another self-fertilised plant of the same generation:
  28 :  21.62 :   27 :  12.20 :  56.

Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with the
offspring of a plant self-fertilised for eight generations, and then
intercrossed with another self-fertilised plant of the same generation,
in fertility:
  .. :     .. :   .. :     .. :   4.

Brassica oleracea--offspring of plants self-fertilised for two
generations and then crossed by a fresh stock, compared with plants of
the third self-fertilised generation, by weight:
   6 :        :    6 :        :  22.

Iberis umbellata--offspring from English variety crossed by slightly
different Algerine variety, compared with the self-fertilised offspring
of the English variety:
  30 :  17.34 :   29 :  15.51 :  89.

Iberis umbellata--offspring from English variety crossed by slightly
different Algerine variety, compared with the self-fertilised offspring
of the English variety, in fertility:
  .. :     .. :   .. :     .. :  75.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation:
  19 :  45.92 :   19 :  50.30 : 109.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation, in weight:
  .. :     .. :   .. :     .. : 118.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation, in fertility:
  .. :     .. :   .. :     .. :  40.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in height:
  19 :  45.92 :   18 :  43.38 :  94.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in weight:
  .. :     .. :   .. :     .. : 100.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in fertility:
  .. :     .. :   .. :     .. :  45.

Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with plants of
the fourth self-fertilised generation:
  16 :  32.82 :   10 :  26.55 :  81.

Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with plants of
the fourth self-fertilised generation, in fertility:
  .. :     .. :   .. :     .. :  33.

Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with the
offspring of plants self-fertilised for three generations and then
crossed by plants of the third intercrossed generation:
  16 :  32.82 :   15 :  28.00 :  85.

Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with the
offspring of plants self-fertilised for three generations and then
crossed by plants of the third intercrossed generation, in fertility:
  .. :     .. :   .. :     .. :  45.

Pisum sativum--offspring from a cross between two closely allied
varieties, compared with the self-fertilised offspring of one of the
varieties, or with intercrossed plants of the same stock:
  ?  :        :   ?  :        :  60 to 75.

Lathyrus odoratus--offspring from two varieties, differing only in
colour of their flowers, compared with the self-fertilised offspring of
one of the varieties: in first generation:
   2 :  79.25 :    2 :  63.75 :  80.

Lathyrus odoratus--offspring from two varieties, differing only in
colour of their flowers, compared with the self-fertilised offspring of
one of the varieties: in second generation:
   6 :  62.91 :    6 :  55.31 :  88.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, in height:
  21 :  50.05 :   21 :  33.23 :  66.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, in weight:
  .. :     .. :   .. :     .. :  23.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in height:
  10 :  36.67 :   10 :  23.31 :  63.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in weight:
  .. :     .. :   .. :     .. :  53.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in
fertility:
  .. :     .. :   .. :     .. :  46.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, in height:
  21 :  50.05 :   22 :  54.11 : 108.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, in weight:
  .. :     .. :   .. :     .. : 101.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in height:
  10 :  36.67 :   10 :  38.27 : 104.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in weight:
  .. :     .. :   .. :     .. : 146.

Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in fertility:
  .. :     .. :   .. :     .. :  54.

Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown not much
crowded in pots, in height:
  26 :  63.29 :   26 :  41.67 :  66.

Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown much crowded
in pots, in height:
  12 :  31.53 :   12 :  17.21 :  54.

Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown much crowded
in pots, in weight:
  .. :     .. :   .. :     .. :  37.

Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown in open
ground, in height:
  20 :  48.74 :   20 :  35.20 :  72.

Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown in open
ground, in weight:
  .. :     .. :   .. :     .. :  63.

Anagallis collina--offspring from a red variety crossed by a blue
variety, compared with the self-fertilised offspring of the red variety:
   3 :  27.62 :    3 :  18.21 :  66.

Anagallis collina--offspring from a red variety crossed by a blue
variety, compared with the self-fertilised offspring of the red variety,
in fertility:
  .. :     .. :   .. :     .. :   6.

Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation:
   8 :   7.03 :    8 :   3.21 :  46.

Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation, in fertility:
  .. :     .. :   .. :     .. :   5.

Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation, in fertility
in following year:
  .. :     .. :   .. :     .. :   3.5.

Primula veris--(equal-styled, red-flowered variety)--offspring from
plants self-fertilised for two generations and then crossed by a
different variety, compared with plants of the third self-fertilised
generation:
   3 :   8.66 :    3 :   7.33 :  85.

Primula veris--(equal-styled, red-flowered variety)--offspring from
plants self-fertilised for two generations and then crossed by a
different variety, compared with plants of the third self-fertilised
generation, in fertility:
  .. :     .. :   .. :     .. :  11.

In these three tables the measurements of fifty-seven species, belonging
to fifty-two genera and to thirty great natural families, are given. The
species are natives of various parts of the world. The number of crossed
plants, including those derived from a cross between plants of the same
stock and of two different stocks, amounts to 1,101; and the number of
self-fertilised plants (including a few in Table 7/C derived from a
cross between plants of the same old stock) is 1,076. Their growth was
observed from the germination of the seeds to maturity; and most of them
were measured twice and some thrice. The various precautions taken to
prevent either lot being unduly favoured, have been described in the
introductory chapter. Bearing all these circumstances in mind, it may be
admitted that we have a fair basis for judging of the comparative
effects of cross-fertilisation and of self-fertilisation on the growth
of the offspring.

It will be the most convenient plan first to consider the results given
in Table 7/C, as an opportunity will thus be afforded of incidentally
discussing some important points. If the reader will look down the right
hand column of this table, he will see at a glance what an extraordinary
advantage in height, weight, and fertility the plants derived from a
cross with a fresh stock or with another sub-variety have over the
self-fertilised plants, as well as over the intercrossed plants of the
same old stock. There are only two exceptions to this rule, and these
are hardly real ones. In the case of Eschscholtzia, the advantage is
confined to fertility. In that of Petunia, though the plants derived
from a cross with a fresh stock had an immense superiority in height,
weight, and fertility over the self-fertilised plants, they were
conquered by the intercrossed plants of the same old stock in height and
weight, but not in fertility. It has, however, been shown that the
superiority of these intercrossed plants in height and weight was in all
probability not real; for if the two sets had been allowed to grow for
another month, it is almost certain that those from a cross with the
fresh stock would have been victorious in every way over the
intercrossed plants.

Before we consider in detail the several cases given in Table 7/C, some
preliminary remarks must be made. There is the clearest evidence, as we
shall presently see, that the advantage of a cross depends wholly on the
plants differing somewhat in constitution; and that the disadvantages of
self-fertilisation depend on the two parents, which are combined in the
same hermaphrodite flower, having a closely similar constitution. A
certain amount of differentiation in the sexual elements seems
indispensable for the full fertility of the parents, and for the full
vigour of the offspring. All the individuals of the same species, even
those produced in a state of nature, differ somewhat, though often very
slightly, from one another in external characters and probably in
constitution. This obviously holds good between the varieties of the
same species, as far as external characters are concerned; and much
evidence could be advanced with respect to their generally differing
somewhat in constitution. There can hardly be a doubt that the
differences of all kinds between the individuals and varieties of the
same species depend largely, and as I believe exclusively, on their
progenitors having been subjected to different conditions; though the
conditions to which the individuals of the same species are exposed in a
state of nature often falsely appear to us the same. For instance, the
individuals growing together are necessarily exposed to the same
climate, and they seem to us at first sight to be subjected to
identically the same conditions; but this can hardly be the case, except
under the unusual contingency of each individual being surrounded by
other kinds of plants in exactly the same proportional numbers. For the
surrounding plants absorb different amounts of various substances from
the soil, and thus greatly affect the nourishment and even the life of
the individuals of any particular species. These will also be shaded and
otherwise affected by the nature of the surrounding plants. Moreover,
seeds often lie dormant in the ground, and those which germinate during
any one year will often have been matured during very different seasons.
Seeds are widely dispersed by various means, and some will occasionally
be brought from distant stations, where their parents have grown under
somewhat different conditions, and the plants produced from such seeds
will intercross with the old residents, thus mingling their
constitutional peculiarities in all sorts of proportions.

Plants when first subjected to culture, even in their native country,
cannot fail to be exposed to greatly changed conditions of life, more
especially from growing in cleared ground, and from not having to
compete with many or any surrounding plants. They are thus enabled to
absorb whatever they require which the soil may contain. Fresh seeds are
often brought from distant gardens, where the parent-plants have been
subjected to different conditions. Cultivated plants like those in a
state of nature frequently intercross, and will thus mingle their
constitutional peculiarities. On the other hand, as long as the
individuals of any species are cultivated in the same garden, they will
apparently be subjected to more uniform conditions than plants in a
state of nature, as the individuals have not to compete with various
surrounding species. The seeds sown at the same time in a garden have
generally been matured during the same season and in the same place; and
in this respect they differ much from the seeds sown by the hand of
nature. Some exotic plants are not frequented by the native insects in
their new home, and therefore are not intercrossed; and this appears to
be a highly important factor in the individuals acquiring uniformity of
constitution.

In my experiments the greatest care was taken that in each generation
all the crossed and self-fertilised plants should be subjected to the
same conditions. Not that the conditions were absolutely the same, for
the more vigorous individuals will have robbed the weaker ones of
nutriment, and likewise of water when the soil in the pots was becoming
dry; and both lots at one end of the pot will have received a little
more light than those at the other end. In the successive generations,
the plants were subjected to somewhat different conditions, for the
seasons necessarily varied, and they were sometimes raised at different
periods of the year. But as they were all kept under glass, they were
exposed to far less abrupt and great changes of temperature and moisture
than are plants growing out of doors. With respect to the intercrossed
plants, their first parents, which were not related, would almost
certainly have differed somewhat in constitution; and such
constitutional peculiarities would be variously mingled in each
succeeding intercrossed generation, being sometimes augmented, but more
commonly neutralised in a greater or less degree, and sometimes revived
through reversion; just as we know to be the case with the external
characters of crossed species and varieties. With the plants which were
self-fertilised during the successive generations, this latter important
source of some diversity of constitution will have been wholly
eliminated; and the sexual elements produced by the same flower must
have been developed under as nearly the same conditions as it is
possible to conceive.

In Table 7/C the crossed plants are the offspring of a cross with a
fresh stock, or with a distinct variety; and they were put into
competition either with self-fertilised plants, or with intercrossed
plants of the same old stock. By the term fresh stock I mean a
non-related plant, the progenitors of which have been raised during some
generations in another garden, and have consequently been exposed to
somewhat different conditions. In the case of Nicotiana, Iberis, the red
variety of Primula, the common Pea, and perhaps Anagallis, the plants
which were crossed may be ranked as distinct varieties or sub-varieties
of the same species; but with Ipomoea, Mimulus, Dianthus, and Petunia,
the plants which were crossed differed exclusively in the tint of their
flowers; and as a large proportion of the plants raised from the same
lot of purchased seeds thus varied, the differences may be estimated as
merely individual. Having made these preliminary remarks, we will now
consider in detail the several cases given in Table 7/C, and they are
well worthy of full consideration.

1. Ipomoea purpurea.

Plants growing in the same pots, and subjected in each generation to the
same conditions, were intercrossed for nine consecutive generations.
These intercrossed plants thus became in the later generations more or
less closely inter-related. Flowers on the plants of the ninth
intercrossed generation were fertilised with pollen taken from a fresh
stock, and seedlings thus raised. Other flowers on the same intercrossed
plants were fertilised with pollen from another intercrossed plant,
producing seedlings of the tenth intercrossed generation. These two sets
of seedlings were grown in competition with one another, and differed
greatly in height and fertility. For the offspring from the cross with a
fresh stock exceeded in height the intercrossed plants in the ratio of
100 to 78; and this is nearly the same excess which the intercrossed had
over the self-fertilised plants in all ten generations taken together,
namely, as 100 to 77. The plants raised from the cross with a fresh
stock were also greatly superior in fertility to the intercrossed,
namely, in the ratio of 100 to 51, as judged by the relative weight of
the seed-capsules produced by an equal number of plants of the two sets,
both having been left to be naturally fertilised. It should be
especially observed that none of the plants of either lot were the
product of self-fertilisation. On the contrary, the intercrossed plants
had certainly been crossed for the last ten generations, and probably,
during all previous generations, as we may infer from the structure of
the flowers and from the frequency of the visits of humble-bees. And so
it will have been with the parent-plants of the fresh stock. The whole
great difference in height and fertility between the two lots must be
attributed to the one being the product of a cross with pollen from a
fresh stock, and the other of a cross between plants of the same old
stock.

This species offers another interesting case. In the five first
generations in which intercrossed and self-fertilised plants were put
into competition with one another, every single intercrossed plant beat
its self-fertilised antagonist, except in one instance, in which they
were equal in height. But in the sixth generation a plant appeared,
named by me the Hero, remarkable for its tallness and increased
self-fertility, and which transmitted its characters to the next three
generations. The children of Hero were again self-fertilised, forming
the eighth self-fertilised generation, and were likewise intercrossed
one with another; but this cross between plants which had been subjected
to the same conditions and had been self-fertilised during the seven
previous generations, did not effect the least good; for the
intercrossed grandchildren were actually shorter than the
self-fertilised grandchildren, in the ratio of 100 to 107. We here see
that the mere act of crossing two distinct plants does not by itself
benefit the offspring. This case is almost the converse of that in the
last paragraph, on which the offspring profited so greatly by a cross
with a fresh stock. A similar trial was made with the descendants of
Hero in the following generation, and with the same result. But the
trial cannot be fully trusted, owing to the extremely unhealthy
condition of the plants. Subject to this same serious cause of doubt,
even a cross with a fresh stock did not benefit the great-grandchildren
of Hero; and if this were really the case, it is the greatest anomaly
observed by me in all my experiments.

2. Mimulus luteus.

During the three first generations the intercrossed plants taken
together exceeded in height the self-fertilised taken together, in the
ratio of 100 to 65, and in fertility in a still higher degree. In the
fourth generation a new variety, which grew taller and had whiter and
larger flowers than the old varieties, began to prevail, especially
amongst the self-fertilised plants. This variety transmitted its
characters with remarkable fidelity, so that all the plants in the later
self-fertilised generations belonged to it. These consequently exceeded
the intercrossed plants considerably in height. Thus in the seventh
generation the intercrossed plants were to the self-fertilised in height
as 100 to 137. It is a more remarkable fact that the self-fertilised
plants of the sixth generation had become much more fertile than the
intercrossed plants, judging by the number of capsules spontaneously
produced, in the ratio of 147 to 100. This variety, which as we have
seen appeared amongst the plants of the fourth self-fertilised
generation, resembles in almost all its constitutional peculiarities the
variety called Hero which appeared in the sixth self-fertilised
generation of Ipomoea. No other such case, with the partial exception of
that of Nicotiana, occurred in my experiments, carried on during eleven
years.

Two plants of this variety of Mimulus, belonging to the sixth
self-fertilised generation, and growing in separate pots, were
intercrossed; and some flowers on the same plants were again
self-fertilised. From the seeds thus obtained, plants derived from a
cross between the self-fertilised plants, and others of the seventh
self-fertilised generation, were raised. But this cross did not do the
least good, the intercrossed plants being inferior in height to the
self-fertilised, in the ratio of 100 to 110. This case is exactly
parallel with that given under Ipomoea, of the grandchildren of Hero,
and apparently of its great-grandchildren; for the seedlings raised by
intercrossing these plants were not in any way superior to those of the
corresponding generation raised from the self-fertilised flowers.
Therefore in these several cases the crossing of plants, which had been
self-fertilised for several generations and which had been cultivated
all the time under as nearly as possible the same conditions, was not in
the least beneficial.

Another experiment was now tried. Firstly, plants of the eighth
self-fertilised generation were again self-fertilised, producing plants
of the ninth self-fertilised generation. Secondly, two of the plants of
the eighth self-fertilised generation were intercrossed one with
another, as in the experiment above referred to; but this was now
effected on plants which had been subjected to two additional
generations of self-fertilisation. Thirdly, the same plants of the
eighth self-fertilised generation were crossed with pollen from plants
of a fresh stock brought from a distant garden. Numerous plants were
raised from these three sets of seeds, and grown in competition with one
another. The plants derived from a cross between the self-fertilised
plants exceeded in height by a little the self-fertilised, namely, as
100 to 92; and in fertility in a greater degree, namely, as 100 to 73. I
do not know whether this difference in the result, compared with that in
the previous case, can be accounted for by the increased deterioration
of the self-fertilised plants from two additional generations of
self-fertilisation, and the consequent advantage of any cross whatever,
along merely between the self-fertilised plants. But however this may
be, the effects of crossing the self-fertilised plants of the eighth
generation with a fresh stock were extremely striking; for the seedlings
thus raised were to the self-fertilised of the ninth generation as 100
to 52 in height, and as 100 to 3 in fertility! They were also to the
intercrossed plants (derived from crossing two of the self-fertilised
plants of the eighth generation) in height as 100 to 56, and in
fertility as 100 to 4. Better evidence could hardly be desired of the
potent influence of a cross with a fresh stock on plants which had been
self-fertilised for eight generations, and had been cultivated all the
time under nearly uniform conditions, in comparison with plants
self-fertilised for nine generations continuously, or then once
intercrossed, namely in the last generation.

3. Brassica oleracea.

Some flowers on cabbage plants of the second self-fertilised generation
were crossed with pollen from a plant of the same variety brought from a
distant garden, and other flowers were again self-fertilised. Plants
derived from a cross with a fresh stock and plants of the third
self-fertilised generation were thus raised. The former were to the
self-fertilised in weight as 100 to 22; and this enormous difference
must be attributed in part to the beneficial effects of a cross with a
fresh stock, and in part to the deteriorating effects of
self-fertilisation continued during three generations.

4. Iberis umbellata.

Seedlings from a crimson English variety crossed by a pale-coloured
variety which had been grown for some generations in Algiers, were to
the self-fertilised seedlings from the crimson variety in height as 100
to 89, and as 100 to 75 in fertility. I am surprised that this cross
with another variety did not produce a still more strongly marked
beneficial effect; for some intercrossed plants of the crimson English
variety, put into competition with plants of the same variety
self-fertilised during three generations, were in height as 100 to 86,
and in fertility as 100 to 75. The slightly greater difference in height
in this latter case, may possibly be attributed to the deteriorating
effects of self-fertilisation carried on for two additional generations.

5. Eschscholtzia californica.

This plant offers an almost unique case, inasmuch as the good effects of
a cross are confined to the reproductive system. Intercrossed and
self-fertilised plants of the English stock did not differ in height
(nor in weight, as far as was ascertained) in any constant manner; the
self-fertilised plants usually having the advantage. So it was with the
offspring of plants of the Brazilian stock, tried in the same manner.
The parent-plants, however, of the English stock produced many more
seeds when fertilised with pollen from another plant than when
self-fertilised; and in Brazil the parent-plants were absolutely sterile
unless they were fertilised with pollen from another plant. Intercrossed
seedlings, raised in England from the Brazilian stock, compared with
self-fertilised seedlings of the corresponding second generation,
yielded seeds in number as 100 to 89; both lots of plants being left
freely exposed to the visits of insects. If we now turn to the effects
of crossing plants of the Brazilian stock with pollen from the English
stock,--so that plants which had been long exposed to very different
conditions were intercrossed,--we find that the offspring were, as
before, inferior in height and weight to the plants of the Brazilian
stock after two generations of self-fertilisation, but were superior to
them in the most marked manner in the number of seeds produced, namely,
as 100 to 40; both lots of plants being left freely exposed to the
visits of insects.

In the case of Ipomoea, we have seen that the plants derived from a
cross with a fresh stock were superior in height as 100 to 78, and in
fertility as 100 to 51, to the plants of the old stock, although these
had been intercrossed during the last ten generations. With
Eschscholtzia we have a nearly parallel case, but only as far as
fertility is concerned, for the plants derived from a cross with a fresh
stock were superior in fertility in the ratio of 100 to 45 to the
Brazilian plants, which had been artificially intercrossed in England
for the two last generations, and which must have been naturally
intercrossed by insects during all previous generations in Brazil, where
otherwise they are quite sterile.

6. Dianthus caryophyllus.

Plants self-fertilised for three generations were crossed with pollen
from a fresh stock, and their offspring were grown in competition with
plants of the fourth self-fertilised generation. The crossed plants thus
obtained were to the self-fertilised in height as 100 to 81, and in
fertility (both lots being left to be naturally fertilised by insects)
as 100 to 33.

These same crossed plants were also to the offspring from the plants of
the third generation crossed by the intercrossed plants of the
corresponding generation, in height as 100 to 85, and in fertility as
100 to 45.

We thus see what a great advantage the offspring from a cross with a
fresh stock had, not only over the self-fertilised plants of the fourth
generation, but over the offspring from the self-fertilised plants of
the third generation, when crossed by the intercrossed plants of the old
stock.

7. Pisum sativum.

It has been shown under the head of this species, that the several
varieties in this country almost invariably fertilise themselves, owing
to insects rarely visiting the flowers; and as the plants have been long
cultivated under nearly similar conditions, we can understand why a
cross between two individuals of the same variety does not do the least
good to the offspring either in height or fertility. This case is almost
exactly parallel with that of Mimulus, or that of the Ipomoea named
Hero; for in these two instances, crossing plants which had been
self-fertilised for seven generations did not at all benefit the
offspring. On the other hand, a cross between two varieties of the pea
causes a marked superiority in the growth and vigour of the offspring,
over the self-fertilised plants of the same varieties, as shown by two
excellent observers. From my own observations (not made with great care)
the offspring from crossed varieties were to self-fertilised plants in
height, in one case as 100 to about 75, and in a second case as 100 to
60.

8. Lathyrus odoratus.

The sweet-pea is in the same state in regard to self-fertilisation as
the common pea; and we have seen that seedlings from a cross between two
varieties, which differed in no respect except in the colour of their
flowers, were to the self-fertilised seedlings from the same
mother-plant in height as 100 to 80; and in the second generation as 100
to 88. Unfortunately I did not ascertain whether crossing two plants of
the same variety failed to produce any beneficial effect, but I venture
to predict such would be the result.

9. Petunia violacea.

The intercrossed plants of the same stock in four out of the five
successive generations plainly exceeded in height the self-fertilised
plants. The latter in the fourth generation were crossed by a fresh
stock, and the seedlings thus obtained were put into competition with
the self-fertilised plants of the fifth generation. The crossed plants
exceeded the self-fertilised in height in the ratio of 100 to 66, and in
weight as 100 to 23; but this difference, though so great, is not much
greater than that between the intercrossed plants of the same stock in
comparison with the self-fertilised plants of the corresponding
generation. This case, therefore, seems at first sight opposed to the
rule that a cross with a fresh stock is much more beneficial than a
cross between individuals of the same stock. But as with Eschscholtzia,
the reproductive system was here chiefly benefited; for the plants
raised from the cross with the fresh stock were to the self-fertilised
plants in fertility, both lots being naturally fertilised, as 100 to 46,
whereas the intercrossed plants of the same stock were to the
self-fertilised plants of the corresponding fifth generation in
fertility only as 100 to 86.

Although at the time of measurement the plants raised from the cross
with the fresh stock did not exceed in height or weight the intercrossed
plants of the old stock (owing to the growth of the former not having
been completed, as explained under the head of this species), yet they
exceeded the intercrossed plants in fertility in the ratio of 100 to 54.
This fact is interesting, as it shows that plants self-fertilised for
four generations and then crossed by a fresh stock, yielded seedlings
which were nearly twice as fertile as those from plants of the same
stock which had been intercrossed for the five previous generations. We
here see, as with Eschscholtzia and Dianthus, that the mere act of
crossing, independently of the state of the crossed plants, has little
efficacy in giving increased fertility to the offspring. The same
conclusion holds good, as we have already seen, in the analogous cases
of Ipomoea, Mimulus, and Dianthus, with respect to height.

10. Nicotiana tabacum.

My plants were remarkably self-fertile, and the capsules from the
self-fertilised flowers apparently yielded more seeds than those which
were cross-fertilised. No insects were seen to visit the flowers in the
hothouse, and I suspect that the stock on which I experimented had been
raised under glass, and had been self-fertilised during several previous
generations; if so, we can understand why, in the course of three
generations, the crossed seedlings of the same stock did not uniformly
exceed in height the self-fertilised seedlings. But the case is
complicated by individual plants having different constitutions, so that
some of the crossed and self-fertilised seedlings raised at the same
time from the same parents behaved differently. However this may be,
plants raised from self-fertilised plants of the third generation
crossed by a slightly different sub-variety, exceeded greatly in height
and weight the self-fertilised plants of the fourth generation; and the
trial was made on a large scale. They exceeded them in height when grown
in pots, and not much crowded, in the ratio of 100 to 66; and when much
crowded, as 100 to 54. These crossed plants, when thus subjected to
severe competition, also exceeded the self-fertilised in weight in the
ratio of 100 to 37. So it was, but in a less degree (as may be seen in
Table 7/C), when the two lots were grown out of doors and not subjected
to any mutual competition. Nevertheless, strange as is the fact, the
flowers on the mother-plants of the third self-fertilised generation did
not yield more seed when they were crossed with pollen from plants of
the fresh stock than when they were self-fertilised.

11. Anagallis collina.

Plants raised from a red variety crossed by another plant of the same
variety were in height to the self-fertilised plants from the red
variety as 100 to 73. When the flowers on the red variety were
fertilised with pollen from a closely similar blue-flowered variety,
they yielded double the number of seeds to what they did when crossed by
pollen from another individual of the same red variety, and the seeds
were much finer. The plants raised from this cross between the two
varieties were to the self-fertilised seedlings from the red variety, in
height as 100 to 66, and in fertility as 100 to 6.

12. Primula veris.

Some flowers on long-styled plants of the third illegitimate generation
were legitimately crossed with pollen from a fresh stock, and others
were fertilised with their own pollen. From the seeds thus produced
crossed plants, and self-fertilised plants of the fourth illegitimate
generation, were raised. The former were to the latter in height as 100
to 46, and in fertility during one year as 100 to 5, and as 100 to 3.5
during the next year. In this case, however, we have no means of
distinguishing between the evil effects of illegitimate fertilisation
continued during four generations (that is, by pollen of the same form,
but taken from a distinct plant) and strict self-fertilisation. But it
is probable that these two processes do not differ so essentially as at
first appears to be the case. In the following experiment any doubt
arising from illegitimate fertilisation was completely eliminated.

13. Primula veris. (Equal-styled, red-flowered variety.)

Flowers on plants of the second self-fertilised generation were crossed
with pollen from a distinct variety or fresh stock, and others were
again self-fertilised. Crossed plants and plants of the third
self-fertilised generation, all of legitimate origin, were thus raised;
and the former was to the latter in height as 100 to 85, and in
fertility (as judged by the number of capsules produced, together with
the average number of seeds) as 100 to 11.

SUMMARY OF THE MEASUREMENTS IN TABLE 7/C.

This table includes the heights and often the weights of 292 plants
derived from a cross with a fresh stock, and of 305 plants, either of
self-fertilised origin, or derived from an intercross between plants of
the same stock. These 597 plants belong to thirteen species and twelve
genera. The various precautions which were taken to ensure a fair
comparison have already been stated. If we now look down the right hand
column, in which the mean height, weight, and fertility of the plants
derived from a cross with a fresh stock are represented by 100, we shall
see by the other figures how wonderfully superior they are both to the
self-fertilised and to the intercrossed plants of the same stock. With
respect to height and weight, there are only two exceptions to the rule,
namely, with Eschscholtzia and Petunia, and the latter is probably no
real exception. Nor do these two species offer an exception in regard to
fertility, for the plants derived from the cross with a fresh stock were
much more fertile than the self-fertilised plants. The difference
between the two sets of plants in the table is generally much greater in
fertility than in height or weight. On the other hand, with some of the
species, as with Nicotiana, there was no difference in fertility between
the two sets, although a great difference in height and weight.
Considering all the cases in this table, there can be no doubt that
plants profit immensely, though in different ways, by a cross with a
fresh stock or with a distinct sub-variety. It cannot be maintained that
the benefit thus derived is due merely to the plants of the fresh stock
being perfectly healthy, whilst those which had been long intercrossed
or self-fertilised had become unhealthy; for in most cases there was no
appearance of such unhealthiness, and we shall see under Table 7/A that
the intercrossed plants of the same stock are generally superior to a
certain extent to the self-fertilised,--both lots having been subjected
to exactly the same conditions and being equally healthy or unhealthy.

We further learn from Table 7/C, that a cross between plants that have
been self-fertilised during several successive generations and kept all
the time under nearly uniform conditions, does not benefit the offspring
in the least or only in a very slight degree. Mimulus and the
descendants of Ipomoea named Hero offer instances of this rule. Again,
plants self-fertilised during several generations profit only to a small
extent by a cross with intercrossed plants of the same stock (as in the
case of Dianthus), in comparison with the effects of a cross by a fresh
stock. Plants of the same stock intercrossed during several generations
(as with Petunia) were inferior in a marked manner in fertility to those
derived from the corresponding self-fertilised plants crossed by a fresh
stock. Lastly, certain plants which are regularly intercrossed by
insects in a state of nature, and which were artificially crossed in
each succeeding generation in the course of my experiments, so that they
can never or most rarely have suffered any evil from self-fertilisation
(as with Eschscholtzia and Ipomoea), nevertheless profited greatly by a
cross with a fresh stock. These several cases taken together show us in
the clearest manner that it is not the mere crossing of any two
individuals which is beneficial to the offspring. The benefit thus
derived depends on the plants which are united differing in some manner,
and there can hardly be a doubt that it is in the constitution or nature
of the sexual elements. Anyhow, it is certain that the differences are
not of an external nature, for two plants which resemble each other as
closely as the individuals of the same species ever do, profit in the
plainest manner when intercrossed, if their progenitors have been
exposed during several generations to different conditions. But to this
latter subject I shall have to recur in a future chapter.

TABLE 7/A.

We will now turn to our first table, which relates to crossed and
self-fertilised plants of the same stock. These consist of fifty-four
species belonging to thirty natural orders. The total number of crossed
plants of which measurements are given is 796, and of self-fertilised
809; that is altogether 1,605 plants. Some of the species were
experimented on during several successive generations; and it should be
borne in mind that in such cases the crossed plants in each generation
were crossed with pollen from another crossed plant, and the flowers on
the self-fertilised plants were almost always fertilised with their own
pollen, though sometimes with pollen from other flowers on the same
plant. The crossed plants thus became more or less closely inter-related
in the later generations; and both lots were subjected in each
generation to almost absolutely the same conditions, and to nearly the
same conditions in the successive generations. It would have been a
better plan in some respects if I had always crossed some flowers either
on the self-fertilised or intercrossed plants of each generation with
pollen from a non-related plant, grown under different conditions, as
was done with the plants in Table 7/C; for by this procedure I should
have learnt how much the offspring became deteriorated through continued
self-fertilisation in the successive generations. As the case stands,
the self-fertilised plants of the successive generations in Table 7/A
were put into competition with and compared with intercrossed plants,
which were probably deteriorated in some degree by being more or less
inter-related and grown under similar conditions. Nevertheless, had I
always followed the plan in Table 7/C, I should not have discovered the
important fact that, although a cross between plants which are rather
closely related and which had been subjected to closely similar
conditions, gives during several generations some advantage to the
offspring, yet that after a time they may be intercrossed with no
advantage whatever to the offspring. Nor should I have learnt that the
self-fertilised plants of the later generations might be crossed with
intercrossed plants of the same stock with little or no advantage,
although they profited to an extraordinary degree by a cross with a
fresh stock.

With respect to the greater number of the plants in Table 7/A, nothing
special need here be said; full particulars may be found under the head
of each species by the aid of the Index. The figures in the right-hand
column show the mean height of the self-fertilised plants, that of the
crossed plants with which they competed being represented by 100. No
notice is here taken of the few cases in which crossed and
self-fertilised plants were grown in the open ground, so as not to
compete together. The table includes, as we have seen, plants belonging
to fifty-four species, but as some of these were measured during several
successive generations, there are eighty-three cases in which crossed
and self-fertilised plants were compared. As in each generation the
number of plants which were measured (given in the table) was never very
large and sometimes small, whenever in the right hand column the mean
height of the crossed and self-fertilised plants is the same within five
per cent, their heights may be considered as practically equal. Of such
cases, that is, of self-fertilised plants of which the mean height is
expressed by figures between 95 and 105, there are eighteen, either in
some one or all the generations. There are eight cases in which the
self-fertilised plants exceed the crossed by above five per cent, as
shown by the figures in the right hand column being above 105. Lastly,
there are fifty-seven cases in which the crossed plants exceed the
self-fertilised in a ratio of at least 100 to 95, and generally in a
much higher degree.

If the relative heights of the crossed and self-fertilised plants had
been due to mere chance, there would have been about as many cases of
self-fertilised plants exceeding the crossed in height by above five per
cent as of the crossed thus exceeding the self-fertilised; but we see
that of the latter there are fifty-seven cases, and of the former only
eight cases; so that the cases in which the crossed plants exceed in
height the self-fertilised in the above proportion are more than seven
times as numerous as those in which the self-fertilised exceed the
crossed in the same proportion. For our special purpose of comparing the
powers of growth of crossed and self-fertilised plants, it may be said
that in fifty-seven cases the crossed plants exceeded the
self-fertilised by more than five per cent, and that in twenty-six cases
(18 + 8) they did not thus exceed them. But we shall now show that in
several of these twenty-six cases the crossed plants had a decided
advantage over the self-fertilised in other respects, though not in
height; that in other cases the mean heights are not trustworthy, owing
to too few plants having been measured, or to their having grown
unequally from being unhealthy, or to both causes combined.
Nevertheless, as these cases are opposed to my general conclusion I have
felt bound to give them. Lastly, the cause of the crossed plants having
no advantage over the self-fertilised can be explained in some other
cases. Thus a very small residue is left in which the self-fertilised
plants appear, as far as my experiments serve, to be really equal or
superior to the crossed plants.

We will now consider in some little detail the eighteen cases in which
the self-fertilised plants equalled in average height the crossed plants
within five per cent; and the eight cases in which the self-fertilised
plants exceeded in average height the crossed plants by above five per
cent; making altogether twenty-six cases in which the crossed plants
were not taller than the self-fertilised plants in any marked degree.

[1. Dianthus caryophyllus (third generation).

This plant was experimented on during four generations, in three of
which the crossed plants exceeded in height the self-fertilised
generally by much more than five per cent; and we have seen under Table
7/C that the offspring from the plants of the third self-fertilised
generation crossed by a fresh stock profited in height and fertility to
an extraordinary degree. But in this third generation the crossed plants
of the same stock were in height to the self-fertilised only as 100 to
99, that is, they were practically equal. Nevertheless, when the eight
crossed and eight self-fertilised plants were cut down and weighed, the
former were to the latter in weight as 100 to 49! There can therefore be
not the least doubt that the crossed plants of this species are greatly
superior in vigour and luxuriance to the self-fertilised; and what was
the cause of the self-fertilised plants of the third generation, though
so light and thin, growing up so as almost to equal the crossed in
height, I cannot explain.

2. Lobelia fulgens (first generation).

The crossed plants of this generation were much inferior in height to
the self-fertilised, in the proportion of 100 to 127. Although only two
pairs were measured, which is obviously much too few to be trusted, yet
from other evidence given under the head of this species, it is certain
that the self-fertilised plants were very much more vigorous than the
crossed. As I used pollen of unequal maturity for crossing and
self-fertilising the parent-plants, it is possible that the great
difference in the growth of their offspring may have been due to this
cause. In the next generation this source of error was avoided, and many
more plants were raised, and now the average height of the twenty-three
crossed plants was to that of the twenty-three self-fertilised plants as
100 to 91. We can therefore hardly doubt that a cross is beneficial to
this species.

3. Petunia violacea (third generation).

Eight crossed plants were to eight self-fertilised of the third
generation in average height as 100 to 131; and at an early age the
crossed were inferior even in a still higher degree. But it is a
remarkable fact that in one pot in which plants of both lots grew
extremely crowded, the crossed were thrice as tall as the
self-fertilised. As in the two preceding and two succeeding generations,
as well as with plants raised by a crossed with a fresh stock, the
crossed greatly exceeded the self-fertilised in height, weight, and
fertility (when these two latter points were attended to), the present
case must be looked at as an anomaly not affecting the general rule. The
most probable explanation is that the seeds from which the crossed
plants of the third generation were raised were not well ripened; for I
have observed an analogous case with Iberis. Self-fertilised seedlings
of this latter plant, which were known to have been produced from seeds
not well matured, grew from the first much more quickly than the crossed
plants, which were raised from better matured seeds; so that having thus
once got a great start they were enabled ever afterwards to retain their
advantage. Some of these same seeds of the Iberis were sown on the
opposite sides of pots filled with burnt earth and pure sand, not
containing any organic matter; and now the young crossed seedlings grew
during their short life to double the height of the self-fertilised, in
the same manner as occurred with the above two sets of seedlings of
Petunia which were much crowded and thus exposed to very unfavourable
conditions. We have seen also in the eighth generation of Ipomoea that
the self-fertilised seedlings raised from unhealthy parents grew at
first very much more quickly than the crossed seedlings, so that they
were for a long time much taller, though ultimately beaten by them.

4, 5, 6. Eschscholtzia californica.

Four sets of measurements are given in Table 7/A. In one of these the
crossed plants exceed the self-fertilised in average height, so that
this is not one of the exceptions here to be considered. In two other
cases the crossed equalled the self-fertilised in height within five per
cent; and in the fourth case the self-fertilised exceeded the crossed by
above this limit. We have seen in Table 7/C that the whole advantage of
a cross by a fresh stock is confined to fertility, and so it was with
the intercrossed plants of the same stock compared with the
self-fertilised, for the former were in fertility to the latter as 100
to 89. The intercrossed plants thus have at least one important
advantage over the self-fertilised. Moreover, the flowers on the
parent-plants when fertilised with pollen from another individual of the
same stock yield far more seeds than when self-fertilised; the flowers
in this latter case being often quite sterile. We may therefore conclude
that a cross does some good, though it does not give to the crossed
seedlings increased powers of growth.

7. Viscaria oculata.

The average height of the fifteen intercrossed plants to that of the
fifteen self-fertilised plants was only as 100 to 97; but the former
produced many more capsules than the latter, in the ratio of 100 to 77.
Moreover, the flowers on the parent-plants which were crossed and
self-fertilised, yielded seeds on one occasion in the proportion of 100
to 38, and on a second occasion in the proportion of 100 to 58. So that
there can be no doubt about the beneficial effects of a cross, although
the mean height of the crossed plants was only three per cent above that
of the self-fertilised plants.

8. Specularia speculum.

Only the four tallest of the crossed and the four tallest of the
self-fertilised plants, growing in four pots, were measured; and the
former were to the latter in height as 100 to 98. In all four pots a
crossed plant flowered before any one of the self-fertilised plants, and
this is usually a safe indication of some real superiority in the
crossed plants. The flowers on the parent-plants which were crossed with
pollen from another plant yielded seeds compared with the
self-fertilised flowers in the ratio of 100 to 72. We may therefore draw
the same conclusion as in the last case with respect to a cross being
decidedly beneficial.

9. Borago officinalis.

Only four crossed and four self-fertilised plants were raised and
measured, and the former were to the latter in height as 100 to 102. So
small a number of measurements ought never to be trusted; and in the
present instance the advantage of the self-fertilised over the crossed
plants depended almost entirely on one of the self-fertilised plants
having grown to an unusual height. All four crossed plants flowered
before their self-fertilised opponents. The cross-fertilised flowers on
the parent-plants in comparison with the self-fertilised flowers yielded
seeds in the proportion of 100 to 60. So that here again we may draw the
same conclusion as in the two last cases.

10. Passiflora gracilis.

Only two crossed and two self-fertilised plants were raised; and the
former were to the latter in height as 100 to 104. On the other hand,
fruits from the cross-fertilised flowers on the parent-plants contained
seeds in number, compared with those from the self-fertilised flowers,
in the proportion of 100 to 85.

11. Phaseolus multiflorus.

The five crossed plants were to the five self-fertilised in height as
100 to 96. Although the crossed plants were thus only four per cent
taller than the self-fertilised, they flowered in both pots before them.
It is therefore probable that they had some real advantage over the
self-fertilised plants.

12. Adonis aestivalis.

The four crossed plants were almost exactly equal in height to the four
self-fertilised plants, but as so few plants were measured, and as these
were all “miserably unhealthy,” nothing can be inferred with safety with
respect to their relative heights.

13. Bartonia aurea.

The eight crossed plants were to the eight self-fertilised in height as
100 to 107. This number of plants, considering the care with which they
were raised and compared, ought to have given a trustworthy result. But
from some unknown cause they grew very unequally, and they became so
unhealthy that only three of the crossed and three of the
self-fertilised plants set any seeds, and these few in number. Under
these circumstances the mean height of neither lot can be trusted, and
the experiment is valueless. The cross-fertilised flowers on the
parent-plants yielded rather more seeds than the self-fertilised
flowers.

14. Thunbergia alata.

The six crossed plants were to the six self-fertilised in height as 100
to 108. Here the self-fertilised plants seem to have a decided
advantage; but both lots grew unequally, some of the plants in both
being more than twice as tall as others. The parent-plants also were in
an odd semi-sterile condition. Under these circumstances the superiority
of the self-fertilised plants cannot be fully trusted.

15. Nolana prostrata.

The five crossed plants were to the five self-fertilised in height as
100 to 105; so that the latter seem here to have a small but decided
advantage. On the other hand, the flowers on the parent-plants which
were cross-fertilised produced very many more capsules than the
self-fertilised flowers, in the ratio of 100 to 21; and the seeds which
the former contained were heavier than an equal number from the
self-fertilised capsules in the ratio of 100 to 82.

16. Hibiscus africanus.

Only four pairs were raised, and the crossed were to the self-fertilised
in height as 100 to 109. Excepting that too few plants were measured, I
know of nothing else to cause distrust in the result. The
cross-fertilised flowers on the parent-plants were, on the other hand,
rather more productive than the self-fertilised flowers.

17. Apium petroselinum.

A few plants (number not recorded) derived from flowers believed to have
been crossed by insects and a few self-fertilised plants were grown on
the opposite sides of four pots. They attained to a nearly equal height,
the crossed having a very slight advantage.

18. Vandellia nummularifolia.

Twenty crossed plants raised from the seeds of perfect flowers were to
twenty self-fertilised plants, likewise raised from the seeds of perfect
flowers, in height as 100 to 99. The experiment was repeated, with the
sole difference that the plants were allowed to grow more crowded; and
now the twenty-four tallest of the crossed plants were to the
twenty-four tallest self-fertilised plants in height as 100 to 94, and
in weight as 100 to 97. Moreover, a larger number of the crossed than of
the self-fertilised plants grew to a moderate height. The
above-mentioned twenty crossed plants were also grown in competition
with twenty self-fertilised plants raised from the closed or cleistogene
flowers, and their heights were as 100 to 94. Therefore had it not been
for the first trial, in which the crossed plants were to the
self-fertilised in height only as 100 to 99, this species might have
been classed with those in which the crossed plants exceed the
self-fertilised by above five per cent. On the other hand, the crossed
plants in the second trial bore fewer capsules; and these contained
fewer seeds, than did the self-fertilised plants, all the capsules
having been produced by cleistogene flowers. The whole case therefore
must be left doubtful.

19. Pisum sativum (common pea).

Four-plants derived from a cross between individuals of the same variety
were in height to four self-fertilised plants belonging to the same
variety as 100 to 115. Although this cross did no good, we have seen
under Table 7/C that a cross between distinct varieties adds greatly to
the height and vigour of the offspring; and it was there explained that
the fact of a cross between the individuals of the same variety not
being beneficial, is almost certainly due to their having been
self-fertilised for many generations, and in each generation grown under
nearly similar conditions.

20, 21, 22. Canna warscewiczi.

Plants belonging to three generations were observed, and in all of three
the crossed were approximately equal to the self-fertilised; the average
height of the thirty-four crossed plants being to that of the same
number of self-fertilised plants as 100 to 101. Therefore the crossed
plants had no advantage over the self-fertilised; and it is probable
that the same explanation here holds good as in the case of Pisum
sativum; for the flowers of this Canna are perfectly self-fertile, and
were never seen to be visited by insects in the hothouse, so as to be
crossed by them. This plant, moreover, has been cultivated under glass
for several generations in pots, and therefore under nearly uniform
conditions. The capsules produced by the cross-fertilised flowers on the
above thirty-four crossed plants contained more seeds than did the
capsules produced by the self-fertilised flowers on the self-fertilised
plants, in the proportion of 100 to 85; so that in this respect crossing
was beneficial.

23. Primula sinensis.

The offspring of plants, some of which were legitimately and others
illegitimately fertilised with pollen from a distinct plant, were almost
exactly of the same height as the offspring of self-fertilised plants;
but the former with rare exceptions flowered before the latter. I have
shown in my paper on dimorphic plants that this species is commonly
raised in England from self-fertilised seed, and the plants from having
been cultivated in pots have been subjected to nearly uniform
conditions. Moreover, many of them are now varying and changing their
character, so as to become in a greater or less degree equal-styled, and
in consequence highly self-fertile. Therefore I believe that the cause
of the crossed plants not exceeding in height the self-fertilised is the
same as in the two previous cases of Pisum sativum and Canna.

