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  despressing —> depressing
  presistent —> persistent
  actic —> lactic
  excitabilitty —> excitability
  Zephyanthes —> Zephyranthes
  fal —> fall
  tranmission —> transmission
  substracting —> subtracting
  issue —> tissue
  conducing —> conducting
  ummasking —> unmasking
  be —> been
  end —> and
  flexture —> flexure
  tentanising —> tetanising
  anisotrophy —> anisotropy

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                       LIFE MOVEMENTS IN PLANTS

                                  BY

      SIR JAGADIS CHUNDER BOSE, Kt., M.A., D.Sc., C.S.I., C.I.E.,

                PROFESSOR EMERITUS, PRESIDENCY COLLEGE,
                  DIRECTOR, BOSE RESEARCH INSTITUTE.

                         WITH 92 ILLUSTRATIONS

                        _B.R. Publishing Corp.
                                Delhi_


Cataloging in Publication Data-DK

Bose, Jagadish Chandra, 1858-1937.

  Life movements in plants.

  Reprint.

  1. Plants--Irritability and movements. 2. Growth (Plants).
  3. Plants--Development. 4. Botany. I. Title.

_First Published_ 1918

_Reprinted_ 1985

_Published in India by_

  B.R. PUBLISHING CORPORATION
  461, VIVEKANAND NAGAR,
  DELHI-110052 (INDIA)

_Distributed by_

  D.K. PUBLISHERS’ DISTRIBUTORS
  1, ANSARI ROAD, DARYA GANJ,
  NEW DELHI-110002 (INDIA)
  PHONE: 27-8368

_Printed at_:

  BRITE PRINTERS
  NEW DELHI-110005 (INDIA)




CONTENTS


PART I.

RESPONSE OF PLANT ORGANS.

I.--THE PROBLEM OF MOVEMENT IN PLANTS.

                                                                  PAGE

  Complexity of the problem--Effects of different forms
    of stimuli--Diverse responses under identical
    stimulus--Modification of response determined by intensity
    and point of application of stimulus, and tonic condition of
    organ--Response of pulvinated and growing organs--Necessity
    for shortening the period of experiment                          1


II.--THE “PRAYING” PALM TREE.

  Description of phenomenon--The Recording apparatus--Record
    of diurnal movement of the tree--Universality of tree
    movement--Cause of periodic movement--Periodic movement
    of trees, and diurnal variation of moto-excitability
    in _Mimosa pudica_--Relative effects of light
    and temperature--Physiological character of the
    movement--Transpiration and diurnal movement--Diurnal
    movement in inverted position--Effect of variation of
    temperature on geotropic curvature--Reversal of natural
    rhythm by artificial variation of temperature                    5


III.--ACTION OF STIMULUS ON VEGETABLE TISSUES.

  Different types of Response Recorders--Response of a radial
    organ--Response of an anisotropic organ--Response of pulvinus
    of _Mimosa pudica_--Tabular statement of apex time and
    period of recovery in different plants--Response of pulvinus
    of _Mimosa_ to variation of turgor--Different modes of
    stimulation                                                     31


IV.--THE DIURNAL VARIATION OF EXCITABILITY IN _MIMOSA_.

  Apparatus for study of variation of excitability--Uniform
    periodic stimulation--The Response Recorder--Effects of
    external condition on excitability--Effects of light
    and darkness--Effect of excessive turgor--Influence of
    temperature--Diurnal variation of excitability--Effect of
    physiological inertia                                           43


V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA_.

  Effect of wound or section in modification of normal
    excitability--The change of excitability after immersion in
    water--Quantitative determination of the rate of decay of
    excitability in an isolated preparation--Effect of amputation
    of upper half of the pulvinus--Effect of removal of the lower
    half--Influence of weight of leaf on rapidity of responsive
    fall--The action of chemical agents--Effect of “fatigue” on
    response--The action of light and darkness on excitability      73


VI.--CONDUCTION OF EXCITATION IN PLANTS.

  Hydro-dynamic _versus_ physiological theory of conduction
    of excitation--Arrest of conductivity by physiological
    blocks--Convection and conduction of excitation--Effect
    of temperature on velocity--Effect of season--Effect of
    age--Effect of dessication of conducting tissue--Influence
    of tonic condition on conduction--Effect of intensity
    of stimulus on velocity of transmission--Effect
    of stimulus on sub-tonic tissues and tissues in
    optimum condition--Canalisation of conducting path by
    stimulus--Effect of injury on conductivity                      97


VII.--ELECTRIC CONTROL OF EXCITATORY IMPULSE.

  Method of conductivity-balance--Control of transmitted
    excitation in _Averrhoa bilimbi_ by electric
    current--‘Uphill’ transmission--Transmission
    ‘downhill’--Electric control of nervous impulse in
    animal--Directive action of current on conduction of
    excitation--Effects of direction of current on velocity
    of transmission in _Mimosa_--Determination of variation
    of conductivity by method of Minimal Stimulus and
    Response--Influence of direction of current on conduction
    of excitation in animal nerve--Variation of velocity of
    transmission--After-effects on Heterodromous and Homodromous
    currents--Laws of variation of nervous conduction under
    electric current                                               107


VIII.--EFFECT OF INDIRECT STIMULUS ON PULVINATED ORGANS.

  Conduction of excitation--Dual character of the transmitted
    impulse--Effect of distance of application of
    stimulus--Periods of transmission of positive and negative
    impulses--Effects of Direct and Indirect stimulus              135


IX.--MODIFYING INFLUENCE OF TONIC CONDITION ON RESPONSE.

  Theory of assimilation and dissimilation--Unmasking of positive
    effect--Modification of response under artificial depression
    of tonic condition--Positive response in sub-tonic specimen    141


PART II.

GROWTH AND ITS RESPONSIVE VARIATIONS.

X.--THE HIGH MAGNIFICATION CRESCOGRAPH FOR RESEARCHES ON GROWTH.

  Method of high magnification--Automatic record
    of the rate of growth--Determination of the
    absolute rate of growth--Stationary method of
    record--Moving plate method--Precaution against
    physical disturbance--Determination of latent period
    and time-relations of response--Advantages of the
    Crescograph--Magnetic amplification--The Demonstration
    Crescograph                                                    151


XI.--EFFECT OF TEMPERATURE ON GROWTH.

  Method of discontinuous observation--Method of continuous
    observation--Determination of the cardinal points of
    growth--The Thermocrescent curve--Relation between
    temperature and growth                                         173


XXII.--EFFECT OF CHEMICAL AGENTS ON GROWTH.

  Effect of stimulants--Effect of anæsthetics--Action of
    different gases--Action of poisons                             183


XIII.--EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH.

  Response to positive variation of turgor--Method of
    irrigation--Effect of artificial increase of internal
    hydrostatic pressure--Response to negative variation of
    turgor--Method of plasmolysis--Effect of alternative
    variations of turgor on growth--Response of motile and
    growing organs to variation of turgor--Effect of external
    tension                                                        188


XIV.--EFFECT OF ELECTRICAL STIMULUS ON GROWTH.

  Effect of intensity--Effect of continuous
    stimulation--Continuity between ‘incipient’ and actual
    contraction--Immediate effect and after-effect                 195


XV.--EFFECT OF MECHANICAL STIMULUS ON GROWTH.

  Effect of mechanical irritation--Effect of wound                 200


XVI.--ACTION OF LIGHT ON GROWING ORGANS.

  Method of experiment--Normal effect of light--Determination of
    the latent period--Effect of intensity of light--Effect of
    continuous light--Effects of different rays of the spectrum    205


XVII.--EFFECT OF INDIRECT STIMULUS ON GROWTH.

  Mechanical and electrical response to Indirect
    Stimulus--Variation of growth under Indirect
    Stimulus--Effects of Direct and Indirect Stimulus              213


XVIII.--RESPONSE OF GROWING ORGANS IN STATE OF SUB-TONICITY.

  Theory of assimilation and dissimilation--Unmasking of
    positive effect--Modification of response under artificial
    depression of tonic condition--Positive response in
    sub-tonic specimen--Abnormal acceleration of growth
    under stimulus--Continuity between abnormal and normal
    responses--Positive response to sub-minimal stimulus           219


XIX.--RESUMPTION OF AUTONOMOUS PULSATION AND OF GROWTH UNDER
    STIMULUS.

  Resumption of pulsatory activity of _Desmodium_ leaflet at
    standstill--Renewal of growth under stimulus--General laws of
    effects of Direct and Indirect Stimulus                        227


XX.--ACTION OF LIGHT AND WARMTH ON AUTONOMOUS ACTIVITY.

  The Oscillating Recorder--Record of pulsation of _Desmodium
    gyrans_--Effect of diffuse light in diminution of amplitude
    and reduction of diastolic limit of pulsation--Antagonistic
    action of warmth in reduction of systolic limit                233


XXI.--A COMPARISON OF RESPONSES IN GROWING AND NON-GROWING
    ORGANS.

  Contractile response of growing and non-growing
    organs--Time-relations of mechanical response of pulvinated
    and growing organs--Similar modification of response under
    condition of sub-tonicity--Opposite effects of Direct and
    Indirect stimulus--Exhibition of negative electric response
    under Direct, and positive electric response under Indirect
    stimulus--Similar modification of autonomous activity in
    _Desmodium gyrans_ and in growing organs under parallel
    conditions--Similar excitatory effects of various stimuli
    on pulvinated and growing organs--Similar discriminative
    excitatory effects of various rays in excitation of motile
    and growing organs--Action of white light--Action of
    red and yellow lights--Action of blue light--Action of
    ultra-violet rays--Action of infra-red rays--Diverse modes
    of response to stimulus--Mechanical response--Electromotive
    response--Response by variation of electric resistance         239




ILLUSTRATIONS.

  FIGURE.                                                        PAGE.

   1. Photographs of morning and evening positions of the
      ‘Praying Palm’                                                 7

   2. The Recording Apparatus                                        9

   3. Record of diurnal movement of the ‘Praying Palm’              11

   4.    "                  "           Sijbaria Palm               12

   5. Curve of variation of moto-excitability in _Mimosa pudica_    17

   6. Effect of physiological depression on diurnal movement of
      _Arenga saccharifera_                                         19

   7. Record of diurnal movements of young procumbent stem of
      _Mimosa pudica_                                               26

   8. Erectile response of _Basella_ to gradual fall of
      temperature                                                   28

   9. Responsive fall of _Basella_ to gradual rise of temperature    "

  10. Response of a straight tendril of _Passiflora_                33

  11. Response of a hooked tendril of _Passiflora_                  35

  12. Response of pulvinus of _Mimosa pudica_                       36

  13.     "           "       _Mimosa_ to variations of turgor      40

  14. Diagram of complete apparatus for record of diurnal
      variation                                                     46

  15. The Oscillator                                                50

  16. Effect of cloud on excitability of _Mimosa_                   52

  17. Effect of sudden darkness                                     53

  18. Effect of change from darkness to light                       54

  19. Effect of enhanced turgor                                     55

  20. Effect of moderate cooling                                    56

  21. Effect of application of intense cold                         58

  22. Effect of temperature above the optimum                       58

  23. Twenty-four hours’ record of excitability of _Mimosa_         59

  24. Midday record from noon to 3 P.M.                             62

  25. Evening record from 6 to 10 P.M.                              63

  26. Morning record from 8 A.M. to 12 noon                         64

  27. Diurnal variation of excitability showing marked
      nyctitropic movement                                          65

  28. Diurnal curves of temperature and of corresponding
      variation of excitability of _Mimosa_                         68

  29. Diurnal variation of excitability of a summer specimen        70

  30. The Resonant Recorder                                         76

  31. Variation of excitability after section                       80

  32. Effect of amputation of upper half of pulvinus of _Mimosa_    84

  33. Response of _Mimosa_ after amputation of lower half of
      pulvinus                                                      86

  34. Effect of weight on rapidity of fall                          87

  35. Stimulating action of Hydrogen peroxide                       88

  36. Incomplete recovery under the action of BaCl_{2} and
      transient restoration under tetanisation                      89

  37. Antagonistic action of alkali and acid                        90

  38. Fatigue due to shortening of recovery-period                  91

  39. Effect of constant current in removal of fatigue              92

  40. Stimulating action of light and depressing action of
      darkness                                                      94

  41. Action of glycerine in enhancing speed and intensity of
      transmitted excitation in _Mimosa_                           102

  42. Effect of injury in depressing conductivity in normal
      specimen                                                     104

  43. Effect of injury in enhancing conductivity in a subtonic
      specimen                                                     105

  44. Diagram of experimental arrangement for conductivity
      control in _Averrhoa bilimbi_                                109

  45. Diagram of complete experimental arrangement for
      conductivity control in _Mimosa pudica_                      116

  46. Record showing enhanced velocity in ‘up-hill’ and retarded
      velocity in ‘down-hill’ transmission                         121

  47. Direct and after-effect of heterodromous and homodromous
      currents                                                     124

  48. Diagram of experimental arrangement for variation of
      conductivity of animal nerve                                 126

  49. Effect of heterodromous and homodromous current in inducing
      variation of conductivity in nerve                           127

  50. Record of ineffectively transmitted salt-tetanus becoming
      effective under heterodromous current                        129

  51. Direct and after-effect of homodromous current               131

  52. Effect of indirect electric stimulus on the responding
      leaflet of _Averrhoa_                                        136

  53. Staircase responses of sub-tonic specimen of _Mimosa_ to
      electric shock                                               145

  54. Staircase responses of sub-tonic specimen of _Mimosa_ to
      light                                                        147

  55. Positive, diphasic, and negative responses of extremely
      sub-tonic specimen of _Mimosa_ to successive light stimuli   147

  56. The compound Lever                                           154

  57. The crank arrangement for oscillation                        156

  58. Photograph of the High Magnification Crescograph             157

  59. Crescographic record of absolute rate of growth of
      _Kysoor_, and of effects of cold and warmth on stationary
      and moving plates                                            161

  60. Record of physical change                                    164

  61. Records of latent period and time relations of growth
      response                                                     165

  62. Record of a single growth-pulse of _Zephyranthes_            167

  63. Records of growth-rate at different temperatures             175

  64. Continuous record of growth, showing temperature minimum     178

  65. Continuous record of growth, showing temperature maximum      "

  66. The Thermo-Crescent Curve                                    180

  67. Curve showing the relation between growth and temperature    181

  68. Effects of H_{2}O_{2}, NH_{3}, and ether on growth           184

  69. Effect of CO_{2} on growth                                   185

  70. Effect of irrigation on growth                               189

  71. Effect of plasmolysis on growth                              191

  72. Effect of increasing intensity of electric stimulus on
      growth                                                       196

  73. Effect of continuous electric stimulation on growth          197

  74. Immediate and after-effects of friction, and of wound on
      growth                                                       200

  75. Normal retarding effect of light on growth                   206

  76. Record showing latent period of growth in response to light  207

  77. Effect of light of increasing intensities                    208

  78. Continuous effect of light and of electric stimulus on
      growth                                                       209

  79. Effects of different rays of the spectrum on growth          210

  80. Photographic records of positive, diphasic and negative
      electric responses of petiole of _Musa_                      214

  81. Record of growth variation of _Crinum_ under Direct and
      Indirect stimulus                                            216

  82. Effect of electric stimulus on sub-tonic specimen of wheat
      seedling                                                     221

  83. Acceleration of growth under sub-minimal stimulus of light   224

  84. Revival by stimulus of light of autonomous pulsations of
      _Desmodium gyrans_ at stand still                            228

  85. Renewal of growth in the mature style of a flower by the
      action of stimulus                                           229

  86. Effect of light in diminution of amplitude and reduction of
      diastolic limit of pulsation of _Desmodium_                  236

  87. Antagonistic effect of warmth in reduction of systolic
      limit                                                        237

  88. Contractile response of a growing bud of _Crinum_            241

  89. Response of _Mimosa_ pulvinus to white light                 245

  90. Response of _Mimosa_ pulvinus to blue light                  246

  91. Response of _Mimosa_ pulvinus to ultra-violet rays           247

  92. Response of _Mimosa_ pulvinus to thermal radiation           248




I.--THE PROBLEM OF MOVEMENT IN PLANTS

_By_

PROF. SIR J. C. BOSE.


The phenomenon of movement in plants under the action of external
stimuli presents innumerable difficulties and complications. The
responding organs are very different: they may be the pulvini of the
‘sensitive’ or those of the less excitable leguminous plants; the
petioles of leaves, which often act as pulvinoids; and organs of plants
in a state of active growth.

Taking first the case of the pulvinus of _Mimosa_, we find that it
responds to mechanical stimulation, to constant electric current, to
induction shock, to the action of chemical agents, to light, and to
warmth as differentiated from thermal radiation. The reactions induced
by these agents may be similar or dissimilar. An identical agent,
again, may give rise to movements which are not merely different, but
sometimes even of diametrically opposite characters. Certain organs,
for example, direct themselves towards light, others away from it.
Some plants close their leaflets on the approach of darkness, in the
so-called position of ‘sleep’; apparently similar ‘sleep’ movement is
induced in others by the action of the midday sun.

In _Mimosa_, the responsive movement is brought about by a sudden
diminution of turgor in the pulvinus. But very little is definitely
known about the responsive reaction in growing organs. Thus in a
tendril, one-sided contraction causes a shortening of the concave side
and a sudden increase of growth on the convex. No explanation of this
difference has hitherto been forthcoming. Under the action of light
of different intensities a growing organ may approach the source of
light, or place itself at right angles or move away from it. Again
under the identical stimulus of gravity, the root moves downwards,
and the shoot upwards. The sign of response in different organs thus
changes, apparently without any reason. It is thus seen, that there
is hardly any responsive movement that has been observed of which an
example directly to the contrary may not be found. For this reason it
has appeared hopeless to unify these very diverse phenomena, and there
has been a tendency towards a belief that it was not any definite
physiological reaction, but the individuality of the plant that
determines the choice of its movement.

The complexities which baffle us may, however, arise from the
combination of factors whose individual reactions are unknown to us. I
shall show, for example, how the movement of a pulvinus under a given
stimulus is determined by the point of application, direct stimulus
producing one effect, and indirect the diametrically opposite. The
normal reaction is again modified by the tonic condition of the plant.
There is again the likelihood of the presence of other modifying
factors. It is clear how very different the results would become by the
permutation and combination of these diverse factors.

For a comprehensive study of the phenomenon of plant movement,
it is therefore necessary to investigate in detail the effect
of a given stimulus under definite changes of the environmental
condition. With regard to a given stimulus we have to determine the
effects of intensity, of duration, and of the point of application.
The investigation has to include the effects exhibited not merely
by the pulvinated but also by growing organs. As a result of such
a comprehensive study, it may perhaps be possible to discover some
fundamental reaction operative in bringing about the responsive
movement in all plant organs.

I shall in the course of the following series of Papers, describe the
different apparatus by which the movement of pulvinated organ and its
time-relations are automatically recorded. In a growing organ the
induced movement under stimulus is brought about by the change in
its rate of growth. That the change is solely due to the particular
stimulus can only be assured by strict maintenance of constancy of
external conditions, during the period of experiment; this constancy
can, in practice, be secured only for a short time. The necessity
for shortening the period of experiment also arises from a different
consideration; for numerous and varied are the stimulating and
mechanical interactions between neighbouring organs. These effects,
however, come into play after a certain lapse of time. They may be
eliminated by reduction of the period of experiment.

In order to shorten the period of experiment for the study of growth
movements, the rate of growth has to be very highly magnified, so as
to determine the absolute rate and its variations in the course of a
minute or so. I shall in a subsequent Paper give full account of an
apparatus I have been able to devise, by which it is possible to record
automatically the rate of growth magnified many thousand times.

I stated that anomalies of plant movements would disappear, if we
succeeded in carrying out in detail investigations of effects of the
different individual factors in operation. In illustration of this
I shall, in the first Paper of the series, give an account of the
mysterious movement of the ‘Praying’ Palm of Faridpur, and describe the
investigations by which the problem found its solution.




II.--THE “PRAYING” PALM TREE

_By_

SIR J. BOSE,

_Assisted by_

NARENDRA NATH NEOGI, M.Sc.


Perhaps no phenomenon is so remarkable and shrouded with greater
mystery as the performances of a particular Date Palm near Faridpur
in Bengal. In the evening, while the temple bells ring calling upon
people to prayer, this tree bows down as if to prostrate itself. It
erects its head again in the morning, and this process is repeated
every day of the year. This extraordinary phenomenon has been regarded
as miraculous, and pilgrims have been attracted in large numbers.
It is alleged that offerings made to the tree have been the means
of effecting marvellous cures. It is not necessary to pronounce any
opinion on the subject; these cures may be taken as effective as other
faith-cures now prevalent in the West.

This particular Date Palm, _Phœnix dactylifera_, is a full-grown rigid
tree, its trunk being 5 metres in length and 25 cm. in diameter. It
must have been displaced by storm from the vertical and is now at an
inclination of about 60° to the vertical. In consequence of the diurnal
movement, the trunk throughout its entire length is erected in the
morning, and depressed in the afternoon. The highest point of the trunk
thus moves up and down through one metre; the ‘neck,’ above the trunk,
is concave to the sky in the morning; in the afternoon the curvature
disappears, or is even slightly reversed. The large leaves which point
high up against the sky in the morning are thus swung round in the
afternoon through a vertical distance of about five metres. To the
popular imagination the tree appears like a living giant, more than
twice the height of a human being, which leans forward in the evening
from its towering height and bends its neck till the crown of leaves
press against the ground in an apparent attitude of devotion (Fig. 1).
Two vertical stakes, each one metre high, give a general idea of the
size of the tree and movements of the different parts of the trunk.

[Illustration: Fig. 1. The Faridpur ‘Praying’ Palm; the upper
photograph shows position in the morning; the lower, position in the
afternoon. The two fixed stakes are one metre in height. In front is
seen erect trunk of a different Palm.]

For an investigation in elucidation of this phenomenon it was
necessary:--

  1. To obtain an accurate record of the movement of the tree day and
       night, and determine the time of its maximum erection and fall.

  2. To find whether this particular instance of movement was unique,
       or whether the phenomenon was universal.

  3. To discover the cause of the periodic movement of the tree.

  4. To find the reason of the remarkable similarity between the
       diurnal movement of the tree, and the diurnal variation of
       moto-excitability in _Mimosa pudica_.

  5. To determine the relative effects of light and temperature on
       the movement.

  6. To demonstrate the physiological character of the movement of
       the tree.

  7. To discover the physiological factor whose variation determines
       the directive movement.


THE RECORDING APPARATUS.

I shall now describe the principle and construction of my recording
apparatus (Fig. 2) seen attached to a horizontally growing stem of
_Mimosa pudica_. When used to trace the movement of the palm tree,
a reducing device is employed to keep the record within the plate. A
lever, R′, records the movement of the attached tree or plant on
a moving plate of smoked glass. The plate is not in contact with the
tip of the recording lever, but separated from it by a distance of
about 3 mm. A special oscillating device, actuated by clock-work,
C, makes the plate move forwards and backwards. The forward movement
brings about a momentary contact of the recording tip with the smoked
plate inscribing a dot. These single dots are made at intervals of
15 minutes; at the expiration of the hour, however, contact is made
three times in rapid succession, printing a thick dot. It is thus
easy to determine the movement of the tree at all times of the day
and night. A second lever, R, placed above, gives on the same plate,
thermographic record of the diurnal variation of temperature. For
this I use a differential thermometer, T, made of a compound strip of
brass and steel. Curvature is induced by the differential expansion
of the two pieces of metal. The up or down movement of the free end
of the compound strip is further magnified by the recording lever.
This arrangement was extremely sensitive and gave accurate record of
variation of temperature. By the forward movement of the oscillating
plate two dots are made at the same time,--one for the temperature
and the other for the corresponding movement of the tree. As the two
recorders do not move vertically up or down, but describe a circle,
the dots vertically one above the other may not correspond as regards
time. Any possibility of error in calculation is obviated by the fact
that the thick dots in both the records are made every hour, and the
subsequent thin dots at intervals of 15 minutes.

[Illustration: Fig. 2. Apparatus for automatic record of movement of
trees and plants; T, differential metallic thermometer; R, recording
lever for temperature; R′, for recording plant movement; C,
clock-work for oscillation of recording plate. The same clock-work
moves plate laterally in 24 hours.]

A difficulty arose at the beginning in obtaining sanction of the
proprietor to attach the recorder to the tree. He was apprehensive
that its miraculous power might disappear by profane contact with
foreign-looking instruments. His misgivings were removed on the
assurance that the instrument was made in my laboratory in India, and
that it would be attached to the tree by one of my assistants, who was
the son of a priest.

From results of observation it is found that the tree moves through
its entire length; the fall of the highest point of the trunk is one
metre. The movement is not passive, but an active force is exerted;
the force necessary to counteract this movement is equivalent to the
weight of 47 kilograms: in other words, the force is sufficient to
lift a man off the ground. But far greater force would be required to
restrain the change of curvature of the neck of the hard and rigid tree.

[Illustration: Fig. 3. Record of diurnal movement of the ‘Praying’ Palm
(_Phœnix dactylifera_). Thermographic curve for 24 hours commencing
at 9 in the evening is given in the upper record; the corresponding
diurnal curve of movement of the tree is given in the lower. Successive
dots at intervals of 15 minutes; thick dots at intervals of an hour.]

Before entering into the investigation of the cause of periodic
movement I shall give a general account of its characteristics. A
casual observation would lead one to conclude that the tree lifted
itself at sunrise and prostrated at sunset. But continuous record
obtained with my recorder attached to the upper part of the trunk shows
that the tree was never at rest, but in a state of continuous movement,
which underwent periodic reversals (Fig. 3). The tree attained its
maximum erection at 7 in the morning, after which there is a rapid
movement of fall. The down movement reached its maximum at 3-15 P.M.,
after which it was reversed and the tree erected itself to its greatest
height at 7 next morning. This diurnal periodicity was maintained day
after day.


UNIVERSALITY OF TREE MOVEMENT.

The next question which I wished to investigate was whether the
movement of the particular Faridpur tree was a unique phenomenon.
It appeared more likely that similar movement would, under careful
observation, be detected in all trees. The particular palm tree was
growing at a considerable inclination to the vertical; the movement
of the tree and its leaves became easily noticeable, since the ground
afforded a fixed and striking object of reference. In a tree growing
more or less erect, the movement, if any, would escape notice, since
such movements would be executed with only the empty space as the
background.

[Illustration: Fig. 4. Record of the Sijbaria Palm from noon for 24
hours. Successive dots, at intervals of 15 minutes.]

_Experiment 1._--Believing the phenomenon to be universal I
experimented with a different Date Palm that was growing at my research
station at Sijbaria on the Ganges, situated at a distance of about 200
miles from Faridpur. The surrounding conditions were very different.
The tree was much younger; it was 2 metres in height and inclined
20° to the vertical. The curve obtained with this tree (Fig. 4) was
very similar to that of the Faridpur Palm, though this extent to the
movement was much reduced. The tree attained the highest erect position
at 7-15 A.M. and the lowest at 3-45 P.M. Hence the movement of the
Faridpur Palm is not a solitary phenomenon.


THE CAUSE OF PERIODIC MOVEMENT.

The recurrent daily movement of the tree must be due to some diurnal
changes in the environment,--either the recurrent changes of light
and darkness, or the diurnal changes of temperature. These changes
synchronise to a certain extent; for, as the sun rises, light appears
and the temperature begins to rise. It is therefore difficult to
discriminate the effect of light from that of temperature. The only
satisfactory method of discrimination would have been in the erection
of a large structure with screens to cut off light. The effect
of fluctuation of temperature under constant darkness would have
demonstrated the effect of one agent without complication arising from
the other. Unfortunately screening the tree was impracticable. I shall
presently describe other experiments where the action of light was
completely excluded.

The curve of movement of the tree, however, affords us material
for correct inference as regards the relative effects of light and
temperature. The experiment was commenced in March; light appeared
at about 5 A.M., the sunrise being at 6-15 A.M.; the sun set at 6-15
P.M., and it became dark by 7 P.M. The incident light would be the
most intense at about noon; after this it would decline continuously
till night time. If the movement was due to light, its climax, either
in up or down movement, would be reached at or about noon, and the
opposite climax at midnight. But instead of this we find (Fig. 3)
the up-movement reaching its highest point not at noon, but at 7 in
the morning; after this the fall is rapid and continuous, and the
lowest position was reached not in the evening but at 3-15 P.M. The
fluctuation of light has, therefore, little to do with the movement of
the tree.

Turning next to the element of variation of temperature we are at
once struck by the fact that the curve of movement of the tree is
practically a replica of the thermographic curve (Fig. 3). The _fall_
of temperature is seen to induce a _rise_ in the tree and _vice versâ_.
There is a lag in the turning points of the two curves; thus while
temperature began to rise at 6 A.M., the tree did not begin to fall
till 7 A.M. There is in this case a lag of an hour; but the _latent_
period may, sometimes, be as long as three hours. The delay is due
to two reasons; it must take some time for the thick trunk of the
tree to attain the temperature of the surrounding, and secondly, the
physiological inertia will delay the reaction. As a result of other
investigations, I find that the induced effect always lags behind the
inducing cause. It is interesting in this connection to draw attention
to the parallel phenomenon, which is described below, of lag in the
variation of sensibility of _Mimosa_ in response to variation of
temperature. In this case the lag was found to be about three hours.
Returning to the Palm, the tree continues to fall in the forenoon with
rising temperature. At about 2-30 P.M. the temperature was at its
maximum after which it began to decline; the movement of the tree was
not reversed into erection till after 3-15 P.M., the lag being now 45
minutes nearly.

I may state here that the movement of the tree is not primarily
affected by the periodicity of day and night, but by variation of
temperature. In spring and in early summer the rise of temperature
during the early part of the day and the fall of the temperature
from afternoon to next morning, are regular and continuous; the
corresponding movements of the tree are also regular. But at other
seasons, owing to the sudden change of direction of the wind, the
fluctuations of temperature are irregular. Thus at night there may
be a sudden rise, and in the earlier part of the day sudden fall of
temperature. And the record of movement of the tree is found to follow
these fluctuations with astonishing fidelity, the rise of temperature
being followed by a fall of the tree and _vice versâ_. That the
movement is determined by the temperature variation is exhibited in a
striking manner in Fig. 4, where, between 8 and 9 A.M., a common twitch
will be noticed in the two curves.

While trying to obtain some clue to the mysterious movement of
the tree, my attention was strongly attracted by certain striking
similarities which the record of the movement of the tree showed to the
curve of the diurnal variation of moto-excitability, of the pulvinus
of _Mimosa pudica_, an account of which will be found in a subsequent
Paper of the series.[A]

[A] _See_ also Bose--Diurnal Variation of Moto-Excitability in
_Mimosa_--Annals of Botany, Vol. XXVII, No. CVIII, October, 1918.


PERIODIC MOVEMENT OF TREES AND DIURNAL VARIATION OF MOTO-EXCITABILITY
IN _MIMOSA PUDICA_.