24, 25, 26. Nicotiana tabacum.

Four sets of measurements were made; in one, the self-fertilised plants
greatly exceeded in height the crossed, in two others they were
approximately equal to the crossed, and in the fourth were beaten by
them; but this latter case does not here concern us. The individual
plants differ in constitution, so that the descendants of some profit by
their parents having been intercrossed, whilst others do not. Taking all
three generations together, the twenty-seven crossed plants were in
height to the twenty-seven self-fertilised plants as 100 to 96. This
excess of height in the crossed plants, is so small compared with that
displayed by the offspring from the same mother-plants when crossed by a
slightly different variety, that we may suspect (as explained under
Table 7/C) that most of the individuals belonging to the variety which
served as the mother-plants in my experiments, had acquired a nearly
similar constitution, so as not to profit by being mutually
intercrossed.]

Reviewing these twenty-six cases, in which the crossed plants either do
not exceed the self-fertilised by above five per cent in height, or are
inferior to them, we may conclude that much the greater number of the
cases do not form real exceptions to the rule,--that a cross between two
plants, unless these have been self-fertilised and exposed to nearly the
same conditions for many generations, gives a great advantage of some
kind to the offspring. Of the twenty-six cases, at least two, namely,
those of Adonis and Bartonia, may be wholly excluded, as the trials were
worthless from the extreme unhealthiness of the plants. Inn twelve other
cases (three trials with Eschscholtzia here included) the crossed plants
either were superior in height to the self-fertilised in all the other
generations excepting the one in question, or they showed their
superiority in some different manner, as in weight, fertility, or in
flowering first; or again, the cross-fertilised flowers on the
mother-plant were much more productive of seed than the self-fertilised.

Deducting these fourteen cases, there remain twelve in which the crossed
plants show no well-marked advantage over the self-fertilised. On the
other hand, we have seen that there are fifty-seven cases in which the
crossed plants exceed the self-fertilised in height by at least five per
cent, and generally in a much higher degree. But even in the twelve
cases just referred to, the want of any advantage on the crossed side is
far from certain: with Thunbergia the parent-plants were in an odd
semi-sterile condition, and the offspring grew very unequally; with
Hibiscus and Apium much too few plants were raised for the measurements
to be trusted, and the cross-fertilised flowers of Hibiscus produced
rather more seed than did the self-fertilised; with Vandellia the
crossed plants were a little taller and heavier than the
self-fertilised, but as they were less fertile the case must be left
doubtful. Lastly, with Pisum, Primula, the three generations of Canna,
and the three of Nicotiana (which together complete the twelve cases), a
cross between two plants certainly did no good or very little good to
the offspring; but we have reason to believe that this is the result of
these plants having been self-fertilised and cultivated under nearly
uniform conditions for several generations. The same result followed
with the experimental plants of Ipomoea and Mimulus, and to a certain
extent with some other species, which had been intentionally treated by
me in this manner; yet we know that these species in their normal
condition profit greatly by being intercrossed. There is, therefore, not
a single case in Table 7/A which affords decisive evidence against the
rule that a cross between plants, the progenitors of which have been
subjected to somewhat diversified conditions, is beneficial to the
offspring. This is a surprising conclusion, for from the analogy of
domesticated animals it could not have been anticipated, that the good
effects of crossing or the evil effects of self-fertilisation would have
been perceptible until the plants had been thus treated for several
generations.

The results given in Table 7/A may be looked at under another point of
view. Hitherto each generation has been considered as a separate case,
of which there are eighty-three; and this no doubt is the more correct
method of comparing the crossed and self-fertilised plants.

But in those cases in which plants of the same species were observed
during several generations, a general average of their heights in all
the generations together may be made; and such averages are given in
Table 7/A; for instance, under Ipomoea the general average for the
plants of all ten generations is as 100 for the crossed, to 77 for the
self-fertilised plants. This having been done in each case in which more
than one generation was raised, it is easy to calculate the average of
the average heights of the crossed and self-fertilised plants of all the
species included in Table 7/A. It should however be observed that as
only a few plants of some species, whilst a considerable number of
others, were measured, the value of the mean or average heights of the
several species is very different. Subject to this source of error, it
may be worth while to give the mean of the mean heights of the
fifty-four species in Table 7/A; and the result is, calling the mean of
the mean heights of the crossed plants 100, that of the self-fertilised
plants is 87. But it is a better plan to divide the fifty-four species
into three groups, as was done with the previously given eighty-three
cases. The first group consists of species of which the mean heights of
the self-fertilised plants are within five per cent of 100; so that the
crossed and self-fertilised plants are approximately equal; and of such
species there are twelve about which nothing need be said, the mean of
the mean heights of the self-fertilised being of course very nearly 100,
or exactly 99.58. The second group consists of the species, thirty-seven
in number, of which the mean heights of the crossed plants exceed that
of the self-fertilised plants by more than five per cent; and the mean
of their mean heights is to that of the self-fertilised plants as 100 to
78. The third group consists of the species, only five in number, of
which the mean heights of the self-fertilised plants exceed that of the
crossed by more than five per cent; and here the mean of the mean
heights of the crossed plants is to that of the self-fertilised as 100
to 109. Therefore if we exclude the species which are approximately
equal, there are thirty-seven species in which the mean of the mean
heights of the crossed plants exceeds that of the self-fertilised by
twenty-two per cent; whereas there are only five species in which the
mean of the mean heights of the self-fertilised plants exceeds that of
the crossed, and this only by nine per cent.

The truth of the conclusion--that the good effects of a cross depend on
the plants having been subjected to different conditions or to their
belonging to different varieties, in both of which cases they would
almost certainly differ somewhat in constitution--is supported by a
comparison of the Tables 7/A and 7/C. The latter table gives the results
of crossing plants with a fresh stock or with a distinct variety; and
the superiority of the crossed offspring over the self-fertilised is
here much more general and much more strongly marked than in Table 7/A,
in which plants of the same stock were crossed. We have just seen that
the mean of the mean heights of the crossed plants of the whole
fifty-four species in Table 7/A is to that of the self-fertilised plants
as 100 to 87; whereas the mean of the mean heights of the plants crossed
by a fresh stock is to that of the self-fertilised in Table 7/C as 100
to 74. So that the crossed plants beat the self-fertilised plants by
thirteen per cent in Table 7/A, and by twenty-six per cent, or double as
much, in Table 7/C, which includes the results of the cross by a fresh
stock.

TABLE 7/B.

A few words must be added on the weights of the crossed plants of the
same stock, in comparison with the self-fertilised. Eleven cases are
given in Table 7/B, relating to eight species. The number of plants
which were weighed is shown in the two left columns, and their relative
weights in the right column, that of the crossed plants being taken as
100. A few other cases have already been recorded in Table 7/C in
reference to plants crossed by a fresh stock. I regret that more trials
of this kind were not made, as the evidence of the superiority of the
crossed over the self-fertilised plants is thus shown in a more
conclusive manner than by their relative heights. But this plan was not
thought of until a rather late period, and there were difficulties
either way, as the seeds had to be collected when ripe, by which time
the plants had often begun to wither. In only one out of the eleven
cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants
exceed the crossed in weight; and we have already seen they are likewise
superior to them in height, though inferior in fertility, the whole
advantage of a cross being here confined to the reproductive system.
With Vandellia the crossed plants were a little heavier, as they were
also a little taller than the self-fertilised; but as a greater number
of more productive capsules were produced by the cleistogene flowers on
the self-fertilised plants than by those on the crossed plants, the case
must be left, as remarked under Table 7/A, altogether doubtful. The
crossed and self-fertilised offspring from a partially self-sterile
plant of Reseda odorata were almost equal in weight, though not in
height. In the remaining eight cases, the crossed plants show a
wonderful superiority over the self-fertilised, being more than double
their weight, except in one case, and here the ratio is as high as 100
to 67. The results thus deduced from the weights of the plants confirm
in a striking manner the former evidence of the beneficial effects of a
cross between two plants of the same stock; and in the few cases in
which plants derived from a cross with a fresh stock were weighed, the
results are similar or even more striking.



CHAPTER VIII.

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL
VIGOUR AND IN OTHER RESPECTS.

Greater constitutional vigour of crossed plants.
The effects of great crowding.
Competition with other kinds of plants.
Self-fertilised plants more liable to premature death.
Crossed plants generally flower before the self-fertilised.
Negative effects of intercrossing flowers on the same plant.
Cases described.
Transmission of the good effects of a cross to later generations.
Effects of crossing plants of closely related parentage.
Uniform colour of the flowers on plants self-fertilised during several
generations and cultivated under similar conditions.

GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS.

As in almost all my experiments an equal number of crossed and
self-fertilised seeds, or more commonly seedlings just beginning to
sprout, were planted on the opposite sides of the same pots, they had to
compete with one another; and the greater height, weight, and fertility
of the crossed plants may be attributed to their possessing greater
innate constitutional vigour. Generally the plants of the two lots
whilst very young were of equal height; but afterwards the crossed
gained insensibly on their opponents, and this shows that they possessed
some inherent superiority, though not displayed at a very early period
in life. There were, however, some conspicuous exceptions to the rule of
the two lots being at first equal in height; thus the crossed seedlings
of the broom (Sarothamnus scoparius) when under three inches in height
were more than twice as tall as the self-fertilised plants.

After the crossed or the self-fertilised plants had once grown decidedly
taller than their opponents, a still increasing advantage would tend to
follow from the stronger plants robbing the weaker ones of nourishment
and overshadowing them. This was evidently the case with the crossed
plants of Viola tricolor, which ultimately quite overwhelmed the
self-fertilised. But that the crossed plants have an inherent
superiority, independently of competition, was sometimes well shown when
both lots were planted separately, not far distant from one another, in
good soil in the open ground. This was likewise shown in several cases,
even with plants growing in close competition with one another, by one
of the self-fertilised plants exceeding for a time its crossed opponent,
which had been injured by some accident or was at first sickly, but
being ultimately conquered by it. The plants of the eighth generation of
Ipomoea were raised from small seeds produced by unhealthy parents, and
the self-fertilised plants grew at first very rapidly, so that when the
plants of both lots were about three feet in height, the mean height of
the crossed to that of the self-fertilised was as 100 to 122; when they
were about six feet high the two lots were very nearly equal, but
ultimately when between eight and nine feet in height, the crossed
plants asserted their usually superiority, and were to the
self-fertilised in height as 100 to 85.

The constitutional superiority of the crossed over the self-fertilised
plants was proved in another way in the third generation of Mimulus, by
self-fertilised seeds being sown on one side of a pot, and after a
certain interval of time crossed seeds on the opposite side. The
self-fertilised seedlings thus had (for I ascertained that the seeds
germinated simultaneously) a clear advantage over the crossed in the
start for the race. Nevertheless they were easily beaten (as may be seen
under the head of Mimulus) when the crossed seeds were sown two whole
days after the self-fertilised. But when the interval was four days, the
two lots were nearly equal throughout life. Even in this latter case the
crossed plants still possessed an inherent advantage, for after both
lots had grown to their full height they were cut down, and without
being disturbed were transferred to a larger pot, and when in the
ensuing year they had again grown to their full height they were
measured; and now the tallest crossed plants were to the tallest
self-fertilised plants in height as 100 to 75, and in fertility (i.e.,
by weight of seeds produced by an equal number of capsules from both
lots) as 100 to 34.

My usual method of proceeding, namely, to plant several pairs of crossed
and self-fertilised seeds in an equal state of germination on the
opposite sides of the same pots, so that the plants were subjected to
moderately severe mutual competition, was I think the best that could
have been followed, and was a fair test of what occurs in a state of
nature. For plants sown by nature generally come up crowded, and are
almost always exposed to very severe competition with one another and
with other kinds of plants. This latter consideration led me to make
some trials, chiefly but not exclusively with Ipomoea and Mimulus, by
sowing crossed and self-fertilised seeds on the opposite sides of large
pots in which other plants had long been growing, or in the midst of
other plants out of doors. The seedlings were thus subjected to very
severe competition with plants of other kinds; and in all such cases,
the crossed seedlings exhibited a great superiority in their power of
growth over the self-fertilised.

After the germinating seedlings had been planted in pairs on the
opposite sides of several pots, the remaining seeds, whether or not in a
state of germination, were in most cases sown very thickly on the two
sides of an additional large pot; so that the seedlings came up
extremely crowded, and were subjected to extremely severe competition
and unfavourable conditions. In such cases the crossed plants almost
invariably showed a greater superiority over the self-fertilised, than
did the plants which grew in pairs in the pots.

Sometimes crossed and self-fertilised seeds were sown in separate rows
in the open ground, which was kept clear of weeds; so that the seedlings
were not subjected to any competition with other kinds of plants. Those
however in each row had to struggle with the adjoining ones in the same
row. When fully grown, several of the tallest plants in each row were
selected, measured, and compared. The result was in several cases (but
not so invariably as might have been expected) that the crossed plants
did not exceed in height the self-fertilised in nearly so great a degree
as when grown in pairs in the pots. Thus with the plants of Digitalis,
which competed together in pots, the crossed were to the self-fertilised
in height as 100 to 70; whilst those which were grown separately were
only as 100 to 85. Nearly the same result was observed with Brassica.
With Nicotiana the crossed were to the self-fertilised plants in height,
when grown extremely crowded together in pots, as 100 to 54; when grown
much less crowded in pots as 100 to 66, and when grow in the open
ground, so as to be subjected to but little competition, as 100 to 72.
On the other hand with Zea, there was a greater difference in height
between the crossed and self-fertilised plants growing out of doors,
than between the pairs which grew in pots in the hothouse; but this may
be attributed to the self-fertilised plants being more tender, so that
they suffered more than the crossed, when both lots were exposed to a
cold and wet summer. Lastly, with one out of two series of Reseda
odorata, grown out of doors in rows, as well as with Beta vulgaris, the
crossed plants did not at all exceed the self-fertilised in height, or
exceeded them by a mere trifle.

The innate power of the crossed plants to resist unfavourable conditions
far better than did the self-fertilised plants, was shown on two
occasions in a curious manner, namely, with Iberis and in the third
generation of Petunia, by the great superiority in height of the crossed
over the self-fertilised seedlings, when both sets were grown under
extremely unfavourable conditions; whereas owing to special
circumstances exactly the reverse occurred with the plants raised from
the same seeds and grown in pairs in pots. A nearly analogous case was
observed on two other occasions with plants of the first generation of
Nicotiana.

The crossed plants always withstood the injurious effects of being
suddenly removed into the open air after having been kept in the
greenhouse better than did the self-fertilised. On several occasions
they also resisted much better cold and intemperate weather. This was
manifestly the case with some crossed and self-fertilised plants of
Ipomoea, which were suddenly moved from the hothouse to the coldest part
of a cool greenhouse. The offspring of plants of the eighth
self-fertilised generation of Mimulus crossed by a fresh stock, survived
a frost which killed every single self-fertilised and intercrossed plant
of the same old stock. Nearly the same result followed with some crossed
and self-fertilised plants of Viola tricolor. Even the tips of the
shoots of the crossed plants of Sarothamnus scoparius were not touched
by a very severe winter; whereas all the self-fertilised plants were
killed halfway down to the ground, so that they were not able to flower
during the next summer. Young crossed seedlings of Nicotiana withstood a
cold and wet summer much better than the self-fertilised seedlings. I
have met with only one exception to the rule of crossed plants being
hardier than the self-fertilised: three long rows of Eschscholtzia
plants, consisting of crossed seedlings from a fresh stock, of
intercrossed seedlings of the same stock, and of self-fertilised ones,
were left unprotected during a severe winter, and all perished except
two of the self-fertilised. But this case is not so anomalous as it at
first appears, for it should be remembered that the self-fertilised
plants of Eschscholtzia always grow taller and are heavier than the
crossed; the whole benefit of a cross with this species being confined
to increased fertility.

Independently of any external cause which could be detected, the
self-fertilised plants were more liable to premature death than were the
crossed; and this seems to me a curious fact. Whilst the seedlings were
very young, if one died its antagonist was pulled up and thrown away,
and I believe that many more of the self-fertilised died at this early
age than of the crossed; but I neglected to keep any record. With Beta
vulgaris, however, it is certain that a large number of the
self-fertilised seeds perished after germinating beneath the ground,
whereas the crossed seeds sown at the same time did not thus suffer.
When a plant died at a somewhat more advanced age the fact was recorded;
and I find in my notes that out of several hundred plants, only seven of
the crossed died, whilst of the self-fertilised at least twenty-nine
were thus lost, that is more than four times as many. Mr. Galton, after
examining some of my tables, remarks: “It is very evident that the
columns with the self-fertilised plants include the larger number of
exceptionally small plants;” and the frequent presence of such puny
plants no doubt stands in close relation with their liability to
premature death. The self-fertilised plants of Petunia completed their
growth and began to wither sooner than did the intercrossed plants; and
these latter considerably before the offspring from a cross with a fresh
stock.

PERIOD OF FLOWERING.

In some cases, as with Digitalis, Dianthus, and Reseda, a larger number
of the crossed than of the self-fertilised plants threw up flower-stems;
but this probably was merely the result of their greater power of
growth; for in the first generation of Lobelia fulgens, in which the
self-fertilised plants greatly exceeded in height the crossed plants,
some of the latter failed to throw up flower-stems. With a large number
of species, the crossed plants exhibited a well-marked tendency to
flower before the self-fertilised ones growing in the same pots. It
should however be remarked that no record was kept of the flowering of
many of the species; and when a record was kept, the flowering of the
first plant in each pot was alone observed, although two or more pairs
grew in the same pot. I will now give three lists,--one of the species
in which the first plant that flowered was a crossed one,--a second in
which the first that flowered was a self-fertilised plant,--and a third
of those which flowered at the same time.

[SPECIES, OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF CROSSED
PARENTAGE.

Ipomoea purpurea.

I record in my notes that in all ten generations many of the crossed
plants flowered before the self-fertilised; but no details were kept.

Mimulus luteus (First Generation).

Ten flowers on the crossed plants were fully expanded before one on the
self-fertilised.

Mimulus luteus (Second and Third Generation).

In both these generations a crossed plant flowered before one of the
self-fertilised in all three pots.

Mimulus luteus (Fifth Generation).

In all three pots a crossed plant flowered first; yet the
self-fertilised plants, which belonged to the new tall variety, were in
height to the crossed as 126 to 100.

Mimulus luteus.

Plants derived from a cross with a fresh stock as well as the
intercrossed plants of the old stock, flowered before the
self-fertilised plants in nine out of the ten pots.

Salvia coccinea.

A crossed plant flowered before any one of the self-fertilised in all
three pots.

Origanum vulgare.

During two successive seasons several crossed plants flowered before the
self-fertilised.

Brassica oleracea (First Generation).

All the crossed plants growing in pots and in the open ground flowered
first.

Brassica oleracea (Second Generation).

A crossed plant in three out of the four pots flowered before any one of
the self-fertilised.

Iberis umbellata.

In both pots a crossed plant flowered first.

Eschscholtzia californica.

Plants derived from the Brazilian stock crossed by the English stock
flowered in five out of the nine pots first; in four of them a
self-fertilised plant flowered first; and not in one pot did an
intercrossed plant of the old stock flower first.

Viola tricolor.

A crossed plant in five out of the six pots flowered before any one of
the self-fertilised.

Dianthus caryophyllus (First Generation).

In two large beds of plants, four of the crossed plants flowered before
any one of the self-fertilised.

Dianthus caryophyllus (Second Generation).

In both pots a crossed plant flowered first.

Dianthus caryophyllus (Third Generation).

In three out of the four pots a crossed plant flowered first; yet the
crossed were to the self-fertilised in height only as 100 to 99, but in
weight as 100 to 49.

Dianthus caryophyllus.

Plants derived from a cross with a fresh stock, and the intercrossed
plants of the old stock, both flowered before the self-fertilised in
nine out of the ten pots.

Hibiscus africanus.

In three out of the four pots a crossed plant flowered before any one of
the self-fertilised; yet the latter were to the crossed in height as 109
to 100.

Tropaeolum minus.

A crossed plant flowered before any one of the self-fertilised in three
out of the four pots, and simultaneously in the fourth pot.

Limnanthes douglasii.

A crossed plant flowered before any one of the self-fertilised in four
out of the five pots.

Phaseolus multiflorus.

In both pots a crossed plant flowered first.

Specularia speculum.

In all four pots a crossed plant flowered first.

Lobelia ramosa (First Generation).

In all four pots a crossed plant flowered before any one of the
self-fertilised.

Lobelia ramosa (Second Generation).

In all four pots a crossed plant flowered some days before any one of
the self-fertilised.

Nemophila insignis.

In four out of the five pots a crossed plant flowered first.

Borago officinalis.

In both pots a crossed plant flowered first.

Petunia violacea (Second Generation).

In all three pots a crossed plant flowered first.

Nicotiana tabacum.

A plant derived from a cross with a fresh stock flowered before any one
of the self-fertilised plants of the fourth generation, in fifteen out
of the sixteen pots.

Cyclamen persicum.

During two successive seasons a crossed plant flowered some weeks before
any one of the self-fertilised in all four pots.

Primula veris (equal-styled var.)

In all three pots a crossed plant flowered first.

Primula sinensis.

In all four pots plants derived from an illegitimate cross between
distinct plants flowered before any one of the self-fertilised plants.

Primula sinensis.

A legitimately crossed plant flowered before any one of the
self-fertilised plants in seven out of the eight pots.

Fagopyrum esculentum.

A legitimately crossed plant flowered from one to two days before any
one of the self-fertilised plants in all three pots.

Zea mays.

In all four pots a crossed plant flowered first.

Phalaris canariensis.

The crossed plants flowered before the self-fertilised in the open
ground, but simultaneously in the pots.

SPECIES OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF SELF-FERTILISED
PARENTAGE.

Eschscholtzia californica (First Generation).

The crossed plants were at first taller than the self-fertilised, but on
their second growth during the following year the self-fertilised
exceeded the crossed in height, and now they flowered first in three out
of the four pots.

Lupinus luteus.

Although the crossed plants were to the self-fertilised in height as 100
to 82; yet in all three pots the self-fertilised plants flowered first.

Clarkia elegans.

Although the crossed plants were, as in the last case, to the
self-fertilised in height as 100 to 82, yet in the two pots the
self-fertilised flowered first.

Lobelia fulgens (First Generation).

The crossed plants were to the self-fertilised in height only as 100 to
127, and the latter flowered much before the crossed.

Petunia violacea (Third Generation).

The crossed plants were to the self-fertilised in height as 100 to 131,
and in three out of the four pots a self-fertilised plant flowered
first; in the fourth pot simultaneously.

Petunia violacea (Fourth generation).

Although the crossed plants were to the self-fertilised in height as 100
to 69, yet in three out of the five pots a self-fertilised plant
flowered first; in the fourth pot simultaneously, and only in the fifth
did a crossed plant flower first.

Nicotiana tabacum (First Generation).

The crossed plants were to the self-fertilised in height only as 100 to
178, and a self-fertilised plant flowered first in all four pots.

Nicotiana tabacum (Third Generation).

The crossed plants were to the self-fertilised in height as 100 to 101,
and in four out of the five pots a self-fertilised plant flowered first.

Canna warscewiczi.

In the three generations taken together the crossed were to the
self-fertilised in height as 100 to 101; in the first generation the
self-fertilised plants showed some tendency to flower first, and in the
third generation they flowered first in nine out of the twelve pots.

SPECIES IN WHICH THE CROSSED AND SELF-FERTILISED PLANTS FLOWERED ALMOST
SIMULTANEOUSLY.

Mimulus luteus (Sixth Generation).

The crossed plants were inferior in height and vigour to the
self-fertilised plants, which all belonged to the new white-flowered
tall variety, yet in only half the pots did the self-fertilised plants
flower first, and in the other half the crossed plants.

Viscaria oculata.

The crossed plants were only a little taller than the self-fertilised
(namely, as 100 to 97), but considerably more fertile, yet both lots
flowered almost simultaneously.

Lathyrus odoratus (Second Generation).

Although the crossed plants were to the self-fertilised in height as 100
to 88, yet there was no marked difference in their period of flowering.

Lobelia fulgens (Second Generation).

Although the crossed plants were to the self-fertilised in height as 100
to 91, yet they flowered simultaneously.

Nicotiana tabacum (Third Generation).

Although the crossed plants were to the self-fertilised in height as 100
to 83, yet in half the pots a self-fertilised plant flowered first, and
in the other half a crossed plant.]

These three lists include fifty-eight cases, in which the period of
flowering of the crossed and self-fertilised plants was recorded. In
forty-four of them a crossed plant flowered first either in a majority
of the pots or in all; in nine instances a self-fertilised plant
flowered first, and in five the two lots flowered simultaneously. One of
the most striking cases is that of Cyclamen, in which the crossed plants
flowered some weeks before the self-fertilised in all four pots during
two seasons. In the second generation of Lobelia ramosa, a crossed plant
flowered in all four pots some days before any one of the
self-fertilised. Plants derived from a cross with a fresh stock
generally showed a very strongly marked tendency to flower before the
self-fertilised and the intercrossed plants of the old stock; all three
lots growing in the same pots. Thus with Mimulus and Dianthus, in only
one pot out of ten, and in Nicotiana in only one pot out of sixteen, did
a self-fertilised plant flower before the plants of the two crossed
kinds,--these latter flowering almost simultaneously.

A consideration of the two first lists, especially of the second one,
shows that a tendency to flower first is generally connected with
greater power of growth, that is, with greater height. But there are
some remarkable exceptions to this rule, proving that some other cause
comes into play. Thus the crossed plants both of Lupinus luteus and
Clarkia elegans were to the self-fertilised plants in height as 100 to
82, and yet the latter flowered first. In the third generation of
Nicotiana, and in all three generations of Canna, the crossed and
self-fertilised plants were of nearly equal height, yet the
self-fertilised tended to flower first. On the other hand, with Primula
sinensis, plants raised from a cross between two distinct individuals,
whether these were legitimately or illegitimately crossed, flowered
before the illegitimately self-fertilised plants, although all the
plants were of nearly equal height in both cases. So it was with respect
to height and flowering with Phaseolus, Specularia, and Borago. The
crossed plants of Hibiscus were inferior in height to the
self-fertilised, in the ratio of 100 to 109, and yet they flowered
before the self-fertilised in three out of the four pots. On the whole,
there can be no doubt that the crossed plants exhibit a tendency to
flower before the self-fertilised, almost though not quite so strongly
marked as to grow to a greater height, to weigh more, and to be more
fertile.

A few other cases not included in the above three lists deserve notice.
In all three pots of Viola tricolor, naturally crossed plants the
offspring of crossed plants flowered before naturally crossed plants the
offspring of self-fertilised plants. Flowers on two plants, both of
self-fertilised parentage, of the sixth generation of Mimulus luteus
were intercrossed, and other flowers on the same plants were fertilised
with their own pollen; intercrossed seedlings and seedlings of the
seventh self-fertilised generation were thus raised, and the latter
flowered before the intercrossed in three out of the five pots. Flowers
on a plant both of Mimulus luteus and of Ipomoea purpurea were crossed
with pollen from other flowers on the same plant, and other flowers were
fertilised with their own pollen; intercrossed seedlings of this
peculiar kind, and others strictly self-fertilised being thus raised. In
the case of the Mimulus the self-fertilised plants flowered first in
seven out of the eight pots, and in the case of the Ipomoea in eight out
of the ten pots; so that an intercross between the flowers on the same
plant was very far from giving to the offspring thus raised, any
advantage over the strictly self-fertilised plants in their period of
flowering.

EFFECTS OF CROSSING FLOWERS ON THE SAME PLANT.

In the discussion on the results of a cross with a fresh stock, given
under Table 7/C in the last chapter, it was shown that the mere act of
crossing by itself does no good; but that the advantages thus derived
depend on the plants which are crossed, either consisting of distinct
varieties which will almost certainly differ somewhat in constitution,
or on the progenitors of the plants which are crossed, though identical
in every external character, having been subjected to somewhat different
conditions and having thus acquired some slight difference in
constitution. All the flowers produced by the same plant have been
developed from the same seed; those which expand at the same time have
been exposed to exactly the same climatic influences; and the stems have
all been nourished by the same roots. Therefore in accordance with the
conclusion just referred to, no good ought to result from crossing
flowers on the same plant. (8/1. It is, however, possible that the
stamens which differ in length or construction in the same flower may
produce pollen differing in nature, and in this manner a cross might be
made effective between the several flowers on the same plant. Mr. Macnab
states in a communication to M. Verlot ‘La Production des Varietes’ 1865
page 42, that seedlings raised from the shorter and longer stamens of
rhododendron differ in character; but the shorter stamens apparently are
becoming rudimentary, and the seedlings are dwarfs, so that the result
may be simply due to a want of fertilising power in the pollen, as in
the case of the dwarfed plants of Mirabilis raised by Naudin by the use
of too few pollen-grains. Analogous statements have been made with
respect to the stamens of Pelargonium. With some of the Melastomaceae,
seedlings raised by me from flowers fertilised by pollen from the
shorter stamens, certainly differed in appearance from those raised from
the longer stamens, with differently coloured anthers; but here, again,
there is some reason for believing that the shorter stamens are tending
towards abortion. In the very different case of trimorphic heterostyled
plants, the two sets of stamens in the same flower have widely different
fertilising powers.) In opposition to this conclusion is the fact that a
bud is in one sense a distinct individual, and is capable of
occasionally or even not rarely assuming new external characters, as
well as new constitutional peculiarities. Plants raised from buds which
have thus varied may be propagated for a great length of time by grafts,
cuttings, etc., and sometimes even by seminal generation. (8/2. I have
given numerous cases of such bud-variations in my ‘Variation of Animals
and Plants under Domestication’ chapter 11 2nd edition volume 1 page
448.) There exist also numerous species in which the flowers on the same
plant differ from one another,--as in the sexual organs of monoecious
and polygamous plants,--in the structure of the circumferential flowers
in many Compositae, Umbelliferae, etc.,--in the structure of the central
flower in some plants,--in the two kinds of flowers produced by
cleistogene species,--and in several other such cases. These instances
clearly prove that the flowers on the same plant have often varied
independently of one another in many important respects, such variations
having been fixed, like those on distinct plants during the development
of species.

It was therefore necessary to ascertain by experiment what would be the
effect of intercrossing flowers on the same plant, in comparison with
fertilising them with their own pollen or crossing them with pollen from
a distinct plant. Trials were carefully made on five genera belonging to
four families; and in only one case, namely, Digitalis, did the
offspring from a cross between the flowers on the same plant receive any
benefit, and the benefit here was small compared with that derived from
a cross between distinct plants. In the chapter on Fertility, when we
consider the effects of cross-fertilisation and self-fertilisation on
the productiveness of the parent-plants we shall arrive at nearly the
same result, namely, that a cross between the flowers on the same plant
does not at all increase the number of the seeds, or only occasionally
and to a slight degree. I will now give an abstract of the results of
the five trials which were made.

1. Digitalis purpurea.

Seedlings raised from intercrossed flowers on the same plant, and others
from flowers fertilised with their own pollen, were grown in the usual
manner in competition with one another on the opposite sides of ten
pots. In this and the four following cases, the details may be found
under the head of each species. In eight pots, in which the plants did
not grow much crowded, the flower-stems on sixteen intercrossed plants
were in height to those on sixteen self-fertilised plants, as 100 to 94.
In the two other pots on which the plants grew much crowded, the
flower-stems on nine intercrossed plants were in height to those on nine
self-fertilised plants, as 100 to 90. That the intercrossed plants in
these two latter pots had a real advantage over their self-fertilised
opponents, was well shown by their relative weights when cut down, which
was as 100 to 78. The mean height of the flower-stems on the twenty-five
intercrossed plants in the ten pots taken together, was to that of the
flower-stems on the twenty-five self-fertilised plants, as 100 to 92.
Thus the intercrossed plants were certainly superior to the
self-fertilised in some degree; but their superiority was small compared
with that of the offspring from a cross between distinct plants over the
self-fertilised, this being in the ratio of 100 to 70 in height. Nor
does this latter ratio show at all fairly the great superiority of the
plants derived from a cross between distinct individuals over the
self-fertilised, as the former produced more than twice as many
flower-stems as the latter, and were much less liable to premature
death.

2. Ipomoea purpurea.

Thirty-one intercrossed plants raised from a cross between flowers on
the same plants were grown in ten pots in competition with the same
number of self-fertilised plants, and the former were to the latter in
height as 100 to 105. So that the self-fertilised plants were a little
taller than the intercrossed; and in eight out of the ten pots a
self-fertilised plant flowered before any one of the crossed plants in
the same pots. The plants which were not greatly crowded in nine of the
pots (and these offer the fairest standard of comparison) were cut down
and weighed; and the weight of the twenty-seven intercrossed plants was
to that of the twenty-seven self-fertilised as 100 to 124; so that by
this test the superiority of the self-fertilised was strongly marked. To
this subject of the superiority of the self-fertilised plants in certain
cases, I shall have to recur in a future chapter. If we now turn to the
offspring from a cross between distinct plants when put into competition
with self-fertilised plants, we find that the mean height of
seventy-three such crossed plants, in the course of ten generations, was
to that of the same number of self-fertilised plants as 100 to 77; and
in the case of the plants of the tenth generation in weight as 100 to
44. Thus the contrast between the effects of crossing flowers on the
same plant, and of crossing flowers on distinct plants, is wonderfully
great.

3. Mimulus luteus.

Twenty-two plants raised by crossing flowers on the same plant were
grown in competition with the same number of self-fertilised plants; and
the former were to the latter in height as 100 to 105, and in weight as
100 to 103. Moreover, in seven out of the eight pots a self-fertilised
plant flowered before any of the intercrossed plants. So that here again
the self-fertilised exhibit a slight superiority over the intercrossed
plants. For the sake of comparison, I may add that seedlings raised
during three generations from a cross between distinct plants were to
the self-fertilised plants in height as 100 to 65.

4. Pelargonium zonale.

Two plants growing in separate pots, which had been propagated by
cuttings from the same plant, and therefore formed in fact parts of the
same individual, were intercrossed, and other flowers on one of these
plants were self-fertilised; but the seedlings obtained by the two
processes did not differ in height. When, on the other hand, flowers on
one of the above plants were crossed with pollen taken from a distinct
seedling, and other flowers were self-fertilised, the crossed offspring
thus obtained were to the self-fertilised in height as 100 to 74.

5. Origanum vulgare.

A plant which had been long cultivated in my kitchen garden, had spread
by stolons so as to form a large bed or clump. Seedlings raised by
intercrossing flowers on these plants, which strictly consisted of the
same plant, and other seedlings raised from self-fertilised flowers,
were carefully compared from their earliest youth to maturity; and they
did not differ at all in height or in constitutional vigour. Some
flowers on these seedlings were then crossed with pollen taken from a
distinct seedling, and other flowers were self-fertilised; two fresh
lots of seedlings being thus raised, which were the grandchildren of the
plant that had spread by stolons and formed a large clump in my garden.
These differed much in height, the crossed plants being to the
self-fertilised as 100 to 86. They differed, also, to a wonderful degree
in constitutional vigour. The crossed plants flowered first, and
produced exactly twice as many flower-stems; and they afterwards
increased by stolons to such an extent as almost to overwhelm the
self-fertilised plants.

Reviewing these five cases, we see that in four of them, the effect of a
cross between flowers on the same plant (even on offsets of the same
plant growing on separate roots, as with the Pelargonium and Origanum)
does not differ from that of the strictest self-fertilisation. Indeed,
in two of the cases the self-fertilised plants were superior to such
intercrossed plants. With Digitalis a cross between the flowers on the
same plant certainly did do some good, yet very slight compared with
that from a cross between distinct plants. On the whole the results here
arrived at, if we bear in mind that the flower-buds are to a certain
extent distinct individuals and occasionally vary independently of one
another, agree well with our general conclusion, that the advantages of
a cross depend on the progenitors of the crossed plants possessing
somewhat different constitutions, either from having been exposed to
different conditions, or to their having varied from unknown causes in a
manner which we in our ignorance are forced to speak of as spontaneous.
Hereafter I shall have to recur to this subject of the inefficiency of a
cross between the flowers on the same plant, when we consider the part
which insects play in the cross-fertilisation of flowers.

ON THE TRANSMISSION OF THE GOOD EFFECTS FROM A CROSS AND OF THE EVIL
EFFECTS FROM SELF-FERTILISATION.

We have seen that seedlings from a cross between distinct plants almost
always exceed their self-fertilised opponents in height, weight, and
constitutional vigour, and, as will hereafter be shown, often in
fertility. To ascertain whether this superiority would be transmitted
beyond the first generation, seedlings were raised on three occasions
from crossed and self-fertilised plants, both sets being fertilised in
the same manner, and therefore not as in the many cases given in Tables
7/A, 7/B, 7/C, in which the crossed plants were again crossed and the
self-fertilised again self-fertilised.

Firstly, seedlings were raised from self-fertilised seeds produced under
a net by crossed and self-fertilised plants of Nemophila insignis; and
the latter were to the former in height as 133 to 100. But these
seedlings became very unhealthy early in life, and grew so unequally
that some of them in both lots were five times as tall as the others.
Therefore this experiment was quite worthless; but I have felt bound to
give it, as opposed to my general conclusion. I should state that in
this and the two following trials, both sets of plants were grown on the
opposite sides of the same pots, and treated in all respects alike. The
details of the experiments may be found under the head of each species.

Secondly, a crossed and a self-fertilised plant of Heartsease (Viola
tricolor) grew near together in the open ground and near to other plants
of heartsease; and as both produced an abundance of very fine capsules,
the flowers on both were certainly cross-fertilised by insects. Seeds
were collected from both plants, and seedlings raised from them. Those
from the crossed plants flowered in all three pots before those from the
self-fertilised plants; and when fully grown the former were to the
latter in height as 100 to 82. As both sets of plants were the product
of cross-fertilisation, the difference in their growth and period of
flowering was clearly due to their parents having been of crossed and
self-fertilised parentage; and it is equally clear that they transmitted
different constitutional powers to their offspring, the grandchildren of
the plants which were originally crossed and self-fertilised.

Thirdly, the Sweet Pea (Lathyrus odoratus) habitually fertilises itself
in this country. As I possessed plants, the parents and grandparents of
which had been artificially crossed and other plants descended from the
same parents which had been self-fertilised for many previous
generations, these two lots of plants were allowed to fertilise
themselves under a net, and their self-fertilised seeds saved. The
seedlings thus raised were grown in competition with each other in the
usual manner, and differed in their powers of growth. Those from the
self-fertilised plants which had been crossed during the two previous
generations were to those from the plants self-fertilised during many
previous generations in height as 100 to 90. These two lots of seeds
were likewise tried by being sown under very unfavourable conditions in
poor exhausted soil, and the plants whose grandparents and
great-grandparents had been crossed showed in an unmistakable manner
their superior constitutional vigour. In this case, as in that of the
heartsease, there could be no doubt that the advantage derived from a
cross between two plants was not confined to the offspring of the first
generation. That constitutional vigour due to cross-parentage is
transmitted for many generations may also be inferred as highly
probable, from some of Andrew Knight’s varieties of the common pea,
which were raised by crossing distinct varieties, after which time they
no doubt fertilised themselves in each succeeding generation. These
varieties lasted for upwards of sixty years, “but their glory is now
departed.” (8/3. See the evidence on this head in my ‘Variation under
Domestication’ chapter 9 volume 1 2nd edition page 397.) On the other
hand, most of the varieties of the common pea, which there is no reason
to suppose owe their origin to a cross, have had a much shorter
existence. Some also of Mr. Laxton’s varieties produced by artificial
crosses have retained their astonishing vigour and luxuriance for a
considerable number of generations; but as Mr. Laxton informs me, his
experience does not extend beyond twelve generations, within which
period he has never perceived any diminution of vigour in his plants.

An allied point may be here noticed. As the force of inheritance is
strong with plants (of which abundant evidence could be given), it is
almost certain that seedlings from the same capsule or from the same
plant would tend to inherit nearly the same constitution; and as the
advantage from a cross depends on the plants which are crossed differing
somewhat in constitution, it may be inferred as probable that under
similar conditions a cross between the nearest relations would not
benefit the offspring so much as one between non-related plants. In
support of this conclusion we have some evidence, as Fritz Muller has
shown by his valuable experiments on hybrid Abutilons, that the union of
brothers and sisters, parents and children, and of other near relations
is highly injurious to the fertility of the offspring. In one case,
moreover, seedlings from such near relations possessed very weak
constitutions. (8/4. ‘Jenaische Zeitschrift fur Naturw.’ B. 7 pages 22
and 45 1872 and 1873 pages 441-450.) This same observer also found three
plants of a Bignonia growing near together. (8/5. ‘Botanische Zeitung’
1868 page 626.) He fertilised twenty-nine flowers on one of them with
their own pollen, and they did not set a single capsule. Thirty flowers
were then fertilised with pollen from a distinct plant, one of the three
growing together, and they yielded only two capsules. Lastly, five
flowers were fertilised with pollen from a fourth plant growing at a
distance, and all five produced capsules. It seems therefore probable,
as Fritz Muller suggests, that the three plants growing near together
were seedlings from the same parent, and that from being closely related
they had little power of fertilising one another. (8/6. Some remarkable
cases are given in my ‘Variation under Domestication’ chapter 17 2nd
edition volume 2 page 121, of hybrids of Gladiolus and Cistus, any one
of which could be fertilised by pollen from any other, but not by its
own pollen.)

Lastly, the fact of the intercrossed plants in Table 7/A not exceeding
in height the self-fertilised plants in a greater and greater degree in
the later generations, is probably the result of their having become
more and more closely inter-related.

UNIFORM COLOUR OF THE FLOWERS ON PLANTS, SELF-FERTILISED AND GROWN UNDER
SIMILAR CONDITIONS FOR SEVERAL GENERATIONS.

At the commencement of my experiments, the parent-plants of Mimulus
luteus, Ipomoea purpurea, Dianthus caryophyllus, and Petunia violacea,
raised from purchased seeds, varied greatly in the colour of their
flowers. This occurs with many plants which have been long cultivated as
an ornament for the flower-garden, and which have been propagated by
seeds. The colour of the flowers was a point to which I did not at first
in the least attend, and no selection whatever was practised.
Nevertheless, the flowers produced by the self-fertilised plants of the
above four species became absolutely uniform in tint, or very nearly so,
after they had been grown for some generations under closely similar
conditions. The intercrossed plants, which were more or less closely
inter-related in the later generations, and which had been likewise
cultivated all the time under similar conditions, became more uniform in
the colour of their flowers than were the original parent-plants, but
much less so than the self-fertilised plants. When self-fertilised
plants of one of the later generations were crossed with a fresh stock,
and seedlings thus raised, these presented a wonderful contrast in the
diversified tints of their flowers compared with those of the
self-fertilised seedlings. As such cases of flowers becoming uniformly
coloured without any aid from selection seem to me curious, I will give
a full abstract of my observations.

Mimulus luteus.

A tall variety, bearing large, almost white flowers blotched with
crimson, appeared amongst the intercrossed and self-fertilised plants of
the third and fourth generations. This variety increased so rapidly,
that in the sixth generation of self-fertilised plants every single one
consisted of it. So it was with all the many plants which were raised,
up to the last or ninth self-fertilised generation. Although this
variety first appeared amongst the intercrossed plants, yet from their
offspring being intercrossed in each succeeding generation, it never
prevailed amongst them; and the flowers on the several intercrossed
plants of the ninth generation differed considerably in colour. On the
other hand, the uniformity in colour of the flowers on the plants of all
the later self-fertilised generations was quite surprising; on a casual
inspection they might have been said to be quite alike, but the crimson
blotches were not of exactly the same shape, or in exactly the same
position. Both my gardener and myself believe that this variety did not
appear amongst the parent-plants, raised from purchased seeds, but from
its appearance amongst both the crossed and self-fertilised plants of
the third and fourth generations; and from what I have seen of the
variation of this species on other occasions, it is probable that it
would occasionally appear under any circumstances. We learn, however,
from the present case that under the peculiar conditions to which my
plants were subjected, this particular variety, remarkable for its
colouring, largeness of the corolla, and increased height of the whole
plant, prevailed in the sixth and all the succeeding self-fertilised
generations to the complete exclusion of every other variety.

Ipomoea purpurea.

My attention was first drawn to the present subject by observing that
the flowers on all the plants of the seventh self-fertilised generation
were of a uniform, remarkably rich, dark purple tint. The many plants
which were raised during the three succeeding generations, up to the
last or tenth, all produced flowers coloured in the same manner. They
were absolutely uniform in tint, like those of a constant species living
in a state of nature; and the self-fertilised plants might have been
distinguished with certainty, as my gardener remarked, without the aid
of labels, from the intercrossed plants of the later generations. These,
however, had more uniformly coloured flowers than those which were first
raised from the purchased seeds. This dark purple variety did not
appear, as far as my gardener and myself could recollect, before the
fifth or sixth self-fertilised generation. However this may have been,
it became, through continued self-fertilisation and the cultivation of
the plants under uniform conditions, perfectly constant, to the
exclusion of every other variety.

Dianthus caryophyllus.

The self-fertilised plants of the third generation all bore flowers of
exactly the same pale rose-colour; and in this respect they differed
quite remarkably from the plants growing in a large bed close by and
raised from seeds purchased from the same nursery garden. In this case
it is not improbable that some of the parent-plants which were first
self-fertilised may have borne flowers thus coloured; but as several
plants were self-fertilised in the first generation, it is extremely
improbable that all bore flowers of exactly the same tint as those of
the self-fertilised plants of the third generation. The intercrossed
plants of the third generation likewise produced flowers almost, though
not quite so uniform in tint as those of the self-fertilised plants.

Petunia violacea.

In this case I happened to record in my notes that the flowers on the
parent-plant which was first self-fertilised were of a “dingy purple
colour.” In the fifth self-fertilised generation, every one of the
twenty-one self-fertilised plants growing in pots, and all the many
plants in a long row out of doors, produced flowers of absolutely the
same tint, namely, of a dull, rather peculiar and ugly flesh colour;
therefore, considerably unlike those on the parent-plant. I believe that
this change of colour supervened quite gradually; but I kept no record,
as the point did not interest me until I was struck with the uniform
tint of the flowers on the self-fertilised plants of the fifth
generation. The flowers on the intercrossed plants of the corresponding
generation were mostly of the same dull flesh colour, but not nearly so
uniform as those on the self-fertilised plants, some few being very
pale, almost white. The self-fertilised plants which grew in a long row
in the open ground were also remarkable for their uniformity in height,
as were the intercrossed plants in a less degree, both lots being
compared with a large number of plants raised at the same time under
similar conditions from the self-fertilised plants of the fourth
generation crossed by a fresh stock. I regret that I did not attend to
the uniformity in height of the self-fertilised seedlings in the later
generations of the other species.

These few cases seem to me to possess much interest. We learn from them
that new and slight shades of colour may be quickly and firmly fixed,
independently of any selection, if the conditions are kept as nearly
uniform as is possible, and no intercrossing be permitted. With Mimulus,
not only a grotesque style of colouring, but a larger corolla and
increased height of the whole plant were thus fixed; whereas with most
plants which have been long cultivated for the flower-garden, no
character is more variable than that of colour, excepting perhaps that
of height. From the consideration of these cases we may infer that the
variability of cultivated plants in the above respects is due, firstly,
to their being subjected to somewhat diversified conditions, and,
secondly, to their being often intercrossed, as would follow from the
free access of insects. I do not see how this inference can be avoided,
as when the above plants were cultivated for several generations under
closely similar conditions, and were intercrossed in each generation,
the colour of their flowers tended in some degree to change and to
become uniform. When no intercrossing with other plants of the same
stock was allowed,--that is, when the flowers were fertilised with their
own pollen in each generation--their colour in the later generations
became as uniform as that of plants growing in a state of nature,
accompanied at least in one instance by much uniformity in the height of
the plants. But in saying that the diversified tints of the flowers on
cultivated plants treated in the ordinary manner are due to differences
in the soil, climate, etc., to which they are exposed, I do not wish to
imply that such variations are caused by these agencies in any more
direct manner than that in which the most diversified illnesses, as
colds, inflammation of the lungs or pleura, rheumatism, etc., may be
said to be caused by exposure to cold. In both cases the constitution of
the being which is acted on is of preponderant importance.



CHAPTER IX.

THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE
PRODUCTION OF SEEDS.

Fertility of plants of crossed and self-fertilised parentage, both lots
being fertilised in the same manner.
Fertility of the parent-plants when first crossed and self-fertilised,
and of their crossed and self-fertilised offspring when again crossed
and self-fertilised.
Comparison of the fertility of flowers fertilised with their own pollen
and with that from other flowers on the same plant.
Self-sterile plants.
Causes of self-sterility.
The appearance of highly self-fertile varieties.
Self-fertilisation apparently in some respects beneficial, independently
of the assured production of seeds.
Relative weights and rates of germination of seeds from crossed and
self-fertilised flowers.

The present chapter is devoted to the Fertility of plants, as influenced
by cross-fertilisation and self-fertilisation. The subject consists of
two distinct branches; firstly, the relative productiveness or fertility
of flowers crossed with pollen from a distinct plant and with their own
pollen, as shown by the proportional number of capsules which they
produce, together with the number of the contained seeds. Secondly, the
degree of innate fertility or sterility of the seedlings raised from
crossed and self-fertilised seeds; such seedlings being of the same age,
grown under the same conditions, and fertilised in the same manner.
These two branches of the subject correspond with the two which have to
be considered by any one treating of hybrid plants; namely, in the first
place the comparative productiveness of a species when fertilised with
pollen from a distinct species and with its own pollen; and in the
second place, the fertility of its hybrid offspring. These two classes
of cases do not always run parallel; thus some plants, as Gartner has
shown, can be crossed with great ease, but yield excessively sterile
hybrids; while others are crossed with extreme difficulty, but yield
fairly fertile hybrids.

The natural order to follow in this chapter would have been first to
consider the effects on the fertility of the parent-plants of crossing
them, and of fertilising them with their own pollen; but as we have
discussed in the two last chapters the relative height, weight, and
constitutional vigour of crossed and self-fertilised plants--that is, of
plants raised from crossed and self-fertilised seeds--it will be
convenient here first to consider their relative fertility. The cases
observed by me are given in Table 9/D, in which plants of crossed and
self-fertilised parentage were left to fertilise themselves, being
either crossed by insects or spontaneously self-fertilised. It should be
observed that the results cannot be considered as fully trustworthy, for
the fertility of a plant is a most variable element, depending on its
age, health, nature of the soil, amount of water given, and temperature
to which it is exposed. The number of the capsules produced and the
number of the contained seeds, ought to have been ascertained on a large
number of crossed and self-fertilised plants of the same age and treated
in every respect alike. In these two latter respects my observations may
be trusted, but a sufficient number of capsules were counted only in a
few instances. The fertility, or as it may perhaps better be called the
productiveness, of a plant depends on the number of capsules produced,
and on the number of seeds which these contain. But from various causes,
chiefly from the want of time, I was often compelled to rely on the
number of the capsules alone. Nevertheless, in the more interesting
cases, the seeds were also counted or weighed. The average number of
seeds per capsule is a more valuable criterion of fertility than the
number of capsules produced. This latter circumstance depends partly on
the size of the plant; and we know that crossed plants are generally
taller and heavier than the self-fertilised; but the difference in this
respect is rarely sufficient to account for the difference in the number
of the capsules produced. It need hardly be added that in Table 9/D the
same number of crossed and self-fertilised plants are always compared.
Subject to the foregoing sources of doubt I will now give the table, in
which the parentage of the plants experimented on, and the manner of
determining their fertility are explained. Fuller details may be found
in the previous part of this work, under the head of each species.

TABLE 9/D.--RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED
PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY
JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression, as 100 to x.

Ipomoea purpurea--first generation: seeds per capsule on crossed and
self-fertilised plants, not growing much crowded, spontaneously
self-fertilised under a net, in number: 99.

Ipomoea purpurea--seeds per capsule on crossed and self-fertilised
plants from the same parents as in the last case, but growing much
crowded, spontaneously self-fertilised under a net, in number: 93.

Ipomoea purpurea--productiveness of the same plants, as judged by the
number of capsules produced, and average number of seeds per capsule:
45.

Ipomoea purpurea--third generation: seeds per capsule on crossed and
self-fertilised plants, spontaneously self-fertilised under a net, in
number: 94.

Ipomoea purpurea--productiveness of the same plants, as judged by the
number of capsules produced, and the average number of seeds per
capsule: 35.

Ipomoea purpurea--fifth generation: seeds per capsule on crossed and
self-fertilised plants, left uncovered in the hothouse, and
spontaneously fertilised: 89.

Ipomoea purpurea--ninth generation: number of capsules on crossed plants
to those on self-fertilised plants, spontaneously self-fertilised under
a net: 26.

Mimulus luteus--an equal number of capsules on plants descended from
self-fertilised plants of the 8th generation crossed by a fresh stock,
and on plants of the 9th self-fertilised generation, both sets having
been left uncovered and spontaneously fertilised, contained seeds, by
weight: 30.

Mimulus luteus--productiveness of the same plants, as judged by the
number of capsules produced, and the average weight of seeds per
capsule: 3.

Vandellia nummularifolia--seeds per capsule from cleistogene flowers on
the crossed and self-fertilised plants, in number: 106.

Salvia coccinea--crossed plants, compared with self-fertilised plants,
produced flowers, in number: 57.

Iberis umbellata--plants left uncovered in greenhouse; intercrossed
plants of the 3rd generation, compared with self-fertilised plants of
the 3rd generation, yielded seeds, in number: 75.

Iberis umbellata--plants from a cross between two varieties, compared
with self-fertilised plants of the 3rd generation, yielded seeds, by
weight : 75.

Papaver vagum--crossed and self-fertilised plants, left uncovered,
produced capsules, in number: 99.

Eschscholtzia californica--Brazilian stock; plants left uncovered and
cross-fertilised by bees; capsules on intercrossed plants of the 2nd
generation, compared with capsules on self-fertilised plants of 2nd
generation, contained seeds, in number: 78.

Eschscholtzia californica--productiveness of the same plants, as judged
by the number of capsules produced, and the average number of seeds per
capsule: 89.

Eschscholtzia californica--plants left uncovered and cross-fertilised by
bees; capsules on plants derived from intercrossed plants of the 2nd
generation of the Brazilian stock crossed by English stock, compared
with capsules on self-fertilised plants of 2nd generation, contained
seeds, in number: 63.

Eschscholtzia californica--productiveness of the same plants, as judged
by the number of capsules produced, and the average number of seeds per
capsule: 40.

Reseda odorata--crossed and self-fertilised plants, left uncovered and
cross-fertilised by bees; produced capsules in number (about): 100.

Viola tricolor--crossed and self-fertilised plants, left uncovered and
cross-fertilised by bees, produced capsules in number: 10.

Delphinium consolida--crossed and self-fertilised plants, left uncovered
in the greenhouse, produced capsules in number: 56.

Viscaria oculata--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 77.

Dianthus caryophyllus--plants spontaneously self-fertilised under a net;
capsules on intercrossed and self-fertilised plants of the 3rd
generation contained seeds in number: 125.

Dianthus caryophyllus--plants left uncovered and cross-fertilised by
insects: offspring from plants self-fertilised for three generations and
then crossed by an intercrossed plant of the same stock, compared with
plants of the 4th self-fertilised generation, produced seeds by weight:
73.

Dianthus caryophyllus--plants left uncovered and cross-fertilised by
insects: offspring from plants self-fertilised for three generations and
then crossed by a fresh stock, compared with plants of the 4th
self-fertilised generation, produced seeds by weight: 33.

Tropaeolum minus--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced seeds in number: 64.

Limnanthes douglasii--crossed and self-fertilised plants, left uncovered
in the greenhouse, produced capsules in number (about): 100.

Lupinus luteus--crossed and self-fertilised plants of the 2nd
generation, left uncovered in the greenhouse, produced seeds in number
(judged from only a few pods): 88.

Phaseolus multiflorus--crossed and self-fertilised plants, left
uncovered in the greenhouse, produced seeds in number (about): 100.

Lathyrus odoratus--crossed and self-fertilised plants of the 2nd
generation, left uncovered in the greenhouse, but certainly
self-fertilised, produced pods in number: 91.

Clarkia elegans--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 60.

Nemophila insignis--crossed and self-fertilised plants, covered by a net
and spontaneously self-fertilised in the greenhouse, produced capsules
in number: 29.

Petunia violacea--left uncovered and cross-fertilised by insects: plants
of the 5th intercrossed and self-fertilised generations produced seeds,
as judged by the weight of an equal number of capsules: 86.

Petunia violacea--left uncovered as above: offspring of plants
self-fertilised for four generations and then crossed by a fresh stock,
compared with plants of the 5th self-fertilised generation, produced
seeds, as judged by the weight of an equal number of capsules: 46.

Cyclamen persicum--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 12.

Anagallis collina--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 8.

Primula veris--left uncovered in open ground and cross-fertilised by
insects: offspring from plants of the 3rd illegitimate generation
crossed by a fresh stock, compared with plants of the 4th illegitimate
and self-fertilised generation, produced capsules in number: 5.

Same plants in the following year: 3.5.

Primula veris--(equal-styled variety): left uncovered in open ground and
cross-fertilised by insects: offspring from plants self-fertilised for
two generations and then crossed by another variety, compared with
plants of the 3rd self-fertilised generation, produced capsules in
number: 15.

Primula veris--(equal-styled variety) same plants; average number of
seeds per capsule: 71.

Primula veris--(equal-styled variety) productiveness of the same plants,
as judged by the number of capsules produced and the average number of
seeds per capsule: 11.

This table includes thirty-three cases relating to twenty-three species,
and shows the degree of innate fertility of plants of crossed parentage
in comparison with those of self-fertilised parentage; both lots being
fertilised in the same manner. With several of the species, as with
Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots
were certainly cross-fertilised by insects, and so it probably was with
several of the others; but in some of the species, as with Nemophila,
and in some of the trials with Ipomoea and Dianthus, the plants were
covered up, and both lots were spontaneously self-fertilised. This also
was necessarily the case with the capsules produced by the cleistogene
flowers of Vandellia.

The fertility of the crossed plants is represented in Table 9/D by 100,
and that of the self-fertilised by the other figures. There are five
cases in which the fertility of the self-fertilised plants is
approximately equal to that of the crossed; nevertheless, in four of
these cases the crossed plants were plainly taller, and in the fifth
somewhat taller than the self-fertilised. But I should state that in
some of these five cases the fertility of the two lots was not strictly
ascertained, as the capsules were not actually counted, from appearing
equal in number and from all apparently containing a full complement of
seeds. In only two instances in the table, namely, with Vandellia and in
the third generation of Dianthus, the capsules on the self-fertilised
plants contained more seed than those on the crossed plants. With
Dianthus the ratio between the number of seeds contained in the
self-fertilised and crossed capsules was as 125 to 100; both sets of
plants were left to fertilise themselves under a net; and it is almost
certain that the greater fertility of the self-fertilised plants was
here due merely to their having varied and become less strictly
dichogamous, so as to mature their anthers and stigmas more nearly at
the same time than is proper to the species. Excluding the seven cases
now referred to, there remain twenty-six in which the crossed plants
were manifestly much more fertile, sometimes to an extraordinary degree,
than the self-fertilised with which they grew in competition. The most
striking instances are those in which plants derived from a cross with a
fresh stock are compared with plants of one of the later self-fertilised
generations; yet there are some striking cases, as that of Viola,
between the intercrossed plants of the same stock and the
self-fertilised, even in the first generation. The results most to be
trusted are those in which the productiveness of the plants was
ascertained by the number of capsules produced by an equal number of
plants, together with the actual or average number of seeds in each
capsule. Of such cases there are twelve in the table, and the mean of
their mean fertility is as 100 for the crossed plants, to 59 for the
self-fertilised plants. The Primulaceae seem eminently liable to suffer
in fertility from self-fertilisation.

The following short table, Table 9/E, includes four cases which have
already been partly given in the last table.

TABLE 9/E.--INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK,
COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH
THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION.
FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN
EQUAL NUMBER OF PLANTS.

Column 1: Name of plant and feature observed.

Column 2: Plants from a cross with a fresh stock.

Column 3: Intercrossed plants of the same stock.

Column 4: Self-fertilised plants.

Mimulus luteus--the intercrossed plants are derived from a cross between
two plants of the 8th self-fertilised generation. The self-fertilised
plants belong to the 9th generation: 100 : 4 : 3.

Eschscholtzia californica--the intercrossed and self-fertilised plants
belong to the 2nd generation: 100 : 45 : 40.

Dianthus caryophyllus--the intercrossed plants are derived from
self-fertilised of the 3rd generation, crossed by intercrossed plants of
the 3rd generation. The self-fertilised plants belong to the 4th
generation: 100 : 45 : 33.

Petunia violacea--the intercrossed and self-fertilised plants belong to
the 5th generation: 100 : 54 : 46.

NB.--In the above cases, excepting in that of Eschscholtzia, the plants
derived from a cross with a fresh stock belong on the mother-side to the
same stock with the intercrossed and self-fertilised plants, and to the
corresponding generation.

These cases show us how greatly superior in innate fertility the
seedlings from plants self-fertilised or intercrossed for several
generations and then crossed by a fresh stock are, in comparison with
the seedlings from plants of the old stock, either intercrossed or
self-fertilised for the same number of generations. The three lots of
plants in each case were left freely exposed to the visits of insects,
and their flowers without doubt were cross-fertilised by them.

Table 9/E further shows us that in all four cases the intercrossed
plants of the same stock still have a decided though small advantage in
fertility over the self-fertilised plants.

With respect to the state of the reproductive organs in the
self-fertilised plants of Tables 9/D and 9/E, only a few observations
were made. In the seventh and eighth generation of Ipomoea, the anthers
in the flowers of the self-fertilised plants were plainly smaller than
those in the flowers of the intercrossed plants. The tendency to
sterility in these same plants was also shown by the first-formed
flowers, after they had been carefully fertilised, often dropping off,
in the same manner as frequently occurs with hybrids. The flowers
likewise tended to be monstrous. In the fourth generation of Petunia,
the pollen produced by the self-fertilised and intercrossed plants was
compared, and they were far more empty and shrivelled grains in the
former.

RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT
AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE
PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE
FIRST OR A SUCCEEDING GENERATION.

I will first treat of the parent-plants, which were raised from seeds
purchased from nursery-gardens, or taken from plants growing in my
garden, or growing wild, and surrounded in every case by many
individuals of the same species. Plants thus circumstanced will commonly
have been intercrossed by insects; so that the seedlings which were
first experimented on will generally have been the product of a cross.
Consequently any difference in the fertility of their flowers, when
crossed and self-fertilised, will have been caused by the nature of the
pollen employed; that is, whether it was taken from a distinct plant or
from the same flower. The degrees of fertility shown in Table 9/F, were
determined in each case by the average number of seeds per capsule,
ascertained either by counting or weighing.

Another element ought properly to have been taken into account, namely,
the proportion of flowers which yielded capsules when they were crossed
and self-fertilised; and as crossed flowers generally produce a larger
proportion of capsules, their superiority in fertility, if this element
had been taken into account, would have been much more strongly marked
than appears in Table 9/F. But had I thus acted, there would have been
greater liability to error, as pollen applied to the stigma at the wrong
time fails to produce any effect, independently of its greater or less
potency. A good illustration of the great difference in the results
which sometimes follows, if the number of capsules produced relatively
to the number of flowers fertilised be included in the calculation, was
afforded by Nolana prostrata. Thirty flowers on some plants of this
species were crossed and produced twenty-seven capsules, each containing
five seeds; thirty-two flowers on the same plants were self-fertilised
and produced only six capsules, each containing five seeds. As the
number of seeds per capsule is here the same, the fertility of the
crossed and self-fertilised flowers is given in Table 9/F as equal, or
as 100 to 100. But if the flowers which failed to produce capsules be
included, the crossed flowers yielded on an average 4.50 seeds, whilst
the self-fertilised flowers yielded only 0.94 seeds, so that their
relative fertility would have been as 100 to 21. I should here state
that it has been found convenient to reserve for separate discussion the
cases of flowers which are usually quite sterile with their own pollen.

TABLE 9/f.--relative fertility of the flowers on the parent-plants used
in my experiments, when fertilised with pollen from a distinct plant and
with their own pollen. Fertility judged of by the average number of
seeds per capsule. Fertility of crossed flowers taken as 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression 100 to x.

Ipomoea purpurea--crossed and self-fertilised flowers yielded seeds as
(about): 100.

Mimulus luteus--crossed and self-fertilised flowers yielded seeds as (by
weight): 79.

Linaria vulgaris--crossed and self-fertilised flowers yielded seeds as:
14.

Vandellia nummularifolia--crossed and self-fertilised flowers yielded
seeds as: 67?

Gesneria pendulina--crossed and self-fertilised flowers yielded seeds as
(by weight): 100.

Salvia coccinea--crossed and self-fertilised flowers yielded seeds as
(about): 100.

Brassica oleracea--crossed and self-fertilised flowers yielded seeds as:
25.

Eschscholtzia californica--(English stock) crossed and self-fertilised
flowers yielded seeds as (by weight): 71.

Eschscholtzia californica--(Brazilian stock grown in England) crossed
and self-fertilised flowers yielded seeds (by weight) as (about): 15.

Delphinium consolida--crossed and self-fertilised flowers
(self-fertilised capsules spontaneously produced, but result supported
by other evidence) yielded seeds as: 59.

Viscaria oculata--crossed and self-fertilised flowers yielded seeds as
(by weight): 38.

Viscaria oculata--crossed and self-fertilised flowers (crossed capsules
compared on following year with spontaneously self-fertilised capsules)
yielded seeds as : 58.

Dianthus caryophyllus--crossed and self-fertilised flowers yielded seeds
as: 92.

Tropaeolum minus--crossed and self-fertilised flowers yielded seeds as:
92.

Tropaeolum tricolorum--crossed and self-fertilised flowers yielded seeds
as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been
introduced into this table, although seedlings were not raised from
them; but of the Cuphea only six crossed and six self-fertilised
capsules, and of the Tropaeolum only six crossed and eleven
self-fertilised capsules, were compared. A larger proportion of the
self-fertilised than of the crossed flowers of the Tropaeolum produced
fruit.)

Limnanthes douglasii--crossed and self-fertilised flowers yielded seeds
as (about): 100.

Sarothamnus scoparius--crossed and self-fertilised flowers yielded seeds
as: 41.

Ononis minutissima--crossed and self-fertilised flowers yielded seeds
as: 65.

Cuphea purpurea--crossed and self-fertilised flowers yielded seeds as:
113.

Passiflora gracilis--crossed and self-fertilised flowers yielded seeds
as: 85.

Specularia speculum--crossed and self-fertilised flowers yielded seeds
as: 72.

Lobelia fulgens--crossed and self-fertilised flowers yielded seeds as
(about): 100.

Nemophila insignis--crossed and self-fertilised flowers yielded seeds as
(by weight): 69.

Borago officinalis--crossed and self-fertilised flowers yielded seeds
as: 60.

Nolana prostrata--crossed and self-fertilised flowers yielded seeds as:
100.

Petunia violacea--crossed and self-fertilised flowers yielded seeds as
(by weight): 67.

Nicotiana tabacum--crossed and self-fertilised flowers yielded seeds as
(by weight): 150.

Cyclamen persicum--crossed and self-fertilised flowers yielded seeds as:
38.

Anagallis collina--crossed and self-fertilised flowers yielded seeds as:
96.

Canna warscewiczi--crossed and self-fertilised flowers (on three
generations of crossed and self-fertilised plants taken all together)
yielded seeds as: 85.

Table 9/G gives the relative fertility of flowers on crossed plants
again cross-fertilised, and of flowers on self-fertilised plants again
self-fertilised, either in the first or in a later generation. Here two
causes combine to diminish the fertility of the self-fertilised flowers;
namely, the lesser efficacy of pollen from the same flower, and the
innate lessened fertility of plants derived from self-fertilised seeds,
which as we have seen in the previous Table 9/D is strongly marked. The
fertility was determined in the same manner as in Table 9/F, that is, by
the average number of seeds per capsule; and the same remarks as before,
with respect to the different proportion of flowers which set capsules
when they are cross-fertilised and self-fertilised, are here likewise
applicable.

TABLE 9/G.--RELATIVE FERTILITY OF FLOWERS ON CROSSED AND SELF-FERTILISED
PLANTS OF THE FIRST OR SOME SUCCEEDING GENERATION; THE FORMER BEING
AGAIN FERTILISED WITH POLLEN FROM A DISTINCT PLANT, AND THE LATTER AGAIN
WITH THEIR OWN POLLEN. FERTILITY JUDGED OF BY THE AVERAGE NUMBER OF
SEEDS PER CAPSULE. FERTILITY OF CROSSED FLOWERS TAKEN AS 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression, 100 to x.

Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the first generation yielded seeds as: 93.

Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 3rd generation yielded seeds as: 94.

Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as: 94.

Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 5th generation yielded seeds as: 107.

Mimulus luteus--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 3rd generation yielded seeds as (by
weight): 65.

Mimulus luteus--same plants of the 3rd generation treated in the same
manner on the following year yielded seeds as (by weight): 34.

Mimulus luteus--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as (by
weight): 40.

Viola tricolor--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as: 69.

Dianthus caryophyllus--crossed and self-fertilised flowers on the
crossed and self-fertilised plants of the 1st generation yielded seeds
as: 65.

Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd
generation crossed by intercrossed plants, and other flowers again
self-fertilised yielded seeds as: 97.

Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as: 127.

Lathytus odoratus--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of the 1st generation yielded seeds as: 65.

Lobelia ramosa--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as (by
weight): 60.

Petunia violacea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as (by
weight): 68.

Petunia violacea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as (by
weight): 72.

Petunia violacea--flowers on self-fertilised plants of the 4th
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as (by weight): 48.

Nicotiana tabacum--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of the 1st generation yielded seeds as (by
weight): 97.

Nicotiana tabacum--flowers on self-fertilised plants of the 2nd
generation crossed by intercrossed plants, and other flowers again
self-fertilised yielded seeds as (by estimation): 110.

Nicotiana tabacum--flowers on self-fertilised plants of the 3rd
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as (by estimation): 110.

Anagallis collina--flowers on red variety crossed by a blue variety, and
other flowers on the red variety self-fertilised yielded seeds as: 48.

Canna warscewiczi--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of three generations taken together yielded
seeds as: 85.

As both these tables relate to the fertility of flowers fertilised by
pollen from another plant and by their own pollen, they may be
considered together. The difference between them consists in the
self-fertilised flowers in Table 9/G, being produced by self-fertilised
parents, and the crossed flowers by crossed parents, which in the later
generations had become somewhat closely inter-related, and had been
subjected all the time to nearly the same conditions. These two tables
include fifty cases relating to thirty-two species. The flowers on many
other species were crossed and self-fertilised, but as only a few were
thus treated, the results cannot be trusted, as far as fertility is
concerned, and are not here given. Some other cases have been rejected,
as the plants were in an unhealthy condition. If we look to the figures
in the two tables expressing the ratios between the mean relative
fertility of the crossed and self-fertilised flowers, we see that in a
majority of cases (i.e., in thirty-five out of fifty) flowers fertilised
by pollen from a distinct plant yield more, sometimes many more, seeds
than flowers fertilised with their own pollen; and they commonly set a
larger proportion of capsules. The degree of infertility of the
self-fertilised flowers differs extremely in the different species, and
even, as we shall see in the section on self-sterile plants, in the
individuals of the same species, as well as under slightly changed
conditions of life. Their fertility ranges from zero to fertility
equalling that of the crossed flowers; and of this fact no explanation
can be offered. There are fifteen cases in the two tables in which the
number of seeds per capsule produced by the self-fertilised flowers
equals or even exceeds that yielded by the crossed flowers. Some few of
these cases are, I believe, accidental; that is, would not recur on a
second trial. This was apparently the case with the plants of the fifth
generation of Ipomoea, and in one of the experiments with Dianthus.
Nicotiana offers the most anomalous case of any, as the self-fertilised
flowers on the parent-plants, and on their descendants of the second and
third generations, produced more seeds than did the crossed flowers; but
we shall recur to this case when we treat of highly self-fertile
varieties.

It might have been expected that the difference in fertility between the
crossed and self-fertilised flowers would have been more strongly marked
in Table 9/G, in which the plants of one set were derived from
self-fertilised parents, than in Table 9/F, in which flowers on the
parent-plants were self-fertilised for the first time. But this is not
the case, as far as my scanty materials allow of any judgment. There is
therefore no evidence at present, that the fertility of plants goes on
diminishing in successive self-fertilised generations, although there is
some rather weak evidence that this does occur with respect to their
height or growth. But we should bear in mind that in the later
generations the crossed plants had become more or less closely
inter-related, and had been subjected all the time to nearly uniform
conditions.

It is remarkable that there is no close correspondence, either in the
parent-plants or in the successive generations, between the relative
number of seeds produced by the crossed and self-fertilised flowers, and
the relative powers of growth of the seedlings raised from such seeds.
Thus, the crossed and self-fertilised flowers on the parent-plants of
Ipomoea, Gesneria, Salvia, Limnanthes, Lobelia fulgens, and Nolana
produced a nearly equal number of seeds, yet the plants raised from the
crossed seeds exceeded considerably in height those raised from the
self-fertilised seeds. The crossed flowers of Linaria and Viscaria
yielded far more seeds than the self-fertilised flowers; and although
the plants raised from the former were taller than those from the
latter, they were not so in any corresponding degree. With Nicotiana the
flowers fertilised with their own pollen were more productive than those
crossed with pollen from a slightly different variety; yet the plants
raised from the latter seeds were much taller, heavier, and more hardy
than those raised from the self-fertilised seeds. On the other hand, the
crossed seedlings of Eschscholtzia were neither taller nor heavier than
the self-fertilised, although the crossed flowers were far more
productive than the self-fertilised. But the best evidence of a want of
correspondence between the number of seeds produced by crossed and
self-fertilised flowers, and the vigour of the offspring raised from
them, is afforded by the plants of the Brazilian and European stocks of
Eschscholtzia, and likewise by certain individual plants of Reseda
odorata; for it might have been expected that the seedlings from plants,
the flowers of which were excessively self-sterile, would have profited
in a greater degree by a cross, than the seedlings from plants which
were moderately or fully self-fertile, and therefore apparently had no
need to be crossed. But no such result followed in either case: for
instance, the crossed and self-fertilised offspring from a highly
self-fertile plant of Reseda odorata were in average height to each
other as 100 to 82; whereas the similar offspring from an excessively
self-sterile plant were as 100 to 92 in average height.

With respect to the innate fertility of the plants of crossed and
self-fertilised parentage, given in the previous Table 9/D--that is, the
number of seeds produced by both lots when their flowers were fertilised
in the same manner,--nearly the same remarks are applicable, in
reference to the absence of any close correspondence between their
fertility and powers of growth, as in the case of the plants in the
Tables 9/F and 9/G, just considered. Thus the crossed and
self-fertilised plants of Ipomoea, Papaver, Reseda odorata, and
Limnanthes were almost equally fertile, yet the former exceeded
considerably in height the self-fertilised plants. On the other hand,
the crossed and self-fertilised plants of Mimulus and Primula differed
to an extreme degree in innate fertility, but by no means to a
corresponding degree in height or vigour.

In all the cases of self-fertilised flowers included in Tables 9/E, 9/F,
and 9/G, these were fertilised with their own pollen; but there is
another form of self-fertilisation, namely, by pollen from other flowers
on the same plant; but this latter method made no difference in
comparison with the former in the number of seeds produced, or only a
slight difference. Neither with Digitalis nor Dianthus were more seeds
produced by the one method than by the other, to any trustworthy degree.
With Ipomoea rather more seeds, in the proportion of 100 to 91, were
produced from a crossed between flowers on the same plant than from
strictly self-fertilised flowers; but I have reason to suspect that the
result was accidental. With Origanum vulgare, however, a cross between
flowers on plants propagated by stolons from the same stock certainly
increased slightly their fertility. This likewise occurred, as we shall
see in the next section, with Eschscholtzia, perhaps with Corydalis cava
and Oncidium; but not so with Bignonia, Abutilon, Tabernaemontana,
Senecio, and apparently Reseda odorata.

SELF-STERILE PLANTS.

The cases here to be described might have been introduced in Table 9/F,
which gives the relative fertility of flowers fertilised with their own
pollen, and with that from a distinct plant, but it has been found more
convenient to keep them for separate discussion. The present cases must
not be confounded with those to be given in the next chapter relatively
to flowers which are sterile when insects are excluded; for such
sterility depends not merely on the flowers being incapable of
fertilisation with their own pollen, but on mechanical causes, by which
their pollen is prevented from reaching the stigma, or on the pollen and
stigma of the same flower being matured at different periods.

In the seventeenth chapter of my ‘Variation of Animals and Plants under
Domestication’ I had occasion to enter fully on the present subject; and
I will therefore here give only a brief abstract of the cases there
described, but others must be added, as they have an important bearing
on the present work. Kolreuter long ago described plants of Verbascum
phoeniceum which during two years were sterile with their own pollen,
but were easily fertilised by that of four other species; these plants
however afterwards became more or less self-fertile in a strangely
fluctuating manner. Mr. Scott also found that this species, as well as
two of its varieties, were self-sterile, as did Gartner in the case of
Verbascum nigrum. So it was, according to this latter author, with two
plants of Lobelia fulgens, though the pollen and ovules of both were in
an efficient state in relation to other species. Five species of
Passiflora and certain individuals of a sixth species have been found
sterile with their own pollen; but slight changes in their conditions,
such as being grafted on another stock or a change of temperature,
rendered them self-fertile. Flowers on a completely self-impotent plant
of Passiflora alata fertilised with pollen from its own self-impotent
seedlings were quite fertile. Mr. Scott, and afterwards Mr. Munro, found
that some species of Oncidium and of Maxillaria cultivated in a hothouse
in Edinburgh were quite sterile with their own pollen; and Fritz Muller
found this to be the case with a large number of Orchidaceous genera
growing in their native home of South Brazil. (9/2. ‘Botanische Zeitung’
1868 page 114.) He also discovered that the pollen-masses of some
orchids acted on their own stigmas like a poison; and it appears that
Gartner formerly observed indications of this extraordinary fact in the
case of some other plants.

Fritz Muller also states that a species of Bignonia and Tabernaemontana
echinata are both sterile with their own pollen in their native country
of Brazil. (9/3. Ibid 1868 page 626 and 1870 page 274.) Several
Amaryllidaceous and Liliaceous plants are in the same predicament.
Hildebrand observed with care Corydalis cava, and found it completely
self-sterile (9/4. ‘Report of the International Horticultural Congress’
1866.); but according to Caspary a few self-fertilised seeds are
occasionally produced: Corydalis halleri is only slightly self-sterile,
and C. intermedia not at all so. (9/5. ‘Botanische Zeitung’ June 27,
1873.) In another Fumariaceous genus, Hypecoum, Hildebrand observed that
H. grandiflorum was highly self-sterile, whilst H. procumbens was fairly
self-fertile. (9/6. ‘Jahrb. fur wiss. Botanik’ B. 7 page 464.)
Thunbergia alata kept by me in a warm greenhouse was self-sterile early
in the season, but at a later period produced many spontaneously
self-fertilised fruits. So it was with Papaver vagum: another species,
P. alpinum, was found by Professor H. Hoffmann to be quite self-sterile
excepting on one occasion (9/7. ‘Zur Speciesfrage’ 1875 page 47.);
whilst P. somniferum has been with me always completely self-sterile.

Eschscholtzia californica.

This species deserves a fuller consideration. A plant cultivated by
Fritz Muller in South Brazil happened to flower a month before any of
the others, and it did not produce a single capsule. This led him to
make further observations during the next six generations, and he found
that all his plants were completely sterile, unless they were crossed by
insects or were artificially fertilised with pollen from a distinct
plant, in which case they were completely fertile. (9/8. ‘Botanische
Zeitung’ 1868 page 115 and 1869 page 223.) I was much surprised at this
fact, as I had found that English plants, when covered by a net, set a
considerable number of capsules; and that these contained seeds by
weight, compared with those on plants intercrossed by the bees, as 71 to
100. Professor Hildebrand, however, found this species much more
self-sterile in Germany than it was with me in England, for the capsules
produced by self-fertilised flowers, compared with those from
intercrossed flowers, contained seeds in the ratio of only 11 to 100. At
my request Fritz Muller sent me from Brazil seeds of his self-sterile
plants, from which I raised seedlings. Two of these were covered with a
net, and one produced spontaneously only a single capsule containing no
good seeds, but yet, when artificially fertilised with its own pollen,
produced a few capsules. The other plant produced spontaneously under
the net eight capsules, one of which contained no less than thirty
seeds, and on an average about ten seeds per capsule. Eight flowers on
these two plants were artificially self-fertilised, and produced seven
capsules, containing on an average twelve seeds; eight other flowers
were fertilised with pollen from a distinct plant of the Brazilian
stock, and produced eight capsules, containing on an average about
eighty seeds: this gives a ratio of 15 seeds for the self-fertilised
capsules to 100 for the crossed capsules. Later in the season twelve
other flowers on these two plants were artificially self-fertilised; but
they yielded only two capsules, containing three and six seeds. It
appears therefore that a lower temperature than that of Brazil favours
the self-fertility of this plant, whilst a still lower temperature
lessens it. As soon as the two plants which had been covered by the net
were uncovered, they were visited by many bees,and it was interesting to
observe how quickly they became, even the more sterile plant of the two,
covered with young capsules. On the following year eight flowers on
plants of the Brazilian stock of self-fertilised parentage (i.e.,
grandchildren of the plants which grew in Brazil) were again
self-fertilised, and produced five capsules, containing on an average
27.4 seeds, with a maximum in one of forty-two seeds; so that their
self-fertility had evidently increased greatly by being reared for two
generations in England. On the whole we may conclude that plants of the
Brazilian stock are much more self-fertile in this country than in
Brazil, and less so than plants of the English stock in England; so that
the plants of Brazilian parentage retained by inheritance some of their
former sexual constitution. Conversely, seeds from English plants sent
by me to Fritz Muller and grown in Brazil, were much more self-fertile
than his plants which had been cultivated there for several generations;
but he informs me that one of the plants of English parentage which did
not flower the first year, and was thus exposed for two seasons to the
climate of Brazil, proved quite self-sterile, like a Brazilian plant,
showing how quickly the climate had acted on its sexual constitution.

Abutilon darwinii.

Seeds of this plant were sent me by Fritz Muller, who found it, as well
as some other species of the same genus, quite sterile in its native
home of South Brazil, unless fertilised with pollen from a distinct
plant, either artificially or naturally by humming-birds. (9/9.
‘Jenaische Zeitschr. fur Naturwiss’ B. 7 1872 page 22 and 1873 page
441.) Several plants were raised from these seeds and kept in the
hothouse. They produced flowers very early in the spring, and twenty of
them were fertilised, some with pollen from the same flower, and some
with pollen from other flowers on the same plants; but not a single
capsule was thus produced, yet the stigmas twenty-seven hours after the
application of the pollen were penetrated by the pollen-tubes. At the
same time nineteen flowers were crossed with pollen from a distinct
plant, and these produced thirteen capsules, all abounding with fine
seeds. A greater number of capsules would have been produced by the
cross, had not some of the nineteen flowers been on a plant which was
afterwards proved to be from some unknown cause completely sterile with
pollen of any kind. Thus far these plants behaved exactly like those in
Brazil; but later in the season, in the latter part of May and in June,
they began to produce under a net a few spontaneously self-fertilised
capsules. As soon as this occurred, sixteen flowers were fertilised with
their own pollen, and these produced five capsules, containing on an
average 3.4 seeds. At the same time I selected by chance four capsules
from the uncovered plants growing close by, the flowers of which I had
seen visited by humble-bees, and these contained on an average 21.5
seeds; so that the seeds in the naturally intercrossed capsules to those
in the self-fertilised capsules were as 100 to 16. The interesting point
in this case is that these plants, which were unnaturally treated by
being grown in pots in a hothouse, under another hemisphere, with a
complete reversal of the seasons, were thus rendered slightly
self-fertile, whereas they seem always to be completely self-sterile in
their native home.

Senecio cruentus (greenhouse varieties, commonly called Cinerarias,
probably derived from several fruticose or herbaceous species much
intercrossed (9/10. I am much obliged to Mr. Moore and to Mr. Thiselton
Dyer for giving me information with respect to the varieties on which I
experimented. Mr. Moore believes that Senecio cruentas, tussilaginis,
and perhaps heritieri, maderensis and populifolius have all been more or
less blended together in our Cinerarias.))

Two purple-flowered varieties were placed under a net in the greenhouse,
and four corymbs on each were repeatedly brushed with flowers from the
other plant, so that their stigmas were well covered with each other’s
pollen. Two of the eight corymbs thus treated produced very few seeds,
but the other six produced on an average 41.3 seeds per corymb, and
these germinated well. The stigmas on four other corymbs on both plants
were well smeared with pollen from the flowers on their own corymbs;
these eight corymbs produced altogether ten extremely poor seeds, which
proved incapable of germinating. I examined many flowers on both plants,
and found the stigmas spontaneously covered with pollen; but they
produced not a single seed. These plants were afterwards left uncovered
in the same house where many other Cinerarias were in flower; and the
flowers were frequently visited by bees. They then produced plenty of
seed, but one of the two plants less than the other, as this species
shows some tendency to be dioecious.

The trial was repeated on another variety with white petals tipped with
red. Many stigmas on two corymbs were covered with pollen from the
foregoing purple variety, and these produced eleven and twenty-two
seeds, which germinated well. A large number of the stigmas on several
of the other corymbs were repeatedly smeared with pollen from their own
corymb; but they yielded only five very poor seeds, which were incapable
of germination. Therefore the above three plants belonging to two
varieties, though growing vigorously and fertile with pollen from either
of the other two plants, were utterly sterile with pollen from other
flowers on the same plant.

Reseda odorata.

Having observed that certain individuals were self-sterile, I covered
during the summer of 1868 seven plants under separate nets, and will
call these plants A, B, C, D, E, F, G. They all appeared to be quite
sterile with their own pollen, but fertile with that of any other plant.

Fourteen flowers on A were crossed with pollen from B or C, and produced
thirteen fine capsules. Sixteen flowers were fertilised with pollen from
other flowers on the same plant, but yielded not a single capsule.

Fourteen flowers on B were crossed with pollen from A, C or D, and all
produced capsules; some of these were not very fine, yet they contained
plenty of seeds. Eighteen flowers were fertilised with pollen from other
flowers on the same plant, and produced not one capsule.

Ten flowers on C were crossed with pollen from A, B, D or E, and
produced nine fine capsules. Nineteen flowers were fertilised with
pollen from other flowers on the same plant, and produced no capsules.

Ten flowers on D were crossed with pollen from A, B, C or E, and
produced nine fine capsules. Eighteen flowers were fertilised with
pollen from other flowers on the same plant, and produced no capsules.

Seven flowers on E were crossed with pollen from A, C, or D, and all
produced fine capsules. Eight flowers were fertilised with pollen from
other flowers on the same plant, and produced no capsules.

On the plants F and G no flowers were crossed, but very many (number not
recorded) were fertilised with pollen from other flowers on the same
plants, and these did not produce a single capsule.

We thus see that fifty-five flowers on five of the above plants were
reciprocally crossed in various ways; several flowers on each of these
plants being fertilised with pollen from several of the other plants.
These fifty-five flowers produced fifty-two capsules, almost all of
which were of full size and contained an abundance of seeds. On the
other hand, seventy-nine flowers (besides many others not recorded) were
fertilised with pollen from other flowers on the same plants, and these
did not produce a single capsule. In one case in which I examined the
stigmas of the flowers fertilised with their own pollen, these were
penetrated by the pollen-tubes, although such penetration produced no
effect. Pollen falls generally, and I believe always, from the anthers
on the stigmas of the same flower; yet only three out of the above seven
protected plants produced spontaneously any capsules, and these it might
have been thought must have been self-fertilised. There were altogether
seven such capsules; but as they were all seated close to the
artificially crossed flowers, I can hardly doubt that a few grains of
foreign pollen had accidentally fallen on their stigmas. Besides the
above seven plants, four others were kept covered under the SAME large
net; and some of these produced here and there in the most capricious
manner little groups of capsules; and this makes me believe that a bee,
many of which settled on the outside of the net, being attracted by the
odour, had on some one occasion found an entrance, and had intercrossed
a few of the flowers.

In the spring of 1869 four plants raised from fresh seeds were carefully
protected under separate nets; and now the result was widely different
to what it was before. Three of these protected plants became actually
loaded with capsules, especially during the early part of the summer;
and this fact indicates that temperature produces some effect, but the
experiment given in the following paragraph shows that the innate
constitution of the plant is a far more important element. The fourth
plant produced only a few capsules, many of them of small size; yet it
was far more self-fertile than any of the seven plants tried during the
previous year. The flowers on four small branches of this
semi-self-sterile plant were smeared with pollen from one of the other
plants, and they all produced fine capsules.

As I was much surprised at the difference in the results of the trials
made during the two previous years, six fresh plants were protected by
separate nets in the year 1870. Two of these proved almost completely
self-sterile, for on carefully searching them I found only three small
capsules, each containing either one or two seeds of small size, which,
however, germinated. A few flowers on both these plants were
reciprocally fertilised with each other’s pollen, and a few with pollen
from one of the following self-fertile plants, and all these flowers
produced fine capsules. The four other plants whilst still remaining
protected beneath the nets presented a wonderful contrast (though one of
them in a somewhat less degree than the others), for they became
actually covered with spontaneously self-fertilised capsules, as
numerous as, or very nearly so, and as fine as those on the unprotected
plants growing near.

The above three spontaneously self-fertilised capsules produced by the
two almost completely self-sterile plants, contained altogether five
seeds; and from these I raised in the following year (1871) five plants,
which were kept under separate nets. They grew to an extraordinarily
large size, and on August 29th were examined. At first sight they
appeared entirely destitute of capsules; but on carefully searching
their many branches, two or three capsules were found on three of the
plants, half-a-dozen on the fourth, and about eighteen on the fifth
plant. But all these capsules were small, some being empty; the greater
number contained only a single seed, and very rarely more than one.
After this examination the nets were taken off, and the bees immediately
carried pollen from one of these almost self-sterile plants to the
other, for no other plants grew near. After a few weeks the ends of the
branches on all five plants became covered with capsules, presenting a
curious contrast with the lower and naked parts of the same long
branches. These five plants therefore inherited almost exactly the same
sexual constitution as their parents; and without doubt a self-sterile
race of Mignonette could have been easily established.