The excitability of the main pulvinus of _Mimosa pudica_ I find does
not remain constant during the 24 hours, but undergoes a striking
periodic change. At certain hours of the day, the excitability is at
its maximum; at a different period it practically disappears. The
period of insensibility is about 7 A.M., which, strangely enough, is
also the time when the palm tree attains its maximum height. At about
3 in the afternoon the excitability of _Mimosa_ reaches its climax,
and this is the time when the head of the palm tree bends down to its
lowest position. For the determination of the periodic variation of
excitability of _Mimosa_ I devised a special apparatus by which an
electric stimulus of constant intensity was automatically applied
to the plant every hour of the day and night, the responsive moment
being recorded at the same time. The amplitude of responsive fall of
leaf under uniform stimulus gave a measure of excitability of the leaf
at any particular moment. In the lower curve of Fig. 5 is given the
record of diurnal variation of excitability of _Mimosa_. Comparison of
this figure with Figs. 3 and 4, will show the remarkable resemblance
between the curves of diurnal movement of the Palm tree, and of
diurnal variation of moto-excitability of _Mimosa_. The excitability
of _Mimosa_ reached its maximum at about 3 in the afternoon, when
the Palm was at its lowest position. After this hour excitability
fell continuously till 7 or 8 next morning. Corresponding to this is
the continuous erection of the Palm from its lowest position at 3
P.M. to the highest between 7 and 8 A.M. Still more remarkable is the
modifying influence of variation of temperature on the diurnal curve
of excitability in _Mimosa_, and the diurnal curve of movement of the
Palm. This will be quite evident from the inspection of the temperature
curves in Figs. 4 and 5.

[Illustration: Fig. 5. Curve of variation of moto-excitability of
_Mimosa pudica_. The upper curve gives variation of temperature and the
lower, the corresponding variation of excitability.]

I have shown elsewhere[B] that the variation of moto-excitability of
the pulvinus of _Mimosa_ is a physiological function of temperature.
The remarkable similarity between the diurnal variation of
moto-excitability of _Mimosa_ and diurnal movement of the Palm is
due to the fact that both are determined by the physiological action
of temperature. I shall presently describe experiments, which will
establish the physiological character of the movement of the tree in
response to changes of temperature.

[B] Bose--“Irritability of Plants,” d. 60.

The records that have been given show that it is the diurnal variation
of temperature, and not of light that is effective in inducing the
periodic movement of the tree. Further experiments will be given in
support of this conclusion.


RELATIVE EFFECTS OF LIGHT AND TEMPERATURE.

As regards the possibility of light exerting any marked influence
on the movement of the Palm tree, I have shown from study of
time-relations of the movement, that this could not be the case.
Moreover, it is impossible for light to reach the living tissue through
the thick layer of bark that surrounds the tree. That the effect
of light is negligible will appear from the accounts of following
experiments, where the possibility of the effect of changing intensity
of light is excluded by maintaining the plant in constant darkness, or
in constant light.

The employment of the large Palm was obviously impracticable in these
investigations. I, therefore, searched for other plant-organs in which
the movement under variation of temperature was similar to that of the
Date Palm. I found that the horizontally spread leaves of vigorous
specimens of _Arenga saccharifera_ growing in a flower pot executed
movements which were practically the same as that of the Faridpur tree.
The leaf moved downwards with rise of temperature and _vice versâ_.

There are many practical advantages in working with a small specimen.
It can easily be placed under glass cover or taken to a glass house,
thus completely eliminating the troublesome disturbance caused by the
wind.

_Diurnal movement in continued darkness: Experiment 2._--The plant was
placed in a dark room and records taken continuously for three days.
These did not differ in any way from the normal records taken in a
glass house under daily variation of light and darkness. Exposure of
plant to darkness for the very prolonged period of a week or more,
undoubtedly interferes with the healthy photo-tonic condition of the
plant. But such unhealthy condition did not make its appearance in the
first few days.


PHYSIOLOGICAL CHARACTER OF THE MOVEMENT.

There may be a misgiving that the movement of the tree might be due to
physical effect of temperature. If the upper strip of a differential
thermometer be made of the more expansible brass and the lower of
iron, the compound strip bends down with the rise of temperature.
Similarly the movement of the tree might be due to the upper half being
physically more expansible. It would have been possible to discriminate
the physical from the physiological action by causing the death of
the tree; in that case physical movement would have persisted, while
the physiological action would have disappeared. As this test was not
practicable, I tried the effect of physiological depression on the
periodic movement of the leaf of _Arenga saccharifera_.

_Effect of Drought: Experiment 3._--In Fig. 6 is given a series of
records of movement of the leaf-stalk of _Arenga_, first under normal
condition, afterwards under increasing drought, brought about by
withholding water. The uppermost is the thermographic record which
remained practically the same for successive days. Below this are
records of movement of the leaf (_a_) under normal condition, (_b_)
after withholding water for three days, and (_c_) after deprivation for
seven days. It will be noticed how the extent of movement is diminished
under increasing physiological depression brought on by drought. On
the seventh day, the responsive movement disappeared, there being
now a mere fall of the leaf, which was slow and continuous. After
this I supplied the plant with water and the periodic movement was in
consequence nearly restored to its original vigour.

_Effect of poison: Experiment 4._--In another experiment the normal
diurnal record with the leaf was taken and the plant was afterwards
killed by application of poisonous solution of potassium cyanide. The
diurnal movement was found permanently abolished at the death of the
plant.

[Illustration: Fig. 6. Effect of physiological depression on diurnal
movement of the petiole of _Arenga saccharifera_. The uppermost curve
exhibits variation of temperature, (_a_), normal diurnal curve, (_b_),
modification after 3 days’ and (_c_) after 7 days’ withholding of
water.]

These experiments conclusively prove that the periodic movement of
the leaf-stalk induced by variation of temperature is a physiological
phenomenon, and from analogy we are justified in drawing the inference
that the movement of the Faridpur tree is also physiological. The
question, however, was finally settled by the unfortunate death of
the tree which occurred the other day, nearly a year after I commenced
my investigations. While presiding at my lecture on the subject, His
Excellency Lord Ronaldshay, the Governor of Bengal, announced that a
telegram had just reached him from his officer at Faridpur that “the
palm tree was dead, and that its movements had ceased.”

Since my investigation with the Faridpur ‘Praying’ Palm, I have
received information regarding other Palms, which exhibit movements
equally striking. One of the trees is growing by the side of a tank,
the trunk of the tree being inclined towards it. The up-lifted leaves
of this tree are swung round in the afternoon and dipped into the water
of the tank.

The movement of the tree has been shown to be brought about by the
physiological action of temperature variation; in other words the
diurnal movement of the ‘Praying’ Palm is a THERMONASTIC PHENOMENON. I
have found various creeping stems, branches and leaves of many trees,
exhibit this particular movement of fall with a rise of temperature,
and _vice versâ_. Such movements, I shall, for the sake of convenience,
distinguish as belonging to the _negative type_.

Having found that the temperature is the modifying cause, the next
point of inquiry relates to the discovery of the force, whose varying
effects under changing temperature induces the periodic movement. I
shall, in this connection, first discuss the various tentative theories
that may be advanced in explanation of the movement.


TRANSPIRATION AND DIURNAL MOVEMENT.

It may be thought that the fall of the tree during rise of temperature
may be due to passive yielding of the tree to its weight, there being
increased transpiration and general loss of turgor at high temperature.
I shall, however, show that the diurnal movement persists in the
absence of transpiration.

_Diurnal movement in absence of transpiration: Experiment 5._--In the
leaf of _Arenga saccharifera_, I found that the petiole was the organ
of movement. I cut off the transpiring lamina and covered the cut end
with collodion flexile. The plant was now placed in a chamber saturated
with moisture. The petiole continued to give records of its diurnal
movement in every way similar to the record of the intact leaf. In
another experiment with the water plant, _Ipoemia reptans_, immersed in
water, the normal diurnal movement was given by the plant, where there
could be no question of variation of turgor due to transpiration. (See
also _Expt._ 7.)

In the diurnal movement of the ‘Praying’ Palm the concave curvature of
the rigid neck in the morning, became flattened or slightly convex in
the afternoon. The force necessary to cause this is enormously great,
and could on no account result from the passive yielding to the weight
of the upper part of the tree.

From the facts given above it will be seen that the diurnal movement is
not brought about by variation in transpiration. I now turn to another
phenomenon which appeared at first to have some connection with the
movement of the tree. Kraus found that the tissue tensions of a shoot
exhibit a daily periodicity. He, however, found that between 10°C. and
30°C., variation of temperature had no effect on the daily period. But
as regards the diurnal movement of the tree, it is the temperature
which is the principal factor. Kraus also found a daily variation of
bulk in different plant-organs; this variation of bulk is connected
with transpiration, for the removal of the transpiring leaves arrested
this variation. But the periodic movement of the tree, as we have seen,
is independent of transpiration.

Millardet observed a daily periodicity of tension in _Mimosa pudica_.
He found that maximum tension occurs before dawn; the petiole becomes
erected, the movement being upwards or towards the tip of the stem.
Tension decreases during the day, and reaches a minimum early in the
evening; in correspondence with this is the fall of the petiole, the
movement being away from the tip of the stem.[C] If the plant were
placed upside down the periodic movement of the petiole in relation to
the stem will evidently remain the same, but become reversed in space.
Maximum tension in the morning will make the petiole approach the
tip of the stem, _i.e._, the movement will be _downwards_ instead of
upwards as in the normal position. The experiment described below will
show that the diurnal movement induced by variation of temperature is
not reversed by placing the plant in an inverted position.

[C] Vines.--‘Physiology of Plants,’ 1886, pp. 405 and 543.

_Diurnal movement in inverted position: Experiment 6._--I took a
vigorous specimen of _Arenga saccharifera_ growing in a pot, and took
its normal record, which as explained before exhibited down-movement
during rise, and an up-movement during fall of temperature. The plant
was now held inverted, the upper side of the petiole now facing the
earth. The diurnal curve of movement should now show an inversion, if
that movement was solely determined by the anisotropy of the organ.
But the record did not exhibit any such inversion. After being placed
upside down, the leaf did not, on the first day, show any diurnal
movement; there was, on the other hand, a continuous down-movement on
account of the fall of the leaf by its own weight. But in the course
of 24 hours the leaf readjusted itself to its unaccustomed position,
and became somewhat erected under the action of geotropic stimulus.
After the attainment of this new state of geotropic equilibrium, the
leaf gave a very pronounced record of its diurnal movement which did
not show any reversal; the inverted leaf continued to exhibit the
same characteristic movements as in the normal position, that is to
say, a down movement during rise, and an up-movement during fall of
temperature. As the plant in the inverted position did not show any
reversal of the periodic curve, it is clear that the diurnal movement
is determined by the modifying influence of temperature on the
physiological reaction of the plant to some external stimulus which is
constant in direction. I shall presently show that it is the constant
geotropic stimulus modified by the action of temperature, which
determines the diurnal movement of the tree.

This will be better understood if I refer once more to certain
characteristics in the movement of the “Praying” Palm. The neck of
the tree was seen to be concave in the morning. The physiological
effect of raising temperature is virtually to oppose or neutralise the
geotropic curvature as seen in the flattening or slight reversal of
curvature in the afternoon. Similarly, various plant organs, growing at
an inclination to the vertical, are subjected to geotropic action, and
thus assume different characteristic angles. This state of equilibrium
is not static but may better be described as dynamic; for it will be
shown that this state of geotropic balance is upset in a definite way,
by variation of temperature.

That geotropism is an important factor in the diurnal movement is
supported by the fact that the Sijbaria Palm with an inclination of 20°
to the vertical exhibited a daily movement which was only moderate
in extent. But the Faridpur Palm growing at an inclination of 60° was
subjected more effectively to geotropic action, and exhibited movements
which were far more pronounced. I shall now proceed to describe crucial
experiments which will demonstrate the effect of change of temperature
on geotropic curvature.


EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC CURVATURE.

In the instances of diurnal movement already described the trees or
their leaves were already at an inclination to the vertical. I now took
a radial and erect shoot of _Basella cordifolia_ growing in a pot and
laid it horizontally for two weeks. The procumbent stem curved up and
attained a state of equilibrium under the action of geotropic stimulus.

_Diurnal curve of_ Basella cordifolia: _Experiment 7._--The plant
was completely immersed in a vessel of water, and its diurnal curve
recorded. This resembled in all essentials the diurnal curve of the
Palm; the slight deviation was due to the fact that owing to difference
in the season (August) the temperature maximum was attained at 12-25
P.M. and the minimum at 6 A.M. The geotropic curvature was reduced to
its minimum at the maximum temperature, and _vice versâ_. As in the
case of the Palm so also in the procumbent stem of _Basella_ there was
a physiological lag, which was 50 minutes in the morning and about
the same in the afternoon. The free end of the stem thus exhibited a
diurnal movement up and down. The temperature, as stated before, began
to rise from 6 A.M. and the down-movement commenced 50 minutes later,
_i.e._, at 6-50 A.M. The temperature, after reaching the maximum, began
to fall at 12-25 P.M., and the previous movement of fall of the stem
was arrested and reversed into an erectile movement shortly after 1
P.M. There are thus two “turning points,” one at 7 A.M., and the other
at about 1 P.M.; at these periods the movement of the plant remains
more or less arrested for more than half-an-hour.

[Illustration: Fig. 7. Diurnal curve of movement of procumbent young
stem of _Mimosa pudica_. Successive dots at intervals of 15 minutes.]

I obtained records of similar diurnal movements with various procumbent
or creeping stems. Figure 7 gives the diurnal record of the procumbent
stem of a young specimen of _Mimosa pudica_.

The experiment that has just been described shows clearly that
geotropic curvatures of stems is opposed, or neutralised to a greater
or less extent, during rise of temperature, and this antagonistic
reaction is removed during the fall of temperature. The diurnal
movement of the plant completely immersed under water shows once more
that transpiration has little to do with the diurnal movement.


REVERSAL OF NATURAL RHYTHM.

The diurnal rhythm of up and down movement in the particular specimen
_Basella_ had become established under the daily variation of
temperature. I now attempted to reverse this rhythm by artificial
variation of temperature. The plant was placed in water in a
rectangular metallic vessel which was placed within a second outer
vessel. The plant could thus be subjected, without any mechanical
disturbance, to variation of temperature, by circulating warm or
cold water in the outer vessel. In order to reverse the natural
rhythm I subjected the plant to the action of falling temperature
at the “turning” point at 7 A.M., at a time when the plant would
have undergone a down-movement under the daily rise of temperature.
Conversely the plant was subjected to the action of rising temperature
at the second “turning” point at 1 P.M. when the movement under diurnal
fall of temperature would have been one of erection.

_Effect of fall of temperature: Experiment 8._--As stated before
the experiment was carried out in the morning; ice cold water was
circulated in the outer chamber, the fall of temperature was in this
case sudden, and there was an almost immediate responsive movement.
This appeared anomalous, since the latent period of response to slow
variation of temperature was found from the diurnal curve to be as long
as 50 minutes.

As a result of further investigations I found that variation of
temperature produces two different effects which may be distinguished
as transient and persistent. Sudden variation of temperature affects
the superficial tissue, and gives rise to a transient reaction,
while it takes a long time for temperature variation to react on the
geotropically active tissue in the interior. The persistent effect
therefore takes place after a latent period from one to three hours
according to the thickness of the plant.

[Illustration: Fig. 8. Reversal of normal rhythm: Erectile response
_Basella_ to gradual fall of temperature.]

[Illustration: Fig. 9. Responsive fall of _Basella_ to gradual rise of
temperature. (Dots at intervals of 5 minutes).]

The persistent effect of rise of temperature is a movement downwards,
that of fall of temperature is a movement upwards. These definite
reactions will be seen exhibited in Figs. 8 and 9. The plant was
stationary at the turning point in the morning hence the curve at first
was horizontal. The temperature was gradually lowered through 5°C.,
from 29°C., to 24°C. in the course of five minutes and maintained
at the lower temperature. There was no immediate effect, but after
a latent period of 65 minutes the plant responded by a movement of
erection. The natural movement at this period of the day would have
been one of fall, but artificial change of temperature in the opposite
direction effectively reversed the normal diurnal movement. The latent
period for this reverse movement is, as stated before, 65 minutes as
against 50 minutes in the normal diurnal movement. The increase in the
latent period is probably due to the added physiological inertia in
reversing the normal rhythm.

_Effect of rise of temperature: Experiment 9._--The temperature was
raised through 5°C at the second turning point, at 1 P.M. After a
latent period of 50 minutes the plant began to rise steadily (Fig. 9)
thus exhibiting once more the reversal of its normal diurnal movement.

From the experiments described above it will be seen that the movement
of the Palm, and of other organs growing at an inclination to the
vertical, is brought about by the action of temperature in modifying
the geotropic curvature. The ever present tendency of geotropic
movement is opposed or helped by the physiological reaction induced by
rise and fall of temperature respectively. The state of equilibrium is
never permanent, but the dynamic balance is being constantly readjusted
under changing conditions of the environment.

The movement of the tree furnishes an example of the _negative_ type of
THERMONASTIC MOVEMENT. Parallel phenomena are found in floral organs,
where, in the well-known instance of _Crocus_, the perianth leaves
open outwards during rise of temperature and close inwards during the
onset of cold. Looked at from above, the opening outwards during rise
of temperature is a movement downwards, and therefore belongs to the
_negative type_. In such cases the changed rate of growth by variation
of temperature is the most important factor in the movement. It may be
asked whether all thermonastic movements must necessarily belong to the
_negative_ type, where rise of temperature is attended by a movement
downwards. I shall in my Paper on “Thermonastic Phenomena” show that
there is also a _positive type_ where rise of temperature induces an
up-movement or of closure.


SUMMARY.

The ‘Praying’ Palm of Faridpur, growing at an inclination of about
60° to the vertical, exhibited a diurnal movement by which its head
became erected in the morning and depressed towards the afternoon, the
outspread leaves pressing against the ground.

The record of the diurnal movement showed that the head was erected to
the highest position between 7 and 8 in the morning, after which there
was a continuous fall which reached its climax at 3-15 P.M.; after this
the movement was reversed and the maximum erection was again reached
next morning.

This phenomenon is not unique, but is found exhibited, more or less, by
all trees and their branches and leaves.

Diurnal records of temperature, and movement of the tree showed, that
the two curves closely resembled each other. Rise of temperature was
attended by a fall of the tree, and _vice versâ_.

The movement is brought about by the physiological action of
temperature; it may be arrested by artificially induced physiological
depression, and is permanently abolished at death.

The movement is primarily determined by the modifying influence of
temperature on geotropic curvature. Rise of temperature is found to
oppose or neutralise geotropic curvature, the fall of temperature
inducing the opposite effect. The ever present tendency of upwards
geotropic movement is opposed or helped by the effects of rise and fall
of temperature respectively.

The movement of the ‘Praying’ Palm is a thermonastic phenomenon. The
tree, apparently so rigid, responds as a gigantic pulvinoid to the
changes of its environment.




III.--ACTION OF STIMULUS ON VEGETABLE TISSUES

_By_

SIR J. C. BOSE,

_Assisted by_

NARENDRA NATH SEN GUPTA.


The leaf of _Mimosa pudica_ undergoes a rapid fall when subjected
to any kind of shock. This plant has, therefore, been regarded as
“sensitive,” in contradistinction to ordinary plants which remain
apparently immobile under external stimulus. I shall, however, show
in course of this Paper that there is no justification in regarding
ordinary plants as insensitive.

Let us first take any radial organ of a plant and subject it to an
electric shock. It will be found that the organ undergoes a contraction
in length in response to the stimulus. On the cessation of excitation
the specimen gradually recovers its original length. Different organs
of plant may be employed for the experiment, for example, the tendril
of _Cucurbita_, the pistil of _Datura_, or the flower bud of _Crinum_.
The shortening may be observed by means of a low power microscope.
Greater importance is, however, attached to the detailed study of
response and its time relations. The pull exerted by a delicate
organ during its excitatory contraction is slight; hence arises the
necessity of devising a very sensitive apparatus, which would give
records magnified from ten to a hundred times.


RESPONSE RECORDERS.

The magnification of movement is produced by a light lever, the short
arm of which is attached to the plant organ, the long arm tracing the
record on a moving smoked plate of glass. The axis of the lever is
supported by jewel bearings. The principal difficulty in obtaining
accurate record of response of plant lies in the friction of contact of
the recording point against the glass surface. This difficulty I have
been able to overcome by providing a device of intermittent instead
of continuous contact. For this, either the writer is made to vibrate
to and fro, or the recording plate is made to oscillate backwards and
forwards.

1. _The Resonant Recorder._--In this the writing lever is made of a
fine steel wire. One end of this wire is supported at the centre of a
circular electromagnet; this latter is periodically magnetised by a
coercing vibrator, which completes an electric circuit ten hundred,
or two hundred times in a second. The writing lever is exactly tuned
to the vibrating interrupter and is thus thrown into sympathetic
vibration. Successive dots in the record thus measure time from 0.1
to 0.05 second. The employment of the Resonant Recorder enables us to
measure extremely short periods of time for the determination of the
latent period or the velocity of transmission of excitation.[D]

[D] For detailed description _cf._ Bose.--“An Automatic Method
for Investigation of Velocity of Transmission of Excitation in
_Mimosa_.”--Phil. Trans., B. vol. 204, (1913).

2. _The Magnetic Tapper._--Measurement of very short intervals is not
necessary in ordinary records of response. In this type of recorders,
the circular magnet is therefore excited at longer intervals, from
several seconds to several minutes; this is done by completion of the
electric circuit at the required intervals, by means of a key operated
by a clock.

3. _The Mechanical Tapper._--In this, magnetic tapping is discarded in
favour of mechanical tapping. The hinged writing lever is periodically
pressed against the recording plate by a long arm, actuated by
clock-work.

4. _The Oscillating Recorder._--Here the plate itself is made to
oscillate to-and-fro by eccentric worked by a clock. The frame carrying
the plate moves on ball-bearings. The advantage of the Oscillating
Recorder lies in the fact that a long lever, made of fine glass fibre,
or of aluminium wire, may be employed for giving high magnification. A
magnification of a hundred times may be easily obtained by making the
short arm 2.5 mm. and the long arm 25 cm. in length.[E]

[E] Bose--“Researches on Irritability of Plants,” p. 279--Longmans,
Green & Co.


RESPONSE OF A RADIAL ORGAN.

[Illustration: Fig. 10. Response of a straight tendril of _Passiflora_
to electric shock. Successive dots at intervals of 5 seconds. The
vertical lines below are at intervals of a minute. In this and in all
following records (unless stated to the contrary) up-curve represents
contraction, and down-curve expansion or recovery.]

_Experiment 10._--As a typical example I shall describe the response of
a straight tendril of _Passiflora_. A cut specimen was mounted with its
lower end in water. Suitable electric connections were made for sending
a feeble induction shock of short duration through the specimen.
In this and all other records, unless contrary be stated, up-curve
represents contractile movement. On application of stimulus of electric
shock, an excitatory movement of contraction occurred which shortly
reached its maximum; the apex-time was one minute and forty seconds,
and recovery was completed after a further period of five minutes
(Fig. 10). Stronger shocks induce greater contraction with prolongation
of the period of recovery. The specimen was afterwards killed by
application of poisonous solution of potassium cyanide; this brought
about a permanent abolition of response. The experiment just described
may be taken as typical of response of radial organs.

In a radial organ contraction takes place equally in all directions;
it therefore shortens in length, there being no movement in a lateral
plane. But if any agency renders one side less excitable than its
opposite, diffuse stimulation will then induce greater contraction on
the more excitable side which will therefore become concave.


RESPONSE OF AN ANISOTROPIC ORGAN.

Excessive stimulation is found to reduce the excitability of an organ.
Under unilateral mechanical stimulation a tendril of _Passiflora_
becomes hooked or coiled, the concave being the excited side. From what
has been said, the unexcited convex side will relatively be the more
excitable.

[Illustration: Fig. 11. Response of a hooked tendril of _Passiflora_ to
electric shock. Successive dots at intervals of 5 seconds.]

_Experiment 11._--I took a specimen of hooked tendril, and excited it
by an electric shock. The response was by the greater contraction of
the more excitable convex side, on account of which the curved specimen
tended to open out. The record of this response is seen in Fig. 11; the
apex-time was nearly two minutes, and the recovery was completed in the
further course of 15 minutes.

From the responses of organs rendered anisotropic by the differential
action of the environment we pass to others which show certain amount
of anatomical and physiological differentiation between their upper
and lower sides. I find that many petioles of leaves show movement in
response to stimulus. Many pulvini, generally regarded as insensitive,
are also found to exhibit responsive movements.


RESPONSE OF THE PULVINUS OF _MIMOSA PUDICA_.

[Illustration: Fig. 12. Response of the main pulvinus of _Mimosa
pudica_.]

The most striking and familiar example of response is afforded by the
main pulvinus of _Mimosa pudica_ of which a record is given in Fig. 12.
It is generally assumed that sensibility is confined to the lower half
of the organ. It will be shown in a subsequent Paper that this is not
the case. The upper half of the pulvinus is also sensitive though in a
feeble degree, its excitability being about 80 times less than that of
the lower half. On diffuse stimulation the predominant contraction of
the lower half causes the fall of the leaf, the antagonistic reaction
of the upper half being, in practice, negligible. In order to avoid
unnecessary repetition, I shall ignore the feeble antagonistic reaction
of the less excitable half of the organ, and shall use the word
‘contraction’ for ‘relatively greater contraction.’

It is interesting in this connection to refer to the response of the
leaf of Water Mimosa (_Neptunia oleracea_). Here the reaction is
very sluggish in comparison with that of _Mimosa pudica_. A tabular
statement of contractile response of various radial, anisotropic and
pulvinated organs will show a continuity in the contractile reaction;
the difference exhibited is a question of degree and not of kind.

TABLE 1--PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF DIFFERENT
PLANTS.

  +-------------------------------------+-------------+------------+
  |                                     | Period of   | Period of  |
  |        Specimen                     | maximum     | recovery.  |
  |                                     | contraction |            |
  +-------------------------------------+-------------+------------+
  | Radial organ:                       |             |            |
  |   Tendril of _Passiflora_           | 100 seconds |  4 minutes.|
  |                                     |             |            |
  | Anisotropic organ:                  |             |            |
  |   Hooked tendril of _Passiflora_    | 120    "    | 13    "    |
  |                                     |             |            |
  | Pulvinated organ:                   |             |            |
  |   Pulvinus of _Neptunia Oleracea_   | 180    "    | 57    "    |
  |                                     |             |            |
  |   Pulvinus of _Mimosa pudica_       |   3    "    | 16    "    |
  +-------------------------------------+-------------+------------+

As regards the excitatory fall of the leaf of _Mimosa pudica_, Pfeffer
and Haberlandt are of opinion that this is due to the sudden diminution
of turgor in the excited lower half of the pulvinus. The weight of
the leaf, no longer supported by the distended lower cells, causes it
to fall. This is accentuated by the expansion of the upper half of
the pulvinus which is normally in a state of compression. According
to this view the excitatory fall of the leaf is a passive, rather
than an active, movement. I have, however, found that in determining
the rapidity of the fall of _Mimosa_ leaf the factors of expansive
force of the upper half of the pulvinus and the weight of the leaf are
negligible compared to the active force of contraction exerted by the
lower half of the pulvinus (p. 87).

With regard to the fall of turgor, it is not definitely known whether
excitation causes a sudden diminution in the osmotic strength of the
cell-sap or an increase in the permeability of the ectoplast to the
osmotic constituents of the cell. Pfeffer favours the former view,
while others support the theory of variation of permeability.[F]

[F] With reference to the fall of _Mimosa_ leaf Jost says: “When the
pressure of the cell decreases we naturally assume this to be due to a
decreasing _osmotic pressure_ due to alterations in the permeability
of the plasma, and an excretion of materials from the cell. It is a
remarkable fact that plasmolytic research (Hilburg 1881) affords no
evidence of any decrease in osmotic pressure. No complete insight
into the mechanism of the stimulus movement in _Mimosa_ has yet been
obtained, although one thing is certain, that there is a decrease in
the expansive power on the under side of the articulation.”--Jost,
“Plant Physiology”--English Translation, p. 515. Clarendon Press
(1907). Blackman and Paine think that the loss of turgor on excitation
“is probably due to the disappearance or inactivation of a considerable
portion of the osmotic substances of the cells.”--Annals of Botany,
Vol. XXXII, No. CXXXV, Jan. 1918.


RESPONSE OF PULVINUS OF _MIMOSA_ TO VARIATION OF TURGOR.

Whatever difference of opinion there may be in regard to the theories
of osmotic and permeability variations, we have the indubitable fact
of diminution of turgor and contractile fall of the pulvinus of
_Mimosa_ under excitation. The restoration of the original turgor
brings about recovery and erection of the leaf. In connection with this
the following experiments on responsive movements of the leaf under
artificial variation of turgor will be found of interest:--

_Effect of Increased Turgor: Experiment 12._--A young _Mimosa_ plant
was carefully transplanted and the root embedded in soil placed in a
linen bag. This was held securely by a clamp, and one of the leaves
of the plant attached to the recorder. Withholding of water for a day
caused a general loss of turgor of the plant. A vessel full of water
was now raised from below so that the linen bag containing the roots
was now in water. The effect of increased turgor by suction of water
by the roots became apparent by the _upward_ movement of the leaf. The
distance between the immersed portion of the plant and the leaf was
2 cm. and the up-movement of the leaf was indicated within 10 seconds
of application of water (Fig. 13). The velocity with which the effect
of increased turgor travelled was thus 2 mm. per second. The leaf
exhibited increasing erection with absorption of water.

[Illustration: Fig. 13. Response of _Mimosa_ pulvinus to variation
of turgor. Increased turgor by application of water at point marked
with vertical arrow induced erectile movement. Diminution of turgor by
application of KNO_{3} solution at the point marked with the horizontal
arrow, brought about the fall of the leaf within 80 seconds. Successive
dots at intervals of 5 seconds (The down curve represents up-movement
and _vice versâ_.)]

_Effect of Diminution of Turgor: Experiment 13._--While the leaf in the
above experiment was in process of erection, a quick change was made by
substituting KNO_3 solution for the water of the vessel in which the
roots were immersed. The plasmolytic withdrawal of water at the roots
gave rise to a wave of diminished turgor, the effect of which became
perceptible within 40 seconds by the movement of _fall_ of the leaf.
(Fig. 13.)


DIFFERENT MODES OF STIMULATION.

In _Mimosa_ excitation is manifested by the contraction of the pulvinus
and the consequent movement of the leaf. But in most plants, excitatory
movement cannot be realized on account of the rigidity of the plant
structure, the thickness of the cell-wall and the want of facility
for escape of water from the excited cells. I shall show later how
excitation may be detected in the absence of mechanical movement.

As regards stimulation of vegetable tissues, there are various agencies
besides electric shock, which induce excitatory contraction; these
agencies I shall designate as stimuli. Excitation is detected in
_Mimosa_ by the downward movement of the leaf. It will be found that
such excitatory movement is caused by a mechanical blow, by a prick or
a cut, by the application of certain chemical agents, by the action of
electric current and by the action of strong light. The study of the
action of these stimuli will be given in greater detail in subsequent
Papers.

I shall give below a general classification of different stimuli which
cause excitation in vegetable tissues.

_Electric Stimulus._--Induction shock, condenser discharge, the make of
kathode and the break of anode.