Reseda lutea.

Plants of this species were raised from seeds gathered from a group of
wild plants growing at no great distance from my garden. After casually
observing that some of these plants were self-sterile, two plants taken
by hazard were protected under separate nets. One of these soon became
covered with spontaneously self-fertilised capsules, as numerous as
those on the surrounding unprotected plants; so that it was evidently
quite self-fertile. The other plant was partially self-sterile,
producing very few capsules, many of which were of small size. When,
however, this plant had grown tall, the uppermost branches became
pressed against the net and grew crooked, and in this position the bees
were able to suck the flowers through the meshes, and brought pollen to
them from the neighbouring plants. These branches then became loaded
with capsules; the other and lower branches remaining almost bare. The
sexual constitution of this species is therefore similar to that of
Reseda odorata.

CONCLUDING REMARKS ON SELF-STERILE PLANTS.

In order to favour as far as possible the self-fertilisation of some of
the foregoing plants, all the flowers on Reseda odorata and some of
those on the Abutilon were fertilised with pollen from other flowers on
the same plant, instead of with their own pollen, and in the case of the
Senecio with pollen from other flowers on the same corymb; but this made
no difference in the result. Fritz Muller tried both kinds of
self-fertilisation in the case of Bignonia, Tabernaemontana and
Abutilon, likewise with no difference in the result. With Eschscholtzia,
however, he found that pollen from other flowers on the same plant was a
little more effective than pollen from the same flower. So did
Hildebrand in Germany; as thirteen out of fourteen flowers of
Eschscholtzia thus fertilised set capsules, these containing on an
average 9.5 seeds; whereas only fourteen flowers out of twenty-one
fertilised with their own pollen set capsules, these containing on an
average 9.0 seeds. (9/11. ‘Pringsheim’s Jahrbuch fur wiss. Botanik’ 7
page 467.) Hildebrand found a trace of a similar difference with
Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. ‘Variation
under Domestication’ chapter 17 2nd edition volume 2 pages 113-115.)

In considering the several cases above given of complete or almost
complete self-sterility, we are first struck with their wide
distribution throughout the vegetable kingdom. Their number is not at
present large, for they can be discovered only by protecting plants from
insects and then fertilising them with pollen from another plant of the
same species and with their own pollen; and the latter must be proved to
be in an efficient state by other trials. Unless all this be done, it is
impossible to know whether their self-sterility may not be due to the
male or female reproductive organs, or to both, having been affected by
changed conditions of life. As in the course of my experiments I have
found three new cases, and as Fritz Muller has observed indications of
several others, it is probable that they will hereafter be proved to be
far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal
in the United States quoted in ‘Gardeners’ Chronicle’ 1868 page 1286,
states that Lilium auratum, Impatiens pallida and fulva, and Forsythia
viridissima, cannot be fertilised with their own pollen.)

As with plants of the same species and parentage, some individuals are
self-sterile and others self-fertile, of which fact Reseda odorata
offers the most striking instances, it is not at all surprising that
species of the same genus differ in this same manner. Thus Verbascum
phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis
are quite self-fertile, as I know by trial. There is the same difference
between some of the species of Papaver, Corydalis, and of other genera.
Nevertheless, the tendency to self-sterility certainly runs to a certain
extent in groups, as we see in the genus Passiflora, and with the
Vandeae amongst Orchids.

Self-sterility differs much in degree in different plants. In those
extraordinary cases in which pollen from the same flower acts on the
stigma like a poison, it is almost certain that the plants would never
yield a single self-fertilised seed. Other plants, like Corydalis cava,
occasionally, though very rarely, produce a few self-fertilised seeds. A
large number of species, as may be seen in Table 9/F, are less fertile
with their own pollen than with that from another plant; and lastly,
some species are perfectly self-fertile. Even with the individuals of
the same species, as just remarked, some are utterly self-sterile,
others moderately so, and some perfectly self-fertile. The cause,
whatever it may be, which renders many plants more or less sterile with
their own pollen, that is, when they are self-fertilised, must be
different, at least to a certain extent, from that which determines the
difference in height, vigour, and fertility of the seedlings raised from
self-fertilised and crossed seeds; for we have already seen that the two
classes of cases do not by any means run parallel. This want of
parallelism would be intelligible, if it could be shown that
self-sterility depended solely on the incapacity of the pollen-tubes to
penetrate the stigma of the same flower deeply enough to reach the
ovules; whilst the greater or less vigorous growth of the seedlings no
doubt depends on the nature of the contents of the pollen-grains and
ovules. Now it is certain that with some plants the stigmatic secretion
does not properly excite the pollen-grains, so that the tubes are not
properly developed, if the pollen is taken from the same flower. This is
the case according to Fritz Muller with Eschscholtzia, for he found that
the pollen-tubes did not penetrate the stigma deeply; and with the
Orchidaceous genus Notylia they failed altogether to penetrate it.
(9/14. ‘Botanische Zeitung’ 1868 pages 114, 115.)

With dimorphic and trimorphic species, an illegitimate union between
plants of the same form presents the closest analogy with
self-fertilisation, whilst a legitimate union closely resembles
cross-fertilisation; and here again the lessened fertility or complete
sterility of an illegitimate union depends, at least in part, on the
incapacity for interaction between the pollen-grains and stigma. Thus
with Linum grandiflorum, as I have elsewhere shown, not more than two or
three out of hundreds of pollen-grains, either of the long-styled or
short-styled form, when placed on the stigma of their own form, emit
their tubes, and these do not penetrate deeply; nor does the stigma
itself change colour, as occurs when it is legitimately fertilised.
(9/15. ‘Journal of the Linnean Society Botany’ volume 7 1863 pages
73-75.)

On the other hand the difference in innate fertility, as well as in
growth between plants raised from crossed and self-fertilised seeds, and
the difference in fertility and growth between the legitimate and
illegitimate offspring of dimorphic and trimorphic plants, must depend
on some incompatibility between the sexual elements contained within the
pollen-grains and ovules, as it is through their union that new
organisms are developed.

If we now turn to the more immediate cause of self-sterility, we clearly
see that in most cases it is determined by the conditions to which the
plants have been subjected. Thus Eschscholtzia is completely
self-sterile in the hot climate of Brazil, but is perfectly fertile
there with the pollen of any other individual. The offspring of
Brazilian plants became in England in a single generation partially
self-fertile, and still more so in the second generation. Conversely,
the offspring of English plants, after growing for two seasons in
Brazil, became in the first generation quite self-sterile. Again,
Abutilon darwinii, which is self-sterile in its native home of Brazil,
became moderately self-fertile in a single generation in an English
hothouse. Some other plants are self-sterile during the early part of
the year, and later in the season become self-fertile. Passiflora alata
lost its self-sterility when grafted on another species. With Reseda,
however, in which some individuals of the same parentage are
self-sterile and others are self-fertile, we are forced in our ignorance
to speak of the cause as due to spontaneous variability; but we should
remember that the progenitors of these plants, either on the male or
female side, may have been exposed to somewhat different conditions. The
power of the environment thus to affect so readily and in so peculiar a
manner the reproductive organs, is a fact which has many important
bearings; and I have therefore thought the foregoing details worth
giving. For instance, the sterility of many animals and plants under
changed conditions of life, such as confinement, evidently comes within
the same general principle of the sexual system being easily affected by
the environment. It has already been proved, that a cross between plants
which have been self-fertilised or intercrossed during several
generations, having been kept all the time under closely similar
conditions, does not benefit the offspring; and on the other hand, that
a cross between plants that have been subjected to different conditions
benefits the offspring to an extraordinary degree. We may therefore
conclude that some degree of differentiation in the sexual system is
necessary for the full fertility of the parent-plants and for the full
vigour of their offspring. It seems also probable that with those plants
which are capable of complete self-fertilisation, the male and female
elements and organs already differ to an extent sufficient to excite
their mutual interaction; but that when such plants are taken to another
country, and become in consequence self-sterile, their sexual elements
and organs are so acted on as to be rendered too uniform for such
interaction, like those of a self-fertilised plant long cultivated under
the same conditions. Conversely, we may further infer that plants which
are self-sterile in their native country, but become self-fertile under
changed conditions, have their sexual elements so acted on, that they
become sufficiently differentiated for mutual interaction.

We know that self-fertilised seedlings are inferior in many respects to
those from a cross; and as with plants in a state of nature pollen from
the same flower can hardly fail to be often left by insects or by the
wind on the stigma, it seems at first sight highly probable that
self-sterility has been gradually acquired through natural selection in
order to prevent self-fertilisation. It is no valid objection to this
belief that the structure of some flowers, and the dichogamous condition
of many others, suffice to prevent the pollen reaching the stigma of the
same flower; for we should remember that with most species many flowers
expand at the same time, and that pollen from the same plant is equally
injurious or nearly so as that from the same flower. Nevertheless, the
belief that self-sterility is a quality which has been gradually
acquired for the special purpose of preventing self-fertilisation must,
I believe, be rejected. In the first place, there is no close
correspondence in degree between the sterility of the parent-plants when
self-fertilised, and the extent to which their offspring suffer in
vigour by this process; and some such correspondence might have been
expected if self-sterility had been acquired on account of the injury
caused by self-fertilisation. The fact of individuals of the same
parentage differing greatly in their degree of self-sterility is
likewise opposed to such a belief; unless, indeed, we suppose that
certain individuals have been rendered self-sterile to favour
intercrossing, whilst other individuals have been rendered self-fertile
to ensure the propagation of the species. The fact of self-sterile
individuals appearing only occasionally, as in the case of Lobelia, does
not countenance this latter view. But the strongest argument against the
belief that self-sterility has been acquired to prevent
self-fertilisation, is the immediate and powerful effect of changed
conditions in either causing or in removing self-sterility. We are not
therefore justified in admitting that this peculiar state of the
reproductive system has been gradually acquired through natural
selection; but we must look at it as an incidental result, dependent on
the conditions to which the plants have been subjected, like the
ordinary sterility caused in the case of animals by confinement, and in
the case of plants by too much manure, heat, etc. I do not, however,
wish to maintain that self-sterility may not sometimes be of service to
a plant in preventing self-fertilisation; but there are so many other
means by which this result might be prevented or rendered difficult,
including as we shall see in the next chapter the prepotency of pollen
from a distinct individual over a plant’s own pollen, that
self-sterility seems an almost superfluous acquirement for this purpose.

Finally, the most interesting point in regard to self-sterile plants is
the evidence which they afford of the advantage, or rather of the
necessity, of some degree or kind of differentiation in the sexual
elements, in order that they should unite and give birth to a new being.
It was ascertained that the five plants of Reseda odorata which were
selected by chance, could be perfectly fertilised by pollen taken from
any one of them, but not by their own pollen; and a few additional
trials were made with some other individuals, which I have not thought
worth recording. So again, Hildebrand and Fritz Muller frequently speak
of self-sterile plants being fertile with the pollen of any other
individual; and if there had been any exceptions to the rule, these
could hardly have escaped their observation and my own. We may therefore
confidently assert that a self-sterile plant can be fertilised by the
pollen of any one out of a thousand or ten thousand individuals of the
same species, but not by its own. Now it is obviously impossible that
the sexual organs and elements of every individual can have been
specialised with respect to every other individual. But there is no
difficulty in believing that the sexual elements of each differ slightly
in the same diversified manner as do their external characters; and it
has often been remarked that no two individuals are absolutely alike.
Therefore we can hardly avoid the conclusion, that differences of an
analogous and indefinite nature in the reproductive system are
sufficient to excite the mutual action of the sexual elements, and that
unless there be such differentiation fertility fails.

THE APPEARANCE OF HIGHLY SELF-FERTILE VARIETIES.

We have just seen that the degree to which flowers are capable of being
fertilised with their own pollen differs much, both with the species of
the same genus, and sometimes with the individuals of the same species.
Some allied cases of the appearance of varieties which, when
self-fertilised, yield more seed and produce offspring growing taller
than their self-fertilised parents, or than the intercrossed plants of
the corresponding generation, will now be considered.

Firstly, in the third and fourth generations of Mimulus luteus, a tall
variety, often alluded to, having large white flowers blotched with
crimson, appeared amongst both the intercrossed and self-fertilised
plants. It prevailed in all the later self-fertilised generations to the
exclusion of every other variety, and transmitted its characters
faithfully, but disappeared from the intercrossed plants, owing no doubt
to their characters being repeatedly blended by crossing. The
self-fertilised plants belonging to this variety were not only taller,
but more fertile than the intercrossed plants; though these latter in
the earlier generations were much taller and more fertile than the
self-fertilised plants. Thus in the fifth generation the self-fertilised
plants were to the intercrossed in height as 126 to 100. In the sixth
generation they were likewise much taller and finer plants, but were not
actually measured; they produced capsules compared with those on the
intercrossed plants, in number, as 147 to 100; and the self-fertilised
capsules contained a greater number of seeds. In the seventh generation
the self-fertilised plants were to the crossed in height as 137 to 100;
and twenty flowers on these self-fertilised plants fertilised with their
own pollen yielded nineteen very fine capsules,--a degree of
self-sterility which I have not seen equalled in any other case. This
variety seems to have become specially adapted to profit in every way by
self-fertilisation, although this process was so injurious to the
parent-plants during the first four generations. It should however be
remembered that seedlings raised from this variety, when crossed by a
fresh stock, were wonderfully superior in height and fertility to the
self-fertilised plants of the corresponding generation.

Secondly, in the sixth self-fertilised generation of Ipomoea a single
plant named the Hero appeared, which exceeded by a little in height its
intercrossed opponent,--a case which had not occurred in any previous
generation. Hero transmitted the peculiar colour of its flowers, as well
as its increased tallness and a high degree of self-fertility, to its
children, grandchildren, and great-grandchildren. The self-fertilised
children of Hero were in height to other self-fertilised plants of the
same stock as 100 to 85. Ten self-fertilised capsules produced by the
grandchildren contained on an average 5.2 seeds; and this is a higher
average than was yielded in any other generation by the capsules of
self-fertilised flowers. The great-grandchildren of Hero derived from a
cross with a fresh stock were so unhealthy, from having been grown at an
unfavourable season, that their average height in comparison with that
of the self-fertilised plants cannot be judged of with any safety; but
it did not appear that they had profited even by a cross of this kind.

Thirdly, the plants of Nicotiana on which I experimented appear to come
under the present class of cases; for they varied in their sexual
constitution and were more or less highly self-fertile. They were
probably the offspring of plants which had been spontaneously
self-fertilised under glass for several generations in this country. The
flowers on the parent-plants which were first fertilised by me with
their own pollen yielded half again as many seeds as did those which
were crossed; and the seedlings raised from these self-fertilised seeds
exceeded in height those raised from the crossed seeds to an
extraordinary degree. In the second and third generations, although the
self-fertilised plants did not exceed the crossed in height, yet their
self-fertilised flowers yielded on two occasions considerably more seeds
than the crossed flowers, even than those which were crossed with pollen
from a distinct stock or variety.

Lastly, as certain individual plants of Reseda odorata and lutea are
incomparably more self-fertile than other individuals, the former might
be included under the present heading of the appearance of new and
highly self-fertile varieties. But in this case we should have to look
at these two species as normally self-sterile; and this, judging by my
experience, appears to be the correct view.

We may therefore conclude from the facts now given, that varieties
sometimes arise which when self-fertilised possess an increased power of
producing seeds and of growing to a greater height, than the
intercrossed or self-fertilised plants of the corresponding
generation--all the plants being of course subjected to the same
conditions. The appearance of such varieties is interesting, as it bears
on the existence under nature of plants which regularly fertilise
themselves, such as Ophrys apifera and a few other orchids, or as
Leersia oryzoides, which produces an abundance of cleistogene flowers,
but most rarely flowers capable of cross-fertilisation.

Some observations made on other plants lead me to suspect that
self-fertilisation is in some respects beneficial; although the benefit
thus derived is as a rule very small compared with that from a cross
with a distinct plant. Thus we have seen in the last chapter that
seedlings of Ipomoea and Mimulus raised from flowers fertilised with
their own pollen, which is the strictest possible form of
self-fertilisation, were superior in height, weight, and in early
flowering to the seedlings raised from flowers crossed with pollen from
other flowers on the same plant; and this superiority apparently was too
strongly marked to be accidental. Again, the cultivated varieties of the
common pea are highly self-fertile, although they have been
self-fertilised for many generations; and they exceeded in height
seedlings from a cross between two plants belonging to the same variety
in the ratio of 115 to 100; but then only four pairs of plants were
measured and compared. The self-fertility of Primula veris increased
after several generations of illegitimate fertilisation, which is a
process closely analogous to self-fertilisation, but only as long as the
plants were cultivated under the same favourable conditions. I have also
elsewhere shown that with Primula veris and sinensis, equal-styled
varieties occasionally appear which possess the sexual organs of the two
forms combined in the same flower. (9/16. ‘Journal of the Linnean
Society Botany’ volume 10 1867 pages 417, 419.) Consequently they
fertilise themselves in a legitimate manner and are highly self-fertile;
but the remarkable fact is that they are rather more fertile than
ordinary plants of the same species legitimately fertilised by pollen
from a distinct individual. Formerly it appeared to me probable, that
the increased fertility of these dimorphic plants might be accounted for
by the stigma lying so close to the anthers that it was impregnated at
the most favourable age and time of the day; but this explanation is not
applicable to the above given cases, in which the flowers were
artificially fertilised with their own pollen.

Considering the facts now adduced, including the appearance of those
varieties which are more fertile and taller than their parents and than
the intercrossed plants of the corresponding generation, it is difficult
to avoid the suspicion that self-fertilisation is in some respects
advantageous; though if this be really the case, any such advantage is
as a rule quite insignificant compared with that from a cross with a
distinct plant, and especially with one of a fresh stock. Should this
suspicion be hereafter verified, it would throw light, as we shall see
in the next chapter, on the existence of plants bearing small and
inconspicuous flowers which are rarely visited by insects, and therefore
are rarely intercrossed.

RELATIVE WEIGHT AND PERIOD OF GERMINATION OF SEEDS FROM CROSSED AND
SELF-FERTILISED FLOWERS.

An equal number of seeds from flowers fertilised with pollen from
another plant, and from flowers fertilised with their own pollen, were
weighed, but only in sixteen cases. Their relative weights are given in
the following list; that of the seeds from the crossed flowers being
taken as 100.

Column 1: Name of Plant.

Column 2: x, in the expression, 100 to x.

Ipomoea purpurea (parent plants): 127.
Ipomoea purpurea (third generation):  87.
Salvia coccinea: 100.
Brassica oleracea: 103.
Iberis umbellata (second generation): 136.
Delphinium consolida:  45.
Hibiscus africanus: 105.
Tropaeolum minus: 115.
Lathyrus odoratus (about): 100.
Sarothamnus scoparius:  88.
Specularia speculum:  86.
Nemophila insignis: 105.
Borago officinalis: 111.
Cyclamen persicum (about):  50.
Fagopyrum esculentum:  82.
Canna warscewiczi (3 generations): 102.

It is remarkable that in ten out of these sixteen cases the
self-fertilised seeds were either superior or equal to the crossed in
weight; nevertheless, in six out of the ten cases (namely, with Ipomoea,
Salvia, Brassica, Tropaeolum, Lathyrus, and Nemophila) the plants raised
from these self-fertilised seeds were very inferior in height and in
other respects to those raised from the crossed seeds. The superiority
in weight of the self-fertilised seeds in at least six out of the ten
cases, namely, with Brassica, Hibiscus, Tropaeolum, Nemophila, Borago,
and Canna, may be accounted for in part by the self-fertilised capsules
containing fewer seeds; for when a capsule contains only a few seeds,
these will be apt to be better nourished, so as to be heavier, than when
many are contained in the same capsule. It should, however, be observed
that in some of the above cases, in which the crossed seeds were the
heaviest, as with Sarothamnus and Cyclamen, the crossed capsules
contained a larger number of seeds. Whatever may be the explanation of
the self-fertilised seeds being often the heaviest, it is remarkable in
the case of Brassica, Tropaeolum, Nemophila, and of the first generation
of Ipomoea, that the seedlings raised from them were inferior in height
and in other respects to the seedlings raised from the crossed seeds.
This fact shows how superior in constitutional vigour the crossed
seedlings must have been, for it cannot be doubted that heavy and fine
seeds tend to yield the finest plants. Mr. Galton has shown that this
holds good with Lathyrus odoratus; as has Mr. A.J. Wilson with the
Swedish turnip, Brassica campestris ruta baga. Mr. Wilson separated the
largest and smallest seeds of this latter plant, the ratio between the
weights of the two lots being as 100 to 59, and he found that the
seedlings “from the larger seeds took the lead and maintained their
superiority to the last, both in height and thickness of stem.” (9/17.
‘Gardeners’ Chronicle’ 1867 page 107. Loiseleur-Deslongchamp ‘Les
Cereales’ 1842 pages 208-219, was led by his observations to the
extraordinary conclusion that the smaller grains of cereals produce as
fine plants as the large. This conclusion is, however, contradicted by
Major Hallet’s great success in improving wheat by the selection of the
finest grains. It is possible, however, that man, by long-continued
selection, may have given to the grains of the cereals a greater amount
of starch or other matter, than the seedlings can utilise for their
growth. There can be little doubt, as Humboldt long ago remarked, that
the grains of cereals have been rendered attractive to birds in a degree
which is highly injurious to the species.) Nor can this difference in
the growth of the seedling turnips be attributed to the heavier seeds
having been of crossed, and the lighter of self-fertilised origin, for
it is known that plants belonging to this genus are habitually
intercrossed by insects.

With respect to the relative period of germination of crossed and
self-fertilised seeds, a record was kept in only twenty-one cases; and
the results are very perplexing. Neglecting one case in which the two
lots germinated simultaneously, in ten cases or exactly one-half many of
the self-fertilised seeds germinated before the crossed, and in the
other half many of the crossed before the self-fertilised. In four out
of these twenty cases, seeds derived from a cross with a fresh stock
were compared with self-fertilised seeds from one of the later
self-fertilised generations; and here again in half the cases the
crossed seeds, and in the other half the self-fertilised seeds,
germinated first. Yet the seedlings of Mimulus raised from such
self-fertilised seeds were inferior in all respects to the crossed
seedlings, and in the case of Eschscholtzia they were inferior in
fertility. Unfortunately the relative weight of the two lots of seeds
was ascertained in only a few instances in which their germination was
observed; but with Ipomoea and I believe with some of the other species,
the relative lightness of the self-fertilised seeds apparently
determined their early germination, probably owing to the smaller mass
being favourable to the more rapid completion of the chemical and
morphological changes necessary for germination. On the other hand, Mr.
Galton gave me seeds (no doubt all self-fertilised) of Lathyrus
odoratus, which were divided into two lots of heavier and lighter seeds;
and several of the former germinated first. It is evident that many more
observations are necessary before anything can be decided with respect
to the relative period of germination of crossed and self-fertilised
seeds.



CHAPTER X.

MEANS OF FERTILISATION.

Sterility and fertility of plants when insects are excluded.
The means by which flowers are cross-fertilised.
Structures favourable to self-fertilisation.
Relation between the structure and conspicuousness of flowers, the
visits of insects, and the advantages of cross-fertilisation.
The means by which flowers are fertilised with pollen from a distinct
plant.
Greater fertilising power of such pollen.
Anemophilous species.
Conversion of anemophilous species into entomophilous.
Origin of nectar.
Anemophilous plants generally have their sexes separated.
Conversion of diclinous into hermaphrodite flowers.
Trees often have their sexes separated.

In the introductory chapter I briefly specified the various means by
which cross-fertilisation is favoured or ensured, namely, the separation
of the sexes,--the maturity of the male and female sexual elements at
different periods,--the heterostyled or dimorphic and trimorphic
condition of certain plants,--many mechanical contrivances,--the more or
less complete inefficiency of a flower’s own pollen on the stigma,--and
the prepotency of pollen from any other individual over that from the
same plant. Some of these points require further consideration; but for
full details I must refer the reader to the several excellent works
mentioned in the introduction. I will in the first place give two lists:
the first, of plants which are either quite sterile or produce less than
about half the full complement of seeds, when insects are excluded; and
a second list of plants which, when thus treated, are fully fertile or
produce at least half the full complement of seeds. These lists have
been compiled from the several previous tables, with some additional
cases from my own observations and those of others. The species are
arranged nearly in the order followed by Lindley in his ‘Vegetable
Kingdom.’ The reader should observe that the sterility or fertility of
the plants in these two lists depends on two wholly distinct causes;
namely, the absence or presence of the proper means by which pollen is
applied to the stigma, and its less or greater efficiency when thus
applied. As it is obvious that with plants in which the sexes are
separate, pollen must be carried by some means from flower to flower,
such species are excluded from the lists; as are likewise dimorphic and
trimorphic plants, in which the same necessity occurs to a limited
extent. Experience has proved to me that, independently of the exclusion
of insects, the seed-bearing power of a plant is not lessened by
covering it while in flower under a thin net supported on a frame; and
this might indeed have been inferred from the consideration of the two
following lists, as they include a considerable number of species
belonging to the same genera, some of which are quite sterile and others
quite fertile when protected by a net from the access of insects.

[LIST OF PLANTS WHICH, WHEN INSECTS ARE EXCLUDED, ARE EITHER QUITE
STERILE, OR PRODUCE, AS FAR AS I COULD JUDGE, LESS THAN HALF THE NUMBER
OF SEEDS PRODUCED BY UNPROTECTED PLANTS.

Passiflora alata, racemosa, coerulea, edulis, laurifolia, and some
individuals of P. quadrangularis (Passifloraceae), are quite sterile
under these conditions: see ‘Variation of Animals and Plants under
Domestication’ chapter 17 2nd edition volume 2 page 118.

Viola canina (Violaceae).--Perfect flowers quite sterile unless
fertilised by bees, or artificially fertilised.

Viola tricolor.--Sets very few and poor capsules.

Reseda odorata (Resedaceae).--Some individuals quite sterile.

Reseda lutea.--Some individuals produce very few and poor capsules.

Abutilon darwinii (Malvaceae).--Quite sterile in Brazil: see previous
discussion on self-sterile plants.

Nymphaea (Nymphaeaceae).--Professor Caspary informs me that some of the
species are quite sterile if insects are excluded.

Euryale amazonica (Nymphaeaceae).--Mr. J. Smith, of Kew, informs me that
capsules from flowers left to themselves, and probably not visited by
insects, contained from eight to fifteen seeds; those from flowers
artificially fertilised with pollen from other flowers on the same plant
contained from fifteen to thirty seeds; and that two flowers fertilised
with pollen brought from another plant at Chatsworth contained
respectively sixty and seventy-five seeds. I have given these statements
because Professor Caspary advances this plant as a case opposed to the
doctrine of the necessity or advantage of cross-fertilisation: see
Sitzungsberichte der Phys.-okon. Gesell.zu Konigsberg, B.6 page 20.)

Delphinium consolida (Ranunculaceae).--Produces many capsules, but these
contain only about half the number of seeds compared with capsules from
flowers naturally fertilised by bees.

Eschscholtzia californica (Papaveraceae).--Brazilian plants quite
sterile: English plants produce a few capsules.

Papaver vagum (Papaveraceae).--In the early part of the summer produced
very few capsules, and these contained very few seeds.

Papaver alpinum.--H. Hoffmann (‘Speciesfrage’ 1875 page 47) states that
this species produced seeds capable of germination only on one occasion.

Corydalis cava (Fumariaceae).--Sterile: see the previous discussion on
self-sterile plants.

Corydalis solida.--I had a single plant in my garden (1863), and saw
many hive-bees sucking the flowers, but not a single seed was produced.
I was much surprised at this fact, as Professor Hildebrand’s discovery
that C. cava is sterile with its own pollen had not then been made. He
likewise concludes from the few experiments which he made on the present
species that it is self-sterile. The two foregoing cases are
interesting, because botanists formerly thought (see, for instance,
Lecoq, ‘De la Fecondation et de l’Hybridation’ 1845 page 61 and Lindley
‘Vegetable Kingdom’ 1853 page 436) that all the species of the
Fumariaceae were specially adapted for self-fertilisation.

Corydalis lutea.--A covered-up plant produced (1861) exactly half as
many capsules as an exposed plant of the same size growing close
alongside. When humble-bees visit the flowers (and I repeatedly saw them
thus acting) the lower petals suddenly spring downwards and the pistil
upwards; this is due to the elasticity of the parts, which takes effect,
as soon as the coherent edges of the hood are separated by the entrance
of an insect. Unless insects visit the flowers the parts do not move.
Nevertheless, many of the flowers on the plants which I had protected
produced capsules, notwithstanding that their petals and pistils still
retained their original position; and I found to my surprise that these
capsules contained more seeds than those from flowers, the petals of
which had been artificially separated and allowed to spring apart. Thus,
nine capsules produced by undisturbed flowers contained fifty-three
seeds; whilst nine capsules from flowers, the petals of which had been
artificially separated, contained only thirty-two seeds. But we should
remember that if bees had been permitted to visit these flowers, they
would have visited them at the best time for fertilisation. The flowers,
the petals of which had been artificially separated, set their capsules
before those which were left undisturbed under the net. To show with
what certainty the flowers are visited by bees, I may add that on one
occasion all the flowers on some unprotected plants were examined, and
every single one had its petals separated; and, on a second occasion,
forty-one out of forty-three flowers were in this state. Hildebrand
states (Pring. Jahr. f. wiss. Botanik, B. 7 page 450) that the mechanism
of the parts in this species is nearly the same as in C. ochroleuca,
which he has fully described.

Hypecoum grandiflorum (Fumariaceae).--Highly self-sterile (Hildebrand,
ibid.).

Kalmia latifolia (Ericaceae).--Mr. W.J. Beal says (‘American Naturalist’
1867) that flowers protected from insects wither and drop off, with
“most of the anthers still remaining in the pockets.”

Pelargonium zonale (Geraniaceae).--Almost sterile; one plant produced
two fruits. It is probable that different varieties would differ in this
respect, as some are only feebly dichogamous.

Dianthus caryophyllus (Caryophyllaceae).--Produces very few capsules
which contain any good seeds.

Phaseolus multiflorus (Leguminosae).--Plants protected from insects
produced on two occasions about one-third and one-eighth of the full
number of seeds: see my article in ‘Gardeners’ Chronicle’ 1857 page 225
and 1858 page 828; also ‘Annals and Magazine of Natural History’ 3rd
series volume 2 1858 page 462. Dr. Ogle (‘Popular Science Review’ 1870
page 168) found that a plant was quite sterile when covered up. The
flowers are not visited by insects in Nicaragua, and, according to Mr.
Belt, the species is there quite sterile: ‘The Naturalist in Nicaragua’
page 70.

Vicia faba (Leguminosae).--Seventeen covered-up plants yielded 40 beans,
whilst seventeen plants left unprotected and growing close alongside
produced 135 beans; these latter plants were, therefore, between three
and four times more fertile than the protected plants: see ‘Gardeners’
Chronicle’ for fuller details, 1858 page 828.

Erythrina (sp.?) (Leguminosae).--Sir W. MacArthur informed me that in
New South Wales the flowers do not set, unless the petals are moved in
the same manner as is done by insects.

Lathyrus grandiflorus (Leguminosae).--Is in this country more or less
sterile. It never sets pods unless the flowers are visited by
humble-bees (and this happens only rarely), or unless they are
artificially fertilised: see my article in ‘Gardeners’ Chronicle’ 1858
page 828.

Sarothamnus scoparius (Leguminosae).--Extremely sterile when the flowers
are neither visited by bees, nor disturbed by being beaten by the wind
against the surrounding net.

Melilotus officinalis (Leguminosae).--An unprotected plant visited by
bees produced at least thirty times more seeds than a protected one. On
this latter plant many scores of racemes did not produce a single pod;
several racemes produced each one or two pods; five produced three; six
produced four; and one produced six pods. On the unprotected plant each
of several racemes produced fifteen pods; nine produced between sixteen
and twenty-two pods, and one produced thirty pods.

Lotus corniculatus (Leguminosae).--Several covered-up plants produced
only two empty pods, and not a single good seed.

Trifolium repens (Leguminosae).--Several plants were protected from
insects, and the seeds from ten flowers-heads on these plants, and from
ten heads on other plants growing outside the net (which I saw visited
by bees), were counted; and the seeds from the latter plants were very
nearly ten times as numerous as those from the protected plants. The
experiment was repeated on the following year; and twenty protected
heads now yielded only a single aborted seed, whilst twenty heads on the
plants outside the net (which I saw visited by bees) yielded 2290 seeds,
as calculated by weighing all the seed, and counting the number in a
weight of two grains.

Trifolium pratense.--One hundred flower-heads on plants protected by a
net did not produce a single seed, whilst 100 heads on plants growing
outside, which were visited by bees, yielded 68 grains weight of seeds;
and as eighty seeds weighed two grains, the 100 heads must have yielded
2720 seeds. I have often watched this plant, and have never seen
hive-bees sucking the flowers, except from the outside through holes
bitten by humble-bees, or deep down between the flowers, as if in search
of some secretion from the calyx, almost in the same manner as described
by Mr. Farrer, in the case of Coronilla (‘Nature’ 1874 July 2 page 169).
I must, however, except one occasion, when an adjoining field of
sainfoin (Hedysarum onobrychis) had just been cut down, and when the
bees seemed driven to desperation. On this occasion most of the flowers
of the clover were somewhat withered, and contained an extraordinary
quantity of nectar, which the bees were able to suck. An experienced
apiarian, Mr. Miner, says that in the United States hive-bees never suck
the red clover; and Mr. R. Colgate informs me that he has observed the
same fact in New Zealand after the introduction of the hive-bee into
that island. On the other hand, H. Muller (‘Befruchtung’ page 224) has
often seen hive-bees visiting this plant in Germany, for the sake both
of pollen and nectar, which latter they obtained by breaking apart the
petals. It is at least certain that humble-bees are the chief
fertilisers of the common red clover.

Trifolium incarnatum.--The flower-heads containing ripe seeds, on some
covered and uncovered plants, appeared equally fine, but this was a
false appearance; 60 heads on the latter yielded 349 grains weight of
seeds, whereas 60 on the covered-up plants yielded only 63 grains, and
many of the seeds in the latter lot were poor and aborted. Therefore the
flowers which were visited by bees produced between five and six times
as many seeds as those which were protected. The covered-up plants not
having been much exhausted by seed-bearing, bore a second considerable
crop of flower-stems, whilst the exposed plants did not do so.

Cytisus laburnum (Leguminosae).--Seven flower-racemes ready to expand
were enclosed in a large bag made of net, and they did not seem in the
least injured by this treatment. Only three of them produced any pods,
each a single one; and these three pods contained one, four, and five
seeds. So that only a single pod from the seven racemes included a fair
complement of seeds.

Cuphea purpurea (Lythraceae).--Produced no seeds. Other flowers on the
same plant artificially fertilised under the net yielded seeds.

Vinca major (Apocynaceae).--Is generally quite sterile, but sometimes
sets seeds when artificially cross-fertilised: see my notice ‘Gardeners’
Chronicle’ 1861 page 552.

Vinca rosea.--Behaves in the same manner as the last species:
‘Gardeners’ Chronicle’ 1861 page 699, 736, 831.

Tabernaemontana echinata (Apocynaceae).--Quite sterile.

Petunia violacea (Solanaceae).--Quite sterile, as far as I have
observed.

Solanum tuberosum (Solanaceae).--Tinzmann says (‘Gardeners’ Chronicle’
1846 page 183) that some varieties are quite sterile unless fertilised
by pollen from another variety.

Primula scotica (Primulaceae).--A non-dimorphic species, which is
fertile with its own pollen, but is extremely sterile if insects are
excluded. J. Scott in ‘Journal of the Linnean Society Botany’ volume 8
1864 page 119.

Cortusa matthioli (Primulaceae).--Protected plants completely sterile;
artificially self-fertilised flowers perfectly fertile. J. Scott ibid.
page 84.

Cyclamen persicum (Primulaceae).--During one season several covered-up
plants did not produce a single seed.

Borago officinalis (Boraginaceae).--Protected plants produced about half
as many seeds as the unprotected.

Salvia tenori (Labiatae).--Quite sterile; but two or three flowers on
the summits of three of the spikes, which touched the net when the wind
blew, produced a few seeds. This sterility was not due to the injurious
effects of the net, for I fertilised five flowers with pollen from an
adjoining plant, and these all yielded fine seeds. I removed the net,
whilst one little branch still bore a few not completely faded flowers,
and these were visited by bees and yielded seeds.

Salvia coccinea.--Some covered-up plants produced a good many fruits,
but not, I think, half as many as did the uncovered plants; twenty-eight
of the fruits spontaneously produced by the protected plant contained on
an average only 1.45 seeds, whilst some artificially self-fertilised
fruits on the same plant contained more than twice as many, namely 3.3
seeds.

Bignonia (unnamed species) (Bignoniaceae).--Quite sterile: see my
account of self-sterile plants.

Digitalis purpurea (Scrophulariaceae).--Extremely sterile, only a few
poor capsules being produced.

Linaria vulgaris (Scrophulariaceae).--Extremely sterile.

Antirrhinum majus, red var. (Scrophulariaceae).--Fifty pods gathered
from a large plant under a net contained 9.8 grains weight of seeds; but
many (unfortunately not counted) of the fifty pods contained no seeds.
Fifty pods on a plant fully exposed to the visits of humble-bees
contained 23.1 grains weight of seed, that is, more than twice the
weight; but in this case again, several of the fifty pods contained no
seeds.

Antirrhinum majus (white var., with a pink mouth to the corolla).--Fifty
pods, of which only a very few were empty, on a covered-up plant
contained 20 grains weight of seed; so that this variety seems to be
much more self-fertile than the previous one. With Dr. W. Ogle (‘Popular
Science Review’ January 1870 page 52) a plant of this species was much
more sterile when protected from insects than with me, for it produced
only two small capsules. As showing the efficiency of bees, I may add
that Mr. Crocker castrated some young flowers and left them uncovered;
and these produced as many seeds as the unmutilated flowers.

Antirrhinum majus (peloric var.).--This variety is quite fertile when
artificially fertilised with its own pollen, but is utterly sterile when
left to itself and uncovered, as humble-bees cannot crawl into the
narrow tubular flowers.

Verbascum phoeniceum (Scrophulariaceae).--Quite sterile. See my account
of self-sterile plants.

Verbascum nigrum.--Quite sterile. See my account of self-sterile plants.

Campanula carpathica (Lobeliaceae).--Quite sterile.

Lobelia ramosa (Lobeliaceae).--Quite sterile.

Lobelia fulgens.--This plant is never visited in my garden by bees, and
is quite sterile; but in a nursery-garden at a few miles’ distance I saw
humble-bees visiting the flowers, and they produced some capsules.

Isotoma (a white-flowered var.) (Lobeliaceae).--Five plants left
unprotected in my greenhouse produced twenty-four fine capsules,
containing altogether 12.2 grains weight of seed, and thirteen other
very poor capsules, which were rejected. Five plants protected from
insects, but otherwise exposed to the same conditions as the above
plants, produced sixteen fine capsules, and twenty other very poor and
rejected ones. The sixteen fine capsules contained seeds by weight in
such proportion that twenty-four would have yielded 4.66 grains. So that
the unprotected plants produced nearly thrice as many seeds by weight as
the protected plants.

Leschenaultia formosa (Goodeniaceae).--Quite sterile. My experiments on
this plant, showing the necessity of insect aid, are given in the
‘Gardeners’ Chronicle’ 1871 page 1166.

Senecio cruentus (Compositae).--Quite sterile: see my account of
self-sterile plants.

Heterocentron mexicanum (Malastomaceae).--Quite sterile; but this
species and the following members of the group produce plenty of seed
when artificially self-fertilised.

Rhexia glandulosa (Melastomaceae).--Set spontaneously only two or three
capsules.

Centradenia floribunda (Melastomaceae).--During some years produced
spontaneously two or three capsules, sometimes none.

Pleroma (unnamed species from Kew) (Melastomaceae).--During some years
produced spontaneously two or three capsules, sometimes none.

Monochaetum ensiferum (Melastomaceae).--During some years produced
spontaneously two or three capsules, sometimes none.

Hedychium (unnamed species) (Marantaceae).--Almost self-sterile without
aid.

Orchideae.--An immense proportion of the species sterile, if insects are
excluded.

LIST OF PLANTS, WHICH WHEN PROTECTED FROM INSECTS ARE EITHER QUITE
FERTILE, OR YIELD MORE THAN HALF THE NUMBER OF SEEDS PRODUCED BY
UNPROTECTED PLANTS.

Passiflora gracilis (Passifloraceae).--Produces many fruits, but these
contain fewer seeds than fruits from intercrossed flowers.

Brassica oleracea (Cruciferae).--Produces many capsules, but these
generally not so rich in seed as those on uncovered plants.

Raphanus sativus (Cruciferae).--Half of a large branching plant was
covered by a net, and was as thickly covered with capsules as the other
and unprotected half; but twenty of the capsules on the latter contained
on an average 3.5 seeds, whilst twenty of the protected capsules
contained only 1.85 seeds, that is, only a little more than half the
number. This plant might perhaps have been more properly included in the
former list.

Iberis umbellata (Cruciferae).--Highly fertile.

Iberis amara.--Highly fertile.

Reseda odorata and lutea (Resedaceae).--Certain individuals completely
self-fertile.

Euryale ferox (Nymphaeaceae).--Professor Caspary informs me that this
plant is highly self-fertile when insects are excluded. He remarks in
the paper before referred to, that his plants (as well as those of the
Victoria regia) produce only one flower at a time; and that as this
species is an annual, and was introduced in 1809, it must have been
self-fertilised for the last fifty-six generations; but Dr. Hooker
assures me that to his knowledge it has been repeatedly introduced, and
that at Kew the same plant both of the Euryale and of the Victoria
produce several flowers at the same time.

Nymphaea (Nymphaeaceae).--Some species, as I am informed by Professor
Caspary, are quite self-fertile when insects are excluded.

Adonis aestivalis (Ranunculaceae).--Produces, according to Professor H.
Hoffmann (‘Speciesfrage’ page 11), plenty of seeds when protected from
insects.

Ranunculus acris (Ranunculaceae).--Produces plenty of seeds under a net.

Papaver somniferum (Papaveraceae).--Thirty capsules from uncovered
plants yielded 15.6 grains weight of seed, and thirty capsules from
covered-up plants, growing in the same bed, yielded 16.5 grains weight;
so that the latter plants were more productive than the uncovered.
Professor H. Hoffmann (‘Speciesfrage’ 1875 page 53) also found this
species self-fertile when protected from insects.

Papaver vagum.--Produced late in the summer plenty of seeds, which
germinated well.

Papaver argemonoides.--According to Hildebrand (‘Jahrbuch fur w. Bot.’
B.7 page 466), spontaneously self-fertilised flowers are by no means
sterile.

Glaucium luteum (Papaveraceae).--According to Hildebrand (‘Jahrbuch fur
w. Bot.’ B.7 page 466), spontaneously self-fertilised flowers are by no
means sterile.

Argemone ochroleuca (Papaveraceae).--According to Hildebrand (‘Jahrbuch
fur w. Bot.’ B.7 page 466), spontaneously self-fertilised flowers are by
no means sterile.

Adlumia cirrhosa (Fumariaceae).--Sets an abundance of capsules.

Hypecoum procumbens (Fumariaceae).--Hildebrand says (idem), with respect
to protected flowers, that “eine gute Fruchtbildung eintrete.”

Fumaria officinalis (Fumariaceae).--Covered-up and unprotected plants
apparently produced an equal number of capsules, and the seeds of the
former seemed to the eye equally good. I have often watched this plant,
and so has Hildebrand, and we have never seen an insect visit the
flowers. Hermann Muller has likewise been struck with the rarity of the
visits of insects to it, though he has sometimes seen hive-bees at work.
The flowers may perhaps be visited by small moths, as is probably the
case with the following species.

Fumaria capreolata.--Several large beds of this plant growing wild were
watched by me during many days, but the flowers were never visited by
any insects, though a humble-bee was once seen closely to inspect them.
Nevertheless, as the nectary contains much nectar, especially in the
evening, I felt convinced that they were visited, probably by moths. The
petals do not naturally separate or open in the least; but they had been
opened by some means in a certain proportion of the flowers, in the same
manner as follows when a thick bristle is pushed into the nectary; so
that in this respect they resemble the flowers of Corydalis lutea.
Thirty-four heads, each including many flowers, were examined, and
twenty of them had from one to four flowers, whilst fourteen had not a
single flower thus opened. It is therefore clear that some of the
flowers had been visited by insects, while the majority had not; yet
almost all produced capsules.

Linum usitatissimum (Linaceae).--Appears to be quite fertile. H.
Hoffmann ‘Botanische Zeitung’ 1876 page 566.

Impatiens barbigerum (Balsaminaceae).--The flowers, though excellently
adapted for cross-fertilisation by the bees which freely visit them, set
abundantly under a net.

Impatiens noli-me-tangere (Balsaminaceae).--This species produces
cleistogene and perfect flowers. A plant was covered with a net, and
some perfect flowers, marked with threads, produced eleven spontaneously
self-fertilised capsules, which contained on an average 3.45 seeds. I
neglected to ascertain the number of seeds produced by perfect flowers
exposed to the visits of insects, but I believe it is not greatly in
excess of the above average. Mr. A.W. Bennett has carefully described
the structure of the flowers of I. fulva in ‘Journal of the Linnean
Society’ volume 13 Bot. 1872 page 147. This latter species is said to be
sterile with its own pollen (‘Gardeners’ Chronicle’ 1868 page 1286), and
if so, it presents a remarkable contrast with I. barbigerum and
noli-me-tangere.

Limnanthes douglasii (Geraniaceae).--Highly fertile.

Viscaria oculata (Caryophyllaceae).--Produces plenty of capsules with
good seeds.

Stellaria media (Caryophyllaceae).--Covered-up and uncovered plants
produced an equal number of capsules, and the seeds in both appeared
equally numerous and good.

Beta vulgaris (Chenopodiaceae).--Highly self-fertile.

Vicia sativa (Leguminosae).--Protected and unprotected plants produced
an equal number of pods and equally fine seeds. If there was any
difference between the two lots, the covered-up plants were the most
productive.

Vicia hirsuta.--This species bears the smallest flowers of any British
leguminous plant. The result of covering up plants was exactly the same
as in the last species.

Pisum sativum (Leguminosae).--Fully fertile.

Lathyrus odoratus (Leguminosae).--Fully fertile.

Lathyrus nissolia.--Fully fertile.

Lupinus luteus (Leguminosae).--Fairly productive.

Lupinus pilosus.--Produced plenty of pods.

Ononis minutissima (Leguminosae).--Twelve perfect flowers on a plant
under a net were marked by threads, and produced eight pods, containing
on an average 2.38 seeds. Pods produced by flowers visited by insects
would probably have contained on an average 3.66 seeds, judging from the
effects of artificial cross-fertilisation.

Phaseolus vulgaris (Leguminosae).--Quite fertile.

Trifolium arvense (Leguminosae).--The excessively small flowers are
incessantly visited by hive and humble-bees. When insects were excluded
the flower-heads seemed to produce as many and as fine seeds as the
exposed heads.

Trifolium procumbens.--On one occasion covered-up plants seemed to yield
as many seeds as the uncovered. On a second occasion sixty uncovered
flower-heads yielded 9.1 grains weight of seeds, whilst sixty heads on
protected plants yielded no less than 17.7 grains; so that these latter
plants were much more productive; but this result I suppose was
accidental. I have often watched this plant, and have never seen the
flowers visited by insects; but I suspect that the flowers of this
species, and more especially of Trifolium minus, are frequented by small
nocturnal moths which, as I hear from Mr. Bond, haunt the smaller
clovers.

Medicago lupulina (Leguminosae).--On account of the danger of losing the
seeds, I was forced to gather the pods before they were quite ripe; 150
flower-heads on plants visited by bees yielded pods weighing 101 grains;
whilst 150 heads on protected plants yielded pods weighing 77 grains.
The inequality would probably have been greater if the mature seeds
could have been all safely collected and compared. Ig. Urban (Keimung,
Bluthen, etc., bei Medicago 1873) has described the means of
fertilisation in this genus, as has the Reverend G. Henslow in the
‘Journal of the Linnean Society Botany’ volume 9 1866 pages 327 and 355.

Nicotiana tabacum (Solanaceae).--Fully self-fertile.

Ipomoea purpurea (Convolvulaceae).--Highly self-fertile.

Leptosiphon androsaceus (Polemoniacae).--Plants under a net produced a
good many capsules.

Primula mollis (Primulaceae).--A non-dimorphic species, self-fertile: J.
Scott, in ‘Journal of the Linnean Society Botany’ volume 8 1864 page
120.

Nolana prostrata (Nolanaceae).--Plants covered up in the greenhouse,
yielded seeds by weight compared with uncovered plants, the flowers of
which were visited by many bees, in the ratio of 100 to 61.

Ajuga reptans (Labiatae).--Set a good many seeds; but none of the stems
under a net produced so many as several uncovered stems growing closely
by.

Euphrasia officinalis (Scrophulariaceae).--Covered-up plants produced
plenty of seed; whether less than the exposed plants I cannot say. I saw
two small Dipterous insects (Dolichopos nigripennis and Empis chioptera)
repeatedly sucking the flowers; as they crawled into them, they rubbed
against the bristles which project from the anthers, and became dusted
with pollen.

Veronica agrestis (Scrophulariaceae).--Covered-up plants produced an
abundance of seeds. I do not know whether any insects visit the flowers;
but I have observed Syrphidae repeatedly covered with pollen visiting
the flowers of V. hederaefolia and chamoedrys.

Mimulus luteus (Scrophulariaceae).--Highly self-fertile.

Calceolaria (greenhouse variety) (Scrophulariaceae).--Highly
self-fertile.

Verbascum thapsus (Scrophulariaceae).--Highly self-fertile.

Verbascum lychnitis.--Highly self-fertile.

Vandellia nummularifolia (Scrophulariaceae).--Perfect flowers produce a
good many capsules.

Bartsia odontites (Scrophulariaceae).--Covered-up plants produced a good
many seeds; but several of these were shrivelled, nor were they so
numerous as those produced by unprotected plants, which were incessantly
visited by hive and humble-bees.

Specularia speculum (Lobeliaceae).--Covered plants produced almost as
many capsules as the uncovered.

Lactuca sativa (Compositae).--Covered plants produced some seeds, but
the summer was wet and unfavourable.

Galium aparine (Rubiaceae).--Covered plants produced quite as many seeds
as the uncovered.

Apium petroselinum (Umbelliferae).--Covered plants apparently were as
productive as the uncovered.

Zea mays (Gramineae).--A single plant in the greenhouse produced a good
many grains.

Canna warscewiczi (Marantaceae).--Highly self-fertile.

Orchidaceae.--In Europe Ophrys apifera is as regularly self-fertilised
as is any cleistogene flower. In the United States, South Africa, and
Australia there are a few species which are perfectly self-fertile.
These several cases are given in the second edition of my work on the
Fertilisation of Orchids.

Allium cepa (blood red var.) (Liliaceae).--Four flower-heads were
covered with a net, and they produced somewhat fewer and smaller
capsules than those on the uncovered heads. The capsules were counted on
one uncovered head, and were 289 in number; whilst those on a fine head
from under the net were only 199.]

Each of these lists contains by a mere accident the same number of
genera, namely, forty-nine. The genera in the first list include
sixty-five species, and those in the second sixty species; the Orchideae
in both being excluded. If the genera in this latter order, as well as
in the Asclepiadae and Apocynaceae, had been included, the number of
species which are sterile if insects are excluded would have been
greatly increased; but the lists are confined to species which were
actually experimented on. The results can be considered as only
approximately accurate, for fertility is so variable a character, that
each species ought to have been tried many times. The above number of
species, namely, 125, is as nothing to the host of living plants; but
the mere fact of more than half of them being sterile within the
specified degree, when insects are excluded, is a striking one; for
whenever pollen has to be carried from the anthers to the stigma in
order to ensure full fertility, there is at least a good chance of
cross-fertilisation. I do not, however, believe that if all known plants
were tried in the same manner, half would be found to be sterile within
the specified limits; for many flowers were selected for experiment
which presented some remarkable structure; and such flowers often
require insect-aid. Thus out of the forty-nine genera in the first list,
about thirty-two have flowers which are asymmetrical or present some
remarkable peculiarity; whilst in the second list, including species
which are fully or moderately fertile when insects were excluded, only
about twenty-one out of the forty-nine are asymmetrical or present any
remarkable peculiarity.

MEANS OF CROSS-FERTILISATION.

The most important of all the means by which pollen is carried from the
anthers to the stigma of the same flower, or from flower to flower, are
insects, belonging to the orders of Hymenoptera, Lepidoptera, and
Diptera; and in some parts of the world, birds. (10/1. I will here give
all the cases known to me of birds fertilising flowers. In South Brazil,
humming-birds certainly fertilise the various species of Abutilon, which
are sterile without their aid (Fritz Muller ‘Jenaische Zeitschrift f.
Naturwiss.’ B. 7 1872 page 24.) Long-beaked humming-birds visit the
flowers of Brugmansia, whilst some of the short-beaked species often
penetrate its large corolla in order to obtain the nectar in an
illegitimate manner, in the same manner as do bees in all parts of the
world. It appears, indeed, that the beaks of humming-birds are specially
adapted to the various kinds of flowers which they visit: on the
Cordillera they suck the Salviae, and lacerate the flowers of the
Tacsoniae; in Nicaragua, Mr. Belt saw them sucking the flowers of
Marcgravia and Erythina, and thus they carried pollen from flower to
flower. In North America they are said to frequent the flowers of
Impatiens: (Gould ‘Introduction to the Trochilidae’ 1861 pages 15, 120;
‘Gardeners’ Chronicle’ 1869 page 389; ‘The Naturalist in Nicaragua’ page
129; ‘Journal of the Linnean Society Botany’ volume 13 1872 page 151.) I
may add that I often saw in Chile a Mimus with its head yellow with
pollen from, as I believe, a Cassia. I have been assured that at the
Cape of Good Hope, Strelitzia is fertilised by the Nectarinidae. There
can hardly be a doubt that many Australian flowers are fertilised by the
many honey-sucking birds of that country. Mr. Wallace remarks (address
to the Biological Section, British Association 1876) that he has “often
observed the beaks and faces of the brush-tongued lories of the Moluccas
covered with pollen.” In New Zealand, many specimens of the Anthornis
melanura had their heads coloured with pollen from the flowers of an
endemic species of Fuchsia (Potts ‘Transactions of the New Zealand
Institute’ volume 3 1870 page 72.) Next in importance, but in a quite
subordinate degree, is the wind; and with some aquatic plants, according
to Delpino, currents of water. The simple fact of the necessity in many
cases of extraneous aid for the transport of the pollen, and the many
contrivances for this purpose, render it highly probable that some great
benefit is thus gained; and this conclusion has now been firmly
established by the proved superiority in growth, vigour, and fertility
of plants of crossed parentage over those of self-fertilised parentage.
But we should always keep in mind that two somewhat opposed ends have to
be gained; the first and more important one being the production of
seeds by any means, and the second, cross-fertilisation.

The advantages derived from cross-fertilisation throw a flood of light
on most of the chief characters of flowers. We can thus understand their
large size and bright colours, and in some cases the bright tints of the
adjoining parts, such as the peduncles, bracteae, etc. By this means
they are rendered conspicuous to insects, on the same principle that
almost every fruit which is devoured by birds presents a strong contrast
in colour with the green foliage, in order that it may be seen, and its
seeds freely disseminated. With some flowers conspicuousness is gained
at the expense even of the reproductive organs, as with the ray-florets
of many Compositae, the exterior flowers of Hydrangea, and the terminal
flowers of the Feather-hyacinth or Muscari. There is also reason to
believe, and this was the opinion of Sprengel, that flowers differ in
colour in accordance with the kinds of insects which frequent them.

Not only do the bright colours of flowers serve to attract insects, but
dark-coloured streaks and marks are often present, which Sprengel long
ago maintained served as guides to the nectary. These marks follow the
veins in the petals, or lie between them. They may occur on only one, or
on all excepting one or more of the upper or lower petals; or they may
form a dark ring round the tubular part of the corolla, or be confined
to the lips of an irregular flower. In the white varieties of many
flowers, such as of Digitalis purpurea, Antirrhinum majus, several
species of Dianthus, Phlox, Myosotis, Rhododendron, Pelargonium, Primula
and Petunia, the marks generally persist, whilst the rest of the corolla
has become of a pure white; but this may be due merely to their colour
being more intense and thus less readily obliterated. Sprengel’s notion
of the use of these marks as guides appeared to me for a long time
fanciful; for insects, without such aid, readily discover and bite holes
through the nectary from the outside. They also discover the minute
nectar-secreting glands on the stipules and leaves of certain plants.
Moreover, some few plants, such as certain poppies, which are not
nectariferous, have guiding marks; but we might perhaps expect that some
few plants would retain traces of a former nectariferous condition. On
the other hand, these marks are much more common on asymmetrical
flowers, the entrance into which would be apt to puzzle insects, than on
regular flowers. Sir J. Lubbock has also proved that bees readily
distinguish colours, and that they lose much time if the position of
honey which they have once visited be in the least changed. (10/2.
‘British Wild Flowers in relation to Insects’ 1875 page 44.) The
following case affords, I think, the best evidence that these marks have
really been developed in correlation with the nectary. The two upper
petals of the common Pelargonium are thus marked near their bases; and I
have repeatedly observed that when the flowers vary so as to become
peloric or regular, they lose their nectaries and at the same time the
dark marks. When the nectary is only partially aborted, only one of the
upper petals loses its mark. Therefore the nectary and these marks
clearly stand in some sort of close relation to one another; and the
simplest view is that they were developed together for a special
purpose; the only conceivable one being that the marks serve as a guide
to the nectary. It is, however, evident from what has been already said,
that insects could discover the nectar without the aid of guiding marks.
They are of service to the plant, only by aiding insects to visit and
suck a greater number of flowers within a given time than would
otherwise be possible; and thus there will be a better chance of
fertilisation by pollen brought from a distinct plant, and this we know
is of paramount importance.

The odours emitted by flowers attract insects, as I have observed in the
case of plants covered by a muslin net. Nageli affixed artificial
flowers to branches, scenting some with essential oils and leaving
others unscented; and insects were attracted to the former in an
unmistakable manner. (10/3. ‘Enstehung etc. der Naturhist. Art.’ 1865
page 23.) Not a few flowers are both conspicuous and odoriferous. Of all
colours, white is the prevailing one; and of white flowers a
considerably larger proportion smell sweetly than of any other colour,
namely, 14.6 per cent; of red, only 8.2 per cent are odoriferous. (10/4.
The colours and odours of the flowers of 4200 species have been
tabulated by Landgrabe and by Schubler and Kohler. I have not seen their
original works, but a very full abstract is given in Loudon’s
‘Gardeners’ Magazine’ volume 13 1837 page 367.) The fact of a larger
proportion of white flowers smelling sweetly may depend in part on those
which are fertilised by moths requiring the double aid of
conspicuousness in the dusk and of odour. So great is the economy of
nature, that most flowers which are fertilised by crepuscular or
nocturnal insects emit their odour chiefly or exclusively in the
evening. Some flowers, however, which are highly odoriferous depend
solely on this quality for their fertilisation, such as the
night-flowering stock (Hesperis) and some species of Daphne; and these
present the rare case of flowers which are fertilised by insects being
obscurely coloured.

The storage of a supply of nectar in a protected place is manifestly
connected with the visits of insects. So is the position which the
stamens and pistils occupy, either permanently or at the proper period
through their own movements; for when mature they invariably stand in
the pathway leading to the nectary. The shape of the nectary and of the
adjoining parts are likewise related to the particular kinds of insects
which habitually visit the flowers; this has been well shown by Hermann
Muller by his comparison of lowland species which are chiefly visited by
bees, with alpine species belonging to the same genera which are visited
by butterflies. (10/5. ‘Nature’ 1874 page 110, 1875 page 190, 1876 pages
210, 289.) Flowers may also be adapted to certain kinds of insects, by
secreting nectar particularly attractive to them, and unattractive to
other kinds; of which fact Epipactis latifolia offers the most striking
instance known to me, as it is visited exclusively by wasps. Structures
also exist, such as the hairs within the corolla of the fox glove
(Digitalis), which apparently serve to exclude insects that are not well
fitted to bring pollen from one flower to another. (10/6. Belt ‘The
Naturalist in Nicaragua’ 1874 page 132.) I need say nothing here of the
endless contrivances, such as the viscid glands attached to the
pollen-masses of the Orchideae and Asclepiadae, or the viscid or
roughened state of the pollen-grains of many plants, or the irritability
of their stamens which move when touched by insects etc.--as all these
contrivances evidently favour or ensure cross-fertilisation.

All ordinary flowers are so far open that insects can force an entrance
into them, notwithstanding that some, like the Snapdragon (Antirrhinum),
various Papilionaceous and Fumariaceous flowers, are in appearance
closed. It cannot be maintained that their openness is necessary for
fertility, as cleistogene flowers which are permanently closed yield a
full complement of seeds. Pollen contains much nitrogen and
phosphorus--the two most precious of all the elements for the growth of
plants--but in the case of most open flowers, a large quantity of pollen
is consumed by pollen-devouring insects, and a large quantity is
destroyed during long-continued rain. With many plants this latter evil
is guarded against, as far as is possible, by the anthers opening only
during dry weather (10/7. Mr. Blackley observed that the ripe anthers of
rye did not dehisce whilst kept under a bell-glass in a damp atmosphere,
whilst other anthers exposed to the same temperature in the open air
dehisced freely. He also found much more pollen adhering to the sticky
slides, which were attached to kites and sent high up in the atmosphere,
during the first fine and dry days after wet weather, than at other
times: ‘Experimental Researches on Hay Fever’ 1873 page 127.)--by the
position and form of some or all of the petals,--by the presence of
hairs, etc., and as Kerner has shown in his interesting essay, by the
movements of the petals or of the whole flower during cold and wet
weather. (10/8. ‘Die Schutzmittel des Pollens’ 1873.) In order to
compensate the loss of pollen in so many ways, the anthers produce a far
larger amount than is necessary for the fertilisation of the same
flower. I know this from my own experiments on Ipomoea, given in the
Introduction; and it is still more plainly shown by the astonishingly
small quantity produced by cleistogene flowers, which lose none of their
pollen, in comparison with that produced by the open flowers borne by
the same plants; and yet this small quantity suffices for the
fertilisation of all their numerous seeds. Mr. Hassall took pains in
estimating the number of pollen-grains produced by a flower of the
Dandelion (Leontodon), and found the number to be 243,600, and in a
Paeony 3,654,000 grains. (10/9. ‘Annals and Magazine of Natural History’
volume 8 1842 page 108.) The editor of the ‘Botanical Register’ counted
the ovules in the flowers of Wistaria sinensis, and carefully estimated
the number of pollen-grains, and he found that for each ovule there were
7000 grains. (10/10. Quoted in ‘Gardeners’ Chronicle’ 1846 page 771.)
With Mirabilis, three or four of the very large pollen-grains are
sufficient to fertilise an ovule; but I do not know how many grains a
flower produces. With Hibiscus, Kolreuter found that sixty grains were
necessary to fertilise all the ovules of a flower, and he calculated
that 4863 grains were produced by a single flower, or eighty-one times
too many. With Geum urbanum, however, according to Gartner, the pollen
is only ten times too much. (10/11. Kolreuter ‘Vorlaufige Nachricht’
1761 page 9. Gartner ‘Beitrage zur Kenntniss’ etc. page 346.) As we thus
see that the open state of all ordinary flowers, and the consequent loss
of much pollen, necessitate the development of so prodigious an excess
of this precious substance, why, it may be asked, are flowers always
left open? As many plants exist throughout the vegetable kingdom which
bear cleistogene flowers, there can hardly be a doubt that all open
flowers might easily have been converted into closed ones. The graduated
steps by which this process could have been effected may be seen at the
present time in Lathyrus nissolia, Biophytum sensitivum, and several
other plants. The answer to the above question obviously is, that with
permanently closed flowers there could be no cross-fertilisation.

The frequency, almost regularity, with which pollen is transported by
insects from flower to flower, often from a considerable distance, well
deserves attention. (10/12. An experiment made by Kolreuter ‘Forsetsung’
etc. 1763 page 69, affords good evidence on this head. Hibiscus
vesicarius is strongly dichogamous, its pollen being shed before the
stigmas are mature. Kolreuter marked 310 flowers, and put pollen from
other flowers on their stigmas every day, so that they were thoroughly
fertilised; and he left the same number of other flowers to the agency
of insects. Afterwards he counted the seeds of both lots: the flowers
which he had fertilised with such astonishing care produced 11,237
seeds, whilst those left to the insects produced 10,886; that is, a less
number by only 351; and this small inferiority is fully accounted for by
the insects not having worked during some days, when the weather was
cold with continued rain.) This is best shown by the impossibility in
many cases of raising two varieties of the same species pure, if they
grow at all near together; but to this subject I shall presently return;
also by the many cases of hybrids which have appeared spontaneously both
in gardens and a state of nature. With respect to the distance from
which pollen is often brought, no one who has had any experience would
expect to obtain pure cabbage-seed, for instance, if a plant of another
variety grew within two or three hundred yards. An accurate observer,
the late Mr. Masters of Canterbury, assured me that he once had his
whole stock of seeds “seriously affected with purple bastards,” by some
plants of purple kale which flowered in a cottager’s garden at the
distance of half a mile; no other plant of this variety growing any
nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of
a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis
(Habenaria chlorantha) sticking to their eyes, and, therefore, in the
proper position for fertilising the flowers of this species, on an
island in Derwentwater, at the distance of half a mile from any place
where this plant grew: ‘Nature’ 1872 page 393.) But the most striking
case which has been recorded is that by M. Godron, who shows by the
nature of the hybrids produced that Primula grandiflora must have been
crossed with pollen brought by bees from P. officinalis, growing at the
distance of above two kilometres, or of about one English mile and a
quarter. (10/14. ‘Revue des Sc. Nat.’ 1875 page 331.)

All those who have long attended to hybridisation, insist in the
strongest terms on the liability of castrated flowers to be fertilised
by pollen brought from distant plants of the same species. (10/15. See,
for instance, the remarks by Herbert ‘Amaryllidaceae’ 1837 page 349.
Also Gartner’s strong expressions on this subject in his
‘Bastarderzeugung’ 1849 page 670 and ‘Kenntniss der Befruchtung’ 1844
pages 510, 573. Also Lecoq ‘De la Fecondation’ etc. 1845 page 27. Some
statements have been published during late years of the extraordinary
tendency of hybrid plants to revert to their parent forms; but as it is
not said how the flowers were protected from insects, it may be
suspected that they were often fertilised with pollen brought from a
distance from the parent-species.) The following case shows this in the
clearest manner: Gartner, before he had gained much experience,
castrated and fertilised 520 flowers on various species with pollen of
other genera or other species, but left them unprotected; for, as he
says, he thought it a laughable idea that pollen should be brought from
flowers of the same species, none of which grew nearer than between 500
and 600 yards. (10/16. ‘Kenntniss der Befruchtung’ pages 539, 550, 575,
576.) The result was that 289 of these 520 flowers yielded no seed, or
none that germinated; the seed of 29 flowers produced hybrids, such as
might have been expected from the nature of the pollen employed; and
lastly, the seed of the remaining 202 flowers produced perfectly pure
plants, so that these flowers must have been fertilised by pollen
brought by insects from a distance of between 500 and 600 yards. (10/17.
Henschel’s experiments quoted by Gartner ‘Kenntniss’ etc. page 574,
which are worthless in all other respects, likewise show how largely
flowers are intercrossed by insects. He castrated many flowers on
thirty-seven species, belonging to twenty-two genera, and put on their
stigmas either no pollen, or pollen from distinct genera, yet they all
seeded, and all the seedlings raised from them were of course pure.) It
is of course possible that some of these 202 flowers might have been
fertilised by pollen left accidentally in them when they were castrated;
but to show how improbable this is, I may add that Gartner, during the
next eighteen years, castrated no less than 8042 flowers and hybridised
them in a closed room; and the seeds from only seventy of these, that is
considerably less than 1 per cent, produced pure or unhybridised
offspring. (10/18. ‘Kenntniss’ etc. pages 555, 576.)

From the various facts now given, it is evident that most flowers are
adapted in an admirable manner for cross-fertilisation. Nevertheless,
the greater number likewise present structures which are manifestly
adapted, though not in so striking a manner, for self-fertilisation. The
chief of these is their hermaphrodite condition; that is, their
including within the same corolla both the male and female reproductive
organs. These often stand close together and are mature at the same
time; so that pollen from the same flower cannot fail to be deposited at
the proper period on the stigma. There are also various details of
structure adapted for self-fertilisation. (10/19. Hermann Muller ‘Die
Befruchtung’ etc. page 448.) Such structures are best shown in those
curious cases discovered by Hermann Muller, in which a species exists
under two forms,--one bearing conspicuous flowers fitted for
cross-fertilisation, and the other smaller flowers fitted for
self-fertilisation, with many parts in the latter slightly modified for
this special purpose. (10/20. ‘Nature’ 1873 pages 44, 433.)

As two objects in most respects opposed, namely, cross-fertilisation and
self-fertilisation, have in many cases to be gained, we can understand
the co-existence in so many flowers of structures which appear at first
sight unnecessarily complex and of an opposed nature. We can thus
understand the great contrast in structure between cleistogene flowers,
which are adapted exclusively for self-fertilisation, and ordinary
flowers on the same plant, which are adapted so as to allow of at least
occasional cross-fertilisation. (10/21. Fritz Muller has discovered in
the animal kingdom ‘Jenaische Zeitschr.’ B. 4 page 451, a case curiously
analogous to that of the plants which bear cleistogene and perfect
flowers. He finds in the nests of termites in Brazil, males and females
with imperfect wings, which do not leave the nests and propagate the
species in a cleistogene manner, but only if a fully-developed queen
after swarming does not enter the old nest. The fully-developed males
and females are winged, and individuals from distinct nests can hardly
fail often to intercross. In the act of swarming they are destroyed in
almost infinite numbers by a host of enemies, so that a queen may often
fail to enter an old nest; and then the imperfectly developed males and
females propagate and keep up the stock.) The former are always minute,
completely closed, with their petals more or less rudimentary and never
brightly coloured; they never secrete nectar, never are odoriferous,
have very small anthers which produce only a few grains of pollen, and
their stigmas are but little developed. Bearing in mind that some
flowers are cross-fertilised by the wind (called anemophilous by
Delpino), and others by insects (called entomophilous), we can further
understand, as was pointed out by me several years ago, the great
contrast in appearance between these two classes of flowers. (10/22.
‘Journal of the Linnean Society’ volume 7 Botany 1863 page 77.)
Anemophilous flowers resemble in many respects cleistogene flowers, but
differ widely in not being closed, in producing an extraordinary amount
of pollen which is always incoherent, and in the stigma often being
largely developed or plumose. We certainly owe the beauty and odour of
our flowers and the storage of a large supply of honey to the existence
of insects.

ON THE RELATION BETWEEN THE STRUCTURE AND CONSPICUOUSNESS OF FLOWERS,
THE VISITS OF INSECTS, AND THE ADVANTAGES OF CROSS-FERTILISATION.

It has already been shown that there is no close relation between the
number of seeds produced by flowers when crossed and self-fertilised,
and the degree to which their offspring are aaffected by the two
processes. I have also given reasons for believing that the inefficiency
of a plant’s own pollen is in most cases an incidental result, or has
not been specially acquired for the sake of preventing
self-fertilisation. On the other hand, there can hardly be a doubt that
dichogamy, which prevails according to Hildebrand in the greater number
of species (10/23. ‘Die Geschlecter Vertheiling’ etc. page 32.),--that
the heterostyled condition of certain plants,--and that many mechanical
structures--have all been acquired so as both to check
self-fertilisation and to favour cross-fertilisation. The means for
favouring cross-fertilisation must have been acquired before those which
prevent self-fertilisation; as it would manifestly be injurious to a
plant that its stigma should fail to receive its own pollen, unless it
had already become well adapted for receiving pollen from another
individual. It should be observed that many plants still possess a high
power of self-fertilisation, although their flowers are excellently
constructed for cross-fertilisation--for instance, those of many
papilionaceous species.

It may be admitted as almost certain that some structures, such as a
narrow elongated nectary, or a long tubular corolla, have been developed
in order that certain kinds of insects alone should obtain the nectar.
These insects would thus find a store of nectar preserved from the
attacks of other insects; and they would thus be led to visit frequently
such flowers and to carry pollen from one to the other. (10/24. See the
interesting discussion on this subject by Hermann Muller, ‘Die
Befruchtung’ etc. page 431.) It might perhaps have been expected that
plants having their flowers thus peculiarly constructed would profit in
a greater degree by being crossed, than ordinary or simple flowers; but
this does not seem to hold good. Thus Tropaeolum minus has a long
nectary and an irregular corolla, whilst Limnanthes douglasii has a
regular flower and no proper nectary, yet the crossed seedlings of both
species are to the self-fertilised in height as 100 to 79. Salvia
coccinea has an irregular corolla, with a curious apparatus by which
insects depress the stamens, while the flowers of Ipomoea are regular;
and the crossed seedlings of the former are in height to the
self-fertilised as 100 to 76, whilst those of the Ipomoea are as 100 to
77. Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and
the crossed seedlings of both are in height to the self-fertilised as
100 to 69.

With all European plants, excepting the comparatively rare anemophilous
kinds, the possibility of distinct individuals intercrossing depends on
the visits of insects; and Hermann Muller has proved by his valuable
observations, that large conspicuous flowers are visited much more
frequently and by many more kinds of insects, than are small
inconspicuous flowers. He further remarks that the flowers which are
rarely visited must be capable of self-fertilisation, otherwise they
would quickly become extinct. (10/25. ‘Die Befruchtung’ etc. page 426.
‘Nature’ 1873 page 433.) There is, however, some liability to error in
forming a judgment on this head, from the extreme difficulty of
ascertaining whether flowers which are rarely or never visited during
the day (as in the above given case of Fumaria capreolata) are not
visited by small nocturnal Lepidoptera, which are known to be strongly
attracted by sugar. (10/26. In answer to a question by me, the editor of
an entomological journal writes--“The Depressariae, as is notorious to
every collector of Noctuae, come very freely to sugar, and no doubt
naturally visit flowers:” the ‘Entomologists’ Weekly Intelligencer’ 1860
page 103.) The two lists given in the early part of this chapter support
Muller’s conclusion that small and inconspicuous flowers are completely
self-fertile: for only eight or nine out of the 125 species in the two
lists come under this head, and all of these were proved to be highly
fertile when insects were excluded. The singularly inconspicuous flowers
of the Fly Ophrys (O. muscifera), as I have elsewhere shown, are rarely
visited by insects; and it is a strange instance of imperfection, in
contradiction to the above rule, that these flowers are not
self-fertile, so that a large proportion of them do not produce seeds.
The converse of the rule that plants bearing small and inconspicuous
flowers are self-fertile, namely, that plants with large and conspicuous
flowers are self-sterile, is far from true, as may be seen in our second
list of spontaneously self-fertile species; for this list includes such
species as Ipomoea purpurea, Adonis aestivalis, Verbascum thapsus, Pisum
sativum, Lathyrus odoratus, some species of Papaver and of Nymphaea, and
others.

The rarity of the visits of insects to small flowers, does not depend
altogether on their inconspicuousness, but likewise on the absence of
some sufficient attraction; for the flowers of Trifolium arvense are
extremely small, yet are incessantly visited by hive and humble-bees, as
are the small and dingy flowers of the asparagus. The flowers of Linaria
cymbalaria are small and not very conspicuous, yet at the proper time
they are freely visited by hive-bees. I may add that, according to Mr.
Bennett, there is another and quite distinct class of plants which
cannot be much frequented by insects, as they flower either exclusively
or often during the winter, and these seem adapted for
self-fertilisation, as they shed their pollen before the flowers expand.
(10/27. ‘Nature’ 1869 page 11.)

That many flowers have been rendered conspicuous for the sake of guiding
insects to them is highly probable or almost certain; but it may be
asked, have other flowers been rendered inconspicuous so that they may
not be frequently visited, or have they merely retained a former and
primitive condition? If a plant were much reduced in size, so probably
would be the flowers through correlated growth, and this may possibly
account for some cases; but the size and colour of the corolla are both
extremely variable characters, and it can hardly be doubted that if
large and brightly-coloured flowers were advantageous to any species,
these could be acquired through natural selection within a moderate
lapse of time, as indeed we see with most alpine plants. Papilionaceous
flowers are manifestly constructed in relation to the visits of insects,
and it seems improbable, from the usual character of the group, that the
progenitors of the genera Vicia and Trifolium produced such minute and
unattractive flowers as those of V. hirsuta and T. procumbens. We are
thus led to infer that some plants either have not had their flowers
increased in size, or have actually had them reduced and purposely
rendered inconspicuous, so that they are now but little visited by
insects. In either case they must also have acquired or retained a high
degree of self-fertility.

If it became from any cause advantageous to a species to have its
capacity for self-fertilisation increased, there is little difficulty in
believing that this could readily be effected; for three cases of plants
varying in such a manner as to be more fertile with their own pollen
than they originally were, occurred in the course of my few experiments,
namely, with Mimulus, Ipomoea, and Nicotiana. Nor is there any reason to
doubt that many kinds of plants are capable under favourable
circumstances of propagating themselves for very many generations by
self-fertilisation. This is the case with the varieties of Pisum sativum
and of Lathyrus odoratus which are cultivated in England, and with
Ophrys apifera and some other plants in a state of nature. Nevertheless,
most or all of these plants retain structures in an efficient state
which cannot be of the least use excepting for cross-fertilisation. We
have also seen reason to suspect that self-fertilisation is in some
peculiar manner beneficial to certain plants; but if this be really the
case, the benefit thus derived is far more than counter-balanced by a
cross with a fresh stock or with a slightly different variety.

Notwithstanding the several considerations just advanced, it seems to me
highly improbable that plants bearing small and inconspicuous flowers
have been or should continue to be subjected to self-fertilisation for a
long series of generations. I think so, not from the evil which
manifestly follows from self-fertilisation, in many cases even in the
first generation, as with Viola tricolor, Sarothamnus, Nemophila,
Cyclamen, etc.; nor from the probability of the evil increasing after
several generations, for on this latter head I have not sufficient
evidence, owing to the manner in which my experiments were conducted.
But if plants bearing small and inconspicuous flowers were not
occasionally intercrossed, and did not profit by the process, all their
flowers would probably have been rendered cleistogene, as they would
thus have largely benefited by having to produce only a small quantity
of safely-protected pollen. In coming to this conclusion, I have been
guided by the frequency with which plants belonging to distinct orders
have been rendered cleistogene. But I can hear of no instance of a
species with all its flowers rendered permanently cleistogene. Leersia
makes the nearest approach to this state; but as already stated, it has
been known to produce perfect flowers in one part of Germany. Some other
plants of the cleistogene class, for instance Aspicarpa, have failed to
produce perfect flowers during several years in a hothouse; but it does
not follow that they would fail to do so in their native country, any
more than with Vandellia, which with me produced only cleistogene
flowers during certain years. Plants belonging to this class commonly
bear both kinds of flowers every season, and the perfect flowers of
Viola canina yield fine capsules, but only when visited by bees. We have
also seen that the seedlings of Ononis minutissima, raised from the
perfect flowers fertilised with pollen from another plant, were finer
than those from self-fertilised flowers; and this was likewise the case
to a certain extent with Vandellia. As therefore no species which at one
time bore small and inconspicuous flowers has had all its flowers
rendered cleistogene, I must believe that plants now bearing small and
inconspicuous flowers profit by their still remaining open, so as to be
occasionally intercrossed by insects. It has been one of the greatest
oversights in my work that I did not experimentise on such flowers,
owing to the difficulty of fertilising them, and to my not having seen
the importance of the subject. (10/28. Some of the species of Solanum
would be good ones for such experiments, for they are said by Hermann
Muller ‘Befruchtung’ page 434, to be unattractive to insects from not
secreting nectar, not producing much pollen, and not being very
conspicuous. Hence probably it is that, according to Verlot ‘Production
des Varieties’ 1865 page 72, the varieties of “les aubergines et les
tomates” (species of Solanum) do not intercross when they are cultivated
near together; but it should be remembered that these are not endemic
species. On the other hand, the flowers of the common potato (S.
tuberosum), though they do not secrete nectar Kurr ‘Bedeutung der
Nektarien’ 1833 page 40, yet cannot be considered as inconspicuous, and
they are sometimes visited by diptera (Muller), and, as I have seen, by
humble-bees. Tinzmann (as quoted in ‘Gardeners’ Chronicle’ 1846 page
183, found that some of the varieties did not bear seed when fertilised
with pollen from the same variety, but were fertile with that from
another variety.)

It should be remembered that in two of the cases in which highly
self-fertile varieties appeared amongst my experimental plants, namely,
with Mimulus and Nicotiana, such varieties were greatly benefited by a
cross with a fresh stock or with a slightly different variety; and this
likewise was the case with the cultivated varieties of Pisum sativum and
Lathyrus odoratus, which have been long propagated by
self-fertilisation. Therefore until the contrary is distinctly proved, I
must believe that as a general rule small and inconspicuous flowers are
occasionally intercrossed by insects; and that after long-continued
self-fertilisation, if they are crossed with pollen brought from a plant
growing under somewhat different conditions, or descended from one thus
growing, their offspring would profit greatly. It cannot be admitted,
under our present state of knowledge, that self-fertilisation continued
during many successive generations is ever the most beneficial method of
reproduction.

THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN
FROM A DISTINCT PLANT.

We have seen in four cases that seedlings raised from a cross between
flowers on the same plant, even on plants appearing distinct from having
been propagated by stolons or cuttings, were not superior to seedlings
from self-fertilised flowers; and in a fifth case (Digitalis) superior
only in a slight degree. Therefore we might expect that with plants
growing in a state of nature a cross between the flowers on distinct
individuals, and not merely between the flowers on the same plant, would
generally or often be effected by some means. The fact of bees and of
some Diptera visiting the flowers of the same species as long as they
can, instead of promiscuously visiting various species, favours the
intercrossing of distinct plants. On the other hand, insects usually
search a large number of flowers on the same plant before they fly to
another, and this is opposed to cross-fertilisation. The extraordinary
number of flowers which bees are able to search within a very short
space of time, as will be shown in a future chapter, increases the
chance of cross-fertilisation; as does the fact that they are not able
to perceive without entering a flower whether other bees have exhausted
the nectar. For instance, Hermann Muller found that four-fifths of the
flowers of Lamium album which a humble-bee visited had been already
exhausted of their nectar. (10/29. ‘Die Befruchtung’ etc. page 311.) In
order that distinct plants should be intercrossed, it is of course
indispensable that two or more individuals should grow near one another;
and this is generally the case. Thus A. de Candolle remarks that in
ascending a mountain the individuals of the same species do not commonly
disappear near its upper limit quite gradually, but rather abruptly.
This fact can hardly be explained by the nature of the conditions, as
these graduate away in an insensible manner, and it probably depends in
large part on vigorous seedlings being produced only as high up the
mountain as many individuals can subsist together.

With respect to dioecious plants, distinct individuals must always
fertilise each other. With monoecious plants, as pollen has to be
carried from flower to flower, there will always be a good chance of its
being carried from plant to plant. Delpino has also observed the curious
fact that certain individuals of the monoecious walnut (Juglans regia)
are proterandrous, and others proterogynous, and these will reciprocally
fertilise each other. (10/30. ‘Ult. Osservazioni’ etc. part 2 fasc 2
page 337.) So it is with the common nut (Corylus avellana) (10/31.
‘Nature’ 1875 page 26.), and, what is more surprising, with some few
hermaphrodite plants, as observed by Hermann Muller. (10/32. ‘Die
Befruchtung’ etc. pages 285, 339.) These latter plants cannot fail to
act on each other like dimorphic or trimorphic species, in which the
union of two individuals is necessary for full and normal fertility.
With ordinary hermaphrodite species, the expansion of only a few flowers
at the same time is one of the simplest means for favouring the
intercrossing of distinct individuals; but this would render the plants
less conspicuous to insects, unless the flowers were of large size, as
in the case of several bulbous plants. Kerner thinks that it is for this
object that the Australian Villarsia parnassifolia produces daily only a
single flower. (10/33. ‘Die Schutzmittel’ etc page 23.) Mr. Cheeseman
also remarks, that as certain Orchids in New Zealand which require
insect-aid for their fertilisation bear only a single flower, distinct
plants cannot fail to intercross. (10/34. ‘Transactions of the New
Zealand Institute’ volume 5 1873 page 356.)

Dichogamy, which prevails so extensively throughout the vegetable
kingdom, much increases the chance of distinct individuals
intercrossing. With proterandrous species, which are far more ccommon
than proterogynous, the young flowers are exclusively male in function,
and the older ones exclusively female; and as bees habitually alight low
down on the spikes of flowers in order to crawl upwards, they get dusted
with pollen from the uppermost flowers, which they carry to the stigmas
of the lower and older flowers on the next spike which they visit. The
degree to which distinct plants will thus be intercrossed depends on the
number of spikes in full flower at the same time on the same plant. With
proterogynous flowers and with depending racemes, the manner in which
insects visit the flowers ought to be reversed in order that distinct
plants should be intercrossed. But this whole subject requires further
investigation, as the great importance of crosses between distinct
individuals, instead of merely between distinct flowers, has hitherto
been hardly recognised.

In some few cases the special movements of certain organs almost ensure
pollen being carried from plant to plant. Thus with many orchids, the
pollen-masses after becoming attached to the head or proboscis of an
insect do not move into the proper position for striking the stigma,
until ample time has elapsed for the insect to fly to another plant.
With Spiranthes autumnalis, the pollen-masses cannot be applied to the
stigma until the labellum and rostellum have moved apart, and this
movement is very slow. (10/35. ‘The Various Contrivances by which
British and Foreign Orchids are fertilised’ first edition page 128.)
With Posoqueria fragrans (one of the Rubiaceae) the same end is gained
by the movement of a specially constructed stamen, as described by Fritz
Muller.

We now come to a far more general and therefore more important means by
which the mutual fertilisation of distinct plants is effected, namely,
the fertilising power of pollen from another variety or individual being
greater than that of a plant’s own pollen. The simplest and best known
case of prepotent action in pollen, though it does not bear directly on
our present subject, is that of a plant’s own pollen over that from a
distinct species. If pollen from a distinct species be placed on the
stigma of a castrated flower, and then after the interval of several
hours, pollen from the same species be placed on the stigma, the effects
of the former are wholly obliterated, excepting in some rare cases. If
two varieties are treated in the same manner, the result is analogous,
though of directly opposite nature; for pollen from any other variety is
often or generally prepotent over that from the same flower. I will give
some instances: the pollen of Mimulus luteus regularly falls on the
stigma of its own flower, for the plant is highly fertile when insects
are excluded. Now several flowers on a remarkably constant whitish
variety were fertilised without being castrated with pollen from a
yellowish variety; and of the twenty-eight seedlings thus raised, every
one bore yellowish flowers, so that the pollen of the yellow variety
completely overwhelmed that of the mother-plant. Again, Iberis umbellata
is spontaneously self-fertile, and I saw an abundance of pollen from
their own flowers on the stigmas; nevertheless, of thirty seedlings
raised from non-castrated flowers of a crimson variety crossed with
pollen from a pink variety, twenty-four bore pink flowers, like those of
the male or pollen-bearing parent.

In these two cases flowers were fertilised with pollen from a distinct
variety, and this was shown to be prepotent by the character of the
offspring. Nearly similar results often follow when two or more
self-fertile varieties are allowed to grow near one another and are
visited by insects. The common cabbage produces a large number of
flowers on the same stalk, and when insects are excluded these set many
capsules, moderately rich in seeds. I planted a white Kohl-rabi, a
purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a
Sugar-loaf cabbage near together and left them uncovered. Seeds
collected from each kind were sown in separate beds; and the majority of
the seedlings in all five beds were mongrelised in the most complicated
manner, some taking more after one variety, and some after another. The
effects of the Kohl-rabi were particularly plain in the enlarged stems
of many of the seedlings. Altogether 233 plants were raised, of which
155 were mongrelised in the plainest manner, and of the remaining 78 not
half were absolutely pure. I repeated the experiment by planting near
together two varieties of cabbage with purple-green and white-green
lacinated leaves; and of the 325 seedlings raised from the purple-green
variety, 165 had white-green and 160 purple-green leaves. Of the 466
seedlings raised from the white-green variety, 220 had purple-green and
246 white-green leaves. These cases show how largely pollen from a
neighbouring variety of the cabbage effaces the action of the plant’s
own pollen. We should bear in mind that pollen must be carried by the
bees from flower to flower on the same large branching stem much more
abundantly than from plant to plant; and in the case of plants the
flowers of which are in some degree dichogamous, those on the same stem
would be of different ages, and would thus be as ready for mutual
fertilisation as the flowers on distinct plants, were it not for the
prepotency of pollen from another variety. (10/36. A writer in the
‘Gardeners’ Chronicle’ 1855 page 730, says that he planted a bed of
turnips (Brassica rapa) and of rape (B. napus) close together, and sowed
the seeds of the former. The result was that scarcely one seedling was
true to its kind, and several closely resembled rape.)

Several varieties of the radish (Raphanus sativus), which is moderately
self-fertile when insects are excluded, were in flower at the same time
in my garden. Seed was collected from one of them, and out of twenty-two
seedlings thus raised only twelve were true to their kind. (10/37.
Duhamel as quoted by Godron ‘De l’Espece’ tome 2 page 50, makes an
analogous statement with respect to this plant.)

The onion produces a large number of flowers, all crowded together into
a large globular head, each flower having six stamens; so that the
stigmas receive plenty of pollen from their own and the adjoining
anthers. Consequently the plant is fairly self-fertile when protected
from insects. A blood-red, silver, globe and Spanish onion were planted
near together; and seedlings were raised from each kind in four separate
beds. In all the beds mongrels of various kinds were numerous, except
amongst the ten seedlings from the blood-red onion, which included only
two. Altogether forty-six seedlings were raised, of which thirty-one had
been plainly crossed.