_Mechanical Stimulus._--Mechanical blow, friction, prick or cut.

_Chemical Stimulus._--Effect of certain acids and of other chemical
substances.

_Thermal Stimulus._--Sudden variation of temperature; application of
heated wire.

_Radiation Stimulus._--Luminous radiation of the more refrangible
portion of the spectrum; ultra-violet rays; thermal radiation in the
infra-red region.

All these different forms of stimulus induce an excitatory contraction,
a diminution of turgor, and a negative mechanical response or fall of a
motile leaf.


SUMMARY.

A radial organ responds to stimulus by contraction in length; as all
its flanks are equally excitable there is no lateral movement under
diffuse stimulus.

Physiological anisotropy is induced in an organ, originally radial and
isotropic, by the unequal action of the environment on its different
sides. Diffuse stimulus induces a greater contraction of the more
excitable side.

In a curved tendril the concave side is less excitable than the convex.
Diffuse stimulus tends to straighten the curved tendril.

In the pulvinus of _Mimosa pudica_, the lower half is eighty times
more excitable than the upper, and the fall of the leaf is due to the
predominant contraction of the more excitable lower half.

A diminution of turgor takes place in the excited cells. Restoration of
turgor brings about recovery of the leaf to its normal erect position.
Independent experiments show that the fall of the leaf may be brought
about by an artificial diminution of turgor, and the erection of the
leaf by an increase of turgor.




IV.--DIURNAL VARIATION OF MOTO-EXCITABILITY IN _MIMOSA_

BY

SIR J. C. BOSE.


Several phenomena of daily periodicity are known, but the relations
between the recurrent external changes and the resulting periodic
variations are more or less obscure. As an example of this may be
cited the periodic variation of growth. Here the daily periodicity
exhibited by a plant is not only different in varying seasons, but
it also differs in diverse species of plants. The complexity of the
problem is very great, for not only are the direct effects of the
changing environment to be taken into consideration but also their
unknown after-effects. Even in the case of direct effect, different
factors, such as light, temperature, turgor, and so on, are undergoing
independent variations; it may thus happen that their reactions may
sometimes be concordant and at other times discordant. The nyctitropic
movement of plants affords another example of daily periodicity. The
fanciful name of ‘sleep’ is often given to the closure of the leaflets
of certain plants at night. The question whether plants sleep or not
may be put in the form of the definite inquiry: Is the plant equally
excitable throughout day and night? If not, is there any definite
period at which it practically loses its excitability? Is there, again,
another period at which the plant wakes up, as it were, to a condition
of maximum excitability?

In the course of my investigations on the irritability of _Mimosa
pudica_, I became aware of the existence of such a daily periodicity;
that is to say, the moto-excitability of the pulvinus was found to
be markedly diminished or even completely abolished at a certain
definite period of the day; at another equally definite period, the
excitability was observed to have attained its climax. The observations
on the periodic variation of excitability appeared at first to be
extremely puzzling. It might be thought, for example, that light would
prove to be favourable for moto-excitability; in actual experiment
the results apparently contradicted such a supposition: for the
excitability of the plant was found much higher in the evening than
in the morning. Favourable temperature, again, might be regarded as
an important factor for the enhancement of the moto-excitability;
it was, nevertheless, found that though the excitatory response was
only moderate at that period of night when the temperature was at its
minimum, yet the excitability was altogether abolished at another
period when the temperature was several degrees higher. The obscurities
which surrounded the subject were only removed as a result of
protracted investigation and comparison of continuous automatic records
made by the plant itself during several months, beginning with winter
and ending in summer.

The question whether a plant like _Mimosa_ exhibits diurnal variation
of excitability can be experimentally investigated by subjecting the
plant at every hour of the day and night to a test-stimulus of uniform
intensity, and obtaining the corresponding mechanical responses. Under
these circumstances the amplitude of response at any time will serve
as a measure of the excitability of the plant at the particular time.
Any periodic fluctuation of response will then demonstrate the periodic
character of variation of excitability.

The investigation thus resolves itself into:--

  The successful construction of a Response Recorder which will
       automatically record the response of the plant to uniform
       periodic stimulation at all hours of day or night;

  the study of the effects of various external conditions on
       excitability;

  the diurnal variation of excitability and its relation to the
       changes of external conditions.

I will first give a diagrammatic view of the different parts of the
apparatus which I devised for this investigation.[G] The leaf of
_Mimosa_ is attached to one arm of a light aluminium lever, L, by means
of thread. At right angles to the lever is the writing index W, which
traces on a smoked glass plate allowed to fall at a definite rate
by clockwork the responsive movement of the leaf. Under a definite
stimulus of electric shock the leaf falls down, pulling the lever L,
and moving the writer towards the left. (Fig. 14.) The amplitude of
the response-curve measures the intensity of excitation. The leaf
re-erects itself after a time, the corresponding record exhibiting
recovery. A second stimulus is applied after a definite interval, say
an hour, and the corresponding response shows whether the excitability
of the plant has remained constant or undergone any variation.

[G] _See_ also Bose.--The Diurnal Variation of Moto-excitability in
_Mimosa_--Annals of Botany, Oct. 1913.

[Illustration: FIG. 14. Diagrammatic representation of the complete
apparatus for determination of diurnal variation of excitability.
Petiole of _Mimosa_, attached by thread to one arm of lever L;
writing index W traces on smoked glass plate G, the responsive fall
and recovery of leaf. A, primary, and S, secondary, of induction
coil. Exciting shock passes through the plant by electrodes E, E′. A,
accumulator. C, clockwork for regulating duration of tetanizing shock.
Primary circuit of coil completed by plunging rod, V, dipping into cup
of mercury M.]


UNIFORM PERIODIC STIMULATION.

_Electric mode of excitation._--I find that one of the best methods of
stimulating the plant is by means of tetanizing induction shock. The
sensitiveness of _Mimosa_ to electric stimulation is very great; the
plant often responds to a shock which is quite imperceptible to a human
subject. By the employment of a sliding induction coil, the intensity
of the shock can be regulated with great accuracy; the secondary if
gradually brought nearer the primary till a stimulus is found which is
minimally effective. The intensity of stimulus actually employed is
slightly higher than this, but within the sub-maximal range. When the
testing stimulus is maintained constant and of sub-maximal intensity,
then any variation of excitability is attended by a corresponding
variation in the amplitude of response.

The exciting value of a tetanizing electric shock depends (1) on the
intensity, (2) on the duration of shock. The intensity may be rendered
uniform by placing the secondary at a fixed distance from the primary,
and keeping the current in the primary circuit constant. The constancy
of the current in primary circuit is secured by the employment of an
accumulator or storage cell of definite electromotive force. It is
far more difficult to secure the constant duration of the tetanizing
shock in successive stimulations at intervals of, say, one hour during
twenty-four hours. The duration of the induction shock given by the
secondary coil depends on the length of time during which the primary
circuit is completed in successive excitations. I have succeeded in
overcoming the difficulty of securing uniformity of duration of shock
by the employment of a special clockwork device.

_The clockwork plunger._--The alarum clock can be so arranged that a
wheel is suddenly released and allowed to complete one rapid revolution
at intervals of, say, one hour. There is a fan-governor by which the
speed of the revolution can be regulated and maintained constant. This
will specially be the case when the alarum spring is long and fully
wound. The succession of short releases twenty-four times during the
day produce relatively little unwinding of the spring. On account of
this and the presence of the fan-governor, the period of a single
revolution of the wheel remains constant. By means of an eccentric
the circular movement is converted into an up and down movement. The
plunging rod R thus dips into a cup of mercury M, for a definite
short interval and is then lifted off. The duration of closure can be
regulated by raising or lowering the cup of mercury. In practice the
duration of tetanizing shock is about 0.2 second.

The same clock performs three functions. The axis which revolves once
in twelve hours has attached to it a wheel, and round this is wound
a thread which allows the recording glass plate to fall through six
inches in the course of twenty-four hours. A spoke attached to the
minute hand releases the alarum at regular and pre-determined intervals
of time, say once in an hour. The plunging rod R, actuated by the
eccentric, causes a tetanizing shock of uniform intensity and duration
to be given to the plant at specified times.

_Constancy of resistance in the secondary circuit._--In order that the
testing electric stimulus shall remain uniform, another condition has
to be fulfilled, namely, the maintenance of constancy of resistance in
the secondary circuit, including the plant. Electric connections have
to be made with the latter by means of cloth moistened with dilute
salt solution; drying of the salt solution, however, gives rise to a
variation of resistance in the electrolytic contact. This difficulty
is overcome by making the electrolytic resistance negligible compared
to the resistance offered by the plant. Thin and flexible spirals
of silver tinsel attached to the electrodes E, E′ are tied round
the petiole and the stem, respectively. In order to secure better
electric contact, a small strip of cloth moistened with dilute salt
and glycerine is wound round the tinsel. As the resistance of contact
is relatively small, and as drying is to a great extent retarded by
glycerine, the total resistance of the secondary circuit undergoes
practically no variation in the course of twenty-four hours. This will
be seen from the following data. An experiment was commenced one day at
1 P.M., when the resistance offered by 8 cm. length of stem and 2 cm.
length of petiole was found to be 1.5 million ohms. After twenty-four
hours’ record, the resistance was measured the next day and was found
unchanged. The fact that the stimulus remains perfectly uniform will be
quite apparent when the records given in the course of this paper are
examined in detail.


THE RESPONSE RECORDER.

The amplitude of response affords, as we have seen, a measure of the
excitability of the plant. In actual record friction of the writer
against the glass surface becomes a source of error. This difficulty I
have been able to overcome by the two independent devices, the Resonant
Recorder and the Oscillating Recorder. In the former the writer is
maintained by electric means in a state of continuous to and fro
vibration, about ten times in a second. There is thus no continuous
contact between the writer and the smoked glass surface, friction
being thereby practically eliminated. The writer in this case taps
a record, the successive dots occurring at intervals of 0.1 second.
The responsive fall of the leaf is rapid, hence the successive dots
in this part of the record are widely spaced; but the erection of the
leaf during recovery takes place slowly, hence the recovery part of
the curve appears continuous on account of the superposition of the
successive dots. The advantage of the Resonant Recorder is that the
curve exhibits both response and recovery. This apparatus is admirably
suited for experiments which last for a few hours. There is, however,
some drawback to its use in experiments which are continued for days
together. This will be understood when we remember that for the
maintenance of 10 vibrations of the writer in a second, 10 electric
contacts have to be made; in other words, 36,000 intermittent electric
currents have to be kept up per hour. This necessitates the employment
of an electric accumulator having a very large capacity.

In the Oscillating Recorder the recording plate itself moves to and
fro, making intermittent contact with the writer about once in a
minute. The recording smoked glass plate is allowed to fall at a
definite rate by the unwinding of a clock wheel. By an electromagnetic
arrangement the holder of the smoked glass plate is made to oscillate
to and fro, causing periodic contact with the writer.

[Illustration: FIG. 15. The Oscillator. Electromagnet M, M′,
periodically magnetized by completion of electric current by clockwork
C. Periodic attraction of soft iron armature A moves attached glass
plate G to left, making thereby electric contact with writer.]

The Oscillator is diagrammatically shown in Fig. 15. M, M′ are the two
electromagnetic coils, the free ends of the horseshoe being pointed.
Facing them are the conical holes of the soft iron armature A. This
armature carries two rods which slide through hollow tubes. The distal
ends of the rods support the holder H, carrying the smoked glass plate.
Under normal conditions, the plate-holder is held by suitable springs,
somewhat to the right of, and free from contact with, the writer. A
clockwork C carries a rotating arm, which makes periodic contact with
a pool of mercury contained in the vessel V, once in a minute. On the
completion of the electromagnetic circuit, the armature A is attracted,
the recording glass plate being thereby moved to the left making
contact with the writer. The successive dots in the record thus take
place at intervals of a minute. Only a moderate amount of electric
current is thus consumed in maintaining the oscillation of the plate. A
4-volt storage cell of 20 amperes capacity is quite sufficient to work
the apparatus for several days.

The responsive fall of the leaf of _Mimosa_ is completed in the course
of about two seconds. The leaf remains in the fallen or ‘contracted’
position for nearly fifteen seconds; it then begins to recover slowly.
As the successive dots of the Oscillating Recorder are at intervals
of a minute, the maximum fall of leaf is accomplished between two
successive dots. The dotted response record here obtained exhibits
the recovery from maximum fall under stimulation (_cf._ Fig. 23). The
recovery of the leaf in one minute is less than one-tenth the total
amplitude of the fall, and is proportionately the same in all the
response records. Hence the successive amplitudes of response curves
that are recorded at different hours of the day afford us measures
of the relative variations of excitability of the plant at different
times. This enables us to demonstrate the reality of diurnal variation
of excitability. In my experimental investigations on the subject I
have not been content to take my data from any particular method of
obtaining response, but have employed both types of recorders, the
Resonant and Oscillating. It will be shown that the results given by
the different instruments are in complete agreement with each other.


EFFECTS OF EXTERNAL CONDITIONS ON EXCITABILITY.

Before giving the daily records of periodic variation of excitability,
I will give my experimental results on the influence of various
external conditions in modifying excitability. The conditions which
are likely to affect excitability and induce periodicity are, first,
the effects of light and darkness: under natural conditions the plant
is subjected in the morning to the changing condition from darkness
to light; then to the action of continued light during the day; and in
the evening to the changing condition from light to darkness. A second
periodic factor is the change in the condition of turgidity, which
is at its maximum in the morning, as evidenced by the characteristic
erect position of the petiole. Finally, the plant in the course of day
and night is subjected to a great variation of temperature. I will
now describe the effects of these various factors on excitability. It
should be mentioned here that the experiments were carried out about
the middle of the day, when the excitability, generally speaking, is
found to remain constant.


EFFECTS OF LIGHT AND DARKNESS.

[Illustration: FIG. 16. Effect of cloud. Dotted up-curve indicates
responsive fall, and continuous down-line exhibits slow recovery. First
four responses normal; next three show depression due to diminution
of light brought on by cloud, the duration of which is indicated
by horizontal line below. Last three records show restoration of
excitability brought on by clearing of sky. All records read from left
to right.]

I have frequently noticed that a depression of excitability occurred
when the sky was darkened by passing clouds. This is clearly seen
in the above records obtained with the Resonant Recorder. Uniform
sub-maximal stimuli had been applied to a specimen of _Mimosa_ at
intervals of fifteen minutes. The dotted up-line represents the
responsive fall, and the continuous down-line, the slow recovery. The
first four are the normal uniform responses (Fig. 16). The next three
show the depressing effect of relative darkness due to cloudy weather.
The sky cleared after forty-five minutes, and we notice the consequent
restoration of normal excitability.

_Effect of sudden darkness and its continuation. Experiment 14._--In
the next record (Fig. 17) is shown the immediate and continued action
of darkness. The first two are the normal uniform responses in light.
By means of screens, the plant was next subjected to sudden darkness;
this brought about a marked depression of excitability. Subjection to
sudden darkness thus acts as a stimulus inducing a marked but transient
fall of excitability. Under the continuous action of darkness, however,
the excitability is at first restored and then undergoes a persistent
depression.

[Illustration: FIG. 17. Effect of sudden darkness. Plant subjected to
sudden darkness beyond horizontal line seen below. First two responses
normal. Note sudden depression of excitability, revival and final
depression under continued darkness.]

_Effect of transition from darkness to light: Experiment 15._--Here
we have to deal first with the immediate effect of sudden transition,
and then with the persistent effect of continuous light. In the record
given in Fig. 18 the plant had been kept in the dark and the responses
taken in the usual manner. It was then subjected to light; the sudden
change from darkness to light acted as a stimulus, inducing a transient
depression of excitability. In this connection it is interesting to
note that Godlewski found that in the phenomenon of growth, transition
from darkness to light acted as a stimulus, causing a transient
decrease in the normal rate. The effect of continued light on _Mimosa_
is an enhancement of excitability.

[Illustration: FIG. 18. Effect of change from darkness to light. The
first three records are normal under darkness. Horizontal line below
indicates exposure to light. Note preliminary depression followed by
enhancement of excitability.]


EFFECT OF EXCESSIVE TURGOR.

I have often found that the moto-excitability is depressed under
excessive turgor. Thus the “over-turgid” leaf of _Biophytum sensitivum_
does not exhibit any mechanical response on rainy days.

[Illustration: FIG. 19. Effect of enhanced turgor, artificially
induced. First two responses normal. Application of water, at arrow,
induces depression of moto-excitability.]

_Experiment 16._--The effect of excessive turgor on moto-excitability
may be demonstrated in the case of _Mimosa_ by allowing its main
pulvinus to absorb water. The result is seen in the above record
(Fig. 19), where water was applied on the pulvinus after the second
response. It is seen how a depression of moto-excitability results from
excessive turgor brought on by absorption of water. In such cases,
however, the plant is found to accommodate itself to the abnormal
condition and gradually regain its normal excitability in the course of
one or two hours.


INFLUENCE OF TEMPERATURE.

The moto-excitability of the pulvinus of _Mimosa_ is greatly
modified under the influence of temperature. For the purpose
of this investigation I enclosed the plant in a glass chamber,
raising the temperature to the desired degree by means of electric
heating. Responses to identical stimuli were then taken at different
temperatures. It was found that the effect of heightened temperature,
up to an optimum, was to enhance the amplitude of response. Thus with
a given specimen it was found that while at 22°C. the amplitude of
response was 2.5 mm., it became 22 mm. at 27°C., and 52 mm. at 32°C.
The excitability is enhanced under rising, and depressed under falling
temperature. The moto-excitability of _Mimosa_ is practically abolished
at the minimum temperature of about 19°C.

[Illustration: FIG. 20. Effect of moderate cooling during a period
shown by horizontal line below. Moderate depression followed by quick
restoration.]

_Effect of lowering of temperature: Experiment 17._--A simple way of
exhibiting the effect of lowering of temperature is by artificial
cooling of the pulvinus. This cannot very well be done by application
of a stream of cooled water, because, as we have seen, absorption of
water by the pulvinus is attended by a loss of excitability: diluted
glycerine has, however, no such drawback. This fluid at ordinary
temperature was first applied on the pulvinus, and after an interval of
half an hour records were taken in the usual manner. Cooled glycerine
was then applied and the record taken once more; the results are seen
in Figs. 20 and 21. In the former, the first response was normal at
the temperature of the room, which was 32°C.; the next two exhibit
depression of excitability under moderate cooling; the duration of
application of moderately cooled glycerine is there indicated by the
horizontal line below. On the cessation of application, the normal
temperature was quickly restored, with the restoration of normal
excitability.

In the next record (Fig. 21) is shown the effect of a more intense
cold. It will be noticed that the first effect was a depression, and
subsequently, a complete abolition of excitability. Thick dots in the
record represent applications of stimulus which proved ineffective. It
will also be noticed that even on the cessation of cooling, and the
return of the tissue to normal temperature the induced abolition of
excitability persisted as an after-effect for a considerable time. I
have likewise found that the after-effect of cold in abolishing the
conduction of excitation is also very persistent. These experiments
show that owing to physiological inertia, the variations of
excitability in the plant often lag considerably behind the external
changes which induce them.

[Illustration: FIG. 21. Effect of application of more intense cold.
Note sudden depression followed by abolition of excitability, also
persistent after-effect.]

_Effect of high temperature: Experiment 18_.--It has been shown that
the moto-excitability is enhanced by rising temperature; there is,
however, an optimum temperature above which the excitability undergoes
a depression. This is seen in the following record (Fig. 22), where the
normal response at 32°C. was depressed on raising the temperature to
42°C.; the excitability was, however, gradually restored when the plant
was allowed to regain the former temperature.

[Illustration: FIG. 22. Effect of temperature above optimum. Note
depression of excitability induced by high temperature, and gradual
restoration on return to normal.]

I may now briefly recapitulate some of the important results: darkness
depresses and light exalts the moto-excitability. Excessive turgor
depresses motility. Still more marked is the effect of temperature.
Lowering of temperature depresses and finally abolishes the
moto-excitability: rise of temperature enhances it up to an optimum
temperature, but beyond this point the excitability undergoes
depression. _The change in excitability induced by the variation of
external condition is not immediate; the induced effect, generally
speaking, lags behind the inducing cause._


DIURNAL VARIATION OF EXCITABILITY.

I will now give automatic records of responses taken once every hour
for twenty-four hours. They prove conclusively the diurnal variation
of excitability in _Mimosa_. After studying in detail the variations
characteristic of particular times of the day, I will endeavour to
correlate them with the effects brought on by the periodic changes of
the environment.

_Experiment 19._--As a typical example I will first give a record
obtained in the month of February, that is, say, in spring. From this
it will not be difficult to follow the variations which take place
earlier in winter or later in summer.

[Illustration: FIG. 23. Record for twenty-four hours, exhibiting
diurnal variation of excitability (spring specimen). The displacement
of base-line is due to nyctitropic movement.]

The record given in Fig. 23 was commenced at 5 P.M. and continued to
the same hour next day. The first thing noticeable is the periodic
displacement of the base-line. This is due to the nyctitropic movements
of the leaf. It should be remembered that the up-movement of the leaf
is represented by down-curve, and _vice versâ_. After the maximum fall
of the leaf, which in this case was attained at 9 P.M., there followed
a reverse movement: the highest erection, indicative of maximum turgor,
was reached at 6 A.M. The leaf then fell slowly and reached a middle
position at noon. The extent of the nyctitropic movement varies in
individual cases; in some it is slight, in others very large. The
erectile movement began, as stated before, at about 9 P.M.; in some
cases, however, it may occur as early as 6 P.M.

In following the characteristic variations of response occurring
throughout the day, we find that while they are practically uniform
between the hours of 5 and 6 P.M., a continuous decline is manifested
after setting in of darkness (7 P.M.); the fall of excitability
continues even after sunrise (6-30 A.M.), response being practically
abolished at 8 A.M. The excitability is then gradually restored in
a staircase manner, the maximum being reached after 12 noon. After
attaining this, the excitability remains more or less constant till the
evening. It will be noticed that the amplitude of response at 5 P.M.
on the second day was the same as the corresponding response on the
previous day.

The results of this and numerous other records taken in spring may be
summarized as:--

  1. The maximum excitability of _Mimosa_ is attained between 1 and
       3 P.M., and remains constant for several hours. In connection
       with the constancy of response at this period, it should be
       remembered that when the response is at its maximum a slight
       increase of excitability cannot further enhance the amplitude of
       response.

  2. The excitability, generally speaking, undergoes a continuous
       decline from evening to morning, the response being practically
       abolished at or about 8 A.M.

  3. From 8 A.M. to 12 noon, the excitability is gradually enhanced
       in a staircase manner, till the maximum excitability is reached
       after 1 P.M.

I have obtained numerous records in support of these conclusions,
some of which are reproduced in the following figures. In these cases
responses to uniform stimuli at intervals of half an hour were taken at
different parts of the day, the recorder employed being of the Resonant
type.

_Mid-day record: Experiment 20._--The record of daily periodicity
previously given shows that the excitability reaches its maximum after
12 noon, and that it remains constant at the maximum value for several
hours. This fact is fully borne out in the following record obtained
with a different specimen (Fig. 24). The responses were taken here from
noon to 3 P.M., once every half-hour.

[Illustration: FIG. 24. Mid-day record from noon to 3 P.M. exhibiting
uniform excitability. Responses taken once every half-hour.]

_Evening record: Experiment 21._--The record given in Fig. 23 shows
that the amplitude of response falls continuously after 6 P.M. It
might be thought that the diminished amplitude in the first part may
be due to the natural nyctitropic fall of the leaf. The range of the
pulvinar movement being limited, it is clear that the extent of the
responsive fall must become smaller on account of the natural fall of
the leaf during the first part of the night. That this is not the whole
explanation of the decline of response in the evening will be clear
from certain facts which I will presently adduce. It was stated that
the leaf of _Mimosa_ exhibits nyctitropic fall from 6 to 9 P.M., after
which there is a reverse movement of erection. In certain specimens,
however, the erectile movement commenced as early as 6 P.M. It is
obvious that in these latter cases diminution of amplitude of response
cannot be due to the reduction of the range of movement of the leaf.
In Fig. 25 is given a series of records from 6 to 10 P.M. obtained
with a leaf in which erectile movement had commenced early in the
evening. Though the full range of responsive movement was in this case
available, yet the amplitude of successive responses is seen to undergo
continuous diminution.

[Illustration: FIG. 25. Evening record from 6 to 10 P.M., showing
gradual depression of excitability.]

_Record in the morning: Experiment 22._--The excitability is, as we
have seen, nearly abolished about 8 A.M., after which there is a
gradual restoration. This gradual enhancement of excitability to a
maximum in the course of the forenoon is seen well illustrated in the
record below (Fig. 26).

[Illustration: FIG. 26. Morning record from 8 A.M. to 12 noon,
exhibiting gradual enhancement of excitability.]

The record of daily periodicity given in Fig. 23 may be regarded as
a typical example. Modifications may, however, be observed which are
traceable to individual peculiarities. As an example of this, I give
a record (Fig. 27) obtained with a specimen in which nyctitropic
movement was very pronounced. The periodic variation of excitability
exhibited here is practically the same as shown by other specimens.
The interesting variation is in the character of the recovery from
stimulus; the leaf was falling from 6 to 9 P.M.; owing to the shifting
of the base-line upwards the recovery appears to be incomplete. After 9
P.M. the leaf was erected, at first slowly, then at a very rapid rate.
The consequent fall of the base-line late at night is very abrupt;
hence there is an apparent over-shooting in the line of recovery.

[Illustration: FIG. 27. Record of diurnal variation of excitability; it
exhibits marked nyctitropic movement.]


EFFECT OF TEMPERATURE ON VARIATION OF EXCITABILITY.

So far I have merely described the observed diurnal variation of
excitability. We may next inquire whether there is any causal relation
between the change of external conditions and the observed variation of
excitability. It has been shown that the moto-excitability is greatly
influenced by temperature. In order to find in what manner the diurnal
variation of excitability was influenced by the daily variation of
temperature, I took special care to secure by means of the thermograph
a continuous record of temperature variations. The table which follows
shows the relation between the hours of the day, temperature, and
amplitude of response, in a typical case of diurnal variation of
excitability.

TABLE II.--SHOWING THE RELATION BETWEEN HOUR OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SPRING SPECIMEN.)

  +--------------------------------+--------------------------------+
  | _Hours  _Tempera-  _Amplitude  | _Hours  _Tempera-  _Amplitude  |
  |   of      ture._       of      |   of      ture._       of      |
  |  day._              Response._ |  day._              Response._ |
  +--------------------------------+--------------------------------+
  | 5 p.m.   28°   C.    28   mm.  | 5 a.m.   20°   C.     5   mm.  |
  | 6  "     25.5° "     28   "    | 6  "     20.5° "      4.2 "    |
  | 7  "     24.5° "     27   "    | 7  "     21°   "      3.5 "    |
  | 8  "     23°   "     23.5 "    | 8  "     22°   "      2.5 "    |
  | 9  "     22°   "     21.5 "    | 9  "     24°   "      0   "    |
  |10  "     21°   "     18   "    |10  "     26°   "      6   "    |
  |11  "     20.5° "     15   "    |11  "     26.5° "     15.5 "    |
  |12  "     20°   "     13   "    |12  "     28°   "     22.5 "    |
  | 1 a.m.   20°   "     10   "    | 1 p.m.   28°   "     26   "    |
  | 2  "     20°   "      8   "    | 2  "     28.5° "     28   "    |
  | 3  "     20°   "      7.5 "    | 3  "     28.5° "     28   "    |
  | 4  "     19.5° "      6   "    | 4  "     29°   "     28   "    |
  +--------------------------------+--------------------------------+

From the data given in the table, two curves have been obtained. One of
these shows the relation between the hours of the day and temperature;
the other exhibits the relation between the hours of the day and the
excitability as gauged by the amplitude of response (Fig. 28). It
will be seen that there is, broadly speaking, a marked resemblance
between the two curves, which demonstrate the predominant influence of
temperature on diurnal variation of excitability.

[Illustration: FIG. 28. The continuous curve shows the relation between
the hour of the day and temperature. The dotted curve exhibits relation
between the hour of the day and excitability.]


EFFECT OF PHYSIOLOGICAL INERTIA.

It has been shown (page 59) that owing to physiological inertia, the
change of excitability, generally speaking, lags behind the inducing
cause. This fact finds striking illustration in the lag exhibited by
the curve of excitability in reference to the temperature curve. The
minimum temperature was attained at about 4 A.M., but the excitability
was not reduced to a minimum till four hours later and again there is a
marked fall of temperature after 5 P.M., but the excitability did not
become depressed till two hours later.

There is again the factor of variation of light, the effect of which is
not so great as that of temperature. The periods of maximum of light
and temperature are, however, not coincident.

We may now discuss in greater detail the diurnal variation of
excitability in _Mimosa_, taking the typical case, the record of which
is given in Fig. 23. The temperature here is seen to remain almost
constant, and at an optimum, from 1 to 5 P.M., the condition of light
is also favourable. Hence the excitability is found to be constant,
and at its maximum between these hours. The temperature begins to fall
after 6 P.M., and there is, in addition, the depressing action of
gathering darkness. Owing to the time-lag, the fall of excitability
does not commence immediately at 6 P.M., but an hour afterwards,
and continues till the next morning. During this period we have the
cumulative effect of twelve hours’ darkness and the increasing
depression due to cold, the temperature minimum occurring at 4 A.M.
On account of the combined effects of these various factors, and
phenomenon of lag, the period of minimum excitability is in general
reached about 8 A.M. In certain other cases this may occur earlier.
After the attainment of this minimum, the excitability is gradually
and continuously increased, under the action of light and of rising
temperature, till the maximum is reached in the afternoon.


EFFECT OF SEASON.

It was said that temperature exerted a predominant influence in
inducing variation of excitability. We may, therefore, expect that
the diurnal period would be modified in a certain way according to
the season. In winter the night temperature falls very low; hence the
depression of excitability is correspondingly great, and results in
the complete abolition of excitability. The after-effect of intense
cold is seen in the condition of inexcitability persisting for a very
long period in the morning. In summer the prevailing high temperature
modifies the diurnal periodicity in a different manner. When the night
is warm, the fall of excitability is slight. In the day, on the other
hand, the temperature may rise above the optimum, bringing about a
depression. In such a case the excitability in the earlier part of the
evening may actually be greater than in the middle of the day. These
modifications are shown in a very interesting way in the following
record (Fig. 29) taken at the end of April. The temperature of Calcutta
at this season often rises above 100°F. or 38°C. Table III also
exhibits, in the case of the summer specimen, the relation between the
hours of the day, temperature, and excitability.

[Illustration: FIG. 29. Diurnal variation of excitability exhibited by
summer specimen.]

An inspection of the record given in Fig. 29 shows that the amplitude
of response was enhanced after 4 P.M. The temperature up to that time
was unusually high (38°C.), and there was in consequence a depression
of excitability. After that hour there was a mitigation of heat, the
temperature returning towards the optimum. Hence we find that the
maximum excitability was attained between the hours 4 and 6 P.M. The
minimum temperature at night was higher in the present case than that
of the experiment carried out in February; in the former the minimum
was 25.5°C., while in the latter it was 19.5°C. On account of this
difference the night record in summer shows a fall of excitability
which is far more gradual than that obtained in spring. The
excitability is here not totally abolished in the morning, but reaches
a minimum after 8 A.M.; the sensitiveness is then gradually enhanced
in a staircase manner.

TABLE III--SHOWING THE RELATION BETWEEN HOURS OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SUMMER SPECIMEN.)

  +--------------------------------+--------------------------------+
  | _Hours  _Tempera-  _Amplitude  | _Hours  _Tempera-  _Amplitude  |
  |   of      ture._       of      |   of      ture._       of      |
  |  day._              Response._ |  day._              Response._ |
  +--------------------------------+--------------------------------+
  | 1 p.m.   38°   C.    22   mm   | 1 a.m.   26°   C.    21.5 mm.  |
  | 2  "     38°   "     23   "    | 2  "     26°   "     20   "    |
  | 3  "     38°   "     24.5 "    | 3  "     25.5° "     18.5 "    |
  | 4  "     37°   "     28   "    | 4  "     25.5° "     17   "    |
  | 5  "     35.5° "     29   "    | 5  "     25.5° "     16   "    |
  | 6  "     33°   "     27   "    | 6  "     26°   "     15   "    |
  | 7  "     31°   "     26   "    | 7  "     27°   "     14   "    |
  | 8  "     30°   "     26   "    | 8  "     29°   "     13   "    |
  | 9  "     29°   "     25   "    | 9  "     30.5° "     11   "    |
  |10  "     27°   "     24.5 "    |10  "     33°   "     16   "    |
  |11  "     27°   "     24   "    |11  "     35°   "     17   "    |
  |12  "     26.5° "     22.5 "    |12  "     37°   "     21   "    |
  +--------------------------------+--------------------------------+


SUMMARY.

The moto-excitability of _Mimosa_ was gauged every hour of the day
and night, by the amplitude of the response to a testing stimulus.
This is effected by means of automatic devices which excite the plant
periodically by an absolutely constant stimulus, and record the
corresponding mechanical response.

From the record thus obtained, it was found that the excitability
of the plant is not the same throughout the day, but undergoes a
variation characteristically different at different times of the day.
In a typical case in spring the excitability attained its maximum
value after 1 P.M. and remained constant for several hours. There was
then a continuous fall of excitability, the minimum being reached at
about eight in the morning. The plant at this time was practically
insensitive. The moto-excitability was then gradually enhanced in a
staircase manner till it again reached a maximum next afternoon.

The effect of sudden darkness was found to induce a transient
depression, followed by revival of excitability. The effect of
persistent darkness was to induce a depression.

Exposure to light from darkness caused a transient depression, followed
by an enhancement of excitability.

Excessive turgor induced a diminished response.

Lowering of temperature induced a depression of excitability,
culminating in an abolition of response. The after-effect of excessive
cold was a prolonged depression of excitability.

Excitability was enhanced by rising temperature up to an optimum; above
this point a depression was induced.

Owing to physiological inertia the change of excitability induced by
variation of external condition lags behind the inducing cause.

The diurnal variation of excitability is primarily due to diurnal
variation of temperature. The effect is modified in a minor degree by
variation of light.




V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA PUDICA_

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHANDRA DAS, M.A.


The most suitable plant for researches on irritability of plants is
_Mimosa pudica_, which can be obtained in all parts of the world.
An impression unfortunately prevails that the excitatory reaction
of the plant can be obtained only in summer and under favourable
circumstances; this has militated against its extensive use in
physiological experiments, but the misgiving is without any foundation;
for I found no difficulty in demonstrating even the most delicate
experiments on _Mimosa_ before the meeting of the American Association
for the Advancement of Science held during Christmas of 1914. The
prevailing outside temperature at the time was considerably below
the freezing point. With foresight and care it should not be at all
difficult to maintain in a hot-house a large number of these plants in
a sensitive condition all the year round.

In order to remove the drawback connected with the supply of sufficient
material, I commenced an investigation to find whether a detached leaf
preparation could be made as effective for the study of irritability
as the whole plant. Here we have at the central end of the leaf the
pulvinus, which acts as the contractile organ; the conducting strand
in the interior of the petiole, on the other hand, is the vehicle for
transmission of excitation. The problem to be solved is the rendering
of an isolated petiole-and-pulvinus of _Mimosa_ as efficient for
researches on irritability as the nerve-and-muscle preparation of a
frog. On the success of this attempt depended the practical opening
out of an extended field of physiological investigation which would be
unhampered by any scarcity of experimental material.

In connection with this it is well to note the surprising difference in
vegetative growth as exhibited by plants grown in soil and in pots. A
pot-specimen of _Mimosa_ produces relatively few leaves, but one grown
in the open ground is extremely luxuriant. As an instance in point,
I may state that for the last five months I have taken from a plant
grown in a field about 20 leaves a day for experiment, without making
any impression on it. A large box containing soil would be practically
as good as the open ground, and the slower rate of growth in a colder
climate could be easily made up by planting half a dozen specimens. The
protection of the plants from inclemencies of weather can be ensured by
means of a glass cover with simple heat-regulation by electric lamps,
in place of an expensive green-house.

Returning to the question of the employment of an isolated leaf, which
I shall designate as a petiole-pulvinus preparation, instead of the
entire plant, the first attempts which I made proved unsuccessful. The
cut leaf kept in water would sometimes exhibit very feeble response, at
other times all signs of excitability appeared to be totally abolished.
It was impossible to attempt an investigation on the effect of changing
environment on excitability when the normal sensitiveness itself
underwent so capricious a change

These difficulties were ultimately overcome from knowledge derived
through systematic investigation on the relative importance of
the different parts of the motor apparatus, on the immediate and
after-effect of section on the excitability of the leaf, and on the
rate of decay of this excitability on isolation from the plant. The
experience thus gained enabled me to secure long-continued and uniform
sensibility under normal conditions. It was thus possible to study the
physiological effects of changing external conditions by observing the
responsive variation in the isolated petiole-pulvinus preparation. I
propose to deal with the different aspects of the investigation in the
following order:--

  1. The effect of wound or section in modification of normal
       excitability.

  2. The change of excitability after immersion in water.

  3. Quantitative determination of the rate of decay of excitability
       in an isolated preparation.

  4. Effect of amputation of the upper half of pulvinus.

  5. Effect of removal of the lower half.

  6. Influence of the weight of leaf on rapidity of responsive fall.

  7. The action of chemical agents.

  8. Effect of “fatigue” on response.

  9. The influence of constant electric current on recovery.

  10. The action of light and darkness on excitability.

The isolated petiole-pulvinus preparation is made by cutting out a
portion of the stem bearing a single lateral leaf. The four diverging
sub-petioles may also be cut off. In order to prevent rapid drying
the specimen has to be kept in water. Preparations made in this way
often appeared to have lost their sensibility. I was, however, able to
trace this loss to two different factors: first, to the physiological
depression due to injury caused by section, and, second, to the sudden
increase of turgor brought on by excessive absorption of water. I shall
now proceed to show that the loss of sensibility is not permanent, but
is capable of restoration.


EFFECT OF WOUND OR SECTION IN MODIFICATION OF NORMAL EXCITABILITY.

In connection with the question of effect of injury, it is to be borne
in mind that after each excitation the plant becomes temporarily
irresponsive and that the excitability is fully restored after the
completion of protoplasmic recovery. A cut or a section acts as a very
intense stimulus, from the effect of which the recovery is very slow.
If the stem be cut very near the leaf, the excitation of the pulvinus
is very intense, and the consequent loss of excitability becomes more
or less persistent. But if the stem be cut at a greater distance, the
transmitted excitation is less intense, and the cut specimen recovers
its excitability within a moderate time. I have also succeeded in
reducing the excitatory depression by previously benumbing the tissue
by physiological means. The isolated specimen can be made still more
compact by cutting off the sub-petioles bearing the leaflets; the
preparation now consists of a short length of stem of about 2 cm. and
an equally short length of primary petiole, the motile pulvinus being
at the junction of the two.

[Illustration: FIG. 30--The Resonant Recorder, with petiole-pulvinus
preparation. (From a photograph.)]

For the restoration of sensitiveness, and to meet working conditions,
the lower end of the cut stem is mounted on a T-tube, with
funnel-attachment and exit-tube, as shown in Fig. 30. The other two cut
ends--of the stem and of the petiole--may be covered with moist cloth
or may be closed with collodion flexile to prevent rapid evaporation
and drying up of the specimen. A slight hydrostatic pressure maintains
the specimen in a moderately turgid condition. A preparation thus
made is insensitive at the beginning, but if left undisturbed it
slowly recovers its excitability. The history of the depression of
excitability after shock of preparation and its gradual restoration
is graphically illustrated by a series of records made by the plant
(Fig. 31).

The petiole-pulvinus preparation thus made offers all facilities for
experiment. Owing to its small size it can be easily manipulated; it
can be enclosed in a small chamber and subjected to varying conditions
of temperature and to the action of different vapours and gases. Drugs
are easily absorbed at the cut end, and poison and its antidote can
be successively applied through the funnel without any disturbance
of the continuity of record. In fact, many experiments which would
be impossible with the entire plant are quite practicable with the
isolated leaf.

The arrangement for taking records of response is seen in Fig. 30,
which is reproduced from a photograph of the actual apparatus. For
recording the response and recovery of the leaf under stimulation,
I use my Resonant Recorder fully described in the ‘Philosophical
Transactions’ (1913). The petiole is attached to one arm of the
horizontal lever. The writer, made of fine steel wire with a bent tip,
is at right angles to the lever, and is maintained by electromagnetic
means in a state of to-and-fro vibration, say, ten times in a second.
The record, consisting of a series of dots, is free from errors arising
from friction of continuous contact of the writer with the recording
surface. The successive dots in the record at definite intervals of a
tenth of a second also give the time-relations of the response curve.

On account of its small size, the petiole-pulvinus preparation offers
great facilities for mounting in different ways suitable for special
investigations. Ordinarily, the cut stem with its lower end enclosed
in moist cloth is supported below. A very suitable form of stimulus is
that of induction shock from a secondary coil, the intensity of which
is capable of variation in the usual manner by adjusting the distance
between the primary and the secondary coils. The motile pulvinus, P,
may be excited directly. For investigations on velocity of transmission
of excitation, stimulus is applied on the petiole at some distance
from the pulvinus, by means of suitable electrodes. Excitation is now
transmitted along the intervening length of petiole, the conducting
power of which will be found appropriately modified under the action
of chemical and other agents. In this normal method of mounting, the
more excitable lower half of the pulvinus is below; excitatory reaction
produces the fall of the petiole, gravity helping the movement. The
preparation may, however, be mounted in the inverted position, with
the more excitable lower half of the pulvinus facing upwards. The
excitatory movement will now be the erection of the petiole, against
gravity.

Under natural conditions the stem is fixed, and it is the petiole which
moves under excitation. But a very interesting case presents itself
when the petiole is fixed and the stem free. Here is presented the
unusual spectacle of the plant or the stem “wagging” in response to
excitation.


THE CHANGE OF EXCITABILITY AFTER IMMERSION IN WATER.

The isolated specimen can be kept alive for several days immersed in
water. The excitability of the pulvinus, however, undergoes great
depression, or even abolition, by the sudden change of turgor brought
on by excessive absorption of water. The plant gradually accommodates
itself to the changed condition, and the excitability is restored in a
staircase manner from zero to a maximum.

In studying the action of a chemical solution on excitability,
the solution may be applied through the cut end or directly on
the pulvinus. The sudden variation of turgor, due to the liquid,
always induces a depression, irrespective of the stimulating or
the depressing action of the drug. The difficulty may be eliminated
by previous long-continued application of water on the pulvinus and
waiting till the attainment of uniform excitability which generally
takes place in the course of about three hours. Subsequent application
of a chemical solution gives rise to characteristic variation in the
response.


QUANTITATIVE DETERMINATION OF THE RATE OF DECAY OF EXCITABILITY IN AN
ISOLATED PREPARATION.

[Illustration: FIG. 31--Variation of excitability after section. (1)
Immediate effect; (2) variation of excitability in a second specimen
during 50 hours: (_a_) response 4 hours after section; (_b_) response
after 24 hours; (_c_) after 49 hours. Up-line of record represents
responsive fall of the leaf, down-line indicates recovery from
excitation.]

_Variation of excitability after section: Experiment 23._--In order
to test the history of the change of excitability resulting from the
immediate and after-effect of section, I took an intact plant and
fixed the upper half of the stem in a clamp. The response of a given
leaf was now taken to the stimulus of an induction shock of 0.1 unit
intensity, the unit chosen being that which causes a bare perception
of shock in a human being. The specimen was vigorous and the response
obtained was found to be a maximum. The stem bearing the leaf was cut
at the moment marked in the record with a cross, and water was applied
at the cut end. The effect of section was to cause the maximum fall of
the leaf, with subsequent recovery. After this, successive responses
to uniform stimuli at intervals of 15 minutes show, in (1) of Fig. 31,
that a depression of excitability has been induced owing to the shock
caused by section. In course of an hour, however, the excitability
had been restored almost to its original value before the section.
This was the case with a vigorous specimen, but with less vigorous
ones a longer period of about three hours is required for restoration.
In certain other cases the response after section exhibits alternate
fatigue; that is to say, one response is large and the next feeble,
and this alternation goes on for a length of time. The isolated
specimen, generally speaking, attains a uniform sensibility after a few
hours, which is maintained, with very slight decline under constant
external conditions, for about 24 hours. On the third day the fall
of excitability is very rapid, and the sensibility declines to zero
in about 50 hours after isolation [Fig. 31 (2)]. We may describe
the whole cycle of change as follows: by the shock of operation the
isolated preparation is rendered insensitive for nearly an hour, the
excitability is then gradually restored almost to its normal value
before operation. Under constant external conditions, this excitability
remains fairly constant for about 24 hours after which depression sets
in. The rate of fall of excitability becomes very rapid 40 hours after
the operation, being finally abolished after the fiftieth hour. It
is probable that in a colder climate the fall of excitability would
be much slower. The most important outcome of this inquiry is the
demonstration of the possibility of obtaining persistent and uniform
sensibility in isolated preparations. On account of this, not only is
the difficulty of supply of material entirely removed but a very high
degree of accuracy secured for the investigation itself.


EFFECT OF AMPUTATION OF UPPER HALF OF PULVINUS.

_Experiment 24._--The determination of the _rôle_ played by different
parts of the pulvinus in response and recovery is of much theoretical
importance. Our knowledge on this subject is unfortunately very
scanty. The generally accepted view is that on excitation “the actual
downward curvature of the pulvinus is partly due to a contraction of
the walls of the motor cells consequent upon the decrease of turgor,
but is accentuated by expansion of the insensitive adaxial half of the
pulvinus--which was strongly compressed in the unstimulated condition
of the organ--and also by the weight of the leaf.”[H] According to
Pfeffer, after excitation of the organ, “the original condition of
turgor is gradually reproduced in the lower half of the pulvinus, which
expands, raising the leaf and producing compression of the upper half
of the pulvinus, which aids in the rapid curvature of the stimulated
pulvinus.”[I]

[H] Haberlandt, ‘Physiological Plant Anatomy,’ 1914, p. 570. English
Translation, Macmillan & Co.

[I] PFEFFER--‘Physiology of Plants,’ vol. 3, pp. 75 and 76. English
Translation, Clarendon Press.

It was held, then, that the rapidity of the fall of leaf under stimulus
is materially aided (1) by the expansion of the upper half of the
pulvinus, which is normally in a state of compression, and (2) by the
weight of the leaf. So much for theory. The experimental evidence
available regarding the relative importance of the upper and lower
halves of the pulvinus is not very conclusive. Lindsay attempted to
decide the question by his amputation experiments. He showed that when
the upper half was removed the leaf carried out the response, but rigor
set in when the lower half was amputated. Pfeffer’s experiments on the
subject, however, contradicted the above results. He found that “after
the upper half of the pulvinus was carefully removed, no movement
was produced by stimulation, whereas when the lower half is absent a
weakened power of movement is retained.” Pfeffer, however, adds, “since
the operation undoubtedly affects the irritability, it is impossible
to determine from such experiments the exact part played by the active
contraction of the lower half of the pulvinus.”[I]

The cause of uncertainty in this investigation is twofold. First,
it arises from the unknown change in irritability consequent on
amputation; and, secondly, from absence of any quantitative standard
by which the effect of selective amputation of the pulvinus may
be measured. As regards the first, I have been able to reduce the
depressing action caused by injury to a minimum by benumbing the tissue
before operation, through local application of cold, and also allowing
the shock-effect to disappear after a rest of several hours. As regards
the physiological gauge of efficiency of the motor mechanism, such
a measure is afforded by the relation between a definite testing
stimulus and the resulting response with its time-relations, which
is secured by my Resonant Recorder with the standardised electrical
stimulator.

[Illustration: FIG. 32--Effect of amputation of upper half of pulvinus.
Upper record gives normal response before amputation, and the lower,
response after amputation. (Successive dots at intervals of 0.1 sec).
Apex-time 11 sec, in both.]

In carrying out this investigation I first took the record of normal
response of an intact leaf on a fast moving plate. A second record,
with the same stimulus, was taken after the removal of the upper half
of the pulvinus, having taken the necessary precautions that have been
described. Comparison of the two records (Fig. 32) shows that the only
difference between them is in the exhibition of slight diminution of
excitability due to operation. But, as regards the latent period and
the quickness of attaining maximum fall, there is no difference between
the two records before and after the amputation of the upper half. The
upper part of the pulvinus is thus seen practically to have little
influence in hastening the fall.


EFFECT OF REMOVAL OF THE LOWER HALF.

_Experiment 25._--The shock-effect caused by the amputation of the
lower half was found to be very great, and it required a long period of
rest before the upper half regained its excitability. The excitatory
reaction of the upper half is by contraction, and the response is,
therefore, the lifting of the petiole. Thus, in an intact specimen,
excitation causes antagonistic reactions of the two halves. But the
sensibility of the upper half is very feeble and the rate of its
contractile movement, relatively speaking, very slow. The record of
the response of the upper half of the pulvinus, seen in Fig. 33, was
taken with an Oscillating Recorder, where the successive dots are at
intervals of 1 sec.: the magnification employed was about five times
greater than in recording the response of the lower half (Fig. 32). The
intensity of stimulus to evoke response had also to be considerably
increased. Taking into account the factors of magnification and the
intensity of stimulus for effective response, the lower half I find to
be about 80 times more sensitive than the upper. Thus, under feeble
stimulus the upper half exerts practically no antagonistic reaction.
The excitatory response of the upper half is also seen to be very
sluggish.

[Illustration: FIG. 33.--Response after amputation of lower half of
pulvinus. (Successive dots at intervals of a second; vertical lines
mark minutes.) Apex-time, 40 secs.]


INFLUENCE OF THE WEIGHT OF LEAF ON RAPIDITY OF RESPONSIVE FALL.

_Experiment 26._--It is obvious that the mechanical moment exerted by
the weight of the leaf must help its responsive fall under excitation.
But the relative importance of the factors of active contraction
of the lower half of the pulvinus and of the weight, in the rate
of the responsive down-movement, still remains to be determined. A
satisfactory way of solving the problem would lie in the study of
the characteristics of response-records taken under three different
conditions: (1) When the leaf is helped in its fall by its weight; (2)
when the action of the weight is eliminated; and (3) when the fall
has to be executed against an equivalent weight. An approximation to
these conditions was made in the following manner. We may regard the
mechanical moment to be principally due to the weight of the four
sub-petioles applied at the end of the main petiole. In a given case
these sub-petioles were cut off, and their weight found to be 0.5 grm.
The main petiole was now attached to the right arm of the lever, and
three successive records were taken: (1) With no weight attached to
the petiole; (2) with 0.5 grm. attached to its end; and (3) with 0.5
grm. attached to left arm of the lever at an equal distance from the
fulcrum. In the first case, the fall due to the excitatory contraction
will practically have little weight to help it; in the second case,
it will be helped by a weight equivalent to those of the sub-petioles
with their attached leaflets; and in the third case, the fall will be
opposed by an equivalent weight. We find that in these three cases
there is very little difference in the time taken by the leaf to
complete the fall (Fig. 34).

[Illustration: FIG. 34.--Effect of weight on rapidity of fall. N,
without action of weight; W, with weight helping; and A, with weight
opposing.]

It has been shown that the presence or absence of the upper half of
the pulvinus makes practically no difference in the period of fall;
it is now seen that the weight exerts comparatively little effect. We
are thus led to conclude that in determining the rapidity of fall,
the factors of expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the active force of
contraction exerted by the lower half of the pulvinus.


ACTION OF CHEMICAL AGENTS.

In connection with this subject it need hardly be said that the various
experiments which I had previously carried out with the intact plant
can also be repeated with the isolated preparation. I will only give
here accounts of experiments which are entirely new.

The chemical solution may be applied directly to the pulvinus, or it
may be absorbed through the cut end, the absorption being hastened
by hydrostatic pressure. The normal record is taken after observing
precautions which have already been mentioned. The reaction of a
given chemical agent is demonstrated by the changed character of the
record. The effect of the drug is found to depend not merely on its
chemical nature, but also on the dose. There is another very important
factor--that of the tonic condition of the tissue--which is found to
modify the result. The influence of this will be realised from the
account of an experiment to be given presently, where an identical
agent is shown to produce diametrically opposite effects on two
specimens, one of which was in a normal, and the other in a sub-tonic,
condition. The experiments described below relate to reactions of
specimens in a normal condition.

[Illustration: FIG. 35.--Stimulating action of hydrogen peroxide.]

_Hydrogen Peroxide: Experiment 27._--This reagent in dilute solution
exerts a stimulating action. Normal records, were taken after
long-continued application of water on the pulvinus. The peroxide, as
supplied by Messrs. Parke Davis & Co., was diluted to 1 per cent., and
applied to the pulvinus; this gave rise to an enhancement of response.
Re-application of water reduced the amplitude to the old normal value
(Fig. 35).

[Illustration: FIG. 36.--Incomplete recovery under the action BaCl_{2}
and transient restoration under tetanisation at T.]

_Barium Chloride: Experiment 28._--The action of this agent is
very characteristic, inducing great sluggishness in recovery. The
preparation had been kept in 1-per cent. solution of this substance for
two hours. After this the first response to a given test-stimulus was
taken; the response was only moderate, and the recovery incomplete. The
sluggishness was so great that the next stimulation, represented by a
thick dot (Fig. 36), was ineffective. Tetanising electric shock at T,
not only brought about response, but removed for the time being the
induced sluggishness. This is seen in the next two records, which were
taken under the old test-stimulus. There is now an enhanced response
and a complete recovery. Beneficial effect of tetanisation disappeared,
however, on the cessation of stimulus. This is seen in the next two
records which were taken after two hours. The amplitude of response was
not only diminished, but the recovery also was incomplete.

[Illustration: FIG. 37.--Antagonistic action of alkali and acid. Arrest
of response in contraction under NaOH (↑), restoration and final arrest
in expansion under lactic acid. (↑)]

_Antagonistic actions of Alkali and Acid: Experiment 29._--Alkali and
acid are known to exert antagonistic actions on the spontaneous beat
of the heart; dilute solution of NaOH arrests the beat of the heart
in systolic contraction, while dilute lactic acid arrests the beat in
diastolic expansion. I have found identical antagonistic reactions
in the pulsating tissue of _Desmodium gyrans_, the telegraph plant.
It is very interesting to find that these agents also exert their
characteristic effects on the response of _Mimosa_ in a manner which
is precisely the same. This is seen illustrated in Fig. 37, where the
application of NaOH arrested the response in a contracted state; after
this, the antagonistic effect of dilute lactic acid is seen first, in
its power of restoring the excitability; its continued application,
however, causes a second arrest, but this time in a state of relaxed
expansion.

_CuSO_4 Solution._--This agent acts as a poison, causing a gradual
diminution of amplitude of response, culminating in actual arrest at
death. Certain poisons, again, exhibit another striking symptom at the
moment of death, an account of which will be given in a separate paper.


EFFECT OF “FATIGUE” ON RESPONSE.

[Illustration: FIG. 38.--“Fatigue” induced by shortening intervening
period of rest.]

With _Mimosa_, after each excitation the recovery becomes complete
after a resting period of about 15 min. With this interval of rest the
successive responses for a given stimulus are equal, and are at their
maximum.

_Experiment 30._--When the resting interval is diminished the recovery
becomes incomplete, and there is a consequent diminution of amplitude
of response. There is thus an increased fatigue with diminished period
of rest. This is illustrated in Fig. 38, where the first two responses
are at intervals of 15 min.; the resting interval was then reduced to
10 min., the response undergoing a marked diminution. Conversely, by
increasing the resting interval, first to 12 and then to 15 min., the
extent of fatigue was reduced and then abolished.


THE INFLUENCE OF CONSTANT ELECTRIC CURRENT ON RECOVERY.

[Illustration: FIG. 39.--Action of constant current in removal of
fatigue by hastening recovery; N, curve of response in fatigued
specimen; C, after passage of current.]

_Experiment 31._--From the above experiment it would appear that
since the incompleteness of recovery induces fatigue, hastening of
recovery would remove it. With this idea I tried various methods for
quickening the recovery of the excited leaf. The application of a
constant electric current was found to have the desired effect. Two
electrodes for introduction of current were applied, one on the stem
and the other on the petiole, at some distance from the pulvinus. In
order to avoid the excitatory effect of sudden application, the applied
current should be increased gradually; this was secured by means of a
potentiometer slide. In my experiment a current having an intensity of
1.4 micro-ampère was found to be effective. Responses at intervals of
10 min., as we have seen, exhibit marked fatigue. Two responses were
recorded on a fast-moving plate, N before, and C after, the application
of the current. It will be seen (Fig. 39) how the application of
current has, by hastening the recovery, enhanced the amplitude of
response and brought about a diminution of fatigue. In connection with
this, I may state that the tonic condition is, in general, improved as
an after-effect of the passage of current. This is seen in some cases
by a slight increase in excitability; in others, where the responses
had been irregular, the previous passage of a current tends to make the
responses more uniform.


ACTION OF LIGHT AND DARKNESS ON EXCITABILITY.

In taking continuous records of responses I was struck by the marked
change of excitability exhibited by the intact plant under variation
of light. Thus the appearance of a cloud was quickly followed by
an induced depression, and its disappearance by an equally quick
restoration of excitability. This may be explained on the theory that
certain explosive chemical compounds are built up by the photosynthetic
processes in green leaves, and that the intensity of response depends
on the presence of these compounds. But the building up of a chemical
compound must necessarily be a slow process, and it is difficult on the
above hypothesis to connect the rapid variation of excitability with
the production of a chemical compound, or its cessation, concomitant
with changes in the incident light.

[Illustration: FIG. 40.--Stimulating action of light, and depressing
action of darkness. Horizontal line below represents period of
darkness.]

_Experiment 32._--In order to find out whether photo-synthesis had
any effect on excitability, I placed an intact plant in a dark
room and obtained from it a long series of responses under uniform
test-stimulus. While this was being done the green leaflets were
alternately subjected to strong light and to darkness, care being taken
that the pulvinus was shaded all the time. The alternate action of
light and darkness on leaflets induced no variation in the uniformity
of response. This shows that the observed variation of excitability
in _Mimosa_ under the alternate action of light and darkness is not
attributable to the photo-synthetic processes.

I next took a petiole-pulvinus preparation from which the sub-petioles
bearing the leaflets had been cut off, and placed it in a room
illuminated by diffused daylight. The normal responses were taken,
the temperature of the room being 30°C. The room was darkened by
pulling down the blinds, and records were continued in darkness. The
temperature of the room remained unchanged at 30°C. It will be seen
from records given in Fig. 40, that in darkness there is a great
depression of excitability. Blinds were next pulled up and the records
now obtained exhibit the normal excitability under light. The sky
had by this time become brighter, and this accounts for the slight
enhancement of excitability. This experiment proves conclusively that
light has a direct stimulating action on the pulvinus, independent of
photo-synthesis.[J]

[J] See also Bose and Das--‘Physiological Investigations with
Petiole-Pulvinus preparations of _Mimosa pudica_.’ Proc. Roy. Soc. B.
Vol. 89, 1916.


SUMMARY.

On isolation of a petiole-pulvinus preparation, the shock of operation
is found to paralyse its sensibility. After suitable mounting the
excitability is restored, and remains practically uniform for nearly
24 hours. After this a depression sets in, the rate of fall of
excitability becomes rapid 40 hours after the operation, sensibility
being finally abolished after the fiftieth hour.

Experiments carried out on the effect of weight, and the influence of
selective amputation of the upper and lower halves of the pulvinus,
show that in determining the rapidity of fall of leaf, the assumed
factors of the expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the force of active
contraction exerted by the lower half of the pulvinus. The excitability
of the lower half is eighty times greater than that of the upper.

Chemical agents induce characteristic changes in excitability.
Hydrogen peroxide acts as a stimulant. Barium chloride renders the
recovery incomplete: but tetanisation temporarily removes the induced
sluggishness. Acids and alkalis induce antagonistic reactions,
abolition of excitability with alkali taking place in a contracted, and
with acid in an expanded condition of the pulvinus.

The responses exhibit fatigue when the period of rest is diminished.
The passage of constant current is found to remove the fatigue.

Response is enhanced on exposure to light, and diminished in darkness.
Light is shown to exert a direct stimulating action on the pulvinus,
independent of photo-synthesis.




VI.--ON CONDUCTION OF EXCITATION IN PLANTS

_By_

SIR J. C. BOSE.


The plant _Mimosa_ offers the best material for investigation on
conduction of excitation. With regard to this question the prevailing
opinion had been that in plants like _Mimosa_, there is merely a
transmission of hydro-mechanical disturbance and no transmission of
true excitation comparable with the animal nerve. I have, however, been
able to show that the transmission in the plant is not a mechanical
phenomenon, but a propagation of excitatory protoplasmic change. This
has been proved by the arrest of conduction by the application of
various physiological blocks. Thus local application of increasing cold
retards, and finally abolishes the conducting power. The conducting
tissue becomes paralysed for a time as an after-effect of application
of cold; the lost conducting power may, however, be quickly restored
by tetanising electric shocks. The conducting power of an animal nerve
is arrested by an electrotonic block, the conductivity being restored
on the cessation of the current. I have succeeded in inducing similar
electrotonic block of conduction in _Mimosa_. Conductivity of a
selective portion of petiole may also be permanently abolished by local
action, of poisonous solution of potassium cyanide.[K]

[K] BOSE--“An Automatic Method for the Investigation of Velocity of
Transmission of Excitation in _Mimosa_.” ‘Phil. Trans.’ ‘B, Vol. 204
(1913) and also “Irritability of Plants.” Longman’s Green & Co. (1913),
p. 132.

Having thus established the physiological character of the transmitted
impulse in plants I shall now proceed to give some of the principal
results of my earlier and recent investigations on the effects of
various agencies on conduction of excitation in plants.

Apart from any question of hydro-mechanical transmission, it is
important to distinguish two different modes of transmission of
excitation. In a motile tissue contraction of a cell causes a physical
deformation and stimulation of the neighbouring cell. Examples of this
are furnished by the cardiac muscle of the animal, the pulvinus of
_Mimosa_, and the stamen of _Berberis_. This mode of propagation may
better be described as a _convection_ of excitation.