A similar result is known to follow with the varieties of many other
plants, if allowed to flower near together: I refer here only to species
which are capable of fertilising themselves, for if this be not the
case, they would of course be liable to be crossed by any other variety
growing near. Horticulturists do not commonly distinguish between the
effects of variability and intercrossing; but I have collected evidence
on the natural crossing of varieties of the tulip, hyacinth, anemone,
ranunculus, strawberry, Leptosiphon androsaceus, orange, rhododendron
and rhubarb, all of which plants I believe to be self-fertile. (10/38.
With respect to tulips and some other flowers, see Godron ‘De l’Espece’
tome 1 page 252. For anemones ‘Gardeners’ Chronicle’ 1859 page 98. For
strawberries see Herbert in ‘Transactions of the Horticultural Society’
volume 4 page 17. The same observer elsewhere speaks of the spontaneous
crossing of rhododendrons. Gallesio makes the same statement with
respect to oranges. I have myself known extensive crossing to occur with
the common rhubarb. For Leptosiphon, Verlot ‘Des Varieties’ 1865 page
20. I have not included in my list the Carnation, Nemophila, or
Antirrhinum, the varieties of which are known to cross freely, because
these plants are not always self-fertile. I know nothing about the
self-fertility of Trollius Lecoq ‘De la Fecondation’ 1862 page 93,
Mahonia, and Crinum, in which genera the species intercross largely.
With respect to Mahonia it is now scarcely possible to procure in this
country pure specimens of M. aquifolium or repens; and the various
species of Crinum sent by Herbert ‘Amaryllidaceae’ page 32, to Calcutta,
crossed there so freely that pure seed could not be saved.) Much other
indirect evidence could be given with respect to the extent to which
varieties of the same species spontaneously intercross.

Gardeners who raise seed for sale are compelled by dearly bought
experience to take extraordinary precautions against intercrossing. Thus
Messrs. Sharp “have land engaged in the growth of seed in no less than
eight parishes.” The mere fact of a vast number of plants belonging to
the same variety growing together is a considerable protection, as the
chances are strong in favour of plants of the same variety
intercrossing; and it is in chief part owing to this circumstance, that
certain villages have become famous for pure seed of particular
varieties. (10/39. With respect to Messrs. Sharp see ‘Gardeners’
Chronicle’ 1856 page 823. Lindley’s ‘Theory of Horticulture’ page 319.)
Only two trials were made by me to ascertain after how long an interval
of time, pollen from a distinct variety would obliterate more or less
completely the action of a plant’s own pollen. The stigmas in two lately
expanded flowers on a variety of cabbage, called Ragged Jack, were well
covered with pollen from the same plant. After an interval of
twenty-three hours, pollen from the Early Barnes Cabbage growing at a
distance was placed on both stigmas; and as the plant was left
uncovered, pollen from other flowers on the Ragged Jack would certainly
have been left by the bees during the next two or three days on the same
two stigmas. Under these circumstances it seemed very unlikely that the
pollen of the Barnes cabbage would produce any effect; but three out of
the fifteen plants raised from the two capsules thus produced were
plainly mongrelised: and I have no doubt that the twelve other plants
were affected, for they grew much more vigorously than the
self-fertilised seedlings from the Ragged Jack planted at the same time
and under the same conditions. Secondly, I placed on several stigmas of
a long-styled cowslip (Primula veris) plenty of pollen from the same
plant, and after twenty-four hours added some from a short-styled
dark-red Polyanthus, which is a variety of the cowslip. From the flowers
thus treated thirty seedlings were raised, and all these without
exception bore reddish flowers; so that the effect of the plant’s own
pollen, though placed on the stigmas twenty-four hours previously, was
quite destroyed by that of the red variety. It should, however, be
observed that these plants are dimorphic, and that the second union was
a legitimate one, whilst the first was illegitimate; but flowers
illegitimately fertilised with their own pollen yield a moderately fair
supply of seeds.

We have hitherto considered only the prepotent fertilising power of
pollen from a distinct variety over a plants’ own pollen,--both kinds of
pollen being placed on the same stigma. It is a much more remarkable
fact that pollen from another individual of the same variety is
prepotent over a plant’s own pollen, as shown by the superiority of the
seedlings raised from a cross of this kind over seedlings from
self-fertilised flowers. Thus in Tables 7/A, B, and C, there are at
least fifteen species which are self-fertile when insects are excluded;
and this implies that their stigmas must receive their own pollen;
nevertheless, most of the seedlings which were raised by fertilising the
non-castrated flowers of these fifteen species with pollen from another
plant were greatly superior, in height, weight, and fertility, to the
self-fertilised offspring. (10/40. These fifteen species consist of
Brassica oleracea, Reseda odorata and lutea, Limnanthes douglasii,
Papaver vagum, Viscaria oculata, Beta vulgaris, Lupinus luteus, Ipomoea
purpurea, Mimulus luteus, Calceolaria, Verbascum thapsus, Vandellia
nummularifolia, Lactuca sativa, and Zea mays.) For instance, with
Ipomoea purpurea every single intercrossed plant exceeded in height its
self-fertilised opponent until the sixth generation; and so it was with
Mimulus luteus until the fourth generation. Out of six pairs of crossed
and self-fertilised cabbages, every one of the former was much heavier
than the latter. With Papaver vagum, out of fifteen pairs, all but two
of the crossed plants were taller than their self-fertilised opponents.
Of eight pairs of Lupinus luteus, all but two of the crossed were
taller; of eight pairs of Beta vulgaris all but one; and of fifteen
pairs of Zea mays all but two were taller. Of fifteen pairs of
Limnanthes douglasii, and of seven pairs of Lactuca sativa, every single
crossed plant was taller than its self-fertilised opponent. It should
also be observed that in these experiments no particular care was taken
to cross-fertilise the flowers immediately after their expansion; it is
therefore almost certain that in many of these cases some pollen from
the same flower will have already fallen on and acted on the stigma.

There can hardly be a doubt that several other species of which the
crossed seedlings are more vigorous than the self-fertilised, as shown
in Tables 7/A, 7/B and 7/C, besides the above fifteen, must have
received their own pollen and that from another plant at nearly the same
time; and if so, the same remarks as those just given are applicable to
them. Scarcely any result from my experiments has surprised me so much
as this of the prepotency of pollen from a distinct individual over each
plant’s own pollen, as proved by the greater constitutional vigour of
the crossed seedlings. The evidence of prepotency is here deduced from
the comparative growth of the two lots of seedlings; but we have similar
evidence in many cases from the much greater fertility of the
non-castrated flowers on the mother-plant, when these received at the
same time their own pollen and that from a distinct plant, in comparison
with the flowers which received only their own pollen.

From the various facts now given on the spontaneous intercrossing of
varieties growing near together, and on the effects of cross-fertilising
flowers which are self-fertile and have not been castrated, we may
conclude that pollen brought by insects or by the wind from a distinct
plant will generally prevent the action of pollen from the same flower,
even though it may have been applied some time before; and thus the
intercrossing of plants in a state of nature will be greatly favoured or
ensured.

The case of a great tree covered with innumerable hermaphrodite flowers
seems at first sight strongly opposed to the belief in the frequency of
intercrosses between distinct individuals. The flowers which grow on the
opposite sides of such a tree will have been exposed to somewhat
different conditions, and a cross between them may perhaps be in some
degree beneficial; but it is not probable that it would be nearly so
beneficial as a cross between flowers on distinct trees, as we may infer
from the inefficiency of pollen taken from plants which have been
propagated from the same stock, though growing on separate roots. The
number of bees which frequent certain kinds of trees when in full flower
is very great, and they may be seen flying from tree to tree more
frequently than might have been expected. Nevertheless, if we consider
how numerous are the flowers, for instance, on a horse-chestnut or
lime-tree, an incomparably larger number of flowers must be fertilised
by pollen brought from other flowers on the same tree, than from flowers
on a distinct tree. But we should bear in mind that with the
horse-chestnut, for instance, only one or two of the several flowers on
the same peduncle produce a seed; and that this seed is the product of
only one out of several ovules within the same ovarium. Now we know from
the experiments of Herbert and others that if one flower is fertilised
with pollen which is more efficient than that applied to the other
flowers on the same peduncle, the latter often drop off (10/41.
‘Variation under Domestication’ chapter 17 2nd edition volume 2 page
120.); and it is probable that this would occur with many of the
self-fertilised flowers on a large tree, if other and adjoining flowers
were cross-fertilised. Of the flowers annually produced by a great tree,
it is almost certain that a large number would be self-fertilised; and
if we assume that the tree produced only 500 flowers, and that this
number of seeds were requisite to keep up the stock, so that at least
one seedling should hereafter struggle to maturity, then a large
proportion of the seedlings would necessarily be derived from
self-fertilised seeds. But if the tree annually produced 50,000 flowers,
of which the self-fertilised dropped off without yielding seeds, then
the cross-fertilised flowers might yield seeds in sufficient number to
keep up the stock, and most of the seedlings would be vigorous from
being the product of a cross between distinct individuals. In this
manner the production of a vast number of flowers, besides serving to
entice numerous insects and to compensate for the accidental destruction
of many flowers by spring-frosts or otherwise, would be a very great
advantage to the species; and when we behold our orchard-trees covered
with a white sheet of bloom in the spring, we should not falsely accuse
nature of wasteful expenditure, though comparatively little fruit is
produced in the autumn.

ANEMOPHILOUS PLANTS.

The nature and relations of plants which are fertilised by the wind have
been admirably discussed by Delpino and Hermann Muller; and I have
already made some remarks on the structure of their flowers in contrast
with those of entomophilous species. (10/42. Delpino ‘Ult. Osservazioni
sulla Dicogamia’ part 2 fasc. 1 1870 and ‘Studi sopra un Lignaggio
anemofilo’ etc. 1871. Hermann Muller ‘Die Befruchtung’ etc. pages 412,
442. Both these authors remark that plants must have been anemophilous
before they were entomophilous. Hermann Muller further discusses in a
very interesting manner the steps by which entomophilous flowers became
nectariferous and gradually acquired their present structure through
successive beneficial changes.) There is good reason to believe that the
first plants which appeared on this earth were cryptogamic; and judging
from what now occurs, the male fertilising element must either have
possessed the power of spontaneous movement through the water or over
damp surfaces, or have been carried by currents of water to the female
organs. That some of the most ancient plants, such as ferns, possessed
true sexual organs there can hardly be a doubt; and this shows, as
Hildebrand remarks, at how early a period the sexes were separated.
(10/43. ‘Die Geschlechter-Vertheilung’ 1867 pages 84-90.) As soon as
plants became phanerogamic and grew on the dry ground, if they were ever
to intercross, it would be indispensable that the male fertilising
element should be transported by some means through the air; and the
wind is the simplest means of transport. There must also have been a
period when winged insects did not exist, and plants would not then have
been rendered entomophilous. Even at a somewhat later period the more
specialised orders of the Hymenoptera, Lepidoptera, and Diptera, which
are now chiefly concerned with the transport of pollen, did not exist.
Therefore the earliest terrestrial plants known to us, namely, the
Coniferae and Cycadiae, no doubt were anemophilous, like the existing
species of these same groups. A vestige of this early state of things is
likewise shown by some other groups of plants which are anemophilous, as
these on the whole stand lower in the scale than entomophilous species.

There is no great difficulty in understanding how an anemophilous plant
might have been rendered entomophilous. Pollen is a nutritious
substance, and would soon have been discovered and devoured by insects;
and if any adhered to their bodies it would have been carried from the
anthers to the stigma of the same flower, or from one flower to another.
One of the chief characteristics of the pollen of anemophilous plants is
its incoherence; but pollen in this state can adhere to the hairy bodies
of insects, as we see with some Leguminosae, Ericaceae, and
Melastomaceae. We have, however, better evidence of the possibility of a
transition of the above kind in certain plants being now fertilised
partly by the wind and partly by insects. The common rhubarb (Rheum
rhaponticum) is so far in an intermediate condition, that I have seen
many Diptera sucking the flowers, with much pollen adhering to their
bodies; and yet the pollen is so incoherent, that clouds of it are
emitted if the plant be gently shaken on a sunny day, some of which
could hardly fail to fall on the large stigmas of the neighbouring
flowers. According to Delpino and Hermann Muller, some species of
Plantago are in a similar intermediate condition. (10/44. ‘Die
Befruchtung’ etc. page 342.)

Although it is probable that pollen was aboriginally the sole attraction
to insects, and although many plants now exist whose flowers are
frequented exclusively by pollen-devouring insects, yet the great
majority secrete nectar as the chief attraction. Many years ago I
suggested that primarily the saccharine matter in nectar was excreted as
a waste product of chemical changes in the sap; and that when the
excretion happened to occur within the envelopes of a flower, it was
utilised for the important object of cross-fertilisation, being
subsequently much increased in quantity and stored in various ways.
(10/45. Nectar was regarded by De Candolle and Dunal as an excretion, as
stated by Martinet in ‘Annal des Sc. Nat.’ 1872 tome 14 page 211.) This
view is rendered probable by the leaves of some trees excreting, under
certain climatic conditions, without the aid of special glands, a
saccharine fluid, often called honey-dew. This is the case with the
leaves of the lime; for although some authors have disputed the fact, a
most capable judge, Dr. Maxwell Masters, informs me that, after having
heard the discussions on this subject before the Horticultural Society,
he feels no doubt on this head. The leaves, as well as the cut stems, of
the manna ash (Fraxinus ornus) secrete in a like manner saccharine
matter. (10/46. ‘Gardeners’ Chronicle’ 1876 page 242.) According to
Treviranus, so do the upper surfaces of the leaves of Carduus arctioides
during hot weather. Many analogous facts could be given. (10/47. Kurr
‘Untersuchungen uber die Bedeutung der Nektarien’ 1833 page 115.) There
are, however, a considerable number of plants which bear small glands on
their leaves, petioles, phyllodia, stipules, bracteae, or flower
peduncles, or on the outside of their calyx, and these glands secrete
minute drops of a sweet fluid, which is eagerly sought by sugar-loving
insects, such as ants, hive-bees, and wasps. (10/48. A large number of
cases are given by Delpino in the ‘Bulletino Entomologico’ Anno 6 1874.
To these may be added those given in my text, as well as the excretion
of saccharine matter from the calyx of two species of Iris, and from the
bracteae of certain Orchideae: see Kurr ‘Bedeutung der Nektarien’ 1833
pages 25, 28. Belt ‘Nicaragua’ page 224, also refers to a similar
excretion by many epiphytal orchids and passion-flowers. Mr. Rodgers has
seen much nectar secreted from the bases of the flower-peduncles of
Vanilla. Link says that the only example of a hypopetalous nectary known
to him is externally at the base of the flowers of Chironia decussata:
see ‘Reports on Botany, Ray Society’ 1846 page 355. An important memoir
bearing on this subject has lately appeared by Reinke ‘Gottingen
Nachrichten’ 1873 page 825, who shows that in many plants the tips of
the serrations on the leaves in the bud bear glands which secrete only
at a very early age, and which have the same morphological structure as
true nectar-secreting glands. He further shows that the nectar-secreting
glands on the petioles of Prunus avium are not developed at a very early
age, yet wither away on the old leaves. They are homologous with those
on the serrations of the blades of the same leaves, as shown by their
structure and by transition-forms; for the lowest serrations on the
blades of most of the leaves secrete nectar instead of resin (harz).) In
the case of the glands on the stipules of Vicia sativa, the excretion
manifestly depends on changes in the sap, consequent on the sun shining
brightly; for I repeatedly observed that as soon as the sun was hidden
behind clouds the secretion ceased, and the hive-bees left the field;
but as soon as the sun broke out again, they returned to their feast.
(10/49. I published a brief notice of this case in the ‘Gardeners’
Chronicle’ 1855 July 21 page 487, and afterwards made further
observations. Besides the hive-bee, another species of bee, a moth,
ants, and two kinds of flies sucked the drops of fluid on the stipules.
The larger drops tasted sweet. The hive-bees never even looked at the
flowers which were open at the same time; whilst two species of
humble-bees neglected the stipules and visited only the flowers.) I have
observed an analogous fact with the secretion of true nectar in the
flowers of Lobelia erinus.

Delpino, however, maintains that the power of secreting a sweet fluid by
any extra-floral organ has been in every case specially gained, for the
sake of attracting ants and wasps as defenders of the plant against
their enemies; but I have never seen any reason to believe that this is
so with the three species observed by me, namely, Prunus laurocerasus,
Vicia sativa, and V. faba. No plant is so little attacked by enemies of
any kind as the common bracken-fern (Pteris aquilina); and yet, as my
son Francis has discovered, the large glands at the bases of the fronds,
but only whilst young, excrete much sweetish fluid, which is eagerly
sought by innumerable ants, chiefly belonging to Myrmica; and these ants
certainly do not serve as a protection against any enemy. Delpino argues
that such glands ought not to be considered as excretory, because if
they were so, they would be present in every species; but I cannot see
much force in this argument, as the leaves of some plants excrete sugar
only during certain states of the weather. That in some cases the
secretion serves to attract insects as defenders of the plant, and may
have been developed to a high degree for this special purpose, I have
not the least doubt, from the observations of Delpino, and more
especially from those of Mr. Belt on Acacia sphaerocephala, and on
passion-flowers. This acacia likewise produces, as an additional
attraction to ants, small bodies containing much oil and protoplasm, and
analogous bodies are developed by a Cecropia for the same purpose, as
described by Fritz Muller. (10/50. Mr. Belt ‘The Naturalist in
Nicaragua’ 1874 page 218, has given a most interesting account of the
paramount importance of ants as defenders of the above Acacia. With
respect to the Cecropia see ‘Nature’ 1876 page 304. My son Francis has
described the microscopical structure and development of these wonderful
food-bodies in a paper read before the Linnean Society.)

The excretion of a sweet fluid by glands seated outside of a flower is
rarely utilised as a means for cross-fertilisation by the aid of
insects; but this occurs with the bracteae of the Marcgraviaceae, as the
late Dr. Cruger informed me from actual observation in the West Indies,
and as Delpino infers with much acuteness from the relative position of
the several parts of their flowers. (10/51. ‘Ult. Osservaz. Dicogamia’
1868-69 page 188.) Mr. Farrer has also shown that the flowers of
Coronilla are curiously modified, so that bees may fertilise them whilst
sucking the fluid secreted from the outside of the calyx. (10/52.
‘Nature’ 1874 page 169.) It further appears probable from the
observations of the Reverend W.A. Leighton, that the fluid so abundantly
secreted by glands on the phyllodia of the Australian Acacia magnifica,
which stand near the flowers, is connected with their fertilisation.
(10/53. ‘Annals and Magazine of Natural History’ volume 16 1865 page 14.
In my work on the ‘Fertilisation of Orchids’ and in a paper subsequently
published in the ‘Annals and Magazine of Natural History’ it has been
shown that although certain kinds of orchids possess a nectary, no
nectar is actually secreted by it; but that insects penetrate the inner
walls and suck the fluid contained in the intercellular spaces. I
further suggested, in the case of some other orchids which do not
secrete nectar, that insects gnawed the labellum; and this suggestion
has since been proved true. Hermann Muller and Delpino have now shown
that some other plants have thickened petals which are sucked or gnawed
by insects, their fertilisation being thus aided. All the known facts on
this head have been collected by Delpino in his ‘Ult. Osserv.’ part 2
fasc. 2 1875 pages 59-63.)

The amount of pollen produced by anemophilous plants, and the distance
to which it is often transported by the wind, are both surprisingly
great. Mr. Hassall found that the weight of pollen produced by a single
plant of the Bulrush (Typha) was 144 grains. Bucketfuls of pollen,
chiefly of Coniferae and Gramineae, have been swept off the decks of
vessels near the North American shore; and Mr. Riley has seen the ground
near St. Louis, in Missouri, covered with pollen, as if sprinkled with
sulphur; and there was good reason to believe that this had been
transported from the pine-forests at least 400 miles to the south.
Kerner has seen the snow-fields on the higher Alps similarly dusted; and
Mr. Blackley found numerous pollen-grains, in one instance 1200,
adhering to sticky slides, which were sent up to a height of from 500 to
1000 feet by means of a kite, and then uncovered by a special mechanism.
It is remarkable that in these experiments there were on an average
nineteen times as many pollen-grains in the atmosphere at the higher
than at the lower levels. (10/54. For Mr. Hassall’s observations see
‘Annals and Magazine of Natural History’ volume 8 1842 page 108. In the
‘North American Journal of Science’ January 1842, there is an account of
the pollen swept off the decks of a vessel. Riley ‘Fifth Report on the
Noxious Insects of Missouri’ 1873 page 86. Kerner ‘Die Schutzmittel des
Pollens’ 1873 page 6. This author has also seen a lake in the Tyrol so
covered with pollen, that the water no longer appeared blue. Mr.
Blackley ‘Experimental Researches on Hay-fever’ 1873 pages 132,
141-152.) Considering these facts, it is not so surprising as it at
first appears that all, or nearly all, the stigmas of anemophilous
plants should receive pollen brought to them by mere chance by the wind.
During the early part of summer every object is thus dusted with pollen;
for instance, I examined for another purpose the labella of a large
number of flowers of the Fly Ophrys (which is rarely visited by
insects), and found on all very many pollen-grains of other plants,
which had been caught by their velvety surfaces.

The extraordinary quantity and lightness of the pollen of anemophilous
plants are no doubt both necessary, as their pollen has generally to be
carried to the stigmas of other and often distant flowers; for, as we
shall soon see, most anemophilous plants have their sexes separated. The
fertilisation of these plants is generally aided by the stigmas being of
large size or plumose; and in the case of the Coniferae, by the naked
ovules secreting a drop of fluid, as shown by Delpino. Although the
number of anemophilous species is small, as the author just quoted
remarks, the number of individuals is large in comparison with that of
entomophilous species. This holds good especially in cold and temperate
regions, where insects are not so numerous as under a warmer climate,
and where consequently entomophilous plants are less favourably
situated. We see this in our forests of Coniferae and other trees, such
as oaks, beeches, birches, ashes, etc.; and in the Gramineae,
Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all
these trees and plants being fertilised by the wind. As a large quantity
of pollen is wasted by anemophilous plants, it is surprising that so
many vigorous species of this kind abounding with individuals should
still exist in any part of the world; for if they had been rendered
entomophilous, their pollen would have been transported by the aid of
the senses and appetites of insects with incomparably greater safety
than by the wind. That such a conversion is possible can hardly be
doubted, from the remarks lately made on the existence of intermediate
forms; and apparently it has been effected in the group of willows, as
we may infer from the nature of their nearest allies. (10/55. Hermann
Muller ‘Die Befruchtung’ etc. page 149.)

It seems at first sight a still more surprising fact that plants, after
having been once rendered entomophilous, should ever again have become
anemophilous; but this has occasionally though rarely occurred, for
instance, with the common Poterium sanguisorba, as may be inferred from
its belonging to the Rosaceae. Such cases are, however, intelligible, as
almost all plants require to be occasionally intercrossed; and if any
entomiphilous species ceased to be visited by insects, it would probably
perish unless it were rendered anemophilous. A plant would be neglected
by insects if nectar failed to be secreted, unless indeed a large supply
of attractive pollen was present; and from what we have seen of the
excretion of saccharine fluid from leaves and glands being largely
governed in several cases by climatic influences, and from some few
flowers which do not now secrete nectar still retaining coloured
guiding-marks, the failure of the secretion cannot be considered as a
very improbable event. The same result would follow to a certainty, if
winged insects ceased to exist in any district, or became very rare. Now
there is only a single plant in the great order of the Cruciferae,
namely, Pringlea, which is anemophilous, and this plant is an inhabitant
of Kerguelen Land, where there are hardly any winged insects, owing
probably, as was suggested by me in the case of Madeira, to the risk
which they run of being blown out to sea and destroyed. (10/56. The
Reverend A.E. Eaton in ‘Proceedings of the Royal Society’ volume 23 1875
page 351.)

A remarkable fact with respect to anemophilous plants is that they are
often diclinous, that is, they are either monoecious with their sexes
separated on the same plant, or dioecious with their sexes on distinct
plants. In the class Monoecia of Linnaeus, Delpino shows that the
species of twenty-eight genera are anemophilous, and of seventeen genera
entomophilous. (10/57. ‘Studi sopra un Lignaggio anemofilo delle
Compositae’ 1871.) The larger proportion of entomophilous genera in this
latter class is probably the indirect result of insects having the power
of carrying pollen to another and sometimes distant plant much more
securely than the wind. In the above two classes taken together there
are thirty-eight anemophilous and thirty-six entomophilous genera;
whereas in the great mass of hermaphrodite plants the proportion of
anemophilous to entomophilous genera is extremely small. The cause of
this remarkable difference may be attributed to anemophilous plants
having retained in a greater degree than the entomophilous a primordial
condition, in which the sexes were separated and their mutual
fertilisation effected by means of the wind. That the earliest and
lowest members of the vegetable kingdom had their sexes separated, as is
still the case to a large extent, is the opinion of a high authority,
Nageli. (10/58. ‘Entstehung und Begriff der Naturhist. Art’ 1865 page
22.) It is indeed difficult to avoid this conclusion, if we admit the
view, which seems highly probable, that the conjugation of the Algae and
of some of the simplest animals is the first step towards sexual
reproduction; and if we further bear in mind that a greater and greater
degree of differentiation between the cells which conjugate can be
traced, thus leading apparently to the development of the two sexual
forms. (10/59. See the interesting discussion on this whole subject by
O. Butschli in his ‘Studien uber die ersten Entwickelungsvorgange der
Eizelle; etc. 1876 pages 207-219. Also Engelmann “Ueber Entwickelung von
Infusorien” ‘Morphol. Jahrbuch’ B. 1 page 573. Also Dr. A. Dodel “Die
Kraushaar-Algae” ‘Pringsheims Jahrbuch f. Wiss. Bot.’ B. 10.) We have
also seen that as plants became more highly developed and affixed to the
ground, they would be compelled to be anemophilous in order to
intercross. Therefore all plants which have not since been greatly
modified, would tend still to be both diclinous and anemophilous; and we
can thus understand the connection between these two states, although
they appear at first sight quite disconnected. If this view is correct,
plants must have been rendered hermaphrodites at a later though still
very early period, and entomophilous at a yet later period, namely,
after the development of winged insects. So that the relationship
between hermaphroditism and fertilisation by means of insects is
likewise to a certain extent intelligible.

Why the descendants of plants which were originally dioecious, and which
therefore profited by always intercrossing with another individual,
should have been converted into hermaphrodites, may perhaps be explained
by the risk which they ran, especially as long as they were
anemophilous, of not being always fertilised, and consequently of not
leaving offspring. This latter evil, the greatest of all to any
organism, would have been much lessened by their becoming
hermaphrodites, though with the contingent disadvantage of frequent
self-fertilisation. By what graduated steps an hermaphrodite condition
was acquired we do not know. But we can see that if a lowly organised
form, in which the two sexes were represented by somewhat different
individuals, were to increase by budding either before or after
conjugation, the two incipient sexes would be capable of appearing by
buds on the same stock, as occasionally occurs with various characters
at the present day. The organism would then be in a monoecious
condition, and this is probably the first step towards hermaphroditism;
for if very simple male and female flowers on the same stock, each
consisting of a single stamen or pistil, were brought close together and
surrounded by a common envelope, in nearly the same manner as with the
florets of the Compositae, we should have an hermaphrodite flower.

There seems to be no limit to the changes which organisms undergo under
changing conditions of life; and some hermaphrodite plants, descended as
we must believe from aboriginally diclinous plants, have had their sexes
again separated. That this has occurred, we may infer from the presence
of rudimentary stamens in the flowers of some individuals, and of
rudimentary pistils in the flowers of other individuals, for example in
Lychnis dioica. But a conversion of this kind will not have occurred
unless cross-fertilisation was already assured, generally by the agency
of insects; but why the production of male and female flowers on
distinct plants should have been advantageous to the species,
cross-fertilisation having been previously assured, is far from obvious.
A plant might indeed produce twice as many seeds as were necessary to
keep up its numbers under new or changed conditions of life; and if it
did not vary by bearing fewer flowers, and did vary in the state of its
reproductive organs (as often occurs under cultivation), a wasteful
expenditure of seeds and pollen would be saved by the flowers becoming
diclinous.

A related point is worth notice. I remarked in my Origin of Species that
in Britain a much larger proportion of trees and bushes than of
herbaceous plants have their sexes separated; and so it is, according to
Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in
the ‘London Catalogue of British Plants’ that there are thirty-two
indigenous trees and bushes in Great Britain, classed under nine
families; but to err on the safe side, I have counted only six species
of willows. Of the thirty-two trees and bushes, nineteen, or more than
half, have their sexes separated; and this is an enormous proportion
compared with other British plants. New Zealand abounds with diclinous
plants and trees; and Dr. Hooker calculates that out of about 756
phanerogamic plants inhabiting the islands, no less than 108 are trees,
belonging to thirty-five families. Of these 108 trees, fifty-two, or
very nearly half, have their sexes more or less separated. Of bushes
there are 149, of which sixty-one have their sexes in the same state;
whilst of the remaining 500 herbaceous plants only 121, or less than a
fourth, have their sexes separated. Lastly, Professor Asa Gray informs
me that in the United States there are 132 native trees (belonging to
twenty-five families) of which ninety-five (belonging to seventeen
families) “have their sexes more or less separated, for the greater part
decidedly separated.”) It is, however, doubtful how far this rule holds
good generally, and it certainly does not do so in Australia. But I have
been assured that the flowers of the prevailing Australian trees,
namely, the Myrtaceae, swarm with insects, and if they are dichogamous
they would be practically diclinous. (10/61. With respect to the
Proteaceae of Australia, Mr. Bentham ‘Journal of the Linnean Society
Botany’ volume 13 1871 pages 58, 64, remarks on the various contrivances
by which the stigma in the several genera is screened from the action of
the pollen from the same flower. For instance, in Synaphea “the stigma
is held by the eunuch (i.e., one of the stamens which is barren) safe
from all pollution from her brother anthers, and is preserved intact for
any pollen that may be inserted by insects and other agencies.”) As far
as anemophilous plants are concerned, we know that they are apt to have
their sexes separated, and we can see that it would be an unfavourable
circumstance for them to bear their flowers very close to the ground, as
their pollen is liable to be blown high up in the air (10/62. Kerner
‘Schutzmittel des Pollens’ 1873 page 4.); but as the culms of grasses
give sufficient elevation, we cannot thus account for so many trees and
bushes being diclinous. We may infer from our previous discussion that a
tree bearing numerous hermaphrodite flowers would rarely intercross with
another tree, except by means of the pollen of a distinct individual
being prepotent over the plants’ own pollen. Now the separation of the
sexes, whether the plant were anemophilous are entomophilous, would most
effectually bar self-fertilisation, and this may be the cause of so many
trees and bushes being diclinous. Or to put the case in another way, a
plant would be better fitted for development into a tree, if the sexes
were separated, than if it were hermaphrodite; for in the former case
its numerous flowers would be less liable to continued
self-fertilisation. But it should also be observed that the long life of
a tree or bush permits of the separation of the sexes, with much less
risk of evil from impregnation occasionally failing and seeds not being
produced, than in the case of short-lived plants. Hence it probably is,
as Lecoq has remarked, that annual plants are rarely dioecious.

Finally, we have seen reason to believe that the higher plants are
descended from extremely low forms which conjugated, and that the
conjugating individuals differed somewhat from one another,--the one
representing the male and the other the female--so that plants were
aboriginally dioecious. At a very early period such lowly organised
dioecious plants probably gave rise by budding to monoecious plants with
the two sexes borne by the same individual; and by a still closer union
of the sexes to hermaphrodite plants, which are now much the commonest
form. (10/63. There is a considerable amount of evidence that all the
higher animals are the descendants of hermaphrodites; and it is a
curious problem whether such hermaphroditism may not have been the
result of the conjugation of two slightly different individuals, which
represented the two incipient sexes. On this view, the higher animals
may now owe their bilateral structure, with all their organs double at
an early embryonic period, to the fusion or conjugation of two
primordial individuals.) As soon as plants became affixed to the ground,
their pollen must have been carried by some means from flower to flower,
at first almost certainly by the wind, then by pollen-devouring, and
afterwards by nectar-seeking insects. During subsequent ages some few
entomophilous plants have been again rendered anemophilous, and some
hermaphrodite plants have had their sexes again separated; and we can
vaguely see the advantages of such recurrent changes under certain
conditions.

Dioecious plants, however fertilised, have a great advantage over other
plants in their cross-fertilisation being assured. But this advantage is
gained in the case of anemophilous species at the expense of the
production of an enormous superfluity of pollen, with some risk to them
and to entomophilous species of their fertilisation occasionally
failing. Half the individuals, moreover, namely, the males, produce no
seed, and this might possibly be a disadvantage. Delpino remarks that
dioecious plants cannot spread so easily as monoecious and hermaphrodite
species, for a single individual which happened to reach some new site
could not propagate its kind; but it may be doubted whether this is a
serious evil. Monoecious plants can hardly fail to be to a large extent
dioecious in function, owing to the lightness of their pollen and to the
wind blowing laterally, with the great additional advantage of
occasionally or often producing some self-fertilised seeds. When they
are also dichogamous, they are necessarily dioecious in function.
Lastly, hermaphrodite plants can generally produce at least some
self-fertilised seeds, and they are at the same time capable, through
the various means specified in this chapter, of cross-fertilisation.
When their structure absolutely prevents self-fertilisation, they are in
the same relative position to one another as monoecious and dioecious
plants, with what may be an advantage, namely, that every flower is
capable of yielding seeds.



CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they can.
Cause of this habit.
Means by which bees recognise the flowers of the same species.
Sudden secretion of nectar.
Nectar of certain flowers unattractive to certain insects.
Industry of bees, and the number of flowers visited within a short time.
Perforation of the corolla by bees.
Skill shown in the operation.
Hive-bees profit by the holes made by humble-bees.
Effects of habit.
The motive for perforating flowers to save time.
Flowers growing in crowded masses chiefly perforated.

Bees and various other insects must be directed by instinct to search
flowers for nectar and pollen, as they act in this manner without
instruction as soon as they emerge from the pupa state. Their instincts,
however, are not of a specialised nature, for they visit many exotic
flowers as readily as the endemic kinds, and they often search for
nectar in flowers which do not secrete any; and they may be seen
attempting to suck it out of nectaries of such length that it cannot be
reached by them. (11/1. See, on this subject Hermann Muller
‘Befruchtung’ etc. page 427; and Sir J. Lubbock’s ‘British Wild Flowers’
etc. page 20. Muller ‘Bienen Zeitung’ June 1876 page 119, assigns good
reasons for his belief that bees and many other Hymenoptera have
inherited from some early nectar-sucking progenitor greater skill in
robbing flowers than that which is displayed by insects belonging to the
other Orders.) All kinds of bees and certain other insects usually visit
the flowers of the same species as long as they can, before going to
another species. This fact was observed by Aristotle with respect to the
hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper
published in 1736 in the Philosophical Transactions. It may be observed
by any one, both with hive and humble-bees, in every flower-garden; not
that the habit is invariably followed. Mr. Bennett watched for several
hours many plants of Lamium album, L. purpureum, and another Labiate
plant, Nepeta glechoma, all growing mingled together on a bank near some
hives; and he found that each bee confined its visits to the same
species. (11/2. ‘Nature’ 1874 June 4 page 92.) The pollen of these three
plants differs in colour, so that he was able to test his observations
by examining that which adhered to the bodies of the captured bees, and
he found one kind on each bee.

Humble and hive-bees are good botanists, for they know that varieties
may differ widely in the colour of their flowers and yet belong to the
same species. I have repeatedly seen humble-bees flying straight from a
plant of the ordinary red Dictamnus fraxinella to a white variety; from
one to another very differently coloured variety of Delphinium consolida
and of Primula veris; from a dark purple to a bright yellow variety of
Viola tricolor; and with two species of Papaver, from one variety to
another which differed much in colour; but in this latter case some of
the bees flew indifferently to either species, although passing by other
genera, and thus acted as if the two species were merely varieties.
Hermann Muller also has seen hive-bees flying from flower to flower of
Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and
repens; and even from blue hyacinths to blue violets. (11/3. ‘Bienen
Zeitung’ July 1876 page 183.)

Some species of Diptera or flies keep to the flowers of the same species
with almost as much regularity as do bees; and when captured they are
found covered with pollen. I have seen Rhingia rostrata acting in this
manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici
sepium. Volucella plumosa and Empis cheiroptera flew straight from
flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved
in the same manner with Potentilla tormentilla; and other Diptera with
Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica
hederaefolia and chamoedrys; but some flies visited indifferently the
flowers of these two latter species. I have seen more than once a minute
Thrips, with pollen adhering to its body, fly from one flower to another
of the same kind; and one was observed by me crawling about within a
convolvulus with four grains of pollen adhering to its head, which were
deposited on the stigma.

Fabricius and Sprengel state that when flies have once entered the
flowers of Aristolochia they never escape,--a statement which I could
not believe, as in this case the insects would not aid in the
cross-fertilisation of the plant; and this statement has now been shown
by Hildebrand to be erroneous. As the spathes of Arum maculatum are
furnished with filaments apparently adapted to prevent the exit of
insects, they resemble in this respect the flowers of Aristolochia; and
on examining several spathes, from thirty to sixty minute Diptera
belonging to three species were found in some of them; and many of these
insects were lying dead at the bottom, as if they had been permanently
entrapped. In order to discover whether the living ones could escape and
carry pollen to another plant, I tied in the spring of 1842 a fine
muslin bag tightly round a spathe; and on returning in an hour’s time
several little flies were crawling about on the inner surface of the
bag. I then gathered a spathe and breathed hard into it; several flies
soon crawled out, and all without exception were dusted with arum
pollen. These flies quickly flew away, and I distinctly saw three of
them fly to another plant about a yard off; they alighted on the inner
or concave surface of the spathe, and suddenly flew down into the
flower. I then opened this flower, and although not a single anther had
burst, several grains of pollen were lying at the bottom, which must
have been brought from another plant by one of these flies or by some
other insect. In another flower little flies were crawling about, and I
saw them leave pollen on the stigmas.

I do not know whether Lepidoptera generally keep to the flowers of the
same species; but I once observed many minute moths (I believe Lampronia
(Tinea) calthella) apparently eating the pollen of Mercurialis annua,
and they had the whole front of their bodies covered with pollen. I then
went to a female plant some yards off, and saw in the course of fifteen
minutes three of these moths alight on the stigmas. Lepidoptera are
probably often induced to frequent the flowers of the same species,
whenever these are provided with a long and narrow nectary, as in this
case other insects cannot suck the nectar, which will thus be preserved
for those having an elongated proboscis. No doubt the Yucca moth visits
only the flowers whence its name is derived, for a most wonderful
instinct guides this moth to place pollen on the stigma, so that the
ovules may be developed on which the larvae feed. (11/4. Described by
Mr. Riley in the ‘American Naturalist’ volume 7 October 1873.)With
respect to Coleoptera, I have seen Meligethes covered with pollen flying
from flower to flower of the same species; and this must often occur,
as, according to M. Brisout, “many of the species affect only one kind
of plant.” (11/5. As quoted in ‘American Nat.’ May 1873 page 270.)

It must not be supposed from these several statements that insects
strictly confine their visits to the same species. They often visit
other species when only a few plants of the same kind grow near
together. In a flower-garden containing some plants of Œnothera, the
pollen of which can easily be recognised, I found not only single grains
but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum,
and Linaria. Other kinds of pollen were likewise detected in these same
flowers. A large number of the stigmas of a plant of Thyme, in which the
anthers were completely aborted, were examined; and these stigmas,
though scarcely larger than a split needle, were covered not only with
pollen of Thyme brought from other plants by the bees, but with several
other kinds of pollen.

That insects should visit the flowers of the same species as long as
they can, is of great importance to the plant, as it favours the
cross-fertilisation of distinct individuals of the same species; but no
one will suppose that insects act in this manner for the good of the
plant. The cause probably lies in insects being thus enabled to work
quicker; they have just learnt how to stand in the best position on the
flower, and how far and in what direction to insert their proboscides.
(11/6. Since these remarks were written, I find that Hermann Muller has
come to almost exactly the same conclusion with respect to the cause of
insects frequenting as long as they can the flowers of the same species:
‘Bienen Zeitung’ July 1876 page 182.) They act on the same principle as
does an artificer who has to make half-a-dozen engines, and who saves
time by making consecutively each wheel and part for all of them.
Insects, or at least bees, seem much influenced by habit in all their
manifold operations; and we shall presently see that this holds good in
their felonious practice of biting holes through the corolla.

It is a curious question how bees recognise the flowers of the same
species. That the coloured corolla is the chief guide cannot be doubted.
On a fine day, when hive-bees were incessantly visiting the little blue
flowers of Lobelia erinus, I cut off all the petals of some, and only
the lower striped petals of others, and these flowers were not once
again sucked by the bees, although some actually crawled over them. The
removal of the two little upper petals alone made no difference in their
visits. Mr. J. Anderson likewise states that when he removed the
corollas of the Calceolaria, bees never visited the flowers. (11/7.
‘Gardeners’ Chronicle’ 1853 page 534. Kurr cut off the nectaries from a
large number of flowers of several species, and found that the greater
number yielded seeds; but insects probably would not perceive the loss
of the nectary until they had inserted their proboscides into the holes
thus formed, and in doing so would fertilise the flowers. He also
removed the whole corolla from a considerable number of flowers, and
these likewise yielded seeds. Flowers which are self-fertile would
naturally produce seeds under these circumstances; but I am greatly
surprised that Delphinium consolida, as well as another species of
Delphinium, and Viola tricolor, should have produced a fair supply of
seeds when thus treated; but it does not appear that he compared the
number of the seeds thus produced with those yielded by unmutilated
flowers left to the free access of insects: ‘Bedeutung der Nektarien’
1833 pages 123-135.) On the other hand, in some large masses of Geranium
phaeum which had escaped out of a garden, I observed the unusual fact of
the flowers continuing to secrete an abundance of nectar after all the
petals had fallen off; and the flowers in this state were still visited
by humble-bees. But the bees might have learnt that these flowers with
all their petals lost were still worth visiting, by finding nectar in
those with only one or two lost. The colour alone of the corolla serves
as an approximate guide: thus I watched for some time humble-bees which
were visiting exclusively plants of the white-flowered Spiranthes
autumnalis, growing on short turf at a considerable distance apart; and
these bees often flew within a few inches of several other plants with
white flowers, and then without further examination passed onwards in
search of the Spiranthes. Again, many hive-bees which confined their
visits to the common ling (Calluna vulgaris), repeatedly flew towards
Erica tetralix, evidently attracted by the nearly similar tint of their
flowers, and then instantly passed on in search of the Calluna.

That the colour of the flower is not the sole guide, is clearly shown by
the six cases above given of bees which repeatedly passed in a direct
line from one variety to another of the same species, although they bore
very differently coloured flowers. I observed also bees flying in a
straight line from one clump of a yellow-flowered Œnothera to every
other clump of the same plant in the garden, without turning an inch
from their course to plants of Eschscholtzia and others with yellow
flowers which lay only a foot or two on either side. In these cases the
bees knew the position of each plant in the garden perfectly well, as we
may infer by the directness of their flight; so that they were guided by
experience and memory. But how did they discover at first that the above
varieties with differently coloured flowers belonged to the same
species? Improbable as it may appear, they seem, at least sometimes, to
recognise plants even from a distance by their general aspect, in the
same manner as we should do. On three occasions I observed humble-bees
flying in a perfectly straight line from a tall larkspur (Delphinium)
which was in full flower to another plant of the same species at the
distance of fifteen yards which had not as yet a single flower open, and
on which the buds showed only a faint tinge of blue. Here neither odour
nor the memory of former visits could have come into play, and the tinge
of blue was so faint that it could hardly have served as a guide. (11/8.
A fact mentioned by Hermann Muller ‘Die Befruchtung’ etc. page 347,
shows that bees possess acute powers of vision and discrimination; for
those engaged in collecting pollen from Primula elatior invariably
passed by the flowers of the long-styled form, in which the anthers are
seated low down in the tubular corolla. Yet the difference in aspect
between the long-styled and short-styled forms is extremely slight.)

The conspicuousness of the corolla does not suffice to induce repeated
visits from insects, unless nectar is at the same time secreted,
together perhaps with some odour emitted. I watched for a fortnight many
times daily a wall covered with Linaria cymbalaria in full flower, and
never saw a bee even looking at one. There was then a very hot day, and
suddenly many bees were industriously at work on the flowers. It appears
that a certain degree of heat is necessary for the secretion of nectar;
for I observed with Lobelia erinus that if the sun ceased to shine for
only half an hour, the visits of the bees slackened and soon ceased. An
analogous fact with respect to the sweet excretion from the stipules of
Vicia sativa has been already given. As in the case of the Linaria, so
with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some
species of Trifolium, I have watched the flowers day after day without
seeing a bee at work, and then suddenly all the flowers were visited by
many bees. Now how did so many bees discover at once that the flowers
were secreting nectar? I presume that it must have been by their odour;
and that as soon as a few bees began to suck the flowers, others of the
same and of different kinds observed the fact and profited by it. We
shall presently see, when we treat of the perforation of the corolla,
that bees are fully capable of profiting by the labour of other species.
Memory also comes into play, for, as already remarked, bees know the
position of each clump of flowers in a garden. I have repeatedly seen
them passing round a corner, but otherwise in as straight a line as
possible, from one plant of Fraxinella and of Linaria to another and
distant one of the same species; although, owing to the intervention of
other plants, the two were not in sight of each other.