The _conduction_ of excitation, as in a nerve, is a different process
of transmission of protoplasmic change. The conducting tissue in this
case does not itself exhibit any visible change of form. In the plant
the necessary condition for transmission of excitation to a distance
is that the conducting tissue should be possessed of protoplasmic
continuity in a greater or less degree. This condition is fulfilled
by vascular bundles. There being greater facility of transmission
along the bundles than across them, the velocity in the longitudinal
direction is very much greater than in the transverse.

For accurate determination of velocity of transmission the testing
stimulus should be quantitative and capable of repetition. Abnormal
high velocity has been observed in _Mimosa_ by applying crude and
drastic methods of stimulation, by a transverse cut or a burn. This
is apt to give rise to a very strong hydro-dynamic disturbance,
which travelling with great speed, delivers a mechanical blow on the
responding pulvinus. Such hydro-dynamic transmission is not the same as
physiological conduction.

In the primary petiole of _Mimosa_ the highest velocity under electric
stimulation I find to be about 30 mm. per second. This velocity is
considerably lower than the velocity in the nerve of higher animals,
but higher than in the lower animals. As an example of the latter,
mention may be made of the velocity of 10 mm. per second in the nerve
of _Anodon_ and 1 mm. per second in the nerve of _Eledone_.


PREFERENTIAL DIRECTION OF CONDUCTION.

_Experiment 33._--The conduction of excitatory impulse takes place
in both directions. This can be demonstrated by taking a petiole of
_Biophytum sensitivum_ or of _Averrhoa carambola_. These petioles are
provided with a series of motile leaflets. Stimulation at the middle
point of the petiole gives rise to two waves of excitation, one of
which travels towards the central axis of the plant, and the other away
from it. The centrifugal velocity is greater than the centripetal as
will be seen from the following results:

  +---------------+---------------------------+--------------------+
  |               | Velocity in               |                    |
  | _Biophytum_   |     centrifugal direction | 2.9  mm per second.|
  |               |  "  centripetal    "      |   2  mm  "    "    |
  | _Averrhoa_    |  "  centrifugal    "      | 0.5  mm  "    "    |
  |               |  "  centripetal    "      | 0.26 mm  "    "    |
  +---------------+---------------------------+--------------------+


EFFECT OF TEMPERATURE.

Variation of temperature has a marked effect on the velocity of
transmission of excitation. Lowering of temperature diminishes the
velocity, culminating in an arrest. Rise of temperature, on the
other hand, enhances the velocity. This enhancement is considerable
in specimens in which the normal velocity is low, but in plants in
optimum condition, the velocity being already high, cannot be further
enhanced. The following tabular statement gives results of effects of
temperature on velocity of transmission in _Mimosa_ and _Biophytum_:--

TABLE IV.--EFFECT OF TEMPERATURE ON VELOCITY OF TRANSMISSION.

  +---------------------------+--------------+--------------------+
  |         Specimen.         | Temperature. |     Velocity.      |
  +---------------------------+--------------+--------------------+
  |_Mimosa_ (winter specimen) |     22°C     | 3.6 mm. per second.|
  |                           |     28°C     | 6.3 mm.  "   "     |
  |                           |     31°C     | 9.0 mm.  "   "     |
  |                           |              |                    |
  |_Biophytum_                |     30°C     | 3.7 mm.  "   "     |
  |                           |     35°C     | 7.4 mm.  "   "     |
  |                           |     37°C     | 9.1 mm.  "   "     |
  +---------------------------+--------------+--------------------+


EFFECT OF SEASON.

The velocity of transmission is very much lower in winter than in
summer. In the petiole of _Mimosa_, the velocity in summer is as high
as 30 mm. per second; in winter it is reduced to about 4 mm. The
lowering of velocity in winter is partly due to the prevailing low
temperature and also to the depressed state of physiological activity.


EFFECT OF AGE.

In a _Mimosa_ plant, different leaves will be found of different age.
Of these the youngest will be at the top. Lower down, we obtain a fully
grown young leaf, and near the base, leaves which are very old. The
investigation deals with the effect of age on the conducting power of
the petiole.

_Comparison of conducting power in different leaves: Experiment
34._--Selecting three leaves from the same plant we apply an identical
electric stimulus at points 2 cm. from the three responding pulvini.
The electric connections are so made that the same tetanising shock
is applied on the three petioles, very young, fully grown, and very
old. The secondary coil is gradually pushed in till the leaves exhibit
responsive fall. The fully grown leaf was the first to respond, the
velocity of transmission being 23 mm. per second. The secondary coil
had to be pushed nearer the primary through 6 cm. before excitation
could be effectively transmitted through the young petiole; for the
oldest leaf still stronger stimulus was necessary, since in this
case the secondary had to be pushed through an additional distance
of 4 cm. for effective transmission of excitation. I also determined
the relative values of the minimal intensity of stimulus, effective
in causing transmission of excitation in the three cases. Adopting as
before the intensity of electric stimulus which causes bare perception
in a human being as the unit, I find that the effective stimulus for a
fully grown young petiole is 0.3 unit, while the very young required
2.5 units, and the very old 5 units. Hence it may be said that the
conducting power of a very young is an eighth, and of the very old
one-sixteenth of the conductivity of the fully grown young specimen.

It will thus be seen that the conducting power of a very young petiole
is feebler than in a fully grown specimen. The conducting tissue, it
is true, is present, but the power of conduction has not become fully
developed. This power is, as we shall see later, conferred by the
stimulus of the environment. In a very old specimen the diminution of
conducting power is due to the general physiological decline.


EFFECT OF DESICCATION ON CONDUCTING TISSUES.

I have already shown that transmission in the plant is a process
fundamentally similar to that taking place in the animal nerve; it
has also been shown that the effects of various physical and chemical
agents are the same in the conducting tissues of plant and of animal.

[Illustration: FIG. 41--Action of glycerine in enhancing the speed and
intensity of transmitted excitation. Stimulus applied at the vertical
line. Successive dots in record are at intervals of 0.1 sec.]

_Effect of application of glycerine: Experiment 35._--It is known that
desiccation, generally speaking, enhances the excitability of the
animal nerve. As glycerine, by absorption of water, causes partial
desiccation, I tried its effect on conduction of excitation in the
petiole of _Mimosa_. Enhancement of conducting power may be exhibited
in two ways: first, by an increase of velocity of transmission; and,
secondly, by an enhancement of the intensity of the transmitted
excitation, which would give rise to a greater amplitude of response
of the motile indicator. In Fig. 41 are given two records, N, before,
and the other after the application of glycerine on a length of petiole
through which excitation was being transmitted. The time-records
demonstrate conclusively the enhanced rate of transmission after the
application of glycerine. The increased intensity of transmitted
excitation is also seen in the enhanced amplitude of response seen in
the more erect curve in the upper record.


INFLUENCE OF TONIC CONDITION ON CONDUCTIVITY.

Different specimens of _Mimosa_ are found to exhibit differences in
physiological vigour. Some are in an optimum condition, others in an
unfavourable or sub-tonic condition. I shall now describe certain
characteristic differences of conductivity exhibited by tissues in
different conditions.

_Effect of intensity of stimulus on velocity of transmission._--In a
specimen at optimum condition, the velocity remains constant under
varying intensities of stimulus. Thus the velocity of transmission in
a specimen was determined under a stimulus intensity of 0.5 unit; the
next determination was made with a stimulus of four times the previous
intensity, _i.e._, 2 units. In both these cases the velocity remained
constant. But when the specimen is in a sub-tonic condition, the
velocity is found to increase with the intensity of the stimulus. Thus
the velocity of conduction of a specimen of _Mimosa_ in a sub-tonic
condition was found to be 5.9 mm. per second under a stimulus of 0.5
unit; with the intensity raised to 2.5 units, the velocity was enhanced
to 8.3 mm. per second.

_After-effect of stimulus._--In experimenting with a particular
specimen of _Mimosa_ I found that on account of its sub-tonic
condition, the conducting power of the petiole was practically absent.
Previous stimulation was, however, found to confer the power of
conduction as an after-effect. It is thus seen that stimulus canalises
a path for conduction.

The effect of excessive stimulus in a specimen in an optimum condition
is to induce a temporary depression of conductivity; the effect of
strong stimulus on a sub-tonic specimen is precisely the opposite,
namely, an enhancement of conductivity. I give below accounts of two
typical experiments carried out with petiole-pulvinus preparation of
_Mimosa_. Excessive stimulation in these cases was caused by injury.

[Illustration: FIG. 42.--Effect of injury, depressing rate of
conduction in normal specimen; (1) record before, and (2) after injury.
(Dot-intervals, 0.1 sec.).]

_Action of Injury on Normal Specimens: Experiment 36._--A cut stem
with entire leaf was taken, and stimulus applied at a distance of
15 mm. from the pulvinus. From the normal record (1) in Fig. 42 the
velocity of transmission was found to be 18.7 mm. per sec. The end of
the petiole beyond the point of application of the testing stimulus was
now cut off, and record of velocity of transmission taken once more.
It will be seen from record (2) that the excessive stimulus caused by
injury had induced a depression in the conducting power, the velocity
being reduced to 10.7 mm. per sec. Excessive stimulation of normal
specimens is thus seen to depress temporarily the conducting power.

_Action of Injury on Sub-tonic Specimens: Experiment 37._--I will now
describe a very interesting experiment which shows how an identical
agent may, on account of difference in the tonic condition of the
tissue, give rise to diametrically opposite effects. In demonstrating
this, I took a specimen in a sub-tonic condition, in which the
conducting power of the tissue was so far below par, that the
test-stimulus applied at a distance of 15 mm. failed to be transmitted
(Fig. 43). The end of the petiole at a distance of 1 cm. beyond the
point of application of test-stimulus was now cut off. The after-effect
of this injury was found to enhance the conducting power so that the
stimulus previously arrested was now effectively transmitted, the
velocity being 25 mm. per sec. This enhanced conducting power began
slowly to decline, and after half an hour the velocity had declined
to 4.1 mm. per sec. The end of the petiole was cut once more, and the
effect of injury was again found to enhance the conducting power, the
velocity of transmission being restored to 25 mm. per sec.

[Illustration: FIG. 43.--Effect of injury in enhancing the conducting
power of a sub-normal specimen; (1) Ineffective transmission becoming
effective at (2) after section; (3) decline after half an hour, and (4)
increased conductivity after a fresh cut.]


SUMMARY.

There are two different types of propagation of excitation: by
convection, and by conduction. In the former the excited cell undergoes
deformation and causes mechanical stimulation of the next; example
of this type is seen in the stamen of _Berberis_. The conduction of
excitation consists, on the other hand, of propagation of excitatory
protoplasmic change. The transmission in the petiole of _Mimosa_ is a
phenomenon of conduction.

This conduction takes place along vascular elements. The conductivity
is very much greater in the longitudinal than in the transverse
direction.

Rise of temperature enhances, and fall of temperature lowers, the
rate of conduction. Excitation is transmitted in both directions; the
centrifugal velocity is greater than the centripetal.

Dessication of conducting tissue by glycerine enhances the conducting
power. Local application of cold depresses or arrests the conduction.
Application of poison permanently abolishes the power of conduction.

Conductivity is modified by the effect of season, being higher in
summer than in winter.

The power of conduction is also modified by age. In young specimens the
conducting power is low, the conductivity is at its maximum in fully
grown organs; but a decline of conductivity sets in with age.

The tonic condition of a tissue has an influence on conductivity. In
an optimum condition, the velocity is the same for feeble or strong
stimulus. Excessive stimulation induces a temporary depression of the
conducting power.

The effects are different in a sub-tonic tissue: velocity of
transmission increases with intensity of stimulus; after-effect of
stimulus is to initiate or enhance the conducting power. The conducting
path is canalised by stimulus.




VII.--ON ELECTRIC CONTROL OF EXCITATORY IMPULSE

_By_

SIR J. C. BOSE.


I have in my previous works[L] described investigations on the
conduction of excitation in _Mimosa pudica_. It was there shown that
the various characteristics of the propagation of excitation in the
conducting tissue of the plant are in every way similar to those in
the animal nerve. Hence it appeared probable that any newly found
phenomenon in the one case was likely to lead to discovery of a similar
phenomenon in the other.

[L] BOSE--“Comparative Electro-Physiology” (1907). Longmans, Green and
Co.

As the transmission of excitation is a phenomenon of propagation of
molecular disturbance in the conducting vehicle, it appeared that the
excitatory impulse could be controlled by inducing in the conducting
tissue two opposite ‘molecular dispositions’, using that term in the
widest sense. The possibility of accomplishing this by the directive
action of an electric current had attracted my attention for many years.


METHOD OF CONDUCTIVITY BALANCE.

I have previously carried out an electric method of investigation,
dealing with the influence of electric current on conductivity. The
method of Conductivity Balance which I devised for this purpose[M] was
found very suitable. Isolated conducting tissues of certain plants
were found to exhibit transmitted effect of excitatory electric
change of galvanometric negativity, which at the favourable season of
the year was of sufficient intensity to be recorded by a sensitive
galvanometer. A long strand of the conducting tissue was taken and two
electric connections were made with a galvanometer, a few centimetres
from the free ends. Thermal stimulus was applied at the middle, when
two excitatory waves with their concomitant electric changes were
transmitted outwards. By suitably moving the point of application of
stimulus nearer or further away from one of the two electric contacts,
an exact balance was obtained. This was the case when the resultant
galvanometer deflection was reduced to zero. If now an electrical
current be sent along the length of the conducting tissue, the two
excitatory waves sent outwards from the central stimulated point will
encounter the electric current in different ways; one of the excitatory
waves will travel with, and the other against the direction of the
current. If the power of transmitting excitation is modified by the
direction of an electric current then the magnitudes of transmitted
excitations will be different in the two cases, with the result of the
upsetting of the Conductivity Balance. From the results of experiments
carried out by this method on the effect of feeble current on
conductivity, the conclusion was arrived at that _excitation is better
conducted against the direction of the current than with it_. In other
words, the influence of an electric current is to confer a preferential
or selective direction of conductivity for excitation, the tissue
becoming a better conductor in an electric up-hill direction compared
with a down-hill.

[M] _Ibid_, p. 478.

The results were so unexpected that I have for long been desirous
of testing the validity of this conclusion by independent method of
inquiry. I shall presently give full account of the perfected method,
and the various difficulties which had to be overcome to render it
practical. Before doing this I shall describe a simple method which I
have devised for demonstrating the principal results.


CONTROL OF TRANSMITTED EXCITATION IN _AVERRHOA BILIMBI_.

The petiole of _Averrhoa bilimbi_ has a large number of paired
leaflets, which, on excitation, undergo downward closure. Feeble
stimulus is applied at a point in the petiole, and the transmission of
excitation is visibly manifested by the serial fall of the leaflets.
The distance to which the excitation reaches is a measure of normal
power of conduction. Any variation of conductivity, by the passage of
an electric current in one direction or the other is detected by the
enhancement or diminution of the distance through which excitation is
transmitted. I shall describe the special precautions to be taken in
carrying out this investigation.

[Illustration: FIG. 44.--Diagram of experimental arrangement for
control of transmitted excitation in _Averrhoa bilimbi_. For
explanation see text.]

Electric stimulus of induction shock of definite intensity and
duration is supplied at the middle of the petiole at EE′ (Fig. 44).
The leaflets to the left of E, are not necessary for the purpose of
this experiment and therefore removed. The intensity of the induction
shock may be varied in the usual manner by removing the secondary coil
nearer or farther from the primary. The duration of the shock is always
maintained constant. On application of electric stimulus excitation
is transmitted along the petiole, the distance of transmission
depending on the intensity of stimulus. With feeble stimulus two pairs
of leaflets may undergo an excitatory fall; with stronger stimulus
the transmission is extended to the end of the petiole, and all the
leaflets exhibit movements of closure. We shall now study the modifying
influence of a constant current on conduction of excitation. C is
an electric cell, R the reversing key by which the electric current
could be sent from right to left or in the opposite direction. When
the current is sent from right to the left, the excitatory impulse
initiated at EE′ travels against the direction of the current in an
‘up-hill’ direction. When the current is reversed it flows in the
petiole from left to right and the transmitted impulse travels with the
current or in a ‘down-hill’ direction.

Two complications are introduced on the completion of the electric
circuit of the constant current: the first, is the distributing effect
of leakage of the induction current used for excitation, and second,
the polar variation of excitation induced by the constant current.

_Leakage of induction current._--Before completing the constant current
circuit, the alternating induction current goes only through the path
EE′. On completion of the constant current circuit, the alternating
induction current not only passes through the shorter path EE′ but also
by the circuitous path of the constant current circuit. The escaping
induction current would thus excite all the leaflets directly and
not by its transmitted action. This difficulty is fully overcome by
the interposition of a choking coil which will be described below.
A simpler, though less perfect, device may be employed to reduce and
practically eliminate the leakage. This consists of a loop, L, of
silver wire placed outside EE′. The leakage of induction current is
thus diverted along this path of negligible resistance in preference to
the longer circuit through the entire petiole, which has a resistance
of several million ohms.

_Polar action of current on excitability._--It is well known that an
electric current induces a local depression of excitability at the
point of entrance to the tissue, or at the anode, and an enhancement
of excitability at the point of exit, or at the cathode. But the
excitability is unaffected at a point equally distant from anode
and cathode. This is known as the indifferent point. The exciting
electrodes EE′ are placed at the indifferent point. But when the
current enters on the right side, the terminal leaflets to the right
have their excitability depressed by the proximity of anode, but
the leaflets near the electrodes EE′, being at a distance from the
anode are not affected by it. Moreover it will be shown that the
enhanced conductivity conferred by the directive action of the current
overpowers any depression of excitability in the terminal leaflets due
to the proximity of the anode. I shall, for convenience, designate
the transmission as ‘up-hill’, when excitation is propagated against
the direction of the constant electric current, and ‘down-hill’ when
transmitted with the direction of the current.

_Transmission of excitation ‘Up-hill’: Experiment 38._--I shall give
here an account of an experiment which may be taken as typical. I took
a vigorous specimen of _Averrhoa bilimbi_, and applied a stimulus whose
intensity was so adjusted that the propagated impulse brought about
a fall of only two pairs of leaflets. This gave a measure of normal
conduction without the passage of the current. The constant electric
current was now sent from right to left. A necessary precaution is to
increase the current gradually by means of a suitable potentiometer
slide, to its full value. The reason for this will be given later. The
intensity of the constant current employed was 1.4 micro-ampères. Now
on exciting the petiole by the previous stimulus, the conducting power
was found to be greatly enhanced. The excitatory impulse now reached
the end of the petiole, and caused six pairs of leaflets to fall.

_Transmission of excitation ‘Down-hill’: Experiment 39._--In
continuation of the previous experiment, the constant electric current
was reversed, its directions being now from left to right. Transmission
of excitation was now in a down-hill direction. On applying the
induction shock stimulus of the same intensity as before, the
conducting power of the petiole was found to be abolished, none of the
leaflets exhibiting any sign of excitation. This modification of the
conducting power persists during the passage of the constant current.
On cessation of the current the original conducting power is found
to be restored. It will thus be seen that the power of conduction is
capable of modification, and that the passage of an electric current of
moderate intensity induces enhanced power of conduction in an ‘up-hill’
and diminished conductivity in a ‘down-hill’ direction.


ELECTRIC CONTROL OF NERVOUS IMPULSE IN ANIMALS.

In my ‘Researches on Irritability of Plants’ I have shown how
intimately connected are the various physiological reactions in the
plant and in the animal, and I ventured to predict that the recognition
of this unity of response in plant and animal will lead to further
discoveries in physiology in general. This surmise has been fully
justified, as will be seen in the following experiments carried out on
the nerve-and-muscle preparation of a frog. It is best to carry out
the experiments with vigorous specimens; this ensures success, even in
long continued experiments, which can then be repeated with unfailing
certainty for hours. It is also an advantage to use a large frog for
its relatively great length of the nerve.

_Directive action of current on conduction of excitation in a
nerve-and-muscle preparation: Experiment 40._--A preparation was made
with a length of the spine and two nerves leading to the muscles. The
specimen is supported in a suitable manner, and electric connections
made with the toes, one for the entrance and the other for exit of
the constant current. The current thus entered, say, by the left
toe ascended the muscle and went up the nerve on the left side, and
descended through the other nerve on the right side along the muscle
and thence to the right toe. Before the passage of the constant
electric current the spinal nerve was stimulated by an induction
shock of definite intensity. The nervous impulse was conducted by the
two nerves, one to the left and the other to the right, and caused
a feeble twitch of the respective muscles. A feeble current of 1.5
micro-ampère was sent along the nerve-and-muscle circuit, ascending
by the left and descending by the right side. It will be seen that
excitation initiated at the spine is propagated ‘against’ the electric
current on the left side, and ‘with’ the current on the right side.
On repetition of previous electric stimulus the effect of directive
action of current was at once manifested by the left limb being
thrown into a state of strong tetanic contraction, whereas the right
limb remained quiescent. By changing the direction of the constant
current the induced enhancement of conductivity of the nerve was
quickly transferred from the left to the right side, the depression
or arrest of conduction being simultaneously transferred to the left
side. Turning the reversing key one way or the other brought about
_supra_ or _non_-conducting state of the nerve, and this condition was
maintained throughout the duration of the current.

I shall next describe a more perfect method for obtaining quantitative
results both with plant and animal. In order to demonstrate the
universality of the phenomenon, I next used _Mimosa pudica_ instead of
_Averrhoa_, for experiments on plants.

For determination of normal velocity of transmission of excitation
and the induced variation of that velocity, I employed the automatic
method of recording the velocity of transmission of excitation in
_Mimosa_, where the excitatory fall of the motile leaf gave a signal
for the arrival of the excitation initiated at a distant point. In this
method the responding leaf is attached to a light lever, the writer
being placed at right angles to it. The record is taken on a smoked
glass plate, which during its descent makes an instantaneous electric
contact, in consequence of which a stimulating shock is applied at a
given point of the petiole. A mark in the recording plate indicates
the moment of application of stimulus. After a definite interval the
excitation is conducted to the responding pulvinus, when the excitatory
fall of the leaf pulls the writer suddenly to the left. From the
curve traced in this manner the time-interval between the application
of stimulus and the initiation of response can be found, and the
normal rate of transmission of excitation through a given length of
the conducting tissue deduced. The experiment is then repeated with
an electric current flowing along the petiole with or against the
direction of transmission of excitation. The records thus obtained
enable us to determine the influence of the direction of the current
on the rate of transmission. I shall presently describe the various
difficulties which have to be overcome before the method just indicated
can be rendered practical.

The scope of investigation will be best described according to the
following plan[N]:--

[N] For fuller account _see_ Bose--‘The influence of Homodromous and
Heterodromous Electric Current on Transmission of Excitation in Plant
and Animal.’ Proc. R. S. B., Vol. 88, 1915.

PART I.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF
EXCITATION IN PLANTS.

    General method of experiment.

    Effect of feeble current on velocity of transmission of excitation
    ‘up-hill’ or ‘down-hill.’

    Determination of variation of conductivity by the method of minimal
    stimulus and response.

    The after-effect of current.

PART II.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF
EXCITATION IN ANIMAL NERVE.

    The method of experiment.

    Variation of velocity of transmission under the action of current.

    Variation in the intensity of transmitted excitation.


PART I.--INFLUENCE OF DIRECTION OF CURRENT ON TRANSMISSION OF
EXCITATION IN PLANT.

THE METHOD OF EXPERIMENT.

I may here say a few words of the manner in which the period of
transmission can be found from the record given by my Resonant
Recorder, fully described in my previous paper. The writer attached to
the recording lever of this instrument is maintained by electromagnetic
means in a state of to-and-fro vibration. The record thus consists
of a series of dots made by the tapping writer, which is tuned to
vibrate at a definite rate, say, 10 times per second. In a particular
case whose record is given in Curve 1 (Fig. 46), indirect stimulus
of electric shock was applied at a distance of 15 mm. from the
responding pulvinus. There are 15 intervening dots between the
moment of application of stimulus and the beginning of response; the
time-interval is therefore 1.5 seconds. The latent period of the
motile pulvinus is obtained from a record of direct stimulation; the
average value of this in summer is 0.1 second. Hence the true period
of transmission is 1.4 seconds for a distance of 15 mm. The velocity
determined in this particular case is therefore 10.7 mm. per second.

Precaution has to be taken against another source of disturbance,
namely, the excitation caused by the sudden commencement or the
cessation of the constant current. I have shown elsewhere[O] that the
sudden initiation or cessation of the current induces an excitatory
reaction in the plant-tissue similar to that in the animal tissue. This
difficulty is removed by the introduction of a sliding potentiometer,
which allows the applied electromotive force to be gradually increased
from zero to the maximum or decreased from the maximum to zero.

[O] BOSE--‘Plant Response’ (1906); ‘Irritability of Plants’ (1913).

[Illustration: FIG. 45.--Complete apparatus for investigation of
the variation of conducting power in _Mimosa_. A, storage cell; S,
potentiometer slide, which, by alternate movement to right or left,
continuously increases or decreases the applied E. M. F.; K, switch key
for putting current “on” and “off” without variation of resistance; E,
E′, electrodes of induction coil for stimulation; C, choking coil; G,
micro-ammeter.]

The experimental arrangement is diagrammatically shown in Fig. 45.
After attaching the petiole to the recording lever, indirect stimulus
is applied, generally speaking, at a distance of 15 mm. from the
responding pulvinus. Stimulus of electric shock is applied in the
usual manner, by means of a sliding induction coil. The intensity of
the induction shock is adjusted by gradually changing the distance
between the secondary and the primary, till a minimally effective
stimulus is found. In the study of the effect of direction of constant
current on conductivity, non-polarisable electrodes make suitable
electric connections, one with the stem and the other with the tip of
a sub-petiole at a distance from each other of about 95 mm. The point
of stimulation and the responding pulvinus are thus situated at a
considerable distance from the anode or the cathode, in the indifferent
region in which there is no polar variation of excitability. By
means of a Pohl’s commutator or reverser, the constant current can be
maintained either “with” or “against” the direction of transmission of
excitation. The transmission in the former case is “down-hill,” and
in the latter case “up-hill.” Electrical connections are so arranged
that when the commutator is tilted to the right, the transmission is
down-hill, when tilted to the left, up-hill.

The electrical resistance offered by the 95 mm. length of stem and
petiole will be from two to three million ohms. The intensity of the
constant current flowing through the plant can be read by unplugging
the key which short-circuits the micro-ammeter G. The choking coil
C prevents the alternating induction current from flowing into the
polarising circuit and causing direct stimulation of the pulvinus.

Before describing the experimental results, it is as well to enter
briefly into the question of the external indication by which the
conducting power may be gauged. Change of conductivity may be
expected to give rise to a variation in the rate of propagation
or to a variation in the magnitude of the excitatory impulse that
is transmitted. Thus we have several methods at our disposal for
determining the induced variation of conductivity. In the first place
the variation of conductivity may be measured by the induced change
in the velocity of transmission of excitation. In the second place,
the transmitted effect of a sub-maximal stimulus will give rise to
enhanced or diminished amplitude of mechanical response, depending on
the increase or decrease of conductivity brought about by the directive
action of the current. And, finally, the enhancement or depression
of conductivity may be demonstrated by the ineffectively transmitted
stimulus becoming effective, or the effectively transmitted stimulus
becoming ineffective.

_Exclusion of the factor of Excitability._--The object of the enquiry
being the pure effect of variation of conductivity, we have to assure
ourselves that under the particular conditions of the experiment the
complicating factor of polar variation of excitability is eliminated.
It is to be remembered that excitatory transmission in _Mimosa_ takes
place by means of a certain conducting strand of tissue which runs
through the stem and the petiole. In the experiment to be described,
the constant current enters by the tip of the petiole and leaves by
the stem, or _vice versâ_, the length of the intrapolar region being
95 mm. The point of application of stimulus on the petiole is 40 mm.
from the electrode at the tip of the leaf. The responding pulvinus is
also at the same distance from the electrode on the stem. The point of
stimulation and region of response are thus at the relatively great
distance of 40 mm. from either the anode or the cathode, and may
therefore be regarded as situated in the indifferent region. This is
found to be verified in actual experiments.


EFFECTS OF DIRECTION OF CURRENT ON VELOCITY OF TRANSMISSION.

A very convincing method of demonstrating the influence of electric
current on conductivity consists in the determination of changes
induced in the velocity of transmission by the directive action of the
current. For this purpose we have to find out the true time required
by the excitation to travel through a given length of the conducting
tissue (1) in the absence of the current, (2) ‘against’ and (3) ‘with’
the direction of the current. The true time is obtained by subtracting
the latent period of the pulvinus from the observed interval between
the stimulus and response. Now the latent period may not remain
constant, but undergo change under the action of the polarising
current. It has been shown that the excitability of the pulvinus does
not undergo any change when it is situated in the middle or indifferent
region. The following results show that under parallel conditions the
latent period also remains unaffected:--

TABLE V.--SHOWING THE EFFECT OF ELECTRIC CURRENT ON THE LATENT PERIOD.

  +------------------------------------------------+-------+-------+
  |                   Specimens                    |   I.  |  II.  |
  |------------------------------------------------+-------|-------|
  |                                                |  sec. |  sec. |
  | Latent period under normal condition           |  0.10 |  0.09 |
  |       "         "   current from right to left |  0.11 |  0.10 |
  |       "         "   current from left to right |  0.09 |  0.09 |
  +------------------------------------------------+-------+-------+

The results of experiments with two different specimens given above
show that a current applied under the given conditions has practically
no effect on the latent period, the slight variation being of the order
of one-hundredth part of a second. This is quite negligible when the
total period observed for transmission is, as in the following cases,
equal to nearly 2 seconds.

_Induced changes in the Velocity of Transmission._--Having found that
the average value of the latent period in summer is 0.1 second, we
next proceed to determine the influence of the direction of current on
velocity.

_Experiment 41._--As a rule, stimulus of induction shock was applied
in this and in the following experiments on the petiole at a distance
of 15 mm. from the responding pulvinus. The recording writer was
tuned to 10 vibrations per second; the space between two succeeding
dots, therefore, represents a time-interval of 0.1 second. The middle
record, N in Fig. 46, is the normal. There are 17 spaces between the
application of stimulus and the beginning of response. The total time
is therefore 1.7 seconds, and by subtracting from it the latent period
of 0.1 second we obtain the true time, 1.6 seconds. The normal velocity
is found by dividing the distance 15 mm. by the true interval 1.6
seconds. Thus V = 15/1.6 = 9.4 mm. per second. We shall next consider
the effect of current in modifying the normal velocity. The uppermost
record (1) in Fig. 46 was taken under the action of an ‘up-hill,’ or
‘against’ current of the intensity of 1.4 microampères. It will be seen
that the time interval is reduced from 1.7 seconds to 1.4 seconds;
making allowance for the latent period, the velocity of transmission
under ‘up-hill’ current V_{1} = 15/1.3 = 11.5 mm. per second. In the
lowest record (3) we note the effect of ‘down-hill’ current, the
time-interval between stimulus and response being prolonged to 1.95
seconds and the velocity reduced to 8.1 mm. per second. The conclusion
arrived at from this mechanical mode of investigation is thus identical
with that derived from the electric method of conductivity balance
referred to previously.