It would appear that either the taste or the odour of the nectar of
certain flowers is unattractive to hive or to humble-bees, or to both;
for there seems no other reason why certain open flowers which secrete
nectar are not visited by them. The small quantity of nectar secreted by
some of these flowers can hardly be the cause of their neglect, as
hive-bees search eagerly for the minute drops on the glands on the
leaves of the Prunus laurocerasus. Even the bees from different hives
sometimes visit different kinds of flowers, as is said to be the case by
Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9.
‘Gardeners’ Chronicle’ 1844 page 374.) I have known humble-bees to visit
the flowers of Lobelia fulgens in one garden and not in another at the
distance of only a few miles. The cupful of nectar in the labellum of
Epipactis latifolia is never touched by hive- or humble-bees, although I
have seen them flying close by; and yet the nectar has a pleasant taste
to us, and is habitually consumed by the common wasp. As far as I have
seen, wasps seek for nectar in this country only from the flowers of
this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10.
The same fact apparently holds good in Italy, for Delpino says that the
flowers of these three plants are alone visited by wasps: ‘Nettarii
Estranuziali, Bulletino Entomologico’ anno 6.), and Tritoma; the two
former plants being endemic, and the two latter exotic. As wasps are so
fond of sugar and of any sweet fluid, and as they do not disdain the
minute drops on the glands of Prunus laurocerasus, it is a strange fact
that they do not suck the nectar of many open flowers, which they could
do without the aid of a proboscis. Hive-bees visit the flowers of the
Symphoricarpus and Tritoma, and this makes it all the stranger that they
do not visit the flowers of the Epipactis, or, as far as I have seen,
those of the Scrophularia aquatica; although they do visit the flowers
of Scrophularia nodosa, at least in North America. (11/11. ‘Silliman’s
American Journal of Science’ August 1871.)

The extraordinary industry of bees and the number of flowers which they
visit within a short time, so that each flower is visited repeatedly,
must greatly increase the chance of each receiving pollen from a
distinct plant. When the nectar is in any way hidden, bees cannot tell
without inserting their proboscides whether it has lately been exhausted
by other bees, and this, as remarked in a former chapter, forces them to
visit many more flowers than they otherwise would. But they endeavour to
lose as little time as they can; thus in flowers having several
nectaries, if they find one dry they do not try the others, but as I
have often observed, pass on to another flower. They work so
industriously and effectually, that even in the case of social plants,
of which hundreds of thousands grow together, as with the several kinds
of heath, every single flower is visited, of which evidence will
presently be given. They lose no time and fly very quickly from plant to
plant, but I do not know the rate at which hive-bees fly. Humble-bees
fly at the rate of ten miles an hour, as I was able to ascertain in the
case of the males from their curious habit of calling at certain fixed
points, which made it easy to measure the time taken in passing from one
place to another.

With respect to the number of flowers which bees visit in a given time,
I observed that in exactly one minute a humble-bee visited twenty-four
of the closed flowers of the Linaria cymbalaria; another bee visited in
the same time twenty-two flowers of the Symphoricarpus racemosa; and
another seventeen flowers on two plants of a Delphinium. In the course
of fifteen minutes a single flower on the summit of a plant of Œnothera
was visited eight times by several humble-bees, and I followed the last
of these bees, whilst it visited in the course of a few additional
minutes every plant of the same species in a large flower-garden. In
nineteen minutes every flower on a small plant of Nemophila insignis was
visited twice. In one minute six flowers of a Campanula were entered by
a pollen-collecting hive-bee; and bees when thus employed work slower
than when sucking nectar. Lastly, seven flower-stalks on a plant of
Dictamnus fraxinella were observed on the 15th of June 1841 during ten
minutes; they were visited by thirteen humble-bees each of which entered
many flowers. On the 22nd the same flower-stalks were visited within the
same time by eleven humble-bees. This plant bore altogether 280 flowers,
and from the above data, taking into consideration how late in the
evening humble-bees work, each flower must have been visited at least
thirty times daily, and the same flower keeps open during several days.
The frequency of the visits of bees is also sometimes shown by the
manner in which the petals are scratched by their hooked tarsi; I have
seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of
their flowers thus sadly defaced.

PERFORATION OF THE COROLLA BY BEES.

I have already alluded to bees biting holes in flowers for the sake of
obtaining the nectar. They often act in this manner, both with endemic
and exotic species, in many parts of Europe, in the United States, and
in the Himalaya; and therefore probably in all parts of the world. The
plants, the fertilisation of which actually depends on insects entering
the flowers, will fail to produce seed when their nectar is stolen from
the outside; and even with those species which are capable of
fertilising themselves without any aid, there can be no
cross-fertilisation, and this, as we know, is a serious evil in most
cases. The extent to which humble-bees carry on the practice of biting
holes is surprising: a remarkable case was observed by me near
Bournemouth, where there were formerly extensive heaths. I took a long
walk, and every now and then gathered a twig of Erica tetralix, and when
I had got a handful all the flowers were examined through a lens. This
process was repeated many times; but though many hundreds were examined,
I did not succeed in finding a single flower which had not been
perforated. Humble-bees were at the time sucking the flowers through
these perforations. On the following day a large number of flowers were
examined on another heath with the same result, but here hive-bees were
sucking through the holes. This case is all the more remarkable, as the
innumerable holes had been made within a fortnight, for before that time
I saw the bees everywhere sucking in the proper manner at the mouths of
the corolla. In an extensive flower-garden some large beds of Salvia
grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower
perforated, and many scores were examined. I have seen whole fields of
red clover (Trifolium pratense) in the same state. Dr. Ogle found that
90 per cent of the flowers of Salvia glutinosa had been bitten. In the
United States Mr. Bailey says it is difficult to find a blossom of the
native Gerardia pedicularia without a hole in it; and Mr. Gentry, in
speaking of the introduced Wistaria sinensis, says “that nearly every
flower had been perforated.” (11/12. Dr. Ogle ‘Pop. Science Review’ July
1869 page 267. Bailey ‘American Naturalist’ November 1873 page 690.
Gentry ibid May 1875 page 264.)

As far as I have seen, it is always humble-bees which first bite the
holes, and they are well fitted for the work by possessing powerful
mandibles; but hive-bees afterwards profit by the holes thus made. Dr.
Hermann Muller, however, writes to me that hive-bees sometimes bite
holes through the flowers of Erica tetralix. No insects except bees,
with the single exception of wasps in the case of Tritoma, have sense
enough, as far as I have observed, to profit by the holes already made.
Even humble-bees do not always discover that it would be advantageous to
them to perforate certain flowers. There is an abundant supply of nectar
in the nectary of Tropaeolum tricolor, yet I have found this plant
untouched in more than one garden, while the flowers of other plants had
been extensively perforated; but a few years ago Sir J. Lubbock’s
gardener assured me that he had seen humble-bees boring through the
nectary of this Tropaeolum. Muller has observed humble-bees trying to
suck at the mouths of the flowers of Primula elatior and of an
Aquilegia, and, failing in their attempts, they made holes through the
corolla; but they often bite holes, although they could with very little
more trouble obtain the nectar in a legitimate manner by the mouth of
the corolla.

Dr. W. Ogle has communicated to me a curious case. He gathered in
Switzerland 100 flower-stems of the common blue variety of the monkshood
(Aconitum napellus), and not a single flower was perforated; he then
gathered 100 stems of a white variety growing close by, and every one of
the open flowers had been perforated. (11/13. Dr. Ogle ‘Popular Science
Review’ July 1869 page 267. Bailey ‘American Naturalist’ November 1873
page 690. Gentry ibid May 1875 page 264.) This surprising difference in
the state of the flowers may be attributed with much probability to the
blue variety being distasteful to bees, from the presence of the acrid
matter which is so general in the Ranunculaceae, and to its absence in
the white variety in correlation with the loss of the blue tint.
According to Sprengel, this plant is strongly proterandrous (11/14. ‘Das
Entdeckte’ etc. page 278.); it would therefore be more or less sterile
unless bees carried pollen from the younger to the older flowers.
Consequently the white variety, the flowers of which were always bitten
instead of being properly entered by the bees, would fail to yield the
full number of seeds and would be a comparatively rare plant, as Dr.
Ogle informs me was the case.

Bees show much skill in their manner of working, for they always make
their holes from the outside close to the spot where the nectar lies
hidden within the corolla. All the flowers in a large bed of Stachys
coccinea had either one or two slits made on the upper side of the
corolla near the base. The flowers of a Mirabilis and of Salvia coccinea
were perforated in the same manner; whilst those of Salvia grahami, in
which the calyx is much elongated, had both the calyx and the corolla
invariably perforated. The flowers of Pentstemon argutus are broader
than those of the plants just named, and two holes alongside each other
had here always been made just above the calyx. In these several cases
the perforations were on the upper side, but in Antirrhinum majus one or
two holes had been made on the lower side, close to the little
protuberance which represents the nectary, and therefore directly in
front of and close to the spot where the nectar is secreted.

But the most remarkable case of skill and judgment known to me, is that
of the perforation of the flowers of Lathyrus sylvestris, as described
by my son Francis. (11/15. ‘Nature’ January 8, 1874 page 189.) The
nectar in this plant is enclosed within a tube, formed by the united
stamens, which surround the pistil so closely that a bee is forced to
insert its proboscis outside the tube; but two natural rounded passages
or orifices are left in the tube near the base, in order that the nectar
may be reached by the bees. Now my son found in sixteen out of
twenty-four flowers on this plant, and in eleven out of sixteen of those
on the cultivated everlasting pea, which is either a variety of the same
species or a closely allied one, that the left passage was larger than
the right one. And here comes the remarkable point,--the humble-bees
bite holes through the standard-petal, and they always operated on the
left side over the passage, which is generally the larger of the two. My
son remarks: “It is difficult to say how the bees could have acquired
this habit. Whether they discovered the inequality in the size of the
nectar-holes in sucking the flowers in the proper way, and then utilised
this knowledge in determining where to gnaw the hole; or whether they
found out the best situation by biting through the standard at various
points, and afterwards remembered its situation in visiting other
flowers. But in either case they show a remarkable power of making use
of what they have learnt by experience.” It seems probable that bees owe
their skill in biting holes through flowers of all kinds to their having
long practised the instinct of moulding cells and pots of wax, or of
enlarging their old cocoons with tubes of wax; for they are thus
compelled to work on the inside and outside of the same object.

In the early part of the summer of 1857 I was led to observe during some
weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus),
whilst attending to the fertilisation of this plant, and daily saw
humble- and hive-bees sucking at the mouths of the flowers. But one day
I found several humble-bees employed in cutting holes in flower after
flower; and on the next day every single hive-bee, without exception,
instead of alighting on the left wing-petal and sucking the flower in
the proper manner, flew straight without the least hesitation to the
calyx, and sucked through the holes which had been made only the day
before by the humble-bees; and they continued this habit for many
following days. (11/16. ‘Gardeners’ Chronicle’ 1857 page 725.) Mr. Belt
has communicated to me (July 28th, 1874) a similar case, with the sole
difference that less than half of the flowers had been perforated by the
humble-bees; nevertheless, all the hive-bees gave up sucking at the
mouths of the flowers and visited exclusively the bitten ones. Now how
did the hive-bees find out so quickly that holes had been made? Instinct
seems to be out of the question, as the plant is an exotic. The holes
cannot be seen by bees whilst standing on the wing-petals, where they
had always previously alighted. From the ease with which bees were
deceived when the petals of Lobelia erinus were cut off, it was clear
that in this case they were not guided to the nectar by its smell; and
it may be doubted whether they were attracted to the holes in the
flowers of the Phaseolus by the odour emitted from them. Did they
perceive the holes by the sense of touch in their proboscides, whilst
sucking the flowers in the proper manner, and then reason that it would
save them time to alight on the outside of the flowers and use the
holes? This seems almost too abstruse an act of reason for bees; and it
is more probable that they saw the humble-bees at work, and
understanding what they were about, imitated them and took advantage of
the shorter path to the nectar. Even with animals high in the scale,
such as monkeys, we should be surprised at hearing that all the
individuals of one species within the space of twenty-four hours
understood an act performed by a distinct species, and profited by it.

I have repeatedly observed with various kinds of flowers that all the
hive and humble-bees which were sucking through the perforations, flew
to them, whether on the upper or under side of the corolla, without the
least hesitation; and this shows how quickly all the individuals within
the district had acquired the same knowledge. Yet habit comes into play
to a certain extent, as in so many of the other operations of bees. Dr.
Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus
multiflorus that certain individuals went exclusively to the
perforations, while others of the same species visited only the mouths
of the flowers. (11/17. Dr. Ogle ‘Pop. Science Review’ April 1870 page
167. Mr. Farrer ‘Annals and Magazine of Natural History’ 4th series
volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861
exactly the same fact with Trifolium pratense. So persistent is the
force of habit, that when a bee which is visiting perforated flowers
comes to one which has not been bitten, it does not go to the mouth, but
instantly flies away in search of another bitten flower. Nevertheless, I
once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it
entered the mouths of some flowers and cut holes into the others. Dr.
Hermann Muller informs me that in the same district he has seen some
individuals of Bombus mastrucatus boring through the calyx and corolla
of Rhinanthus alecterolophus, and others through the corolla alone.
Different species of bees may, however, sometimes be observed acting
differently at the same time on the same plant. I have seen hive-bees
sucking at the mouths of the flowers of the common bean; humble-bees of
one kind sucking through holes bitten in the calyx, and humble-bees of
another kind sucking the little drops of fluid excreted by the stipules.
Mr. Beal of Michigan informs me that the flowers of the Missouri currant
(Ribes aureum) abound with nectar, so that children often suck them; and
he saw hive-bees sucking through holes made by a bird, the oriole, and
at the same time humble-bees sucking in the proper manner at the mouths
of the flowers. (11/18. The flowers of the Ribes are however sometimes
perforated by humble-bees, and Mr. Bundy says that they were able to
bite through and rob seven flowers of their honey in a minute: ‘American
Naturalist’ 1876 page 238.) This statement about the oriole calls to
mind what I have before said of certain species of humming-birds boring
holes through the flowers of the Brugmansia, whilst other species
entered by the mouth.

The motive which impels bees to gnaw holes through the corolla seems to
be the saving of time, for they lose much time in climbing into and out
of large flowers, and in forcing their heads into closed ones. They were
able to visit nearly twice as many flowers, as far as I could judge, of
a Stachys and Pentstemon by alighting on the upper surface of the
corolla and sucking through the cut holes, than by entering in the
proper way. Nevertheless each bee before it has had much practice, must
lose some time in making each new perforation, especially when the
perforation has to be made through both calyx and corolla. This action
therefore implies foresight, of which faculty we have abundant evidence
in their building operations; and may we not further believe that some
trace of their social instinct, that is, of working for the good of
other members of the community, may here likewise play a part?

Many years ago I was struck with the fact that humble-bees as a general
rule perforate flowers only when these grow in large numbers near
together. In a garden where there were some very large beds of Stachys
coccinea and of Pentstemon argutus, every single flower was perforated,
but I found two plants of the former species growing quite separate with
their petals much scratched, showing that they had been frequently
visited by bees, and yet not a single flower was perforated. I found
also a separate plant of the Pentstemon, and saw bees entering the mouth
of the corolla, and not a single flower had been perforated. In the
following year (1842) I visited the same garden several times: on the
19th of July humble-bees were sucking the flowers of Stachys coccinea
and Salvia grahami in the proper manner, and none of the corollas were
perforated. On the 7th of August all the flowers were perforated, even
those on some few plants of the Salvia which grew at a little distance
from the great bed. On the 21st of August only a few flowers on the
summits of the spikes of both species remained fresh, and not one of
these was now bored. Again, in my own garden every plant in several rows
of the common bean had many flowers perforated; but I found three plants
in separate parts of the garden which had sprung up accidentally, and
these had not a single flower perforated. General Strachey formerly saw
many perforated flowers in a garden in the Himalaya, and he wrote to the
owner to inquire whether this relation between the plants growing
crowded and their perforation by the bees there held good, and was
answered in the affirmative. Hence it follows that the red clover
(Trifolium pratense) and the common bean when cultivated in great masses
in fields,--that Erica tetralix growing in large numbers on
heaths,--rows of the scarlet kidney-bean in the kitchen-garden,--and
masses of any species in the flower-garden,--are all eminently liable to
be perforated.

The explanation of this fact is not difficult. Flowers growing in large
numbers afford a rich booty to the bees, and are conspicuous from a
distance. They are consequently visited by crowds of these insects, and
I once counted between twenty and thirty bees flying about a bed of
Pentstemon. They are thus stimulated to work quickly by rivalry, and,
what is much more important, they find a large proportion of the
flowers, as suggested by my son, with their nectaries sucked dry.
(11/19. ‘Nature’ January 8, 1874 page 189.) They thus waste much time in
searching many empty flowers, and are led to bite the holes, so as to
find out as quickly as possible whether there is any nectar present, and
if so, to obtain it.

Flowers which are partially or wholly sterile unless visited by insects
in the proper manner, such as those of most species of Salvia, of
Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less
completely to produce seeds if the bees confine their visits to the
perforations. The perforated flowers of those species, which are capable
of fertilising themselves, will yield only self-fertilised seeds, and
the seedlings will in consequence be less vigorous. Therefore all plants
must suffer in some degree when bees obtain their nectar in a felonious
manner by biting holes through the corolla; and many species, it might
be thought, would thus be exterminated. But here, as is so general
throughout nature, there is a tendency towards a restored equilibrium.
If a plant suffers from being perforated, fewer individuals will be
reared, and if its nectar is highly important to the bees, these in
their turn will suffer and decrease in number; but, what is much more
effective, as soon as the plant becomes somewhat rare so as not to grow
in crowded masses, the bees will no longer be stimulated to gnaw holes
in the flowers, but will enter them in a legitimate manner. More seed
will then be produced, and the seedlings being the product of
cross-fertilisation will be vigorous, so that the species will tend to
increase in number, to be again checked, as soon as the plant again
grows in crowded masses.



CHAPTER XII.

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.
Allied species differ greatly in the means by which cross-fertilisation
is favoured and self-fertilisation avoided.
The benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.
The evil effects not due to the combination of morbid tendencies in the
parents.
Nature of the conditions to which plants are subjected when growing near
together in a state of nature or under culture, and the effects of such
conditions.
Theoretical considerations with respect to the interaction of
differentiated sexual elements.
Practical lessons.
Genesis of the two sexes.
Close correspondence between the effects of cross-fertilisation and
self-fertilisation, and of the legitimate and illegitimate unions of
heterostyled plants, in comparison with hybrid unions.

The first and most important of the conclusions which may be drawn from
the observations given in this volume, is that cross-fertilisation is
generally beneficial, and self-fertilisation injurious. This is shown by
the difference in height, weight, constitutional vigour, and fertility
of the offspring from crossed and self-fertilised flowers, and in the
number of seeds produced by the parent-plants. With respect to the
second of these two propositions, namely, that self-fertilisation is
generally injurious, we have abundant evidence. The structure of the
flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc.,
renders the aid of insects almost indispensable for their fertilisation;
and bearing in mind the prepotency of pollen from a distinct individual
over that from the same individual, such plants will almost certainly
have been crossed during many or all previous generations. So it must
be, owing merely to the prepotency of foreign pollen, with cabbages and
various other plants, the varieties of which almost invariably
intercross when grown together. The same inference may be drawn still
more surely with respect to those plants, such as Reseda and
Eschscholtzia, which are sterile with their own pollen, but fertile with
that from any other individual. These several plants must therefore have
been crossed during a long series of previous generations, and the
artificial crosses in my experiments cannot have increased the vigour of
the offspring beyond that of their progenitors. Therefore the difference
between the self-fertilised and crossed plants raised by me cannot be
attributed to the superiority of the crossed, but to the inferiority of
the self-fertilised seedlings, due to the injurious effects of
self-fertilisation.

With respect to the first proposition, namely, that cross-fertilisation
is generally beneficial, we likewise have excellent evidence. Plants of
Ipomoea were intercrossed for nine successive generations; they were
then again intercrossed, and at the same time crossed with a plant of a
fresh stock, that is, one brought from another garden; and the offspring
of this latter cross were to the intercrossed plants in height as 100 to
78, and in fertility as 100 to 51. An analogous experiment with
Eschscholtzia gave a similar result, as far as fertility was concerned.
In neither of these cases were any of the plants the product of
self-fertilisation. Plants of Dianthus were self-fertilised for three
generations, and this no doubt was injurious; but when these plants were
fertilised by a fresh stock and by intercrossed plants of the same
stock, there was a great difference in fertility between the two sets of
seedlings, and some difference in their height. Petunia offers a nearly
parallel case. With various other plants, the wonderful effects of a
cross with a fresh stock may be seen in Table 7/C. Several accounts have
also been published of the extraordinary growth of seedlings from a
cross between two varieties of the same species, some of which are known
never to fertilise themselves; so that here neither self-fertilisation
nor relationship even in a remote degree can have come into play. (12/1.
See ‘Variation under Domestication’ chapter 19 2nd edition volume 2 page
159.) We may therefore conclude that the above two propositions are
true,--that cross-fertilisation is generally beneficial and
self-fertilisation injurious to the offspring.

That certain plants, for instance, Viola tricolor, Digitalis purpurea,
Sarothamnus scoparius, Cyclamen persicum, etc., which have been
naturally cross-fertilised for many or all previous generations, should
suffer to an extreme degree from a single act of self-fertilisation is a
most surprising fact. Nothing of the kkind has been observed in our
domestic animals; but then we must remember that the closest possible
interbreeding with such animals, that is, between brothers and sisters,
cannot be considered as nearly so close a union as that between the
pollen and ovules of the same flower. Whether the evil from
self-fertilisation goes on increasing during successive generations is
not as yet known; but we may infer from my experiments that the increase
if any is far from rapid. After plants have been propagated by
self-fertilisation for several generations, a single cross with a fresh
stock restores their pristine vigour; and we have a strictly analogous
result with our domestic animals. (12/2. Ibid chapter 19 2nd edition
volume 2 page 159.) The good effects of cross-fertilisation are
transmitted by plants to the next generation; and judging from the
varieties of the common pea, to many succeeding generations. But this
may merely be that crossed plants of the first generation are extremely
vigorous, and transmit their vigour, like any other character, to their
successors.

Notwithstanding the evil which many plants suffer from
self-fertilisation, they can be thus propagated under favourable
conditions for many generations, as shown by some of my experiments, and
more especially by the survival during at least half a century of the
same varieties of the common pea and sweet-pea. The same conclusion
probably holds good with several other exotic plants, which are never or
most rarely cross-fertilised in this country. But all these plants, as
far as they have been tried, profit greatly by a cross with a fresh
stock. Some few plants, for instance, Ophrys apifera, have almost
certainly been propagated in a state of nature for thousands of
generations without having been once intercrossed; and whether they
would profit by a cross with a fresh stock is not known. But such cases
ought not to make us doubt that as a general rule crossing is
beneficial, any more than the existence of plants which, in a state of
nature, are propagated exclusively by rhizomes, stolons, etc. (their
flowers never producing seeds), (12/3. I have given several cases in my
‘Variation under Domestication’ chapter 18 2nd edition volume 2 page
152.) (their flowers never producing seeds), should make us doubt that
seminal generation must have some great advantage, as it is the common
plan followed by nature. Whether any species has been reproduced
asexually from a very remote period cannot, of course, be ascertained.
Our sole means for forming any judgment on this head is the duration of
the varieties of our fruit trees which have been long propagated by
grafts or buds. Andrew Knight formerly maintained that under these
circumstances they always become weakly, but this conclusion has been
warmly disputed by others. A recent and competent judge, Professor Asa
Gray, leans to the side of Andrew Knight, which seems to me, from such
evidence as I have been able to collect, the more probable view,
notwithstanding many opposed facts. (12/4. ‘Darwiniana: Essays and
Reviews pertaining to Darwinism’ 1876 page 338.)

The means for favouring cross-fertilisation and preventing
self-fertilisation, or conversely for favouring self-fertilisation and
preventing to a certain extent cross-fertilisation, are wonderfully
diversified; and it is remarkable that these differ widely in closely
allied plants,--in the species of the same genus, and sometimes in the
individuals of the same species. (12/5. Hildebrand has insisted strongly
to this effect in his valuable observations on the fertilisation of the
Gramineae: ‘Monatsbericht K. Akad. Berlin’ October 1872 page 763.) It is
not rare to find hermaphrodite plants and others with separated sexes
within the same genus; and it is common to find some of the species
dichogamous and others maturing their sexual elements simultaneously.
The dichogamous genus Saxifraga contains proterandrous and proterogynous
species. (12/6. Dr. Engler ‘Botanische Zeitung’ 1868 page 833.) Several
genera include both heterostyled (dimorphic or trimorphic forms) and
homostyled species. Ophrys offers a remarkable instance of one species
having its structure manifestly adapted for self-fertilisation, and
other species as manifestly adapted for cross-fertilisation. Some
con-generic species are quite sterile and others quite fertile with
their own pollen. From these several causes we often find within the
same genus species which do not produce seeds, while others produce an
abundance, when insects are excluded. Some species bear cleistogene
flowers which cannot be crossed, as well as perfect flowers, whilst
others in the same genus never produce cleistogene flowers. Some species
exist under two forms, the one bearing conspicuous flowers adapted for
cross-fertilisation, the other bearing inconspicuous flowers adapted for
self-fertilisation, whilst other species in the same genus present only
a single form. Even with the individuals of the same species, the degree
of self-sterility varies greatly, as in Reseda. With polygamous plants,
the distribution of the sexes differs in the individuals of the same
species. The relative period at which the sexual elements in the same
flower are mature, differs in the varieties of Pelargonium; and Carriere
gives several cases, showing that the period varies according to the
temperature to which the plants are exposed. (12/7. ‘Des Varieties’ 1865
page 30.)

This extraordinary diversity in the means for favouring or preventing
cross- and self-fertilisation in closely allied forms, probably depends
on the results of both processes being highly beneficial to the species,
but directly opposed in many ways to one another and dependent on
variable conditions. Self-fertilisation assures the production of a
large supply of seeds; and the necessity or advantage of this will be
determined by the average length of life of the plant, which largely
depends on the amount of destruction suffered by the seeds and
seedlings. This destruction follows from the most various and variable
causes, such as the presence of animals of several kinds, and the growth
of surrounding plants. The possibility of cross-fertilisation depends
mainly on the presence and number of certain insects, often of insects
belonging to special groups, and on the degree to which they are
attracted to the flowers of any particular species in preference to
other flowers,--all circumstances likely to change. Moreover, the
advantages which follow from cross-fertilisation differ much in
different plants, so that it is probable that allied plants would often
profit in different degrees by cross-fertilisation. Under these
extremely complex and fluctuating conditions, with two somewhat opposed
ends to be gained, namely, the safe propagation of the species and the
production of cross-fertilised, vigorous offspring, it is not surprising
that allied forms should exhibit an extreme diversity in the means which
favour either end. If, as there is reason to suspect, self-fertilisation
is in some respects beneficial, although more than counterbalanced by
the advantages derived from a cross with a fresh stock, the problem
becomes still more complicated.

As I only twice experimented on more than a single species in a genus, I
cannot say whether the crossed offspring of the several species within
the same genus differ in their degree of superiority over their
self-fertilised brethren; but I should expect that this would often
prove to be the case from what was observed with the two species of
Lobelia and with the individuals of the same species of Nicotiana. The
species belonging to distinct genera in the same family certainly differ
in this respect. The effects of cross- and self-fertilisation may be
confined either to the growth or to the fertility of the offspring, but
generally extends to both qualities. There does not seem to exist any
close correspondence between the degree to which their offspring profit
by this process; but we may easily err on this head, as there are two
means for ensuring cross-fertilisation which are not externally
perceptible, namely, self-sterility and the prepotent fertilising
influence of pollen from another individual. Lastly, it has been shown
in a former chapter that the effect produced by cross and
self-fertilisation on the fertility of the parent-plants does not always
correspond with that produced on the height, vigour, and fertility of
their offspring. The same remark applies to crossed and self-fertilised
seedlings when these are used as the parent-plants. This want of
correspondence probably depends, at least in part, on the number of
seeds produced being chiefly determined by the number of the
pollen-tubes which reach the ovules, and this will be governed by the
reaction between the pollen and the stigmatic secretion or tissues;
whereas the growth and constitutional vigour of the offspring will be
chiefly determined, not only by the number of pollen-tubes reaching the
ovules, but by the nature of the reaction between the contents of the
pollen-grains and ovules.

There are two other important conclusions which may be deduced from my
observations: firstly, that the advantages of cross-fertilisation do not
follow from some mysterious virtue in the mere union of two distinct
individuals, but from such individuals having been subjected during
previous generations to different conditions, or to their having varied
in a manner commonly called spontaneous, so that in either case their
sexual elements have been in some degree differentiated. And secondly,
that the injury from self-fertilisation follows from the want of such
differentiation in the sexual elements. These two propositions are fully
established by my experiments. Thus, when plants of the Ipomoea and of
the Mimulus, which had been self-fertilised for the seven previous
generations and had been kept all the time under the same conditions,
were intercrossed one with another, the offspring did not profit in the
least by the cross. Mimulus offers another instructive case, showing
that the benefit of a cross depends on the previous treatment of the
progenitors: plants which had been self-fertilised for the eight
previous generations were crossed with plants which had been
intercrossed for the same number of generations, all having been kept
under the same conditions as far as possible; seedlings from this cross
were grown in competition with others derived from the same
self-fertilised mother-plant crossed by a fresh stock; and the latter
seedlings were to the former in height as 100 to 52, and in fertility as
100 to 4. An exactly parallel experiment was tried on Dianthus, with
this difference, that the plants had been self-fertilised only for the
three previous generations, and the result was similar though not so
strongly marked. The foregoing two cases of the offspring of Ipomoea and
Eschscholtzia, derived from a cross with a fresh stock, being as much
superior to the intercrossed plants of the old stock, as these latter
were to the self-fertilised offspring, strongly supports the same
conclusion. A cross with a fresh stock or with another variety seems to
be always highly beneficial, whether or not the mother-plants have been
intercrossed or self-fertilised for several previous generations. The
fact that a cross between two flowers on the same plant does no good or
very little good, is likewise a strong corroboration of our conclusion;
for the sexual elements in the flowers on the same plant can rarely have
been differentiated, though this is possible, as flower-buds are in one
sense distinct individuals, sometimes varying and differing from one
another in structure or constitution. Thus the proposition that the
benefit from cross-fertilisation depends on the plants which are crossed
having been subjected during previous generations to somewhat different
conditions, or to their having varied from some unknown cause as if they
had been thus subjected, is securely fortified on all sides.

Before proceeding any further, the view which has been maintained by
several physiologists must be noticed, namely, that all the evils from
breeding animals too closely, and no doubt, as they would say, from the
self-fertilisation of plants, is the result of the increase of some
morbid tendency or weakness of constitution common to the closely
related parents, or to the two sexes of hermaphrodite plants.
Undoubtedly injury has often thus resulted; but it is a vain attempt to
extend this view to the numerous cases given in my Tables. It should be
remembered that the same mother-plant was both self-fertilised and
crossed, so that if she had been unhealthy she would have transmitted
half her morbid tendencies to her crossed offspring. But plants
appearing perfectly healthy, some of them growing wild, or the immediate
offspring of wild plants, or vigorous common garden-plants, were
selected for experiment. Considering the number of species which were
tried, it is nothing less than absurd to suppose that in all these cases
the mother-plants, though not appearing in any way diseased, were weak
or unhealthy in so peculiar a manner that their self-fertilised
seedlings, many hundreds in number, were rendered inferior in height,
weight, constitutional vigour and fertility to their crossed offspring.
Moreover, this belief cannot be extended to the strongly marked
advantages which invariably follow, as far as my experience serves, from
intercrossing the individuals of the same variety or of distinct
varieties, if these have been subjected during some generations to
different conditions.

It is obvious that the exposure of two sets of plants during several
generations to different conditions can lead to no beneficial results,
as far as crossing is concerned, unless their sexual elements are thus
affected. That every organism is acted on to a certain extent by a
change in its environment, will not, I presume, be disputed. It is
hardly necessary to advance evidence on this head; we can perceive the
difference between individual plants of the same species which have
grown in somewhat more shady or sunny, dry or damp places. Plants which
have been propagated for some generations under different climates or at
different seasons of the year transmit different constitutions to their
seedlings. Under such circumstances, the chemical constitution of their
fluids and the nature of their tissues are often modified. (12/8.
Numerous cases together with references are given in my ‘Variation under
Domestication’ chapter 23 2nd edition volume 2 page 264. With respect to
animals, Mr. Brackenridge ‘A Contribution to the Theory of Diathesis’
Edinburgh 1869, has well shown that the different organs of animals are
excited into different degrees of activity by differences of temperature
and food, and become to a certain extent adapted to them.) Many other
such facts could be adduced. In short, every alteration in the function
of a part is probably connected with some corresponding, though often
quite imperceptible change in structure or composition.

Whatever affects an organism in any way, likewise tends to act on its
sexual elements. We see this in the inheritance of newly acquired
modifications, such as those from the increased use or disuse of a part,
and even from mutilations if followed by disease. (12/9. ‘Variation
under Domestication’ chapter 12 2nd edition volume 1 page 466.) We have
abundant evidence how susceptible the reproductive system is to changed
conditions, in the many instances of animals rendered sterile by
confinement; so that they will not unite, or if they unite do not
produce offspring, though the confinement may be far from close; and of
plants rendered sterile by cultivation. But hardly any cases afford more
striking evidence how powerfully a change in the conditions of life acts
on the sexual elements, than those already given, of plants which are
completely self-sterile in one country, and when brought to another,
yield, even in the first generation, a fair supply of self-fertilised
seeds.

But it may be said, granting that changed conditions act on the sexual
elements, how can two or more plants growing close together, either in
their native country or in a garden, be differently acted on, inasmuch
as they appear to be exposed to exactly the same conditions? Although
this question has been already considered, it deserves further
consideration under several points of view. In my experiments with
Digitalis purpurea, some flowers on a wild plant were self-fertilised,
and others were crossed with pollen from another plant growing within
two or three feet’s distance. The crossed and self-fertilised plants
raised from the seeds thus obtained, produced flower-stems in number as
100 to 47, and in average height as 100 to 70. Therefore the cross
between these two plants was highly beneficial; but how could their
sexual elements have been differentiated by exposure to different
conditions? If the progenitors of the two plants had lived on the same
spot during the last score of generations, and had never been crossed
with any plant beyond the distance of a few feet, in all probability
their offspring would have been reduced to the same state as some of the
plants in my experiments,--such as the intercrossed plants of the ninth
generation of Ipomoea,--or the self-fertilised plants of the eighth
generation of Mimulus,--or the offspring from flowers on the same
plant,--and in this case a cross between the two plants of Digitalis
would have done no good. But seeds are often widely dispersed by natural
means, and one of the above two plants or one of their ancestors may
have come from a distance, from a more shady or sunny, dry or moist
place, or from a different kind of soil containing other organic or
inorganic matter. We know from the admirable researches of Messrs. Lawes
and Gilbert that different plants require and consume very different
amounts of inorganic matter. (12/10. ‘Journal of the Royal Agricultural
Society of England’ volume 24 part 1.) But the amount in the soil would
probably not make so great a difference to the several individuals of
any particular species as might at first be expected; for the
surrounding species with different requirements would tend, from
existing in greater or lesser numbers, to keep each species in a sort of
equilibrium, with respect to what it could obtain from the soil. So it
would be even with respect to moisture during dry seasons; and how
powerful is the influence of a little more or less moisture in the soil
on the presence and distribution of plants, is often well shown in old
pasture fields which still retain traces of former ridges and furrows.
Nevertheless, as the proportional numbers of the surrounding plants in
two neighbouring places is rarely exactly the same, the individuals of
the same species will be subjected to somewhat different conditions with
respect to what they can absorb from the soil. It is surprising how the
free growth of one set of plants affects others growing mingled with
them; I allowed the plants on rather more than a square yard of turf
which had been closely mown for several years, to grow up; and nine
species out of twenty were thus exterminated; but whether this was
altogether due to the kinds which grew up robbing the others of
nutriment, I do not know.

Seeds often lie dormant for several years in the ground, and germinate
when brought near the surface by any means, as by burrowing animals.
They would probably be affected by the mere circumstance of having long
lain dormant; for gardeners believe that the production of double
flowers and of fruit is thus influenced. Seeds, moreover, which were
matured during different seasons, will have been subjected during the
whole course of their development to different degrees of heat and
moisture.

It was shown in the last chapter that pollen is often carried by insects
to a considerable distance from plant to plant. Therefore one of the
parents or ancestors of our two plants of Digitalis may have been
crossed by a distant plant growing under somewhat different conditions.
Plants thus crossed often produce an unusually large number of seeds; a
striking instance of this fact is afforded by the Bignonia, previously
mentioned, which was fertilised by Fritz Muller with pollen from some
adjoining plants and set hardly any seed, but when fertilised with
pollen from a distant plant, was highly fertile. Seedlings from a cross
of this kind grow with great vigour, and transmit their vigour to their
descendants. These, therefore, in the struggle for life, will generally
beat and exterminate the seedlings from plants which have long grown
near together under the same conditions, and will thus tend to spread.

When two varieties which present well-marked differences are crossed,
their descendants in the later generations differ greatly from one
another in external characters; and this is due to the augmentation or
obliteration of some of these characters, and to the reappearance of
former ones through reversion; and so it will be, as we may feel almost
sure, with any slight differences in the constitution of their sexual
elements. Anyhow, my experiments indicate that crossing plants which
have been long subjected to almost though not quite the same conditions,
is the most powerful of all the means for retaining some degree of
differentiation in the sexual elements, as shown by the superiority in
the later generations of the intercrossed over the self-fertilised
seedlings. Nevertheless, the continued intercrossing of plants thus
treated does tend to obliterate such differentiation, as may be inferred
from the lessened benefit derived from intercrossing such plants, in
comparison with that from a cross with a fresh stock. It seems probable,
as I may add, that seeds have acquired their endless curious adaptations
for wide dissemination, not only that the seedlings would thus be
enabled to find new and fitting homes, but that the individuals which
have been long subjected to the same conditions should occasionally
intercross with a fresh stock. (12/11. See Professor Hildebrand’s
excellent treatise ‘Verbreitungsmittel der Pflanzen’ 1873.)

From the foregoing several considerations we may, I think, conclude that
in the above case of the Digitalis, and even in that of plants which
have grown for thousands of generations in the same district, as must
often have occurred with species having a much restricted range, we are
apt to over-estimate the degree to which the individuals have been
subjected to absolutely the same conditions. There is at least no
difficulty in believing that such plants have been subjected to
sufficiently distinct conditions to differentiate their sexual elements;
for we know that a plant propagated for some generations in another
garden in the same district serves as a fresh stock and has high
fertilising powers. The curious cases of plants which can fertilise and
be fertilised by any other individual of the same species, but are
altogether sterile with their own pollen, become intelligible, if the
view here propounded is correct, namely, that the individuals of the
same species growing in a state of nature near together, have not really
been subjected during several previous generations to quite the same
conditions.

Some naturalists assume that there is an innate tendency in all beings
to vary and to advance in organisation, independently of external
agencies; and they would, I presume, thus explain the slight differences
which distinguish all the individuals of the same species both in
external characters and in constitution, as well as the greater
differences in both respects between nearly allied varieties. No two
individuals can be found quite alike; thus if we sow a number of seeds
from the same capsule under as nearly as possible the same conditions,
they germinate at different rates and grow more or less vigorously. They
resist cold and other unfavourable conditions differently. They would in
all probability, as we know to be the case with animals of the same
species, be somewhat differently acted on by the same poison, or by the
same disease. They have different powers of transmitting their
characters to their offspring; and many analogous facts could be given.
(12/12. Vilmorin as quoted by Verlot ‘Des Varieties’ pages 32, 38, 39.)
Now, if it were true that plants growing near together in a state of
nature had been subjected during many previous generations to absolutely
the same conditions, such differences as those just specified would be
quite inexplicable; but they are to a certain extent intelligible in
accordance with the views just advanced.

As most of the plants on which I experimented were grown in my garden or
in pots under glass, a few words must be added on the conditions to
which they were exposed, as well as on the effects of cultivation. When
a species is first brought under culture, it may or may not be subjected
to a change of climate, but it is always grown in ground broken up, and
more or less manured; it is also saved from competition with other
plants. The paramount importance of this latter circumstance is proved
by the multitude of species which flourish and multiply in a garden, but
cannot exist unless they are protected from other plants. When thus
saved from competition they are able to get whatever they require from
the soil, probably often in excess; and they are thus subjected to a
great change of conditions. It is probably in chief part owing to this
cause that all plants with rare exceptions vary after being cultivated
for some generations. The individuals which have already begun to vary
will intercross one with another by the aid of insects; and this
accounts for the extreme diversity of character which many of our long
cultivated plants exhibit. But it should be observed that the result
will be largely determined by the degree of their variability and by the
frequency of the intercrosses; for if a plant varies very little, like
most species in a state of nature, frequent intercrosses tend to give
uniformity of character to it.

I have attempted to show that with plants growing naturally in the same
district, except in the unusual case of each individual being surrounded
by exactly the same proportional numbers of other species having certain
powers of absorption, each will be subjected to slightly different
conditions. This does not apply to the individuals of the same species
when cultivated in cleared ground in the same garden. But if their
flowers are visited by insects, they will intercross; and this will give
to their sexual elements during a considerable number of generations a
sufficient amount of differentiation for a cross to be beneficial.
Moreover, seeds are frequently exchanged or procured from other gardens
having a different kind of soil; and the individuals of the same
cultivated species will thus be subjected to a change of conditions. If
the flowers are not visited by our native insects, or very rarely so, as
in the case of the common and sweet pea, and apparently in that of the
tobacco when kept in a hothouse, any differentiation in the sexual
elements caused by intercrosses will tend to disappear. This appears to
have occurred with the plants just mentioned, for they were not
benefited by being crossed one with another, though they were greatly
benefited by a cross with a fresh stock.

I have been led to the views just advanced with respect to the causes of
the differentiation of the sexual elements and of the variability of our
garden plants, by the results of my various experiments, and more
especially by the four cases in which extremely inconstant species,
after having been self-fertilised and grown under closely similar
conditions for several generations, produced flowers of a uniform and
constant tint. These conditions were nearly the same as those to which
plants, growing in a garden clear of weeds, are subjected, if they are
propagated by self-fertilised seeds on the same spot. The plants in pots
were, however, exposed to less severe fluctuations of climate than those
out of doors; but their conditions, though closely uniform for all the
individuals of the same generation, differed somewhat in the successive
generations. Now, under these circumstances, the sexual elements of the
plants which were intercrossed in each generation retained sufficient
differentiation during several years for their offspring to be superior
to the self-fertilised, but this superiority gradually and manifestly
decreased, as was shown by the difference in the result between a cross
with one of the intercrossed plants and with a fresh stock. These
intercrossed plants tended also in a few cases to become somewhat more
uniform in some of their external characters than they were at first.
With respect to the plants which were self-fertilised in each
generation, their sexual elements apparently lost, after some years, all
differentiation, for a cross between them did no more good than a cross
between the flowers on the same plant. But it is a still more remarkable
fact, that although the seedlings of Mimulus, Ipomoea, Dianthus, and
Petunia which were first raised, varied excessively in the colour of
their flowers, their offspring, after being self-fertilised and grown
under uniform conditions for some generations, bore flowers almost as
uniform in tint as those on a natural species. In one case also the
plants themselves became remarkably uniform in height.

The conclusion that the advantages of a cross depend altogether on the
differentiation of the sexual elements, harmonises perfectly with the
fact that an occasional and slight change in the conditions of life is
beneficial to all plants and animals. (12/13. I have given sufficient
evidence on this head in my ‘Variation under Domestication’ chapter 18
volume 2 2nd edition page 127.) But the offspring from a cross between
organisms which have been exposed to different conditions, profit in an
incomparably higher degree than do young or old beings from a mere
change in the conditions. In this latter case we never see anything like
the effect which generally follows from a cross with another individual,
especially from a cross with a fresh stock. This might, perhaps, have
been expected, for the blending together of the sexual elements of two
differentiated beings will affect the whole constitution at a very early
period of life, whilst the organisation is highly flexible. We have,
moreover, reason to believe that changed conditions generally act
differently on the several parts or organs of the same individual
(12/14. See, for instance, Brackenridge ‘Theory of Diathesis’ Edinburgh
1869.); and if we may further believe that these now slightly
differentiated parts react on one another, the harmony between the
beneficial effects on the individual due to changed conditions, and
those due to the interaction of differentiated sexual elements, becomes
still closer.

That wonderfully accurate observer, Sprengel, who first showed how
important a part insects play in the fertilisation of flowers, called
his book ‘The Secret of Nature Displayed;’ yet he only occasionally saw
that the object for which so many curious and beautiful adaptations have
been acquired, was the cross-fertilisation of distinct plants; and he
knew nothing of the benefits which the offspring thus receive in growth,
vigour, and fertility. But the veil of secrecy is as yet far from
lifted; nor will it be, until we can say why it is beneficial that the
sexual elements should be differentiated to a certain extent, and why,
if the differentiation be carried still further, injury follows. It is
an extraordinary fact that with many species, flowers fertilised with
their own pollen are either absolutely or in some degree sterile; if
fertilised with pollen from another flower on the same plant, they are
sometimes, though rarely, a little more fertile; if fertilised with
pollen from another individual or variety of the same species, they are
fully fertile; but if with pollen from a distinct species, they are
sterile in all possible degrees, until utter sterility is reached. We
thus have a long series with absolute sterility at the two ends;--at one
end due to the sexual elements not having been sufficiently
differentiated, and at the other end to their having been differentiated
in too great a degree, or in some peculiar manner.

The fertilisation of one of the higher plants depends, in the first
place, on the mutual action of the pollen-grains and the stigmatic
secretion or tissues, and afterwards on the mutual action of the
contents of the pollen-grains and ovules. Both actions, judging from the
increased fertility of the parent-plants and from the increased powers
of growth in the offspring, are favoured by some degree of
differentiation in the elements which interact and unite so as to form a
new being. Here we have some analogy with chemical affinity or
attraction, which comes into play only between atoms or molecules of a
different nature. As Professor Miller remarks: “Generally speaking, the
greater the difference in the properties of two bodies, the more intense
is their tendency to mutual chemical action...But between bodies of a
similar character the tendency to unite is feeble.” (12/15. ‘Elements of
Chemistry’ 4th edition 1867 part 1 page 11. Dr. Frankland informs me
that similar views with respect to chemical affinity are generally
accepted by chemists.) This latter proposition accords well with the
feeble effects of a plant’s own pollen on the fertility of the
mother-plant and on the growth of the offspring; and the former
proposition accords well with the powerful influence in both ways of
pollen from an individual which has been differentiated by exposure to
changed conditions, or by so-called spontaneous variation. But the
analogy fails when we turn to the negative or weak effects of pollen
from one species on a distinct species; for although some substances
which are extremely dissimilar, for instance, carbon and chlorine, have
a very feeble affinity for each other, yet it cannot be said that the
weakness of the affinity depends in such cases on the extent to which
the substances differ. It is not known why a certain amount of
differentiation is necessary or favourable for the chemical affinity or
union of two substances, any more than for the fertilisation or union of
two organisms.

Mr. Herbert Spencer has discussed this whole subject at great length,
and after stating that all the forces throughout nature tend towards an
equilibrium, remarks, “that the need of this union of sperm-cell and
germ-ccell is the need for overthrowing this equilibrium and
re-establishing active molecular change in the detached germ--a result
which is probably effected by mixing the slightly-different
physiological units of slightly-different individuals.” (12/16.
‘Principles of Biology’ volume 1 page 274 1864. In my ‘Origin of
Species’ published in 1859, I spoke of the good effects from slight
changes in the condition of life and from cross-fertilisation, and of
the evil effects from great changes in the conditions and from crossing
widely distinct forms (i.e., species), as a series of facts “connected
together by some common but unknown bond, which is essentially related
to the principle of life.”) But we must not allow this highly
generalised view, or the analogy of chemical affinity, to conceal from
us our ignorance. We do not know what is the nature or degree of the
differentiation in the sexual elements which is favourable for union,
and what is injurious for union, as in the case of distinct species. We
cannot say why the individuals of certain species profit greatly, and
others very little by being crossed. There are some few species which
have been self-fertilised for a vast number of generations, and yet are
vigorous enough to compete successfully with a host of surrounding
plants. We can form no conception why the advantage from a cross is
sometimes directed exclusively to the vegetative system, and sometimes
to the reproductive system, but commonly to both. It is equally
inconceivable why some individuals of the same species should be
sterile, whilst others are fully fertile with their own pollen; why a
change of climate should either lessen or increase the sterility of
self-sterile species; and why the individuals of some species should be
even more fertile with pollen from a distinct species than with their
own pollen. And so it is with many other facts, which are so obscure
that we stand in awe before the mystery of life.

Under a practical point of view, agriculturists and horticulturists may
learn something from the conclusions at which we have arrived. Firstly,
we see that the injury from the close breeding of animals and from the
self-fertilisation of plants, does not necessarily depend on any
tendency to disease or weakness of constitution common to the related
parents, and only indirectly on their relationship, in so far as they
are apt to resemble each other in all respects, including their sexual
nature. And, secondly, that the advantages of cross-fertilisation depend
on the sexual elements of the parents having become in some degree
differentiated by the exposure of their progenitors to different
conditions, or from their having intercrossed with individuals thus
exposed, or, lastly, from what we call in our ignorance spontaneous
variation. He therefore who wishes to pair closely related animals ought
to keep them under conditions as different as possible. Some few
breeders, guided by their keen powers of observation, have acted on this
principle, and have kept stocks of the same animals at two or more
distant and differently situated farms. They have then coupled the
individuals from these farms with excellent results. (12/17. ‘Variation
of Animals and Plants under Domestication’ chapter 17 2nd edition volume
2 pages 98, 105.) This same plan is also unconsciously followed whenever
the males, reared in one place, are let out for propagation to breeders
in other places. As some kinds of plants suffer much more from
self-fertilisation than do others, so it probably is with animals from
too close interbreeding. The effects of close interbreeding on animals,
judging again from plants, would be deterioration in general vigour,
including fertility, with no necessary loss of excellence of form; and
this seems to be the usual result.

It is a common practice with horticulturists to obtain seeds from
another place having a very different soil, so as to avoid raising
plants for a long succession of generations under the same conditions;
but with all the species which freely intercross by aid of insects or
the wind, it would be an incomparably better plan to obtain seeds of the
required variety, which had been raised for some generations under as
different conditions as possible, and sow them in alternate rows with
seeds matured in the old garden. The two stocks would then intercross,
with a thorough blending of their whole organisations, and with no loss
of purity to the variety; and this would yield far more favourable
results than a mere exchange of seeds. We have seen in my experiments
how wonderfully the offspring profited in height, weight, hardiness, and
fertility, by crosses of this kind. For instance, plants of Ipomoea thus
crossed were to the intercrossed plants of the same stock, with which
they grew in competition, as 100 to 78 in height, and as 100 to 51 in
fertility; and plants of Eschscholtzia similarly compared were as 100 to
45 in fertility. In comparison with self-fertilised plants the results
are still more striking; thus cabbages derived from a cross with a fresh
stock were to the self-fertilised as 100 to 22 in weight.

Florists may learn from the four cases which have been fully described,
that they have the power of fixing each fleeting variety of colour, if
they will fertilise the flowers of the desired kind with their own
pollen for half-a-dozen generations, and grow the seedlings under the
same conditions. But a cross with any other individual of the same
variety must be carefully prevented, as each has its own peculiar
constitution. After a dozen generations of self-fertilisation, it is
probable that the new variety would remain constant even if grown under
somewhat different conditions; and there would no longer be any
necessity to guard against intercrosses between the individuals of the
same variety.

With respect to mankind, my son George has endeavoured to discover by a
statistical investigation whether the marriages of first cousins are at
all injurious, although this is a degree of relationship which would not
be objected to in our domestic animals; and he has come to the
conclusion from his own researches and those of Dr. Mitchell that the
evidence as to any evil thus caused is conflicting, but on the whole
points to its being very small. From the facts given in this volume we
may infer that with mankind the marriages of nearly related persons,
some of whose parents and ancestors had lived under very different
conditions, would be much less injurious than that of persons who had
always lived in the same place and followed the same habits of life. Nor
can I see reason to doubt that the widely different habits of life of
men and women in civilised nations, especially amongst the upper
classes, would tend to counterbalance any evil from marriages between
healthy and somewhat closely related persons.

Under a theoretical point of view it is some gain to science to know
that numberless structures in hermaphrodite plants, and probably in
hermaphrodite animals, are special adaptations for securing an
occasional cross between two individuals; and that the advantages from
such a cross depend altogether on the beings which are united, or their
progenitors, having had their sexual elements somewhat differentiated,
so that the embryo is benefited in the same manner as is a mature plant
or animal by a slight change in its conditions of life, although in a
much higher degree.

Another and more important result may be deduced from my observations.
Eggs and seeds are highly serviceable as a means of dissemination, but
we now know that fertile eggs can be produced without the aid of the
male. There are also many other methods by which organisms can be
propagated asexually. Why then have the two sexes been developed, and
why do males exist which cannot themselves produce offspring? The answer
lies, as I can hardly doubt, in the great good which is derived from the
fusion of two somewhat differentiated individuals; and with the
exception of the lowest organisms this is possible only by means of the
sexual elements, these consisting of cells separated from the body,
containing the germs of every part, and capable of being fused
completely together.

It has been shown in the present volume that the offspring from the
union of two distinct individuals, especially if their progenitors have
been subjected to very different conditions, have an immense advantage
in height, weight, constitutional vigour and fertility over the
self-fertilised offspring from one of the same parents. And this fact is
amply sufficient to account for the development of the sexual elements,
that is, for the genesis of the two sexes.

It is a different question why the two sexes are sometimes combined in
the same individual and are sometimes separated. As with many of the
lowest plants and animals the conjugation of two individuals which are
either quite similar or in some degree different, is a common
phenomenon, it seems probable, as remarked in the last chapter, that the
sexes were primordially separate. The individual which receives the
contents of the other, may be called the female; and the other, which is
often smaller and more locomotive, may be called the male; though these
sexual names ought hardly to be applied as long as the whole contents of
the two forms are blended into one. The object gained by the two sexes
becoming united in the same hermaphrodite form probably is to allow of
occasional or frequent self-fertilisation, so as to ensure the
propagation of the species, more especially in the case of organisms
affixed for life to the same spot. There does not seem to be any great
difficulty in understanding how an organism, formed by the conjugation
of two individuals which represented the two incipient sexes, might have
given rise by budding first to a monoecious and then to an hermaphrodite
form; and in the case of animals even without budding to an
hermaphrodite form, for the bilateral structure of animals perhaps
indicates that they were aboriginally formed by the fusion of two
individuals.

It is a more difficult problem why some plants and apparently all the
higher animals, after becoming hermaphrodites, have since had their
sexes re-separated. This separation has been attributed by some
naturalists to the advantages which follow from a division of
physiological labour. The principle is intelligible when the same organ
has to perform at the same time diverse functions; but it is not obvious
why the male and female glands when placed in different parts of the
same compound or simple individual, should not perform their functions
equally well as when placed in two distinct individuals. In some
instances the sexes may have been re-separated for the sake of
preventing too frequent self-fertilisation; but this explanation does
not seem probable, as the same end might have been gained by other and
simpler means, for instance dichogamy. It may be that the production of
the male and female reproductive elements and the maturation of the
ovules was too great a strain and expenditure of vital force for a
single individual to withstand, if endowed with a highly complex
organisation; and that at the same time there was no need for all the
individuals to produce young, and consequently that no injury, on the
contrary, good resulted from half of them, or the males, failing to
produce offspring.

There is another subject on which some light is thrown by the facts
given in this volume, namely, hybridisation. It is notorious that when
distinct species of plants are crossed, they produce with the rarest
exceptions fewer seeds than the normal number. This unproductiveness
varies in different species up to sterility so complete that not even an
empty capsule is formed; and all experimentalists have found that it is
much influenced by the conditions to which the crossed species are
subjected. The pollen of each species is strongly prepotent over that of
any other species, so that if a plant’s own pollen is placed on the
stigma some time after foreign pollen has been applied to it, any effect
from the latter is quite obliterated. It is also notorious that not only
the parent species, but the hybrids raised from them are more or less
sterile; and that their pollen is often in a more or less aborted
condition. The degree of sterility of various hybrids does not always
strictly correspond with the degree of difficulty in uniting the parent
forms. When hybrids are capable of breeding inter se, their descendants
are more or less sterile, and they often become still more sterile in
the later generations; but then close interbreeding has hitherto been
practised in all such cases. The more sterile hybrids are sometimes much
dwarfed in stature, and have a feeble constitution. Other facts could be
given, but these will suffice for us. Naturalists formerly attributed
all these results to the difference between species being fundamentally
distinct from that between the varieties of the same species; and this
is still the verdict of some naturalists.

The results of my experiments in self-fertilising and cross-fertilising
the individuals or the varieties of the same species, are strikingly
analogous with those just given, though in a reversed manner. With the
majority of species flowers fertilised with their own pollen yield
fewer, sometimes much fewer seeds, than those fertilised with pollen
from another individual or variety. Some self-fertilised flowers are
absolutely sterile; but the degree of their sterility is largely
determined by the conditions to which the parent plants have been
exposed, as was well exemplified in the case of Eschscholtzia and
Abutilon. The effects of pollen from the same plant are obliterated by
the prepotent influence of pollen from another individual or variety,
although the latter may have been placed on the stigma some hours
afterwards. The offspring from self-fertilised flowers are themselves
more or less sterile, sometimes highly sterile, and their pollen is
sometimes in an imperfect condition; but I have not met with any case of
complete sterility in self-fertilised seedlings, as is so common with
hybrids. The degree of their sterility does not correspond with that of
the parent-plants when first self-fertilised. The offspring of
self-fertilised plants suffer in stature, weight, and constitutional
vigour more frequently and in a greater degree than do the hybrid
offspring of the greater number of crossed species. Decreased height is
transmitted to the next generation, but I did not ascertain whether this
applies to decreased fertility.

I have elsewhere shown that by uniting in various ways dimorphic or
trimorphic heterostyled plants, which belong to the same undoubted
species, we get another series of results exactly parallel with those
from crossing distinct species. (12/18. ‘Journal of the Linnean Society
Botany’ volume 10 1867 page 393.) Plants illegitimately fertilised with
pollen from a distinct plant belonging to the same form, yield fewer,
often much fewer seeds, than they do when legitimately fertilised with
pollen from a plant belonging to a distinct form. They sometimes yield
no seed, not even an empty capsule, like a species fertilised with
pollen from a distinct genus. The degree of sterility is much affected
by the conditions to which the plants have been subjected. (12/19.
‘Journal of the Linnean Society Botany’ volume 8 1864 page 180.) The
pollen from a distinct form is strongly prepotent over that from the
same form, although the former may have been placed on the stigma many
hours afterwards. The offspring from a union between plants of the same
form are more or less sterile, like hybrids, and have their pollen in a
more or less aborted condition; and some of the seedlings are as barren
and as dwarfed as the most barren hybrid. They also resemble hybrids in
several other respects, which need not here be specified in
detail,--such as their sterility not corresponding in degree with that
of the parent plants,--the unequal sterility of the latter, when
reciprocally united,--and the varying sterility of the seedlings raised
from the same seed-capsule.

We thus have two grand classes of cases giving results which correspond
in the most striking manner with those which follow from the crossing of
so-called true and distinct species. With respect to the difference
between seedlings raised from cross and self-fertilised flowers, there
is good evidence that this depends altogether on whether the sexual
elements of the parents have been sufficiently differentiated, by
exposure to different conditions or by spontaneous variation. It is
probable that nearly the same conclusion may be extended to heterostyled
plants; but this is not the proper place for discussing the origin of
the long-styled, short-styled and mid-styled forms, which all belong to
the same species as certainly as do the two sexes of the same species.
We have therefore no right to maintain that the sterility of species
when first crossed and of their hybrid offspring, is determined by some
cause fundamentally different from that which determines the sterility
of the individuals both of ordinary and of heterostyled plants when
united in various ways. Nevertheless, I am aware that it will take many
years to remove this prejudice.

There is hardly anything more wonderful in nature than the sensitiveness
of the sexual elements to external influences, and the delicacy of their
affinities. We see this in slight changes in the conditions of life
being favourable to the fertility and vigour of the parents, while
certain other and not great changes cause them to be quite sterile
without any apparent injury to their health. We see how sensitive the
sexual elements of those plants must be, which are completely sterile
with their own pollen, but are fertile with that of any other individual
of the same species. Such plants become either more or less self-sterile
if subjected to changed conditions, although the change may be far from
great. The ovules of a heterostyled trimorphic plant are affected very
differently by pollen from the three sets of stamens belonging to the
same species. With ordinary plants the pollen of another variety or
merely of another individual of the same variety is often strongly
prepotent over its own pollen, when both are placed at the same time on
the same stigma. In those great families of plants containing many
thousand allied species, the stigma of each distinguishes with unerring
certainty its own pollen from that of every other species.

There can be no doubt that the sterility of distinct species when first
crossed, and of their hybrid offspring, depends exclusively on the
nature or affinities of their sexual elements. We see this in the want
of any close correspondence between the degree of sterility and the
amount of external difference in the species which are crossed; and
still more clearly in the wide difference in the results of crossing
reciprocally the same two species;--that is, when species A is crossed
with pollen from B, and then B is crossed with pollen from A. Bearing in
mind what has just been said on the extreme sensitiveness and delicate
affinities of the reproductive system, why should we feel any surprise
at the sexual elements of those forms, which we call species, having
been differentiated in such a manner that they are incapable or only
feebly capable of acting on one another? We know that species have
generally lived under the same conditions, and have retained their own
proper characters, for a much longer period than varieties.
Long-continued domestication eliminates, as I have shown in my
‘Variation under Domestication,’ the mutual sterility which distinct
species lately taken from a state of nature almost always exhibit when
intercrossed; and we can thus understand the fact that the most
different domestic races of animals are not mutually sterile. But
whether this holds good with cultivated varieties of plants is not
known, though some facts indicate that it does. The elimination of
sterility through long-continued domestication may probably be
attributed to the varying conditions to which our domestic animals have
been subjected; and no doubt it is owing to this same cause that they
withstand great and sudden changes in their conditions of life with far
less loss of fertility than do natural species. From these several
considerations it appears probable that the difference in the affinities
of the sexual elements of distinct species, on which their mutual
incapacity for breeding together depends, is caused by their having been
habituated for a very long period each to its own conditions, and to the
sexual elements having thus acquired firmly fixed affinities. However
this may be, with the two great classes of cases before us, namely,
those relating to the self-fertilisation and cross-fertilisation of the
individuals of the same species, and those relating to the illegitimate
and legitimate unions of heterostyled plants, it is quite unjustifiable
to assume that the sterility of species when first crossed and of their
hybrid offspring, indicates that they differ in some fundamental manner
from the varieties or individuals of the same species.



INDEX.

Abutilon darwinii, self-sterile in Brazil.
moderately self-fertile in England.
fertilised by birds.

Acacia sphaerocephala.

Acanthaceae.

Aconitum napellus.

Adlumia cirrhosa.

Adonis aestivalis.
measurements.
relative heights of crossed and self-fertilised plants.
self-fertile.

Ajuga reptans.

Allium cepa (blood-red var.)

Anagallis collina (var. grandiflora).
measurements.
seeds.

Anderson, J., on the Calceolaria.
removing the corollas.

Anemone.

Anemophilous plants.
often diclinous.

Antirrhinum majus (red var.)
perforated corolla.
--(white var.).
--(peloric var.).

Apium petroselinum.
result of experiments.

Argemone ochroleuca.

Aristolochia.

Aristotle on bees frequenting flowers of the same species.

Arum maculatum.

Bailey, Mr., perforation of corolla.

Bartonia aurea.
measurements.
result of experiments.

Bartsia odontites.

Beal, W.J., sterility of Kalmia latifolia.
on nectar in Ribes aureum.

Bean, the common.

Bees distinguish colours.
frequent the flowers of the same species.
guided by coloured corolla.
powers of vision and discrimination.
memory.
unattracted by odour of certain flowers.
industry.
profit by the corolla perforated by humble-bees.
skill in working.
habit.
foresight.

Bees, humble, recognise varieties as of one species.
colour not the sole guide.
rate of flying.
number of flowers visited.
corolla perforated by.
skill and judgment.

Belt, Mr., the hairs of Digitalis purpurea.
Phaseolus multiflorus.
not visited by bees in Nicaragua.
humming-birds carrying pollen.
secretion of nectar.
in Acacia sphaerocephalus and passion-flower.
perforation of corolla.

Bennett, A.W., on Viola tricolor.
structure of Impatiens fulva.
plants flowering in winter.
bees frequenting flowers of same species.

Bentham, on protection of the stigma in Synaphea.

Beta vulgaris.
measurements.
crossed not exceeded by self-fertilised.
prepotency of other pollen.

Bignonia.

Birds, means of fertilisation.

Blackley, Mr., on anthers of rye.
pollen carried by wind, experiments with a kite.

Boraginaceae.

Borago officinalis.
measurements.
early flowering of crossed.
seeds.
partially self-sterile.

Brackenridge, Mr., organism of animals affected by temperature and food.
different effect of changed conditions.

Brassica oleracea.
measurements.
weight.
remarks on experiments.
superiority of crossed.
period of flowering.
seeds.
self-fertile.
--napus.
--rapa.

Brisout, M., insects frequenting flowers of same species.

Broom.

Brugmansia.
humming-birds boring the flower.

Bulrush, weight of pollen produced by one plant.

Bundy, Mr., Ribes perforated by bees.

Butschli, O., sexual relations.

Cabbage.
affected by pollen of purple bastard.
prepotency of other pollen.
--, Ragged Jack.

Calceolaria.

Calluna vulgaris.

Campanula carpathica.

Campanulaceae.

Candolle, A. de, on ascending a mountain the flowers of the same species
disappear abruptly.

Canna warscewiczi.
result of crossed and self-fertilised.
period of flowering.
seeds.
highly self-fertile.

Cannaceae.

Carduus arctioides.

Carnation.

Carriere, relative period of the maturity of the sexual elements on same
flower.

Caryophyllaceae.

Caspary, Professor, on Corydalis cava.
Nymphaeaceae.
Euryale ferox.

Cecropia, food-bodies of.

Centradenia floribunda.

Cereals, grains of.

Cheeseman, Mr., on Orchids in New Zealand.

Chenopodiaceae.

Cineraria.

Clarkia elegans.
measurements.
early flowering of self-fertilised.
seeds.

Cleistogene flowers.

Coe, Mr., crossing Phaseolus vulgaris.

Colgate, R., red clover never sucked by hive-bees in New Zealand.

Colour, uniform, of flowers on plants self-fertilised and grown under
similar conditions for several generations.

Colours of flowers attractive to insects.
not the sole guide to bees.

Compositae.

Coniferae.

Convolvulus major.
-- tricolor.

Corolla, removal of.
perforation by bees.

Coronilla.

Corydalis cava.
-- halleri.
-- intermedia.
-- lutea.
-- ochroleuca.
-- solida.

Corylus avellana.

Cowslip.

Crinum.

Crossed plants, greater constitutional vigour of.

Cross-fertilisation.
see Fertilisation.

Crossing flowers on same plant, effects of.

Cruciferae.

Cruger, Dr., secretion of sweet fluid in Marcgraviaceae.

Cuphea purpurea.

Cycadiae.

Cyclamen persicum.
measurements.
early flowering of crossed.
seeds.
self-sterile.
-- repandum.

Cytisus laburnum.

Dandelion, number of pollen grains.

Darwin, C., self-fertilisation in Pisum sativum.
sexual affinities.
on Primula.
bud variation.
constitutional vigour from cross parentage in common pea.
hybrids of Gladiolus and Cistus.
Phaseolus multiflorus.
nectar in Orchids.
on cross-fertilisation.
inheritance of acquired modifications.
change in the conditions of life beneficial to plants and animals.

Darwin, F., structure of Phaseolus multiflorus.
Pteris aquilina.
perforation of Lathyrus sylvestris.

Darwin, G., on marriages with first cousins.

Decaisne on Delphinium consolida.

De Candolle, nectar as an excretion.

Delphinium consolida.
measurements.
seeds.
partially sterile.
corolla removed.

Delpino, Professor, Viola tricolor.
Phaseolus multiflorus.
intercrossing of sweet-pea.
Lobelia ramosa.
structure of the Cannaceae.
wind and water carrying pollen.
Juglans regia.
anemophilous plants.
fertilisation of Plantago.
excretion of nectar.
secretion of nectar to defend the plant.
anemophilous and entomophilous plants.
dioecious plants.

Denny, Pelargonium zonale.

Diagram showing mean height of Ipomoea purpurea.

Dianthus caryophyllus.
crossed and self-fertilised.
measurements.
cross with fresh stock.
weight of seed.
colour of flowers.
remarks on experiments.
early flowering of crossed.
uniform colour of self-fertilised.
seeds.
few capsules.

Dickie, Dr., self-fertilisation in Cannaceae.

Dictamnus fraxinella.

Digitalis purpurea.
measurements.
effects of intercrossing.
superiority of crossed.
self-sterile.

Dipsaceae.

Dobbs, bees frequenting flowers of same species.

Dodel, Dr. A., sexual reproduction.

Duhamel on Raphanus sativus.

Dunal, nectar as an excretion.

Dyer, Mr., on Lobelia ramosa.
on Cineraria.

Earley, W., self-fertilisation of Lathyrus odoratus.

Eaton, Reverend A.E., on Pringlea.

Engelmann, development of sexual forms.

Engler, Dr., on dichogamous Saxifraga.

Entomophilous plants.

Epipactis latifolia, attractive only to wasps.

Erica tetralix.
perforated corolla.

Erythrina.

Eschscholtzia californica.
measurements.
plants raised from Brazilian seed.
weight.
seeds.
experiments on.
superiority of self-fertilised over crossed.
early flowering.
artificially self-fertilised.
pollen from other flowers more effective.
self-sterile in Brazil.

Euphrasia officinalis.

Euryale amazonica.
-- ferox.

Fabricius on Aristolochia.

Fagopyrum esculentum.
early flowering of crossed plant.

Faivre, Professor, self-fertilisation of Cannaceae.

Farrer, T.H., papilionaceous flowers.
Lupinus luteus.
Phaseolus multiflorus.
Pisum sativum.
cross-fertilisation of Lobelia ramosa.
on Coronilla.

Fermond, M., Phaseolus multiflorus.
Phaseolus coccineus hybridus.

Fertilisation, means of.
plants sterile, or partially so without insect-aid.
plants fertile without insect-aid.
means of cross-fertilisation.
humming-birds.
Australian flowers fertilised by honey-sucking birds.
in New Zealand by the Anthornis melanura.
attraction of bright colours.
of odours.
flowers adapted to certain kinds of insects.
large amount of pollen-grains.
transport of pollen by insects.
structure and conspicuousness of flowers.
pollen from a distinct plant.
prepotent pollen.

Fertility, heights and weights, relative, of plants crossed by a fresh
stock, self-fertilised, or intercrossed (Table 7/C).

Fertility of plants as influenced by cross and self-fertilisation (Table
9/D).
relative, of crossed and self-fertilised parents (Table 9/E).
innate, from a cross with fresh stock (Table 9/F).
relative, of flowers crossed with pollen from a distinct plant and their
own pollen (Table 9/G).
of crossed and self-fertilised flowers.

Flowering, period of, superiority of crossed over self-fertilised.

Flowers, white, larger proportion smelling sweetly.
structure and conspicuousness of.
conspicuous and inconspicuous.
papilionaceous.
fertilised with pollen from a distinct plant.

Forsythia viridissima.

Foxglove.
Frankland, Dr., chemical affinity.

Fraxinus ornus.

Fumaria capreolata.
-- officinalis.

Galium aparine.

Gallesio, spontaneous crossing of oranges.

Galton, Mr., Limnanthes douglasii.
report on the tables of measurements.
self-fertilised plants.
superior vigour of crossed seedlings in Lathyrus odoratus.

Gartner, excess of pollen injurious.
plants fertilising one another at a considerable distance.
Lobelia fulgens.
sterility of Verbascum nigrum.
number of pollen-grains to fertilise Geum urbanum.
experiments with pollen.

Gentry, Mr., perforation of corolla.

Geraniaceae.

Geranium phaeum.

Gerardia pedicularia.

Germination, period of, and relative weight of seeds from crossed and
self-fertilised flowers.

Gesneria pendulina.
measurements.
seeds.

Gesneriaceae.

Geum urbanum, number of pollen-grains for fertilisation.

Glaucium luteum.

Godron, intercrossing of carrot.
Primula grandiflora affected by pollen of Primula officinalis.
tulips.

Gould, humming-birds frequenting Impatiens.

Graminaceae.

Grant, Mr., bees of different hives visiting different kinds of flowers.

Gray, Asa, sexual relations of trees in United States.
on sexual reproduction.

Hallet, Major, on selection of grains of cereals.

Hassall, Mr., number of pollen-grains in Paeony and Dandelion.
weight of pollen produced by one plant of Bulrush.

Heartsease.

Hedychium.

Hedysarum onobrychis.

Heights, relative, of crossed and self-fertilised plants (Table 7/A).

Heights, weights, and fertility, summary.

Henschel’s experiments with pollen.

Henslow, Reverend G., cross-fertilisation in Sarothamnus scoparius.

Herbert on cross-fertilisation.
pollen brought from distant plants.
spontaneous crossing of rhododendrons.

Hero, descendants of the plant.
its self-fertilisation.

Heterocentron mexicanum.

Hibiscus africanus.
measurements.
result of experiments.
early flowering of crossed plant.
number of pollen-grains for fertilisation.

Hildebrand on pollen of Digitalis purpurea.
Thunbergia alata.
experiments on Eschscholtzia californica.
Viola tricolor.
Lobelia ramosa.
Fagopyrum esculentum.
self-fertilisation of Zea mays.
Corydalis cava.
Hypecoum grandiflorum.
and Hypecoum procumbens.
sterility of Eschscholtzia.
experiments on self-fertilisation.
Corydalis lutea.
spontaneously self-fertilised flowers.
various mechanical structure to check self-fertilisation.
early separation of the sexes.
on Aristolochia.
fertilisation of the Gramineae.
wide dissemination of seeds.

Hoffmann, Professor H., self-fertilised capsules of Papaver somniferum.
Adonis aestivalis.
spontaneous variability of Phaseolus multiflorus.
self-fertilisation of kidney-bean.
Papaver alpinum.
sterility of Corydalis solida.
Linum usitatissimum.

Honey-dew.

Hooker, Dr., Euryale ferox and Victoria regia, each producing several
flowers at once.
on sexual relation of trees in New Zealand.

Horse-chestnut.

Humble-bees, see Bees.

Humboldt, on the grains of cereals.

Humming-Birds a means of cross-fertilisation.

Hyacinth.

Hybrid plants, tendency to revert to their parent forms.

Hypecoum grandiflorum.
-- procumbens.

Iberis umbellata (var. kermesiana).
measurement.
cross by fresh stocks.
remarks on experiments.
superiority of crossed over self-fertilised seedlings.
early flowering.
number of seeds.
highly self-fertile.
prepotency of other pollen.
-- amara.

Impatiens frequented by humming-birds.
-- barbigera.
-- fulva.
-- noli-me-tangere.
-- pallida.

Inheritance, force of, in plants.

Insects, means of cross-fertilisation.
attracted by bright colours.
by odours.
by conspicuous flowers.
dark streaks and marks as guides for.
flowers adapted to certain kinds.

Ipomoea purpurea.
measurements.
flowers on same plant crossed.
cross with fresh stock.
descendants of Hero.
summary of measurements.
diagram showing mean heights.
summary of observations.
of experiments.
superiority of crossed.
early flowering.
effects of intercrossing.
uniform colour of self-fertilised.
seeds.
highly self-fertile.
prepotency of other pollen.

Iris, secretion of saccharine matter from calyx.

Isotoma.

Juglans regia.

Kalmia latifolia.

Kerner, on protection of the pollen.
on the single daily flower of Villarsia parnassifolia.
pollen carried by wind.

Kidney-bean.

Kitchener, Mr., on the action of the stigma.
on Viola tricolor.

Knight, A., on the sexual intercourse of plants.
crossing varieties of peas.
sexual reproduction.

Kohl-rabi, prepotency of pollen.

Kolreuter on cross-fertilisation.
number of pollen-grains necessary for fertilisation.
sexual affinities of Nicotiana.
Verbascum phoeniceum.
experiments with pollen of Hibiscus vesicarius.

Kuhn adopts the term cleistogene.

Kurr, on excretion of nectar.
removal of corolla.

Labiatae.

Lactuca sativa.
measurement.
prepotency of other pollen.

Lamium album.
-- purpureum.

Lathyrus odoratus.
measurements.
remarks on experiments.
period of flowering.
cross-fertilisation.
seeds.
self-fertile.
-- grandiflorus.
-- nissolia.
-- sylvestris, perforation of corolla.

Lawes and Gilbert, Messrs., consumption of inorganic matter by plants.

Laxton, Mr., crossing varieties of peas.

Lecoq, Cyclamen repandum.
on Fumariaceae.
annual plants rarely dioecious.

Leersia oryzoides.

Leguminosae.
summary on the.

Leighton, Reverend W.A., on Phaseolus multiflorus.
Acacia magnifica.

Leptosiphon androsaceus.

Leschenaultia formosa.

Lettuce.

Lilium auratum.

Limnanthes douglasii.
measurements.
early flowering of crossed.
seeds.
highly self-fertile.
prepotency of other pollen.

Linaria vulgaris.
seeds.
self-sterile.
-- cymbalaria.

Lindley on Fumariaceae.

Link, hypopetalous nectary in Chironia decussata.

Linum grandiflorum.
-- usitatissimum.

Loasaceae.

Lobelia erinus.
secretion of nectar in sunshine.
experiments with bees.

Lobelia fulgens.
measurements.
summary of experiments.
early flowering of self-fertilised.
seeds.
sterile unless visited by humble-bees.
-- ramosa.
measurements.
early flowering of crossed.
seeds.
self-sterile.
-- tenuior.

Loiseleur-Deslongchamp, on the grains of cereals.

Lotus corniculatus.

Lubbock, Sir J., cross-fertilisation of flowers.
on Viola tricolor.
bees distinguishing colours.
instinct of bees and insects sucking nectar.

Lupinus luteus.
measurements.
early flowering of self-fertilised.
self-fertile.
prepotency of other pollen.
-- pilosus.
self-fertile.

Lychnis dioica.

MacNab, Mr., on the shorter or longer stamens of rhododendrons.

Mahonia aquifolium.
-- repens.

Malvaceae.

Marcgraviaceae.

Masters, Mr., cross-fertilisation in Pisum sativum.
cabbages affected by pollen at a distance.

Masters, Dr. Maxwell, on honey-dew.

Measurements, summary of.
Table 7/A.
Table 7/B.
Table 7/C.

Medicago lupulina.

Meehan, Mr., fertilising Petunia violacea by night moth.

Melastomaceae.

Melilotus officinalis.

Mercurialis annua.

Miller, Professor, on chemical affinity.

Mimulus luteus, effects of crossing.
crossed and self-fertilised plants.
measurements.
cross with a distinct stock.
intercrossed on same plant.
summary of observations.
of experiments.
superiority of crossed plants.
simultaneous flowering.
effects of intercrossing.
uniform colour of self-fertilised.
seeds.
highly self-fertile.
prepotency of other pollen.
-- roseus.

Miner, Mr., red clover never sucked by hive-bees in the United States.

Mirabilis, dwarfed plants raised by using too few pollen-grains.
number of grains necessary for fertilisation.

Mitchell, Dr., on first cousins inter-marrying.

Monochaetum ensiferum.

Moore, Mr., on Cinerarias.

Muller, Fritz, on Posoqueria fragrans.
experiments on hybrid Abutilons and Bignonias.
large number of Orchidaceous genera sterile in their native home, also
Bignonia and Tabernaemontana echinata.
sterility of Eschscholtzia californica.
Abutilon darwinii.
experiments in self-fertilisation.
self-sterile plants.
incapacity of pollen-tubes to penetrate the stigma.
cross-fertilisation by means of birds.
imperfectly developed male and female Termites.
food-bodies in Cecropia.

Muller, Hermann, fertilisation of flowers by insects.
on Digitalis purpurea.
Calceolaria.
Linaria vulgaris.
Verbascum nigrum.
the common cabbage.
Papaver dubium.
Viola tricolor.
structure of Delphinium consolida.
of Lupinus lutea.
flowers of Pisum sativum.
on Sarothamnus scoparius not secreting nectar.
Apium petroselinum.
Borago officinalis.
red clover visited by hive-bees in Germany.
insects rarely visiting Fumaria officinalis.
comparison of lowland and alpine species.
structure of plants adapted to cross and self-fertilisation.
large conspicuous flowers more frequently visited by insects than small
inconspicuous ones.
Solanum generally unattractive to insects.
Lamium album.
on anemophilous plants.
fertilisation of Plantago.
secretion of nectar.
instinct of bees sucking nectar.
bees frequenting flowers of the same species.
cause of it.
powers of vision and discrimination of bees.

Muller, Dr. H., hive-bees occasionally perforate the flower of Erica
tetralix.
calyx and corolla of Rhinanthus alecterolophus bored by Bombus
mastrucatus.

Munro, Mr., some species of Oncidium and Maxillaria sterile with own
pollen.

Myrtaceae.

Nageli on odours attracting insects.
sexual relations.

Natural selection, effect upon self-sterility and self-fertilisation.

Naudin on number of pollen-grains necessary for fertilisation.
Petunia violacea.

Nectar regarded as an excretion.

Nemophila insignis.
measurements.
early flowering of crossed plant.
effects of cross and self-fertilisation.
seeds.

Nepeta glechoma.

Nicotiana glutinosa.
-- tabacum.
measurements.
cross with fresh stock.
measurements.
summary of experiments.
superiority of crossed plants.
early flowering.
seeds.
experiments on.
self-fertile.

Nolana prostrata.
measurements.
crossed and self-fertilised plants.
number of capsules and seeds.
self-fertile.

Nolanaceae.

Nymphaea.

Odours emitted by flowers attractive to insects.

Ogle, Dr., on Digitalis purpurea.
Gesneria.
Phaseolus multiflorus.
perforation of corolla.
case of the Monkshood.

Onagraceae.

Onion, prepotency of other pollen.

Ononis minutissima.
measurements.
seeds.
self-fertile.

Ophrys apifera.
-- muscifera.

Oranges, spontaneous crossing.

Orchideae.
excretion of saccharine matter.

Orchis, fly.

Origanum vulgare.
measurements.
early flowering of crossed plant.
effects of intercrossing.

Paeony, number of pollen-grains.

Papaveraceae.

Papaver alpinum.
-- argemonoides.
-- bracteatum.
-- dubium.
-- orientale.
-- rhoeas.
-- somniferum.
-- vagum.
measurements.
number of capsules.
seeds.
prepotency of other pollen.

Papillae of the Viola tricolor attractive to insects.

Parsley.

Passiflora alata.
-- gracilis.
measurements.
crossed and self-fertilised.
seeds.
self-fertile.

Passifloraceae.

Pea, common.

Pelargonium zonale.
measurements.
effects of intercrossing.
almost self-sterile.

Pentstemon argutus, perforated corolla.

Petunia violacea.
measurements.
weight of seed.
cross with fresh stock.
relative fertility.
colour.
summary of experiments.
superiority of crossed over self-fertilised.
early flowering.
uniform colour of self-fertilised.
seeds.
self-sterile.

Phalaris canariensis.
measurements.
early flowering of crossed.

Phaseolus coccineus.
-- multiflorus.
measurement.
partially sterile.
crossed and self-fertilised.
early flowering of crossed.
seeds.
perforated by humble-bees.
-- vulgaris.
self-fertile.

Pisum sativum.
measurements.
seldom intercross.
summary of experiments.
self-fertile.

Plants, crossed, greater constitutional vigour.

Pleroma.

Polemoniaceae.

Pollen, relative fertility of flowers crossed from a distinct plant, or
with their own.
difference of results in Nolana prostrata.
crossed and self-fertilised plants, again crossed from a distinct plant
and their own pollen.
sterile with their own.
semi-self-sterile.
loss of.
number of grains in Dandelion, Paeony, and Wistaria sinensis.
number necessary for fertilisation.
transported from flower to flower.
prepotency.
aboriginally the sole attraction to insects.
quantity produced by anemophilous plants.

Polyanthus, prepotency over cowslip.

Polygoneae.

Posoqueria fragrans.

Potato.

Poterium sanguisorba.

Potts, heads of Anthornis melanura covered with pollen.

Primrose, Chinese.

Primula elatior.
-- grandiflora.
-- mollis.
-- officinalis.
-- scotica.
-- sinensis.
measurements.
early flowering of crossed.
-- veris (var. officinalis).
measurements.
result of experiments.
early flowering of crossed.
seeds.
self-fertility.
prepotency of dark red polyanthus.

Primulaceae.

Pringlea.

Proteaceae of Australia.

Prunus avium.
-- laurocerasus.

Pteris aquilina.

Radish.

Ranunculaceae.

Ranunculus acris.

Raphanus sativus.

Reinke, nectar-secreting glands of Prunus avium.

Reseda lutea.
measurements.
result of experiments.
self-fertile.
-- odorata.
measurements.
self-fertilised scarcely exceeded by crossed.
seeds.
want of correspondence between seeds and vigour of offspring.
result of experiments.
sterile and self-fertile.

Resedaceae.

Rheum rhaponticum.

Rhexia glandulosa.

Rhododendron, spontaneous crossing.

Rhododendron azaloides.

Rhubarb.

Ribes aureum.

Riley, Mr., pollen carried by wind.
Yucca moth.

Rodgers, Mr., secretion of nectar in Vanilla.

Rye, experiment on pollen of.

Salvia coccinea.
measurements.
early flowering of crossed.
seeds.
partially self-sterile.
-- glutinosa.
-- grahami.
-- tenori.

Sarothamnus scoparius.
measurements.
superiority of crossed seedlings.
seeds.
self-sterile.

Scabiosa atro-purpurea.
measurements.

Scarlet-runner.

Scott, J., Papaver somniferum.
sterility of Verbascum.
Oncidium and Maxillaria.
on Primula scotica and Cortusa matthioli.

Scrophulariaceae.

Self-sterile varieties, appearance of.

Self-fertilisation, mechanical structure to check.

Self-sterile plants.
wide distribution throughout the vegetable kingdom.
difference in plants.
cause of self-sterility.
affected by changed conditions.
necessity of differentiation in the sexual elements.

Senecio cruentus.
-- heritieri.
-- maderensis
-- populifolius.
-- tussilaginis.

Sharpe, Messrs., precautions against intercrossing.

Snow-flake.

Solanaceae.

Solanum tuberosum.

Specularia perfoliata.
-- speculum.
measurements.
crossed and self-fertilised.
early flowering of crossed.
seeds.
self-fertile.

Spencer, Herbert, chemical affinity.

Spiranthes autumnalis.

Sprengel, C.K., fertilisation of flowers by insects.
Viola tricolor.
colours in flowers attract and guide insects.
on Aristolochia.
Aconitum napellus.
importance of insects in fertilising flowers.

Stachys coccinea.

Stellaria media.

Strachey, General, perforated flowers in the Himalaya.

Strawberry.

Strelitzia fertilised by the Nectarinideae.

Structure of plants adapted to cross and self-fertilisation.

Swale, Mr., garden lupine not visited by bees in New Zealand.

Sweet-pea.

Tabernaemontana echinata.

Tables of measurements of heights, weights, and fertility of plants.

Termites, imperfectly developed males and females.

Thunbergia alata.

Thyme.

Tinzmann, on Solanum tuberosum.

Tobacco.

Transmission of the good effects of a cross to later generations.

Trees, separated sexes.

Trifolium arvense.
-- incarnatum.
-- minus.
-- pratense.
-- procumbens.
-- repens.

Tropaeolum minus.
measurements.
early flowering of crossed.
seeds.
-- tricolor.
seeds.

Tulips.

Typha.

Umbelliferae.

Urban, Ig., fertilisation of Medicago lupulina.

Vandellia nummularifolia.
seeds.
self-fertile.

Vanilla, secretion of nectar.

Verbascum lychnitis.
-- nigrum.
-- phoeniceum.
-- thapsus.
measurements.
self-fertile.

Verlot on Convolvulus tricolor.
intercrossing of Nemophila.
of Leptosiphon.

Veronica agrestis.
-- chamaedrys.
-- hederaefolia.

Vicia faba.
-- hirsuta.
-- sativa.

Victoria regia.

Villarsia parnassifolia.

Vilmorin on transmitting character to offspring.

Vinca major.
-- rosea.

Viola canina.
-- tricolor.
measurements.
superiority of crossed plants.
period of flowering.
effects of cross-fertilisation.
seeds.
partially sterile.
corolla removed.

Violaceae.

Viscaria oculata.
measurement.
average height of crossed and self-fertilised.
simultaneous flowering.
seeds.
self-fertile.

Wallace, Mr., the beaks and faces of brush-tongued lories covered with
pollen.

Wasps attracted by Epipactis latifolia.

Weights, relative, of crossed and self-fertilised plants.
and period of germination of seeds.

Wilder, Mr., fertilisation of flowers with their own pollen.

Wilson, A.J., superior vigour of crossed seedlings in Brassica
campestris ruta baga.

Wistaria sinensis.

Yucca moth.

Zea mays.
measurements.
difference of height between crossed and self-fertilised.
early flowering of crossed.
self-fertile.
prepotency of other pollen.