[Illustration: FIG. 46.--Record showing enhancement of velocity of
transmission “up-hill” or against the current (uppermost curve) and
retardation of velocity “down-hill” or with the current (lowest curve).
N, normal record in the absence of current; ← indicates “up-hill” and →
“down-hill” transmission.]

That is to say, _the passage of a feeble current modifies conductivity
for excitation in a selective manner. Conductivity is enhanced_
against_, and diminished_ with_, the direction of the current._

The minimum current which induces a perceptible change of conductivity
varies somewhat in different specimens. The average value of this
minimal current in autumn is 1.4 microampères. The effect of even a
feebler current may be detected by employing a test stimulus which is
barely effective.

TABLE VI.--SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF FEEBLE
INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM.

  +-------+-------------+---------------------+---------------------+
  |Number.|Intensity of |     Period for      |     Period for      |
  |       | current in  |       up-hill       |     down-hill       |
  |       |microampères.|    transmission.    |    transmission.    |
  +-------+-------------+---------------------+---------------------+
  |   1   |     1.4     |14 tenths of a second|16 tenths of a second|
  |   2   |     1.4     |13   "           "   |15   "           "   |
  |   3   |     1.6     |19   "           "   |Arrest.              |
  |   4   |     1.7     |12   "           "   |14 tenths of a second|
  +-------+-------------+---------------------+---------------------+

Having demonstrated the effect of direction of current on the velocity
of transmission, I shall next describe other methods by which induced
variations of conductivity may be exhibited.


DETERMINATION OF VARIATION OF CONDUCTIVITY BY METHOD OF MINIMAL
STIMULUS AND RESPONSE.

In this method we employ a minimal stimulus, the transmitted effect of
which under normal conditions gives rise to a feeble response. If the
passage of a current in a given direction enhances conductivity, then
the intensity of transmitted excitation will also be enhanced; the
minimal response will tend to become maximal. Or excitation which had
hitherto been ineffectively transmitted will now become effectively
transmitted. Conversely, depression of conductivity will result in a
diminution or abolition of response. We may use a single break-shock
of sufficient intensity as the test stimulus. It is, however,
better to employ the additive effect of a definite number of feeble
make-and-break shocks.

We may again employ additive effect of a definite number of induction
shocks, the alternating elements of which are exactly equal and
opposite. This is secured by causing rapid reversals of the primary
current by means of a rotating commutator. The successive induction
shocks of the secondary coil can thus be rendered exactly equal and
opposite.

_Experiment 42._--Working in this way, it is found that the transmitted
excitation against the direction of current becomes effective or
enhanced under ‘up-hill’ current. A current, flowing with the direction
of transmission, on the other hand, diminishes the intensity of
transmitted excitation or blocks it altogether.

Henceforth it would be convenient to distinguish currents in the
two directions: the current in the direction of transmission will
be distinguished as _Homodromous_, and against the direction of
transmission as _Heterodromous_.


AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.

The passage of a current through a conducting tissue in a given
direction causes, as we have seen, an enhanced conductivity in an
opposite direction. We may suppose this to be brought about by a
particular molecular arrangement induced by the current, which
assisted the propagation of the excitatory disturbance in a selected
direction. On the cessation of this inducing force, there may be a
rebound and a temporary reversal of previous molecular arrangement,
with concomitant reversal of the conductivity variation. The immediate
after-effect of a current flowing in a particular direction on
conductivity is likely to be a transient change, the sign of which
would be opposite to that of the direct effect. The after-effect of a
heterodromous current may thus be a temporary depression, that of a
homodromous current, a temporary enhancement of conductivity.

[Illustration: FIG. 47.--Direct and after-effect of heterodromous and
homodromous currents. First two records, N, N, normal. ↓, enhanced
transmission under heterodromous current; ⇣ arrest of conduction is an
after-effect of heterodromous current. Next record ↑ shows arrest under
homodromous current. Last record ⇡ shows enhancement of conduction
greater than normal, as an after-effect of homodromous current. (Dotted
arrow indicates the after-effect on cessation of a given current. ↑
homodromous and ↓ heterodromous current.)]

_Experiment 43._--This inference will be found fully justified in the
following experiment:--The first two responses are normal, after which
the heterodromous current gave rise to an enhanced response. The
depressing after-effect of a heterodromous current rendered the next
response ineffective. The following record taken during the passage
of the homodromous current exhibited an abolition of response due to
induced depression of conductivity. Finally, the after-effect of the
homodromous current is seen to be a response larger than the normal
(Fig. 47). These experiments show that the after-effect of cessation of
a current in a given direction is a transient conductivity variation,
of which the sign is opposite to that induced by the continuation of
the current.


PART II--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF
EXCITATION IN ANIMAL NERVE.

I shall now take up the question whether an electric current induced
any selective variation of conductivity in the animal nerve, similar to
that induced in the conducting tissue of the plant.


THE METHOD OF EXPERIMENT.

In the experiments which I am about to describe, arrangements were
specially made so that (1) the excitation had not to traverse the polar
region, and (2) the point of stimulation was at a relatively great
distance from either pole. The fulfilment of the latter condition
ensured the point of stimulation being placed at the neutral region.

In the choice of experimental specimens I was fortunate enough to
secure frogs of unusually large size, locally known as “golden frogs”
(_Rana tigrina_). A preparation was made of the spine, the attached
nerve, the muscle and the tendon. The electrodes for constant current
were applied at the extreme ends, on the spine and on the tendon
(Fig. 48). The following are the measurements, in a typical case, of
the different parts of the preparation. Length of spine between the
electrode and the nerve = 40 mm. length of nerve = 90 mm. length of
muscle = 50 mm. length of tendon = 30 mm. Stimulus is applied in all
cases on the nerve, midway between the two electrodes this point being
at a minimum distance of 100 mm. from either electrode. The point of
stimulation is, therefore, situated at an indifferent region.

[Illustration: FIG. 48.--Experimental arrangement for study of
variation of conductivity of nerve by the directive action of an
electric current. _n n′_, nerve; S, point of application of stimulus
in the middle or indifferent region.]

Great precautions have to be taken to guard against the leakage of
current. The general arrangement for the experiment on animal nerve is
similar to that employed for the corresponding investigations on the
plant. The choking coil is used to prevent the stimulating induction
current from getting round the circuit of constant current. The
specimen is held on an ebonite support, and every part of the apparatus
insulated with the utmost care.


VARIATION OF VELOCITY OF TRANSMISSION.

In the case of the conducting tissue of the plant a very striking
proof of the influence of the direction of current on conductivity was
afforded by the induced variation of velocity of transmission. Equally
striking is the result which I have obtained with the nerve of the frog.

[Illustration: FIG. 49.--Effect of heterodromous and homodromous
current in inducing variation in velocity of transmission through
nerve. N, normal record, upper record shows enhancement, and lower
record retardation in velocity of transmission under heterodromous and
homodromous currents, respectively.]

_Experiment 44._--The experiments described below were carried out
during the cold weather. The following records (Fig. 49), obtained by
means of the pendulum myograph, exhibit the effect of the direction
of current on the period of transmission through a given length of
nerve. The latent period of muscle being constant, the variations in
the records exhibit changed rates of conduction. The middle record is
the normal, in the absence of any current. The upper record, denoted
by the left-hand arrow, shows the action of a heterodromous current in
shortening the period of transmission and thus enhancing the velocity
above the normal rate. The lower record, denoted by the right-hand
arrow, exhibits the effect of a homodromous current in retarding the
velocity below the normal rate. I find that a very feeble heterodromous
current is enough to induce a considerable increase of velocity,
which soon reaches a limit. For inducing retardation of velocity, a
relatively strong homodromous current is necessary. I give below a
table showing the results of several experiments.

TABLE V--EFFECT OF HETERODROMOUS AND HOMODROMOUS CURRENT OF FEEBLE
INTENSITY ON VELOCITY OF TRANSMISSION.

  +---------+-------------+-------------+------------+-------------+
  |         |Intensity of |             |Intensity of|             |
  |Specimen.|heterodromous|Acceleration |homodromous |Retardation  |
  |         |   current.  |above normal.|  current.  |below normal.|
  +---------+-------------+-------------+------------+-------------+
  |         | microampère |  per cent.  |microampères|  per cent.  |
  |         |             |             |            |             |
  |    1    |     0.35    |     16      |    1       |     20      |
  |    2    |     0.7     |     13      |    1.5     |     19      |
  |    3    |     0.8     |     18      |    2.0     |     14      |
  |    4    |     0.8     |     11      |    2.0     |     13      |
  |    5    |     1.0     |     18      |    2.5     |     12      |
  |    6    |     1.5     |     15      |    3.0     |     40      |
  +---------+-------------+-------------+------------+-------------+


VARIATION OF INTENSITY OF TRANSMITTED EXCITATION UNDER HETERODROMOUS
AND HOMODROMOUS CURRENTS.

In the next method of investigation, the induced variation of intensity
of transmitted excitation is inferred from the varying amplitude of
response of the terminal muscle. Testing stimulus of sub-maximal
intensity is applied at the middle of the nerve, where the constant
current induces no variation of excitability. Stimulation is effected
either by single break-shock or by the summated effects of a definite
number of equi-alternating shocks, or by chemical stimulation

_Experiment 45._--Under the action of feeble heterodromous current the
transmitted excitation was always enhanced, whatever be the form of
stimulation. This is seen illustrated in Fig. 50. Homodromous current
on the other hand inhibited or blocked excitation (Fig. 51).

[Illustration: FIG. 50.--Ineffectively transmitted salt-tetanus
becoming effective under heterodromous current, denoted by
down-pointing arrow.]

_Complication due to variation of Excitability of Muscle_.--In
experiments with the plant, there was the unusual advantage in having
both the point of stimulation and the responding motile organ in the
middle or indifferent region. Unfortunately this ideally perfect
condition cannot be secured in experiments with the nerve-and-muscle
preparation of the frog. It is true that the point of stimulation in
this case is chosen to lie on the nerve at the middle or indifferent
region. But the responding muscle is at one end, not very distant from
the electrode applied on the tendon. It is, therefore, necessary to
find out by separate experiments any variation of excitability that
might be induced in the muscle by the proximity of either the anode or
the cathode, and make allowance for such variation in interpreting the
results obtained from investigations on variation of conductivity.

In the experimental arrangement employed, the heterodromous current
is obtained by making the electrode on the spine cathode and that on
the tendon anode. The depressing influence of the anode in this case
may be expected to lower, to a certain extent, the normal excitability
of the responding muscle. Conversely, with homodromous current, the
tendon is made the cathode and under its influence the muscle might
have its excitability raised above the normal. These anticipations are
fully supported by results of experiments. Sub-maximal stimulus of
equi-alternating induction shock was directly applied to the muscle
and records taken of (_1_) response under normal condition without any
current, (_2_) response under heterodromous current, the tendon being
the anode, and (_3_) response under homodromous current, the tendon
being now made the cathode. It was thus found that under heterodromous
current the excitability of the muscle was depressed, and under
homodromous current the excitability was enhanced.

The effect of current on response to direct stimulation is thus
opposite to that on response to transmitted excitation, as will be seen
in the following Table.

TABLE VIII.--INFLUENCE OF DIRECTION OF CURRENT ON DIRECT AND
TRANSMITTED EFFECTS OF STIMULATION.

  +---------------------+-----------------------+-------------------+
  |Direction of current.|Transmitted excitation.|Direct stimulation.|
  +---------------------+-----------------------+-------------------+
  |Heterodromous current|Enhanced response      |Depressed response |
  |Homodromous current  |Depressed response     |Enhanced response  |
  +---------------------+-----------------------+-------------------+

The passage of a current, therefore, induces opposing effects on the
conductivity of the nerve and the excitability of the muscle, the
resulting response being due to their differential actions. Under
heterodromous current a more intense excitation is transmitted along
the nerve, on account of induced enhancement of conductivity. But
this intense excitation finds the responding muscle in a state of
depressed excitability. In spite of this the resulting response is
enhanced (Fig. 50). The enhancement of conduction under heterodromous
current is, in reality, much greater than is indicated in the record.
Similarly, under homodromous current the depression of conduction in
the nerve may be so great as to cause even an abolition of response, in
spite of the enhanced excitability of the muscle (Fig. 51). The actual
effects of current on conductivity are, thus, far in excess of what are
indicated in the records.


AFTER-EFFECTS OF HETERODROMOUS AND HOMODROMOUS CURRENTS.

On the cessation of a current there is induced in the plant-tissue a
transient conductivity change of opposite sign to that induced by the
direct current (_cf. Expt. 43_). The same I find to be the case as
regards the after-effect of current on conductivity change in animal
nerve. Of this I only give a typical experiment of the direct and
after-effect of homodromous current on salt-tetanus.

[Illustration: FIG. 51.--Direct and after-effect of homodromous
current. Transmitted excitation (salt-tetanus T,) arrested under
homodromous current denoted by up-pointing arrow; on cessation of
current represented by dotted line there is a transient enhancement
above the normal.]

_Experiment 46._--In this experiment sufficient length of time was
allowed to elapse after the application of the salt on the nerve, so
that the muscle, in response to the transmitted excitation, exhibited
an incomplete tetanus T. The homodromous current was next applied,
with the result of inducing a complete block of conduction, with
the concomitant disappearance of tetanus. The homodromous current
was gradually reduced to zero by the appropriate movement of the
potentiometer slide. The after-effect of homodromous current is now
seen in the transient enhancement of transmitted excitation, which
lasted for nearly 40 seconds. After this the normal conductivity was
restored. Repetition of the experiment gave similar results (Fig. 51).

The results that have been given are only typical of a very large
number, which invariably supported the characteristic phenomena that
have been described.

It will thus be seen that with feeble or moderate current, conductivity
is enhanced against the direction of the current and depressed or
blocked with the direction of the current. Under strong current the
normal effect is liable to undergo a reversal.

It has thus been shown that a perfect parallelism exists in the
conductivity variation induced in the plant and in the animal by the
directive action of the current. No explanation could be regarded as
satisfactory which is not applicable to both cases. Now with the plant
we are able to arrange the experimental condition in such a way that
the factor of variation of excitability is completely eliminated. The
various effects described about the plant-tissue are, therefore, due
entirely to variation of conductivity. The parallel phenomena observed
in the case of transmission of excitation in the animal nerve must,
therefore, be due to the induced change of conductivity.

The action of an electrical current in inducing variation of
conductivity may be enunciated under the following laws, which are
equally applicable to the conducting tissue of the plant and the nerve
of the animal:--


LAWS OF VARIATION OF NERVOUS CONDUCTION UNDER THE ACTION OF ELECTRIC
CURRENTS.

  1. The passage of a current induces a variation of conductivity,
       the effect depending on the direction and intensity of current.

  2. Under feeble intensity, heterodromous current enhances, and
       homodromous current depresses, the conduction of excitation.

  3. The after-effect of a feeble current is a transient conductivity
       variation, the sign of which is opposite that induced during the
       continuation of current.


SUMMARY.

The variation of conductivity induced by the directive action of
current has been investigated by two different methods:--

  (1) The method in which the normal speed and its induced variation
       are automatically recorded;

  (2) That in which the variation in the intensity of transmitted
       excitations is gauged by the varying amplitudes of resulting
       responses.

The great difficulty arising from leakage of the exciting induction
current into the polarising circuit was successfully overcome by the
interposition of a choking coil.

The following summarises the effects of direction and intensity of an
electric current, on transmission of excitation through the conducting
tissue of the plant.

The velocity of transmission is enhanced against the direction of a
feeble current, and retarded in the direction of the current.

Feeble heterodromous current enhances conductivity, homodromous
current, on the other hand, depresses it.

Ineffectively transmitted excitation becomes effectively transmitted
under heterodromous current. Effectively transmitted excitation, on
the other hand, becomes ineffectively transmitted under the action of
homodromous current.

The after-effect of a current is a transient conductivity change,
the sign of which is opposite to that induced during the passage
of current. The after-effect of a heterodromous current is, thus,
a transient depression, that of homodromous current, a transient
enhancement of conductivity.

The characteristic variations of conductivity induced in animal nerve
by the direction and intensity of current are in every way similar to
those induced in the conducting tissue of the plant.

These various effects are demonstrated by the employment of not
one, but various kinds of testing stimulus, such as the excitation
caused (1) by a single break-induction shock or (2) by a series of
equi-alternating tetanising shocks or (3) by chemical stimulation.




VIII.--EFFECT OF INDIRECT STIMULUS ON PULVINATED ORGANS

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS, L.M.S.


The leaf of _Mimosa pudica_ undergoes an almost instantaneous fall
when the stimulus is applied directly on the pulvinus which is the
responding organ. The latent period, _i.e._, the interval between the
application of stimulus and the resulting response is about 0.1 second.
Indirect stimulus, _i.e._, application of stimulus at a distance from
the pulvinus, also causes a fall of the leaf; but a longer interval
will elapse between the incidence of stimulus and the response; for it
will take a definite time for the excitation to be conducted through
the intervening tissue. I have already shown that this conduction of
excitation in plant is analogous to the transmission of nervous impulse
in animal.

The power of conduction varies widely in different plants. In the
petiole of _Mimosa pudica_ the velocity may be as high as 30 mm. per
second. In the stem the velocity is considerably less, _i.e._, about
6 mm. per second in the longitudinal direction; but conduction across
the stem is a very much slower process. In the petiole of _Averrhoa_
the longitudinal velocity is of the order of 1 mm. per second.


DUAL CHARACTER OF THE TRANSMITTED IMPULSE.

The record of the transmitted effect of stimulus is found to exhibit
a remarkable preliminary variation. This was detected by my delicate
recorders, which gave magnifications from fifty to hundred times. I
shall give a detailed account of a typical experiment carried out with
_Averrhoa carambola_, which will bring out clearly the characteristic
effects of Indirect Stimulus.

[Illustration: FIG. 52.--Effect of indirect Stimulus on leaflet of
_Averrhoa carambola_. Stimulus was applied at the short vertical
line. Successive dots at intervals of one second. Note the _positive_
response preceding the _negative_.]

_Experiment 47._--Stimulus of electric shock applied at a point on the
long petiole of _Averrhoa_ causes successive fall of pairs of leaflets.
In the experiment to be described one of the leaflets of the plant
was attached to the recorder. Stimulus was applied at a distance of
50 mm. The successive dots in the record are at intervals of a second.
It will be noticed that two distinct impulses--a _positive_ and a
_negative_--were generated by the action of Indirect Stimulus. The
positive impulse reached the responding organ after 1.5 second and
caused an erectile movement. The velocity of the positive impulse in
the present case is 33 mm. per second. The normal excitatory negative
impulse reached the motile organ 44 seconds after the application of
stimulus, and caused a very rapid fall of the leaflet, the fall being
far more pronounced than the positive movement of erection (Fig. 52).
In this and in all subsequent records, the positive and negative
responses offer a great contrast. The movement in response to positive
reaction is slow, whereas that due to negative reaction is very abrupt,
almost ‘explosive,’ the successive dots being now very wide apart. As
regards the velocity of impulse the relation is reversed, the positive
being the quicker of the two. In the present case, the velocity of the
excitatory _negative_ impulse is 1.1 mm. per second, as against 33 mm.
of the _positive_ impulse.

The negative impulse is due to the comparatively slow propagation of
the excitatory protoplasmic change, which brings about a diminution
of turgor in the pulvinus and fall of the responding leaflet. The
erectile movement of the leaflet by the positive impulse must be due
to an increase of turgor, brought on evidently, by the forcing in of
water. This presupposes a forcing out of water somewhere else, probably
at the point of application of stimulus. It may be supposed that an
active contraction occurred in plant cells under direct stimulus, in
consequence of which water was forced out giving rise to a hydraulic
wave. On this supposition the positive impulse is to be regarded as
hydro-mechanical. I have, however, not yet been able to devise a
direct experimental test to settle the question.


EFFECT OF DISTANCE OF APPLICATION OF STIMULUS.

In the last experiment the stimulus was applied at the moderate
distance of 50 mm. Let us now consider the respective effects, first,
of an increase, and second, of a decrease of the intervening distance.
In a tissue whose conducting power is not great, the excitatory
impulse is weakened, even to extinction in transmission through a long
distance. Thus the negative impulse may fail to reach the responding
organ, when the stimulus is feeble or the intervening distance long or
semi-conducting. Hence, under the above conditions, stimulus applied at
a distance will give rise only to a positive response.

A reduction of the intervening distance will give rise to a different
result. As the negative response is the more intense of the two, the
feeble positive will be masked by the superposed negative. The separate
exhibition of the two responses is only possible by a sufficient lag
of the negative impulse behind the positive. This lag increases with
increase of length of transmission and decreases with the diminution of
the length. Hence the application of stimulus near the responding organ
will give rise only to a negative response, in spite of the presence of
the positive, which becomes masked by the predominant negative.[P]

[P] _Cf._ BOSE--“Plant Response,” p. 535; “Comparative
Electro-Physiology,” p. 64; “Irritability of Plants,” p. 196.

These inferences have been fully borne out by results of experiments
carried out with various specimens of plants under the action of
diverse forms of stimuli. In all cases, application of stimulus at
a distance causes a pure positive response; moderate reduction of
the distance induces a diphasic response--a positive followed by a
negative; further diminution of distance gives rise to a resultant
negative response, the positive being masked by the predominant
negative.

From what has been said it will be understood that the exhibition of
positive response is favoured by the conditions, that the transmitting
tissue should be semi-conducting, and the stimulus feeble. It is thus
easier to exhibit the positive effect with the feebly conducting
petiole of _Averrhoa_ than with the better conducting petiole of
_Mimosa_. It is, however, possible to obtain positive response in the
_Mimosa_ by application of indirect stimulus to the stem in which
conduction is less rapid than in the petioles.

TABLE IX.--PERIODS OF TRANSMISSION OF POSITIVE AND NEGATIVE IMPULSES IN
THE PETIOLE OF _AVERRHOA_ AND STEM OF _MIMOSA_.

  +---+----------+-----------+---------------+------------+------------+
  |   |          |           |               |Transmission|Transmission|
  |No.| Specimen |Distance in|   Stimulus    |period for  |period for  |
  |   |          |    mm.    |               |positive    |negative    |
  |   |          |           |               |impulse.    |impulse.    |
  +---+----------+-----------+---------------+------------+------------+
  | 1 |_Averrhoa_|     70    |Thermal        |  22  secs  |  65  secs. |
  | 2 |    "     |    130    |   "           |  40   "    |  95   "    |
  | 3 |    "     |     10    |Induction-shock|   6   "    |  20   "    |
  | 4 |    "     |     20    |       "       |  14   "    |  48   "    |
  | 5 |    "     |     35    |Chemical       |  21   "    |  50   "    |
  | 6 | _Mimosa_ |      5    |Induction-shock|   0.5 "    |  12   "    |
  | 7 |    "     |     10    |       "       |   0.6 "    |   9.4 "    |
  | 8 |    "     |     20    |       "       |   1.1 "    |  10   "    |
  | 9 |    "     |     60    |       "       |   2   "    |  29   "    |
  |10 |    "     |     35    |Chemical       |   5   "    |  17   "    |
  +---+----------+-----------+---------------+------------+------------+


EFFECTS OF DIRECT AND INDIRECT STIMULUS.

From the results given in course of the Paper we are able to formulate
the following laws about the effects of Direct and Indirect Stimulus on
pulvinated organs:--

  1. Effect of all forms of Direct stimulus is a diminution of
       turgor, a contraction and a negative mechanical response.

  2. Effect of Indirect stimulus is an increase of turgor, an
       expansion and a positive mechanical response.

  3. Prolonged application of indirect stimulus of moderate intensity
       gives rise to a diphasic, positive mechanical response followed
       by the negative.

  4. If the intervening tissue be highly conducting, the transmitted
       positive effect becomes masked by the predominant negative.

The laws of Effects of Direct and Indirect stimulus hold good not
merely in the case of sensitive plants, but universally for all plants.
This aspect of the subject will be treated in fuller detail in later
Papers of this series.




IX.--MODIFYING INFLUENCE OF TONIC CONDITION ON RESPONSE

_By_

SIR J. C. BOSE

_Assisted by_

GURUPRASANNA DAS.


In experiments with different pulvinated organs, great difference is
noticed as regards their excitability. If electric shock of increasing
intensity from a secondary coil be passed through the pulvini of
_Mimosa_, _Neptunia_, and _Erythrina_ arranged in series, it would be
found that _Mimosa_ would be the first to respond; a nearer approach
of the secondary coil to the primary would be necessary for _Neptunia_
to show sign of excitation. _Erythrina_ would require a far greater
intensity of electric shock to induce excitatory movement. Organs of
different plants may thus be arranged, according to their excitability,
in a vertical series, the one at the top being the most excitable.
The specific excitability of a given organ is different in different
species.

In addition to this characteristic difference, an identical organ may,
on account of favourable or unfavourable conditions, exhibit wide
variation in excitability. Thus under favourable conditions of light,
warmth and other factors, the excitability of an organ is greatly
enhanced. In the absence of these favourable tonic conditions the
excitability is depressed or even abolished. I shall, for convenience,
distinguish the different tonic conditions of the plant as _normal_,
_hyper-tonic_ and _sub-tonic_. In the first case, stimulus of moderate
intensity will induce excitation; in the second, the excitability being
exceptionally high, very feeble stimulus will be found to precipitate
excitatory reaction. But a tissue in a _sub-tonic_ condition will
require a very strong stimulus to bring about excitation. The
excitability of an organ is thus determined by two factors: the
specific excitability, and the tonic condition of the tissue.


THEORY OF ASSIMILATION AND DISSIMILATION.

A muscle contracts under stimulus; this is assumed to be due to some
explosive chemical change which leaves the tissue in a condition less
capable of functioning, or in a condition below par. Herring designates
this as a process of _dissimilation_. The excitability of the muscle
is restored after suitable periods of rest, by the opposite metabolic
change of _assimilation_. “Assimilation and Dissimilation must be
conceived as two closely interwoven processes, which constitute the
metabolism (unknown to us in its intrinsic nature) of the living
substance. Excitability diminishes in proportion with the duration of
D-stimulus, or, as it is usually expressed, the substance _fatigues_
itself. It is perfectly intelligible that a progressive fatigue and
decrement of the magnitude of contraction must ensue. The only point
that is difficult to elucidate is the initial staircase increment of
the twitches, more especially in excised, bloodless muscle, which seems
in direct contradiction with the previous theory.”[Q]

[Q] BIEDERMANN--Electro-Physiology (English Translation), Vol 1, pp.
83, 84, 85; Macmillan & Co.

With reference to Herring’s theory given above, Bayliss in his
“Principles of General Physiology” (1915), page 377 says, “In the
phenomenon of metabolism, two processes must be distinguished, the
building up of a complex system or substance of high potential energy,
‘anabolism,’ and the breaking down of such a system, ‘catabolism,’
giving off energy in other forms. The tendency of much recent work,
however, is to throw doubt on the universality of this opposition of
anabolism and catabolism as explanatory of physiological activity in
general.”

The results obtained with the response of plants to stimulus may
perhaps throw some light on the obscurities that surround the subject.
They show that the two processes may be present simultaneously, and
that the ‘down’ change induced by stimulus may, in certain instances,
be more than compensated by the ‘up’ change.[R] I shall, for
convenience, designate the physico-chemical modification, associated
with the excitatory negative mechanical and electrical response of
plants, as the “D” change; this is attended by run down of energy. The
positive mechanical and electrical response must therefore connote
opposite physico-chemical change, with increase of potential energy.
This I shall designate as the “A” change, which by increasing the
latent energy, enhances the functional activity of the tissue. That
stimulus may give rise simultaneously to both A, and D, effects, finds
strong support in the dual reactions exhibited in plant-response. Under
indirect stimulus, the two responses are seen separately, the more
intense negative following the feeble positive. When by the reduction
of the intervening distance, stimulus is made direct, the resultant
response, as previously stated, is negative; and this is due not to
the total absence of the positive but to its being masked by the
predominant negative. Let us next consider the question of unmasking
this positive element in the resultant negative response.

[R] In the response of inorganic matter I have obtained records
of positive, diphasic and negative responses. It would perhaps be
advisable to refer the ‘A’ and ‘D’ effects, to physico-chemical change.
The simultaneous double reaction, combination and decomposition, is of
frequent occurrence in many chemical changes.


UNMASKING OF THE POSITIVE EFFECT.

Under favourable conditions of the environment, the excitability of
the organs is at its maximum. A given stimulus will bring about an
intense excitation, and the ‘down’ D-change will therefore be very much
greater than the A-change. Let us now consider the case at the opposite
extreme where, owing to unfavourable condition, the excitability is
at its lowest. Under stimulus the excitatory D-change will now be
relatively feeble compared to the A-change, by which the potential
energy of the system becomes increased. In such a case successive
stimuli will increase the functional activity of the tissue, and bring
about staircase response. Biedermann mentions the staircase response of
_excised bloodless muscle_ as offering difficulty of explanation. It
is obvious that the physiological condition of the excised muscle must
have fallen below par. The staircase response in such a tissue is thus
explained from considerations that have just been adduced.

The results obtained with _Mimosa_ not only corroborate them, but add
incontestable proof of the simultaneous existence of both A and D
changes. The physiological condition of a plant, _Mimosa_ for example,
is greatly modified by the favourable or unfavourable condition of
the environment. In a hyper-tonic condition its excitability becomes
very great; in this condition the plant responds to its maximum even
under very feeble stimulus. Here the D-change is relatively great, and
successive responses are apt to show sign of fatigue.

[Illustration: FIG. 53.--Record showing the effect of stimulus
modifying tonicity and producing staircase effect. (_Mimosa_)]

But the plant in a sub-tonic condition will exhibit feeble or no
excitation. The D-change will be absent while the A-change will take
place under the action of stimulus. This, by increasing the potential
energy, will enhance the functional activity of the tissue.

_Staircase response in Mimosa: Experiment 48._--The theoretical
considerations will be found experimentally verified in the record
obtained with a specimen of _Mimosa_ in a sub-tonic condition
(Fig. 53). Owing to the lack of favourable ‘tone’ the leaf was relaxing
as seen in the first part of the curve. The stimulus of electric shock,
applied at the thick dot in the curve slanting downwards, gave no
response but raised the tone of the tissue by arresting the growing
relaxation. Subsequent stimuli gave rise to staircase responses.
Stimulus has, through the A-effect, raised the functional activity of
the tissue to a maximum.


ARTIFICIAL DEPRESSION OF TONIC CONDITION AND MODIFICATION OF RESPONSE.

It has been shown that while favourable tonic condition has the effect
of raising the excitability and enhancing the negative response with
the associated D-change, a condition of sub-tonicity, on the other
hand, induces depression of excitability, a diminution of negative
response and of the attendant D-change. In this condition the positive
element in the response with the A-change will come into greater
prominence. These considerations led me to experiment with specimens
exhibiting increasing sub-tonicity, with a view of unmasking the
positive element in the response, _i.e._, the A-change. In the last
experiment a specimen was found which happened to be in a sub-tonic
condition on account of the unfavourable condition of its surroundings.
I was next desirous of securing specimens in which I could induce
increasing sub-tonicity at will.

I have shown (_Expt. 23_) that a detached branch of _Mimosa_ can be
kept alive for several days with the cut end immersed in water. In this
condition the pulvinus retains its sensitiveness for more than two
days. The excitability undergoes a continuous decline and is abolished
about the fiftieth hour. Isolation from the parent organism thus causes
a continuous depression of the tonic condition of the specimen. The
case is somewhat analogous to the depression of excitability in an
excised bloodless muscle. It is thus possible to secure specimens of
varying degrees of sub-tonicity. A specimen that has been detached for
six hours will exhibit a slight amount of depression, while a different
specimen isolated for twenty-four hours will occupy a very much lower
position in the scale of tonicity.

_Experiment 49._--The staircase response of _Mimosa_ given in figure
53 was obtained with the stimulus of induction shock. In order to
establish a wider generalisation I now used the stimulus of light given
by an arc lamp. There may be a difficulty on account of the diurnal
movement of _Mimosa_; the leaf, generally speaking, has a movement in
a downward direction from morning till noon, after which there is a
comparative state of rest. It is better to choose the time of noon for
experiment. In any case the response to stimulus is very abrupt and in
strong contrast with the slow diurnal movement. A horizontal pencil of
light was thrown upwards by means of a small mirror and made to fall
on the lower half of a pulvinus of the _Mimosa_ leaf. The excitatory
down movement is followed by recovery on the cessation of light. The
intensity of stimulus can be modified by varying the intensity of
light. I took for my first series of experiments a specimen that had
been isolated for six hours. Stimulation was caused by successive
applications of light for 25 seconds at intervals of 3 minutes.
Figure 54 shows how the functional activity of the sub-tonic specimen
is enhanced by stimulus, the successive responses thus exhibiting the
staircase effect.

[Illustration: FIG. 54.--Staircase response in sub-tonic _Mimosa_.]

[Illustration: FIG. 55.--Positive, diphasic and negative response under
successive stimulation.]


POSITIVE RESPONSE IN SUB-TONIC SPECIMEN.

_Experiment 50._--A still lower degree of sub-tonicity was ensured by
keeping the specimen in an isolated condition for 12 hours. Stimulus of
light for 20 seconds’ duration was applied at intervals of 2 minutes.
In the record (Fig. 55) the first two responses, not shown, were purely
positive. The third exhibited a positive A-effect, followed by the
negative response D-effect. The A-effect is thus seen fully unmasked.
In subsequent responses the A-effect became more and more overshadowed
by the D-effect. At the third response the masking is complete and the
excitatory negative response is at its maximum. The record of staircase
effect (Fig. 54) also exhibits a preliminary positive twitch at the
beginning of the series, which disappeared after the second response.

The modifying influence of tonic condition on response I find to be
of universal occurrence. In vigorous specimens the electric response
to stimulation is _negative_; but tissues in sub-tonic condition
give _positive_ response and after long-continued stimulation the
abnormal positive is converted into the normal _negative_. It is very
interesting that under condition of sub-tonicity diverse expressions
of physiological reaction exhibit similar change of sign of normal
response. Thus in my measurement of the velocity of transmission of
excitation in the conducting tissue of _Mimosa_, I find that, when
the tissue is in an optimum condition, exhibiting high velocity of
transmission, excessive stimulus has the effect of diminishing the
conducting power. But in a depressed condition of the tissue the
effect is precisely the opposite. Thus in a given case the velocity of
transmission was low; strong electric stimulation enhanced the rate by
33 per cent. In extreme cases of sub-tonicity, where the conducting
power was in abeyance, the excessive stimulus caused by wound not
only restored the power of conduction but raised the velocity of
transmission to 25 mm. per second (_Expt. 37_).


SUMMARY.

The excitability of a plant is found to be modified by its tonic
condition.

A sub-tonic specimen of _Mimosa_, like an excised bloodless muscle,
shows a preliminary staircase response. Stimulus induces simultaneously
both “A” and “D” effects, with their attendant positive and negative
reactions.

A tissue in optimum condition exhibits only the resultant negative
response, the comparatively feeble positive being masked by the
predominant negative. With decline of tone, the “D” effect diminishes
and we get “A” effect unmasked.

In extreme sub-tonic specimen, we get first only the “A” effect,
with its positive response. Successive stimulation converts the pure
positive into diphasic and ultimately into normal negative response.




PART II.

GROWTH AND ITS RESPONSIVE VARIATIONS.




X.--THE HIGH MAGNIFICATION CRESCOGRAPH FOR RESEARCHES ON GROWTH[S]

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS, L.M.S.


In discussing the difficulties connected with investigations relating
to longitudinal growth and its variations, special stress must be
laid on the importance of maintaining external conditions absolutely
constant. This constancy can only be maintained in practice for a
short time. Lengthy periods of observation, moreover, introduce the
uncertainty of complication arising from spontaneous variation of
growth. The possibility of accurate investigation, therefore lies in
reducing the period of the experiment to a few minutes during which we
have to determine the normal rate of growth and its variation under
a given changed condition. This would necessitate the devising of a
method of very high magnification for record of the rate of growth.[S]

[S] A short account of my researches with the High Magnification
Crescograph has been published in the _Proceedings_ of the Royal
Society. I shall in the following Papers give a detailed account of my
investigations on growth and on allied phenomena.

With auxanometers now in use, which give a magnification of about
twenty times, it takes nearly four hours to determine the influence of
changed condition in inducing variation of growth. It will be seen
that if we succeeded in enhancing magnification from twenty to ten
thousand times, the necessary period for experiment would be reduced
from four hours to thirty seconds. The importance of securing a
magnification of this order is sufficiently obvious.

The problem of high magnification was first solved by my Optical
Lever.[T] The tip of the growing organ was attached to the short arm of
a lever, the axis of which carried a small mirror; in this way it was
possible to obtain a magnification of a thousand times. The magnified
movement of growth was followed with a pen on a revolving drum. The
record laboured under the disadvantage of not being automatic. This
defect was overcome by the use of the photographic method which however
entailed the inconvenience and discomfort of a dark room.

[T] BOSE--“Plant Response,” p 412.

I have, for the past six years, been working with a different method,
which has now been brought to a great state of perfection. The problem
to be solved was the devising of a direct method of high magnification
and the automatic record of the magnified rate of growth.


METHOD OF HIGH MAGNIFICATION.

The magnification in my Crescograph is obtained by a compound system of
two levers. The growing plant is attached to the short arm of a lever,
the long arm of which is attached to the short arm of the second lever.
If the magnification by the first lever be _m_, and that by the second,
_n_, the resulting magnification would be _mn_.

The practical difficulties met with in carrying out this idea are
very numerous. It will be understood that just as the imperceptible
movement is highly magnified by the compound system of levers, the
various errors and difficulties are likely to be magnified in the
same proportion. The principal difficulties met with were due: (1) to
the weight of the compound lever which exerted a great tension on the
growing plant, (2) to the yielding of flexible connections by which
the plant was attached to the first lever, and the first lever to the
second, and (3) to the friction at the fulcrums.

_Weight of the Lever._--As the first lever is to exert a pull on
the second, it has to be made rigid. The second lever serves as an
index, and can therefore be made of fine glass fibre. The securing of
rigidity of the first lever entails large cross section and consequent
weight, which exerts considerable tension on the plant. Excessive
tension greatly modifies growth; even the weight of the index used
in self-recording auxanometers is found to modify the normal rate of
growth. The weight of the levers introduces an additional difficulty in
the increased friction at the fulcrums, on account of which there is an
obstruction of the free movement of the recording arm of the lever. The
conditions essential for overcoming these difficulties therefore are:
(1) construction of a very light lever possessing sufficient rigidity,
and (2) arranging the levers in such a way that the tension on the
plant may be reduced to any extent, or even eliminated.

I found in _navaldum_, an alloy of aluminium, a light material
possessing sufficient rigidity. The first lever is constructed out of a
thin narrow sheet 25 cm. in length; it has, as explained before, to be
fairly rigid in order to exert a pull on the second without undergoing
any bending; this rigidity is secured by giving the thin narrow plate
of the lever a T-shape. The first lever balances, to a certain extent,
the second. Finer adjustments are made by means of an adjustable
counterpoise B, at the end of the levers. By this means the tension on
the plant can be greatly reduced; or a constant tension may be exerted
by means of a weight T (Fig. 56). In my later type of the apparatus
the plant connection is made to the right, instead of the left side of
the first fulcrum. This gives certain practical advantages. The second
lever is then made practically to balance the first, only a very slight
weight being necessary for exact counterpoise. The reduction of total
weight thus secured reduces materially the friction at the fulcrum with
great enhancement of efficiency of the apparatus.

[Illustration: FIG. 56.--Compound lever. P, plant attached to short arm
of lever L; T, weight exerting tension; C, connecting link; L′, second
lever with bent tip for record; B, B, balancing counterpoise. Fork F,
carries at its side two conical agate cups, on which lever rests by two
pin-points. (From a photograph.)]

The second or the recording lever has a normal excursion through 8 cm.
on the recording surface, which is a very thin sheet of glass 8×8 cm.
coated with a layer of smoke. As the recording lever is about 40 cm.
in length, the curvature in the record is slight, and practically
negligible in the middle portion of 4 cm. The dimensions given allow
a magnification of ten thousand times. A far more compact apparatus
is made with 15 cm. length of levers. This gives a magnification of a
thousand times.


AUTOMATIC RECORD OF THE RATE OF GROWTH.

Another great difficulty in obtaining an accurate record of the curve
of growth arises from the friction of contact of the bent tip of
the writing lever against the recording surface. This I was able to
overcome by an oscillating device by which the contact, instead of
being continuous, was made intermittent. The smoked glass plate, G,
is made to oscillate, to and fro, at regular intervals of time, say
one second. The bent tip of the recording lever comes periodically in
contact with the glass plate during its extreme forward oscillation.
The record would thus consist of a series of dots, the distance between
successive dots representing magnified growth during a second.

The drawback in connection with the obtaining of record on the
oscillating plate lies in the fact that if the plate approaches the
recording point with anything like suddenness, then the stroke on
the flexible lever causes an after-oscillation; the multiple dots,
thus produced, spoil the record. In order to overcome this, a special
contrivance is necessary, by which the speed of approach of the plate
should be gradually reduced to zero at contact with the recording
point. The rate of recession should, on the other hand, continuously
increase from zero to maximum. The recording point will in this manner
be gently pressed against the glass plate, marking the dot, and then
gradually set free. It was only after strict observance of these
conditions that the disturbing effect of after-vibration of the lever
could be obviated.

[Illustration: FIG. 57.--Eccentric for oscillation of plate K, crank;
S, slide; P, holder for glass plate G. A, adjusting screws; L,
recording lever. Clock releases string C for lateral movement of the
plate. (From a photograph.)]

This particular contrivance consists of an eccentric rod actuated by a
rotating wheel. A cylindrical rod is supported eccentrically, so that
semi-rotation of the eccentric causing a pull on the crank K (Fig. 57)
pushes the plate carrier gradually forward. On the return movement of
the eccentric, a light antagonistic spring makes the plate recede.
The rate of the movement of the crank itself is further regulated by
the device of the revolving wheel. This is released periodically by
clockwork at intervals of one, two, five, ten, or fifteen seconds
respectively, according to the requirements of the experiment. The
complete apparatus is shown in figure 58.

[Illustration: FIG. 58.--Complete apparatus. P, plant; S, micrometer
screw for raising or lowering the plant; C, clockwork for periodic
oscillation of plate; W, rotating wheel. V, cylindrical plant-chamber.
(From a photograph.)]

_Connecting Links._--Another puzzling difficulty lay in the fact that
the magnification actually obtained was sometimes very different from
the calculated value. This unreliability I was able to trace to the
defects inherent in thread connections, employed at first to attach the
plant to the first lever, and the first lever to the second. These
flexible connections were found to undergo a variable amount of elastic
yield. Hence it became necessary to use nothing but rigid connections.
The plant attachment, A, of triangular shape is made of a piece of
_navaldum_; its knife-edge rests on a notch at the short arm of the
lever, L. There are several notches at various distances from the
fulcrum. It will be understood how the magnification can be modified
by moving A, nearer or further from the fulcrum. The lower end of the
attachment is bent in the form of a hook. The end of the leaf of the
plant P, is doubled on itself and tied. The loop thus formed is then
slipped over the hooked end of A.

The link, C, connecting L and L′, consists of a pin pointed at both
ends, which rests on two conical agate cups fixed respectively to
the upper and lower surfaces of the levers L and L′. This mode of
frictionless linking is rigid and allows at the same time perfectly
free movement of the levers.

_The fulcrum._--The most serious difficulty was in connection with
frictionless support of the axes of the two levers. The horizontal axis
was at first supported on jewel bearings, with fine screw adjustment
for securing lateral support. Any slight variation from absolute
adjustment made the bearing either too loose or too tight, preventing
free play of the lever. When perfect adjustment was secured by any
chance, the movement of the levers became jerky after a few days. This
I afterwards discovered was due to the deposit of invisible particles
of dust on the bearings. These difficulties forced me to work out a
very perfect and at the same time a much simpler device. The lever now
carries two vertical pin-points which are supported on conical agate
cups. The axis of the lever passes through the points of support.
The friction of support is thus reduced to a minimum. The levers are
kept in place under the constant pressure of their own weight. The
excursion of the end of the recording lever, which represents magnified
movement of growth, was now found to be without jerk and quite uniform.


EXPERIMENTAL ADJUSTMENTS.

The soil in a flower pot is liable to be disturbed by irrigation, and
the record thus vitiated by physical disturbance. This is obviated by
wrapping a piece of cloth round the root imbedded in a small quantity
of soil. The lower end of the plant is held securely by a clamp. In
order to subject the plant to the action of gases and vapours, or to
variation of temperature it is enclosed in a glass cylinder (V) with
an inlet and an outlet pipe (Fig. 58). The chamber is maintained in a
humid condition by means of a sponge soaked in water. Different gases,
warm or cold water vapours, may thus be introduced into the plant
chamber.

Any quick growing organ of a plant will be found suitable for
experiment. In order to avoid all possible disturbing action of
circumnutation, it is preferable to employ either radial organs, such
as flower peduncles and buds of certain flowers, or the limp leaves
of various species of grasses, and the pistils of flowers. It is also
advisable to select specimens in which the growth is uniform. I append
a representative list of various specimens in which, under favourable
conditions of season and temperature, the rates of growth may be as
high as those given below:--

    Peduncle of _Zephyranthes_             0.7  mm. per hour.
    Leaf of grass                          1.10 "    "   "
    Pistil of _Hibiscus_ flower            1.20 "    "   "
    Seedling of wheat                      1.60 "    "   "
    Flower bud of _Crinum_                 2.20 "    "   "
    Seedling of _Scirpus Kysoor_           3.00 "    "   "

The specimen employed for experiment may be an intact plant, rooted in
a flower pot. It is, however, more convenient to employ cut specimens,
the exposed end being wrapped in moist cloth. The shock-effect of
section passes off after several hours, and the isolated organ renews
its growth in a normal manner. Among various specimens I find _S.
Kysoor_ to be very suitable for experiments on growth. The leaves are
much stronger than those of wheat and different grasses, and can bear
a considerable amount of pull without harm. Its rate of growth under
favourable condition of season is considerable. Some specimens were
found to have grown more than 8 cm. in the course of twenty-four hours,
or more than 3 mm. per hour. This was during the rainy season in the
month of August. But a month later the rate of growth fell to about
1 mm. per hour.

I shall now proceed to describe certain typical experiments which will
show: (1) the extreme sensibility of the Crescograph; (2) its wide
applicability in different investigations; and (3) its capability in
determining with great precision the time-relations of responsive
changes in the rate of growth. In describing these typical cases, I
shall give detailed account of the experimental methods employed, and
thus avoid repetition in accounts of subsequent experiments.

_Determination of the absolute rate of growth: Experiment 51._--For
the determination of the absolute rate, I shall interpret the results
of a record of growth obtained with a vigorous specimen _S. Kysoor_ on
a stationary plate. The oscillation frequency of the plate was once
in a second, and the magnification employed was ten thousand times.
The magnified growth movement was so rapid that the record consists
of a series of short dashes instead of dots (Fig. 59A). For securing
regularity in the rate of growth, it is advisable that the plant
should be kept in uniform darkness or in uniformly diffused light. So
sensitive is the recorder that it shows a change of growth-rate due to
the slight increase of illumination by the opening of an additional
window. One-sided light, moreover, gives rise to disturbing phototropic
curvature. With the precautions described the growth-rate in vigorous
specimens is found to be very uniform.

[Illustration: FIG. 59.--Crescographic records: (A) successive records
of growth at intervals of one second (magnification 10,000 times).
(_a_) Effect of temperature on a stationary plate; N, normal rate of
growth; C, retarded rate under cold; H, enhanced rate under warmth;
(_b_) record on moving plate, where diminished slope of curve denotes
retarded rate under cold (Magnification 2,000 times.)]

After the completion of the first vertical series, the recording plate
was moved 1 cm. to the left; the tip of the recorder was brought
once more to the top by the micrometer screw, S, (Fig. 58), and the
record taken once more after an interval of 15 minutes. The magnified
growth for 4 seconds is 38 mm. in the first record; it is precisely
the same in the record taken fifteen minutes after. The successive
growth elongations at intervals of 1 second is practically the same
throughout, being 9.5 mm. This uniformity in the spacings demonstrates
not only the regularity of growth under constant conditions, but also
the precision of the apparatus. It also shows that by keeping the
external condition constant, the normal growth-rate could be maintained
uniform for at least fifteen minutes. The magnified rate of growth
is nearly 1 cm. per second, and since it is quite easy to measure
0.5 mm. the Crescograph enables us to magnify and record a length of
0.0005 mm. that is to say, the sixteenth part of a wave of red light.
The absolute rate of growth, moreover, can be determined in a period as
short as 0.05 of a second. These facts will give some idea of the great
possibilities of the Crescograph for future investigations.

As the period of experiment is very greatly shortened by the method of
high magnification, I shall, in the determination of the absolute rate
of growth, adopt a second as the unit of time, and µ, or _micron_, as
the unit of length,--the micron, being a millionth part of a metre or a
thousandth part of a millimeter.

If _m_ be the magnifying power of the compound lever and _l_, the
average distance between successive dots in mm. at intervals of _t_
seconds then:--

    the rate of growth = _l_/_mt_ × 10^{3}µ per second.

    In the record given _l_ = 9.5 mm.
                        _m_ = 10,000.
                        _t_ = 1 second.

    Hence the rate of growth = 9.5/10,000 × 10^{3}µ per sec.
                             = 0.95µ per sec.

Having demonstrated the extreme sensitiveness and reliability of the
apparatus, in quantitative determination, I shall next proceed to show
its wide applicability for various researches relating to the influence
of external agencies in modification of growth. For this two different
methods are employed. In the first of these methods, the records are
taken on a stationary plate: of these the record is at first taken
under normal condition, the subsequent series being obtained under
the given changed condition; the increase or diminution of intervals
between successive dots, in the two series, at once demonstrates the
stimulating or depressing nature of the changed condition.

In the second method, the record is taken on a plate moving at an
uniform rate by clockwork. A curve is thus obtained, the ordinate
representing growth elongation and the abscissa the time. The increment
of length divided by the increment of time gives the absolute rate of
growth at any part of the curve. As long as the growth is uniform, so
long the slope of the curve remains constant. If a stimulating agency
enhances the rate of growth, there is an immediate upward flexure in
the curve; a depressing agent, on the other hand, lessens the slope of
the curve.

I shall now give a few typical examples of the employment of the
Crescograph for investigations on growth: the first example I shall
take is the demonstration of the influence of variation of temperature.

_Stationary method: Experiment 52._--The records, given in Fig. 59_a_,
were taken on a stationary plate. The specimen was _S. Kysoor_; the
Crescographic magnification was two thousand times, and the successive
dots at intervals of 5 seconds. The middle series, N, was at the
temperature of the room. The next, C, was obtained with the temperature
lowered by a few degrees. Finally H was taken when the plant-chamber
was warmed. It will be seen how under cooling the spaces between
successive dots have become shortened, showing the diminished rate of
growth. Warming, on the other hand, caused a widening of intervals
between successive dots, thus demonstrating an enhancement of the rate
of growth.

Calculating from the data obtained from the figure we find:--

    The absolute value of the normal rate   0.457µ per second.
    Diminished rate under cold              0.101µ  "     "
    Enhanced rate under warmth              0.737µ  "     "

_Moving plate method: Experiment 54._--This was carried out with a
different specimen of _S. Kysoor_, the record being taken on a moving
plate (Fig. 59_b_). The first part of the curve here represents the
normal rate of growth. The plant was then subjected to moderate
cooling, the subsequent curve with its diminished slope denotes the
depression of growth. The question of influence of temperature will be
treated in a subsequent Paper of the present series in much greater
detail.

[Illustration: FIG. 60--Horizontal record shows absence of growth in
a dead branch; physical expansion on application of warmth at arrow
followed by horizontal record on attainment of steady temperature.
(Magnification 2,000 times)]

_Precaution against physical disturbance: Experiment 54._--There may
be some misgiving about the employment of such high magnification: it
may be thought that the accuracy of the record might be vitiated by
physical disturbance, such as vibration. In physical experimentation
far greater difficulties have, however, been overcome, and the problem
of securing freedom from vibration is not at all formidable. The whole
apparatus need only be placed on a heavy bracket screwed on the wall
to ensure against mechanical disturbance. The extent to which this has
been realized will be found from the inspection of the first part of
the record in figure 60, taken on a moving plate. A thin dead twig was
substituted for the growing plant, and the perfectly horizontal record
not only demonstrated the absence of growth movement but also of all
disturbance. There is an element of physical change, against which
precautions have to be taken in experiments on variation of the rate of
growth at different temperatures. In order to determine its character
and extent, a record was taken with the dead twig, of the effect of
raising the temperature of the plant-chamber through ten degrees. The
record (Fig. 60) with a magnification of two thousand times shows that
there is an expansion during the rise of temperature, and that the
variable period lasted for a minute, after which there was a cessation
of physical movement, the record becoming once more horizontal. The
obvious precautions to be taken in such a case, is to wait for several
minutes for the attainment of steady temperature. The movement caused
by physical change abates in a short time whereas the change of rate of
growth brought about by physiological reaction is persistent.


DETERMINATION OF LATENT PERIOD AND TIME-RELATIONS OF RESPONSE.

[Illustration: FIG. 61.--Time-relations of response of growing organ
to electric stimulus of increasing intensities applied at the short
horizontal lines. Successive dots at intervals of 2 seconds.]

_Experiment 55._--In the determination of time-relations of responsive
change in growth under external stimulus, I shall take the typical case
of the effect of electric shock from a secondary coil of one second’s
duration. Two electrodes were applied, one above and the other below
the growing region of a bud of _Crinum_. The record was taken on a
moving plate, the magnification employed being two thousand times, and
successive dots made at intervals of two seconds. It was a matter of
surprise to me to find that the growth of the plant was affected by
an intensity of stimulus far below the limit of our own perception.
As regards the relative sensitiveness of plant and animal, some of
my experiments show that the leaf of _Mimosa pudica_ in a favourable
condition responds to an electric stimulus which is one-tenth the
minimum intensity that causes perception in a human being. For
convenience I shall designate the intensity of electric shock that
is barely perceptible to us as the unit shock. When an intensity of
0.25 unit was applied to the growing organ, it responded to it by a
retardation of growth. Inspection of Fig. 61 shows that there is a
flexure induced in the curve in response to stimulus, the flattening
of the curve denoting retardation of growth. The latent period, in
this case, is 6 seconds. The normal rate was restored after 5 minutes.
The intensity of shock was next raised from 0.25 unit to one unit. The
second record shows that the latent period is reduced to 4 seconds,
and a relatively greater retardation of growth was induced by the
action of the stronger stimulus. The recovery of the normal rate was
effected after the longer period of 10 minutes. I took one more record,
the intensity being three units. The latent period was now reduced
to 1 second, and the induced retardation was so great as to effect a
temporary arrest of growth.

TABLE X.--TIME-RELATIONS OF RESPONSIVE GROWTH-VARIATION UNDER ELECTRIC
SHOCK (_Crinum_).

    +------------+--------------+---------------+----------------+
    |Intensity of|Latent period.| Normal rate.  | Retarded rate. |
    |stimulus.   |              |               |                |
    +------------+--------------+---------------+----------------+
    | 0.25 unit. |  6 seconds.  |0.62 µ per sec.|0.49 µ per sec. |
    | 1     "    |  4    "      |0.62    "      |0.25    "       |
    | 3     "    |  1    "      |0.62    "      |Temporary arrest|
    |            |              |               |of growth.      |
    +------------+--------------+---------------+----------------+

It is thus found that growth in plants is affected by an intensity of
stimulus which is below human perception; that with increasing stimulus
the latent period is diminished and the period of recovery increased;
and that the induced retardation of growth increases continuously with
the stimulus till at a critical intensity there is a temporary arrest
of growth. I shall speak later of the effect induced by stimulus above
this critical point.

[Illustration: FIG. 62.--Record of a single growth-pulse of
_Zephyranthes_ (Magnification 10,000 times.)]

_Experiment 56._--As a further example of the capability of the
Crescograph, I shall give the record of a single pulse of growth
obtained with the peduncle of _Zephyranthes Sulphurea_ (Fig. 62). The
magnification employed was 10,000 times, the successive dots being
at intervals of one second. It will be seen that the growth pulse
commences with a sudden elongation, the maximum rate being 0.4 µ per
sec. The pulse exhausts itself in 15 seconds, after which there is
a partial recovery in the course of 13 seconds. The period of the
complete pulse is 28 seconds. The resultant growth in each pulse is
therefore the difference between elongation and recovery. Had a very
highly magnifying arrangement not been used, the resulting rate would
have appeared continuous. In other specimens, owing probably to greater
frequency of pulsation and co-operation of numerous elements in growth,
the rate appears to be practically uniform.

_Advantages of the Crescograph._--There is no existing method which
enables us to detect and measure such infinitesimal movements and their
time-relations. The only attempt made in measuring minute growth has
been by observing the movement of a mark on a growing plant through
a microscope. The magnification available in practice is about 250
times. The observation of the movement would itself be sufficiently
fatiguing. But a simultaneous estimate of the time-relations of
rapidly fluctuating changes would prove so bewildering, that accurate
results from this method would be altogether impossible. A 1/12″
objective gives a linear enlargement of about 1,200 times. But the
employment of this objective is impracticable in the measurement of
growth elongation of an ordinary plant. With the Crescograph, on the
other hand, we obtain a magnification which far surpasses the highest
powers of a microscope, and it can be used for all plants. It does not
merely detect growth but automatically records the rate of growth and
its slightest fluctuation. The extreme shortness of time required for
an experiment renders the study of the influence of a single factor
at a time possible, the other conditions being kept constant. The
Crescograph thus opens out a very extensive field of inquiry into the
physiology of growth; and the discovery of several important phenomena
mentioned in this Paper is to be ascribed to the extreme sensitiveness
of the apparatus, and the accuracy of the method employed.


MAGNETIC AMPLIFICATION.

The magnification obtained with two levers was, as stated before,
10,000 times. It may be thought that further magnification is possible
by a compound system of three levers. There is, however, a limit to
the number of levers that may be employed with advantage, for the
slight overweight of the last lever becomes multiplied and exerts
very great tension on the plant, which interferes with the normal
rate of its growth. The friction at the bearings also becomes added up
by an increase in the number of levers, and this interferes with the
uniformity of the movement of the last recording lever. For securing
further magnification, additional material contact has, therefore, to
be abandoned. I have recently been successful in devising an ideal
method of magnification without contact. The movement of the lever of
the Crescograph upsets a very delicately balanced magnetic system.
The indicator is a reflected spot of light from a mirror carried by
the deflected magnet. Taking a single lever with the lengths of two
arms 125 mm. and 2.5 mm. respectively we obtain a magnification of 50
times. The magnetic system gives a further magnification of 20,000
the total magnification being thus a million times. This was verified
by moving by means of a micrometer screw the short arm of the lever
through 0.005 mm. The resulting deflection of the spot of light at a
distance of 4 metres was found to be 5,000 mm. or a million times the
movement of the short arm. It is not difficult to produce a further
magnification of 50 times by attaching a second lever to the first. The
total magnification would in this case be 50 million times.

A concrete idea of this will be obtained when we realise that by the
Magnetic Crescograph a magnification can be obtained which is about
50,000 times greater than that produced by the highest power of a
microscope. This order of magnification would lengthen a wave of sodium
light to about 3,000 cm. I am not aware of any existing method by which
it is possible to secure an amplification of this order of magnitude.
The application of this will undoubtedly be of great help in many
physical investigations, some of which I hope to complete in the near
future.

Such an enormous magnification cannot be employed in ordinary
investigations on growth, for the moving spot of light indicating
rate of growth, passes like a flash across the screen. But it is of
signal service in my investigations on growth by the Method of Balance,
to be described in a future Paper. The principle of this method
consists in making the spot of light, which is moving in response to
growth, stationary, by subjecting the plant to a compensating movement
downwards. The slightest variation caused by an external agent would
make the spot of light move either to the right or to the left,
according to the stimulating or depressing character of the agent.
It will be understood, how extremely sensitive this method is for
detection of the most minute variation in the normal rate of growth.


THE DEMONSTRATION CRESCOGRAPH.

Before proceeding with accounts of further investigations, I shall
describe a form of Magnetic Crescograph with which I have been able to
give before a large audience demonstration of a striking character on
various phenomena of growth. The magnification obtained was so great
that I had to take some trouble in reducing it. This was accomplished
by the employment of a single, instead of a compound system of two
levers. The reflected spot of light was thrown on a screen placed at
a distance of 4 metres, and this gave a magnification of a million
times; it is obvious that an increase of the distance of the screen
to 8 metres would have given a magnification of 2 million times. As
it was, even the lower magnification was far too great for use with
quick growing plants like _Kysoor_. I, therefore, employed the slower
growing flower bud of _Crinum_. It will be seen from Table X that the
normal rate of growth of the lily is of the order of 0.0006 mm. per
second. The normal excursion of the spot of light reflected from the
Crescograph exhibiting growth was found to be 3 metres in five seconds
or 60 cm. per second. This is a million times the actual rate of growth
of the _Crinum_ bud. As it is easy to measure 5 mm. in the scale, it
will be seen that with the Demonstration Crescograph it is possible to
detect the growth of a plant for a period shorter than a hundredth part
of a second.

_Experiment 57._--A scale 3 metres long divided into cm. is placed
against the screen. A metronome beating half seconds is started at
the moment when the spot of light transits across the zero division;
the number of beats is counted till the index traverses the 300 cm.
At the normal temperature of the room (30°C.), the index traversed
300 cm. in five seconds. The plant chamber was next cooled to 26°C. by
the blowing in of cooled water vapour; the time taken by the spot of
light to traverse the scale was now 20 seconds, _i.e._, the growth-rate
was depressed to a fourth. Under continuous lowering of temperature
the growth-rate became slowed down till at 21°C. there was an arrest
of growth. Warm vapour was next introduced, gradually raising the
temperature of the chamber to 35°C. The spot of light now rushed across
the scale in a second and a half, _i.e._, the growth was enhanced to
more than three times the normal rate. The entire series of the above
experiments, on the effect of temperature on growth, was thus completed
in the course of 15 minutes.


SUMMARY.

A description is given of the High Magnification Crescograph, which
enables an automatic record of growth magnified ten thousand times. The
absolute rate of growth can be easily determined from the data given in
the record.

A magnification of a million times is obtained by the employment of
Magnetic amplification. An increment of growth so minute as a millionth
part of a mm. or 0.00000004 inch may thus be detected. It is also
possible to detect the growth of a plant for a period shorter than a
hundredth part of a second.

The influence of external conditions on variation of rate of growth is
obtained by two methods of record. In STATIONARY METHOD, the increase
or diminution of the distance between successive dots representing
magnified rate of growth, demonstrates the stimulating or depressing
nature of the changed condition.

In the second, or MOVING PLATE METHOD, a curve is obtained, the
ordinate representing growth elongation, and the abscissa, time. A
stimulating agent causes an upward flexure of the normal curve; a
depressing agent, on the other hand, lessens the slope of the curve.

The action of external stimulus induces a variation of the rate of
growth, the time relations of which are found from the automatic record
of the growth. The latent period is shortened with the intensity of the
stimulus. A responsive variation of growth is induced by an intensity
of stimulus which is below human perception.

It is often possible to obtain record of the pulsatory nature of
growth-elongation. Thus, with the growing peduncle of _Zephyranthes_,
the growth pulse commences with a sudden elongation, the maximum rate
being 0.0004 mm. per second. The pulse exhausts itself in 15 seconds,
after which there is a partial recovery in course of 13 seconds, the
period of complete pulse being 28 seconds. The resultant growth in each
pulse is the difference between elongation and recovery.

The Magnetic Crescograph enables demonstration of principal phenomena
of growth and its variation before a large audience.




XI.--EFFECT OF TEMPERATURE ON GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHUNDER DASS, M.A.


Accurate determination of the effect of temperature on growth presents
many serious difficulties on account of numerous complicating factors.
In nature, the upper part of the plant is exposed to the temperature of
the air, while the root underground is at a very different temperature.
Growth, we shall find, is modified to a certain extent by the ascent
of sap. (See p. 189, _Expt. 70_.) The activity of this latter process
is determined by the temperature to which the roots are subjected. The
difficulty may be removed to a certain extent by placing the plant in
a thermal chamber, with arrangement for regulating the temperature
of the air. The air is a bad conductor of heat, and there is some
uncertainty of the interior of the plant attaining the temperature of
the surrounding air, unless the plant is long exposed to the definite
and constant temperature of the plant chamber. Observation of the
effects of different temperatures then becomes a prolonged process,
with the possibility of vitiation of results by autonomous variation
of growth. Reduction of the period of experiment by rapidly raising
the temperature of the chamber introduces fresh difficulties; for a
sudden variation of temperature often acts like an excitatory shock.
This drawback may to some extent be obviated by ensuring a gradual
change of temperature. This is by no means an easy process, for even
with care the rise of temperature of the air cannot be made perfectly
uniform, and any slight irregularity gives rise to sudden fluctuations
in the magnified record of growth. Another difficulty arises from the
radiation of heat-rays from the sides of the thermal chamber. These
rays, I shall in a different Paper show, induce a retardation of
growth. The effect of rise of temperature in acceleration of growth
is thus antagonised by the action of thermal radiation. This trouble
may be minimised by having the inner surface of the thermal chamber of
bright polished metal, since the radiating power of a polished surface
is relatively feeble.

The contrivance which I employ for ensuring a gradual rise of
temperature, consists of a double-walled cylindrical metallic vessel;
the plant is placed in the inner chamber, the walls of which are coated
with electrically deposited silver and polished afterwards, and at
the bottom of which there is a little water. The space between the
inner and outer cylinder is filled with water, in which is immersed a
coiled copper pipe. Hot water from a small boiler enters the inlet of
the coiled pipe and passes through the outlet at the lower end. The
water in the outer cylinder is thus gradually raised by flow of hot
water in the coiled pipe. The rate of flow of hot water, on which the
rate of rise of temperature depends, is regulated by a stop-cock. The
air of the inner chamber in which the plant is placed, may thus be
adjusted for a definite temperature. The small quantity of water in the
inner chamber keeps its air in a humid condition, since dry hot air by
causing dessication interferes with normal growth.


METHOD OF DISCONTINUOUS OBSERVATION.

[Illustration: FIG. 63.--Effect of temperature on growth, and
determination of optimum temperature.]

_Experiment 58._--High magnification records are taken for successive
periods of ten seconds, for selected temperatures, maintained constant
during the particular observation. In figure 63 is given records of
rate of growth obtained with a specimen of _Kysoor_ at certain selected
temperatures. It will be seen that the rate of growth increases with
the rise of temperature to an optimum, beyond which the growth-rate
undergoes a depression. In the present case the optimum temperature is
in the neighbourhood of 35°C.


METHOD OF CONTINUOUS OBSERVATION.

The method of observation that I have described above is not ideally
perfect, but the best that could be devised under the circumstances.
A very troublesome complication of pulsations in growth, arises at
high temperatures, which render further record extremely difficult.
Growth is undoubtedly a pulsatory phenomenon; but under favourable
circumstances, these merge practically into a continuous average
rate of elongation. At a high temperature the effect of certain
disturbing factors comes into prominence. This may be due to some
slight fluctuation in the temperature of the chamber, or to the effect
of thermal radiation from the side of the chamber. This disturbing
influence is most noticed at about 45°C, rendering the record of growth
above this point a matter of great uncertainty. It will presently be
shown that in plants immersed in water-bath growth is often found to
persist even up to 57°C.

The only way of removing the complication arising from thermal
radiation lies in varying the temperature condition of the plant, by
direct contact with water at different temperatures. This procedure
will also remove uncertainty regarding the body of the plant assuming
the temperature of surrounding non-conducting air. The disturbing
effect of sudden variation of temperature is also obviated by a
more uniform regulation of rise of temperature. The inner cylinder
containing the plant is filled with water; heat from gradually warmed
water in the outer cylinder is conducted across the inner cylinder made
of thin copper and raises the temperature of the water contained in the
inner cylinder with great uniformity. A clock-hand goes round once in
a minute; the experimenter, keeping his hand on the stop-cock, adjusts
the rate of rise of water in the inner cylinder, so that there is a
rise, say, of one-tenth of a degree every 6 seconds or of one degree
every minute. The mass of water acts as a governor, and prevents any
sudden fluctuations of temperature. The adoption of this particular
device eliminated the erratic changes in the rate of growth that had
hitherto proved so baffling.

The elongation recorded by the Crescograph will now be made up of (1)
physical expansion, (2) expansion brought about by absorption of water,
and (3) the pure acceleration of growth. The disentanglement of these
different elements presented many difficulties. I was, however, able to
find out the relative values of the first two factors in reference to
the elongation of growth. This was done by carrying out a preliminary
experiment with a specimen of plant in which growth had been completed.
It was raised through 20°C in temperature, records being taken both
at the beginning and at the end. This was for obtaining a measure of
the physical change due to temperature, and also of the change brought
about by absorption of water. I should state here that for the method
of continuous record of growth which I contemplated, the record had to
be taken for about 18 minutes. The magnification had to be lowered to
250 times to keep the record within the plate. With this magnification,
the fully grown specimen did not show in the record a change even of
1 mm. in length in 18 minutes, while the growing plant under similar
circumstances exhibited an elongation of 100 mm. or more. In records
taken with low magnification, the effect of physical change is quite
negligible.


DETERMINATION OF THE CARDINAL POINTS OF GROWTH.

The cardinal points of growth are not the same in different plants;
they are modified in the same species by the climate to which the
plants are habituated; the results obtained in the tropics may thus
be different from those obtained in colder climates. At the time of
the experiment, the prevailing temperature at Calcutta in day time was
about 30°C.

_Temperature minimum: Experiment 59._--For the determination of the
minimum, I took a specimen of _S. Kysoor_, and subjected it to a
continuous lowering of temperature, by regular flow of ice-cold water
in the outer vessel of the plant-chamber. Record was taken on a moving
plate for every degree fall of temperature; growth was found to be
continuously depressed, till an arrest of growth took place at 22°C.
(Fig. 64).

The arrested growth was feebly revived at 23°C., after which with
further rise of temperature there was increased acceleration. The
optimum point was reached at about 34°C. In some plants the optimum is
reached at about 28°C., and the rate remains constant for the next 10
degrees or more.

[Illustration: FIG. 64.--Record of effect of fall of temperature from
30°C. to arrest of growth at 22°C.]

[Illustration: FIG. 65.--Effect of rise of temperature from 53°C. to
60°C. A sudden contraction, indicative of death-spasm, takes place at
60°C.]

_Temperature maximum: Experiment 60._--For the determination of the
maximum, the temperature was raised much higher. At 55°C. growth was
found to be greatly retarded with practical arrest at 58°C. At 60°C.
there occurred a sudden spasmodic contraction (Fig. 65), which I
have shown elsewhere to be the spasm of death. This mechanical spasm
at 60°C. is also strikingly shown by various pulvinated organs. An
electric spasm of galvanometric negativity, and a sudden diminution of
electrical resistance also take place at the critical temperature of
60°C.[U]

[U] BOSE--“Plant Response,” p. 168; “Comparative Electro-Physiology,”
p. 202, p. 546.

I have described the immediate effect at the critical point. Long
maintenance at a temperature few degrees below 60°C. will no doubt be
attended with the death of the organ. Fatigue is also found to lower
the death-point.


THE THERMO-CRESCENT CURVE.

_Experiment 66._--I was next desirous of devising a method by which
an automatic and continuous record of the plant should enable us to
obtain a curve, which would give the rate of growth at any temperature,
from the arrested growth at the minimum to a temperature as high as
40°C. In order to eliminate the elements of spontaneous variation, the
entire record had to be completed within a reasonable length of time,
say about 18 minutes for a rise of as many degrees in temperature.
This gives a rate of rise of 1°C. for one minute. Separate experiments
showed that at this rate of _continuous_ rise of temperature there is
practically no lag in the temperature assumed by thin specimens of
plants. For observation during a limited range I use the slower rate
of rise at 1°C. per two minutes. But the result obtained by slower
rise was found not to differ from that obtained with one degree rise
per minute. The curve of growth is taken on a moving plate, which
travels 5 mm. per minute. Successive dots are made by the recording
lever at intervals of a minute during which the rise of temperature is
1°C. A _Thermo-crescent Curve_ is thus obtained, the ordinate of which
represents increment of growth, and the abscissa, the time. As the
temperature is made to rise one degree per minute, the abscissa also
represents rise of temperature (Fig. 66). The vertical distance between
two successive dots thus gives increment of growth in one minute for 1
degree rise of temperature from T to T′. If _l_ represents this length,
_t_ the interval of time (here 60 sec.), and _m_ the magnifying power
of the recorder, then the rate of growth for the mean temperature
(T+T′)/2 is found from the formula: rate of growth at (T+T′)/2 =
_l_/(_m × t × 60_)10^{3}µ per sec.

[Illustration: FIG. 66.--The Thermo-crescent Curve.]

TABLE XI.--RATE OF GROWTH FOR DIFFERENT TEMPERATURES.

    +-------------+----------------+-------------+----------------+
    | Temperature.| Growth.        | Temperature.| Growth.        |
    +-------------+----------------+-------------+----------------+
    | 22°C        | 0.00 µ per sec.| 31°C        | 0.45 µ per sec.|
    | 23°C        | 0.02 µ  "   "  | 32°C        | 0.60 µ  "   "  |
    | 24°C        | 0.04 µ  "   "  | 33°C        | 0.80 µ  "   "  |
    | 25°C        | 0.06 µ  "   "  | 34°C        | 0.92 µ  "   "  |
    | 26°C        | 0.08 µ  "   "  | 35°C        | 0.84 µ  "   "  |
    | 27°C        | 0.12 µ  "   "  | 36°C        | 0.64 µ  "   "  |
    | 28°C        | 0.16 µ  "   "  | 37°C        | 0.48 µ  "   "  |
    | 29°C        | 0.22 µ  "   "  | 38°C        | 0.30 µ  "   "  |
    | 30°C        | 0.32 µ  "   "  | 39°C        | 0.16 µ  "   "  |
    +-------------+----------------+-------------+----------------+

I give in figure 67 a curve showing the relation between temperature
and growth.

[Illustration: FIG. 67.--Curve showing relation between temperature and
rate of growth.]

It will thus be seen that, in the course of an experiment lasting
about twenty minutes, data have been obtained which enable us to
determine the rates of growth through a wide range of temperature.
We have likewise been able by the first method to make very accurate
determinations of the temperature maximum and minimum. In short, by
adopting the methods described, the cardinal points of growth and the
rate of growth at any temperature, may be determined with a precision
unattainable by the older methods, of averages or of prolonged
observation.


SUMMARY.

Temperature induces variation in the rate of growth. In accurate
determination of the growth, the disturbing effect of radiation of heat
has not been eliminated.

A continuous record of growth under uniform rise of temperature gives
the Thermo-crescent curve, from which the rate of growth at any
temperature may be deduced.

Different plant-tissues exhibit characteristic differences in their
cardinal points of growth. In _Kysoor_, growth is arrested at the
temperature minimum of 22°C. The optimum temperature is at 34°C., after
which growth-rate declines and becomes completely arrested at 58°C. At
60°C. there is a sudden spasmodic contraction of death.

In other plants the cardinal points are different. In some plants the
optimum growth is attained at 28°C. and remains constant up to 38°C.




XII.--THE EFFECT OF CHEMICAL AGENTS ON GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.


Chemical agents are found to exert characteristic actions on growth.
The method of investigation sketched here opens out an extended field
of investigation. The effect of a chemical substance, I find, to be
modified by (1) the strength of the solution, (2) the duration of
application, and (3) the condition of the tissue. A poisonous substance
in minute doses is often found to exert a stimulating action. Too long
continued action of a stimulant, on the other hand, exerts a depressing
effect. The influence of the tonic condition is shown by the fact
that while a given dilution of a poisonous substance kills a weak
specimen, the same poisonous solution, applied to a vigorous specimen,
actually stimulates and enhances the rate of the growth. I give below
descriptions of a few typical reactions.

The reagent, when in a liquid form, is locally applied on the growing
organ. The records, taken before and after the application, exhibit the
stimulatory or depressing character of the reagent. A different method
of application of the reagent is employed for plants with extended
region of growth. The specimen is then enclosed in a glass cylinder,
with inlet and outlet pipes. The cylinder is first filled with water,
and the normal rate of growth recorded. This rate remains constant for
several hours; but prevention of access of air for too long a time
affects the normal growth. After obtaining normal record, water charged
with the giving chemical agent is passed into the cylinder; and the
subsequent record shows the characteristic effect of the reagent. The
introduction of a gas into the chamber offers no difficulty.


EFFECT OF STIMULANTS.

_Hydrogen Peroxide: Experiment 62._--This reagent, as supplied by
Messrs. Parke Davis & Co., was diluted to 1 per cent. and applied to
the growing plant. Its stimulating action on growth is demonstrated in
the right hand record of Fig. 68_a_, where the rate of growth is seen
enhanced two and a half times the normal rate.

[Illustration: FIG. 68.--Effect of chemical agents: (_a_) Acceleration
of growth under H_{2}O_{2}, (_b_) Effect of NH_{3}, preliminary
acceleration followed by retardation. (_c_) Effect of ether (E) and
recovery (A).]

_Ammonia: Experiment 63._--The immediate effect of dilute vapour of
this reagent is an enhancement of growth, seen in the middle record of
Fig. 68_b_, where the rate is seen to be double the normal. Continued
action, however, caused a depression; the third record of this series
shows this, where the reduction is three-fourths of the normal rate.


EFFECT OF ANÆSTHETICS.

[Illustration: FIG. 69.--Effect of CO_{2}. (_a_) Normal record; (_b_)
immediately after application of CO_{2}, and (_c_) 15 minutes after.]

_Ether: Experiment 64._--In Fig. 68_c_, the records exhibit the effect
of introduction of ether vapour into the plant chamber, and its
recovery after the removal of the vapour. Ether is seen to depress the
rate of growth to a little more than a third of the normal rate. The
recovery is seen to be nearly complete half an hour after the removal
of the vapour.

_Carbonic Acid: Experiment 65._--The action of this gas is very
remarkable. The plant was immersed in water and normal record taken;
the plant chamber was now filled with water, charged with carbonic acid
gas. This induced a very marked acceleration of growth (Fig. 69). In a
seedling of Onion, the increase was found to be two and a half times.
In the flower bud of _Crinum_, the rate was found enhanced threefold
from the normal 0.25 µ to 0.75 µ per second. After this preliminary
enhancement, there was a depression of growth within 15 minutes of the
application, the rate being now reduced to 0.15 µ per second. These
effects were found to take place equally in light or in darkness.


ACTION OF DIFFERENT GASES.

_Coal Gas: Experiment 66._--Coal gas induces a depression. It is
curious that subjection to the action of this gas does not produce so
evil an effect as one would expect. The introduction of the gas had
reduced the growth-rate to more than half; but there was a recovery
half an hour after the introduction of fresh air.

_Sulphuretted Hydrogen: Experiment 67._--This gas not only exerts a
depressing effect, but its after-effect is also very persistent. The
plant experimented on was very vigorous and its rate of growth was
depressed to half by subjection to the action of the gas for a short
time. The record taken half an hour after the introduction of fresh air
did not exhibit any recovery.


ACTION OF POISONS.

_Ammonium Sulphide: Experiment 68._--This reagent in dilute solution
retards growth, and in stronger solution acts as a poison. The
following results were obtained with a wheat seedling under different
strengths of solution:--

    Normal rate                   0.30 µ per sec.
      0.5 per cent. solution      0.15 µ  "   "
      2.0  "   "       "          0.08 µ  "   "

_Copper Sulphate: Experiment 69._--The effect of a solution of this
reagent is far more depressing than the last. One per cent. solution
acting for a short time depressed the rate from 0.45 µ to 0.13 µ per
sec. Long continued action of the poisonous solution kills the plant.


SUMMARY.

The effect of a chemical agent is modified by the strength of the
solution, the duration of application and the tonic condition of the
tissue.

Dilute solution of hydrogen peroxide induces an acceleration of growth.

The action of dilute vapour of ammonia is a preliminary enhancement
followed by depression of growth.

Ether vapour depresses the rate of growth. On the removal of the vapour
there is a recovery of the normal rate.

The effect of carbonic acid is a great enhancement of the rate of
growth; after this preliminary action, growth undergoes a decline. The
effect described takes place equally in light or in darkness.

Coal gas induces a depression of the rate of growth from which there
is a recovery after the removal of the gas. The action of sulphuretted
hydrogen is far more toxic, the after-effect being very persistent.

Solution of ammonium sulphide induces increasing retardation of growth,
with the strength of the solution. Copper sulphate solution acts as a
toxic agent, retarding the rate of growth and ultimately killing the
plant.




XIII.--EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH

_By_

SIR J. C. BOSE.


The movements of leaves of sensitive plants are caused by variation
of turgor in the pulvinus induced by stimulus. The down movement or
_negative_ response of _Mimosa_ is caused by a diminution or negative
variation of turgor, while the erection or _positive_ response is
brought about by an increase, or positive variation of turgor.

We shall now investigate the change induced in a growing organ in the
rate of growth by variation of turgor. Turgor may be increased by
enhancing the rate of ascent of sap or by an artificial increase of
internal hydrostatic pressure. A diminution of turgor may, on the other
hand, be produced by withdrawal of water through plasmolysis. In order
to maintain a constant terminology I shall designate an increase, as
the positive, and a diminution, the negative variation of turgor.


RESPONSE TO POSITIVE VARIATION OF TURGOR.

In experimenting with _Mimosa_ the plant was subjected to the condition
of drought, water being withheld for a day. On supplying water, the
leaf, after a short period, exhibited a positive or erectile movement
(_Expt. 12_). The delay was evidently due to the time taken by the
water absorbed by root to reach the responding organ.

[Illustration: FIG. 70.--Effect of irrigation: D. record of growth
under drought, C. acceleration after irrigation with cold water, H.
enhanced acceleration on irrigation with warm water. (_S. Kysoor_)]

_Method of Irrigation: Experiment 70._--In order to investigate the
effect of enhanced turgor on growth, I took a specimen of _Kysoor_
which had been dug up with an attached quantity of soil; this latter
was enclosed in a small bag. The plant was then securely clamped
and fixed on a stand. This precaution was taken to prevent upward
displacement by the swelling of the soil in flower pot of the plant
under irrigation. The specimen was then subjected to a condition of
drought, water being withheld for a day. The depressed rate of growth
is seen in record (Fig. 70). Ordinary cold water was now applied at the
root, the effect of which is seen in record C. Finally the record (H)
was obtained after irrigation with tepid water. It will be seen that
the spaces between successive dots, representing magnified growth at
intervals of ten seconds, are very different. While a given elongation
took place under drought in 19 × 10 seconds, a similar lengthening took
place, after irrigation with cold water, in 13 × 10 seconds, and after
irrigation with warm water in 3 × 10 seconds. Irrigation with warm
water is thus seen to increase the rate of growth more than six times.

The enhancement of the rate of growth on irrigation with cold water
took place after seventy seconds. The interval will obviously depend
on the distance between the root by which the water is absorbed and
the region of growth. It will further depend on the activity of the
process of the ascent of sap. The time interval is greatly reduced
when this activity is in any way increased. Thus the responsive growth
elongation after application of warm water was very much quicker;
in the case described it was less than 20 seconds. With regard to
application of warm water, the variation of temperature should not be
too sudden; it should commence with tepid, and end with warm water.
_Sudden_ application of hot water brings about certain complications
due to excitatory effect. As regards the persistence of after-effect of
a single application of warm water, it should be remembered that the
absorbed water gradually cools down. In an experiment with a peduncle
of _Zephyranthes_ the growth under partial drought was found to be
0.04 µ per second; application of warm water increased the growth rate
to 0.20 µ per second. After 15 minutes the growth rate fell to 0.13 µ
per second; and after an hour to 0.08 µ per second. It will be noted
that even then the rate was twice the initial rate before irrigation.

TABLE XII.--EFFECT OF IRRIGATION.

    +---------------+---------------------------+------------------+
    |   Specimen.   | Condition of Experiment.  | Rate of growth.  |
    +---------------+---------------------------+------------------+
    |_Kysoor_       |Dry soil                   |0.21 µ per second.|
    |               |Irrigation with cold water |0.30 µ  "     "   |
    |               |Irrigation with warm water |1.33 µ  "     "   |
    |Peduncle of    |Dry soil                   |0.04 µ  "     "   |
    |_Zephyranthes_ |Irrigation with warm water |0.20 µ  "     "   |
    +---------------+---------------------------+------------------+


EFFECT OF ARTIFICIAL INCREASE OF INTERNAL HYDROSTATIC PRESSURE.

Increased turgor was, next, artificially induced by increase of
internal hydrostatic pressure.

_Experiment 71._--The plant was mounted water-tight in the short limb
of an U-tube, and subjected to increased hydrostatic pressure by
increasing the height of the water in the longer limb. Table XIII
shows how increasing pressure enhances the rate of growth till a
critical point is reached, beyond which there is a depression. This
critical point varies in different plants.

TABLE XIII.--EFFECT OF INCREASED INTERNAL HYDROSTATIC PRESSURE
(_Kysoor_).

    +-----------+-----------------------+-------------------+
    | Specimen. | Hydrostatic pressure. | Rate of growth.   |
    +-----------+-----------------------+-------------------+
    |           | Normal                | 0.18 µ per second.|
    | No. I     | 2 cm. pressure        | 0.20 µ  "    "    |
    |           | 4 cm.    "            | 0.11 µ  "    "    |
    |           |                       |                   |
    |           | Normal                | 0.13 µ  "    "    |
    | No. II    | 1 cm. pressure        | 0.20 µ  "    "    |
    |           | 3 cm.    "            | 0.18 µ  "    "    |
    |           | 4 cm.    "            | 0.15 µ  "    "    |
    +-----------+-----------------------+-------------------+


RESPONSE TO NEGATIVE VARIATION OF TURGOR.

I shall now describe the influence of induced diminution of turgor on
the rate of growth.

[Illustration: FIG. 71.--Effect of alternate increase and diminution of
turgor on the same specimen: N, normal rate under drought; H, enhanced
rate under irrigation with warm water; N′, normal permanent rate after
irrigation; P, diminished rate after plasmolysis (_Zephyranthes_).]

_Method of plasmolysis: Experiment 72._--Being desirous of
demonstrating the responsive growth variations of opposite signs in an
identical specimen under alternate increase and diminution of turgor,
I continued the experiment with the same peduncle of _Zephyranthes_
in which the growth acceleration was induced by irrigation with warm
water. In that experiment the growth rate of 0.04 µ per second was
enhanced to 0.20 µ per second after irrigation. A strong solution of
KNO_{3} was now applied at the root; and the growth-rate fell almost
immediately to 0.03 µ per second, or nearly to one-third the previous
rate, the depression induced being thus greater than under condition of
drought (Fig. 71).

TABLE XIV.--EFFECT OF ALTERNATE VARIATION OF TURGOR ON GROWTH
(_Zephyranthes_).

    +--------------------------------+-------------------+
    |    Condition of Experiment.    |  Rate of growth.  |
    +--------------------------------+-------------------+
    |Dry soil                        | 0.04 µ per second.|
    |Application of warm water       | 0.20 µ  "     "   |
    |Steady growth after 1 hour      | 0.08 µ  "     "   |
    |Application of KNO_{3} solution | 0.03 µ  "     "   |
    +--------------------------------+-------------------+

From the series of results that have been given above, it will be seen
that employing very different methods of turgor variation, the rate
of growth, within limits, is enhanced by an increase of turgor. A
diminution or negative variation of turgor, on the other hand, brings
about a retardation or negative variation in the rate of growth. We
should, in this connection, bear in mind the fact that, growth is
dependent on protoplasmic activity, and the variation of turgor itself
is also determined by that activity.


RESPONSE OF MOTILE AND GROWING ORGANS TO VARIATION OF TURGOR.

I have already described (p. 40) the effects of variation of turgor
on the motile pulvinus of _Mimosa_. There is a strict correspondence
between the responsive movement of the leaf of _Mimosa_ and the
movement due to growth, which is summarized as follows:--

  (1) _An increase or positive variation of turgor induces an erection
       or positive response of the leaf of_ Mimosa, _and a positive
       variation or enhancement of the rate of growth_.

  (2) _A diminution or negative variation of turgor induces a fall
       or negative response of the leaf of_ Mimosa, _and a negative
       variation or retardation of the rate of growth_.


EFFECT OF EXTERNAL TENSION.

_Experiment 73._--The recording levers are at first so balanced that
very little tension is exerted on the plant. Record of normal growth
is taken of a specimen of _Crinum_. The tension is gradually increased
from one gram to ten grams. The table given below shows how growth-rate
increases with the tension till a limit is reached, after which there
is a retardation.

TABLE XV.--EFFECT OF TENSION ON GROWTH.

    +--------------+--------------------+
    |  Tension.    |  Rate of growth.   |
    +--------------+--------------------+
    |  0 (Normal)  | 0.41 µ per second. |
    |  4 grams     | 0.44 µ  "     "    |
    |  6    "      | 0.48 µ  "     "    |
    |  8    "      | 0.52 µ  "     "    |
    | 10    "      | 0.40 µ  "     "    |
    +--------------+--------------------+


SUMMARY.

Increase of turgor induced by irrigation enhances the rate of growth.
Irrigation with warm water induces a further augmentation of the rate
of growth.

The latent period for enhancement of growth depends on the distance of
growing region from the root. The latent period is reduced when the
plant is irrigated with warm water.

Artificial increase of internal hydrostatic pressure, up to a critical
degree, enhances the rate of growth.

A diminution or negative variation of turgor depresses the rate of
growth.

There is a strict correspondence between the responsive movement of
the leaf of _Mimosa_, and the movement due to growth. An increase or
positive variation of turgor induces an erection of positive response
of the leaf of _Mimosa_, and a positive variation or enhancement of the
rate of growth. A diminution or negative variation of turgor induces
a fall or negative response of the leaf of _Mimosa_, and a negative
variation or retardation of the rate of growth.

External tension within limits, enhances the rate of growth.




XIV.--EFFECT OF ELECTRIC STIMULUS ON GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.


In plant physiology, the word ‘stimulus’ is often used in a very
indefinite manner. This is probably due to the different meanings which
have been attached to the word. An agent is said to _stimulate_ growth,
when it induces an acceleration. But the normal effect of stimulus is
to cause a retardation of growth. It is probably on account of lack of
precision in the use of the term that we often find it stated, that a
stimulus sometimes accelerates, and at other times, retards growth.
In order to avoid any ambiguity, it is very desirable that the term
stimulus should always be used in the sense as definite as in animal
physiology. An induction shock, a condenser discharge, the make or
break of a constant current, a sudden variation of temperature, and a
mechanical shock bring about an excitatory contraction in a muscle.
These various forms of stimuli cause, as we have seen, a similar
excitatory contraction of the motile pulvinus of _Mimosa pudica_. We
shall enquire whether the diverse forms of stimuli enumerated above,
exert similar or different reactions on the growing organ.


EFFECT OF ELECTRIC STIMULUS OF VARYING INTENSITY AND DURATION.

The form of stimulus which is extensively used in physiological
investigations, is the electric stimulus of induction shock which is
easily graduated by the use of the well known sliding induction coil,
in which the approach of the secondary to the primary coil, indicated
by the higher reading of the scale, gives rise to increasing intensity
of stimulus. The retarding effect of electrical stimulus on growth has
already been demonstrated in record taken on a moving plate (Fig. 61).

I shall adopt for unit stimulus, that intensity of electric shock which
induces a barely perceptible sensation in a human being. It is very
interesting to find, as stated before, that growth is often affected by
an electric stimulus, which is below the range of human perception.

[Illustration: FIG. 72.--Effects of electrical stimulus of increasing
intensities: of 0.25 unit, 1 unit, and 8 units. Short dashes represent
the moments of application of stimulus.]

_Effect of Intensity: Experiment 74._--I shall now describe a typical
experiment on the effect of intensity of stimulus in retarding the
rate of growth. The normal rate of growth of the bud of _Crinum_
was 0.35 µ per second. On the application of electric shock of unit
intensity for 5 seconds, the rate became reduced to 0.22 µ per second.
When the stimulus was increased to 2 units, the retarded rate of growth
was 0.07 µ per second. When the intensity was raised to 4 units, there
was a complete arrest of growth. In figure 72 is given records of a
different experiment which show the effects of increasing intensity of
stimulus in retardation of growth.

[Illustration: FIG. 73.--Effect of continuous electric stimulation of
increasing intensity. The last record exhibits the actual shortening of
the growing organ under strong stimulus.]

_Effect of continuous stimulation: Experiment 75._--The effect of
continuous stimulation of increasing intensity will be seen in the
record (Fig. 73), taken on a moving plate. On application of continuous
stimulus of increasing intensity an increased flexure was produced in
the curve, which denoted greater retardation in the rate of growth.
When the intensity of stimulus was raised to 3 units, there was induced
an actual contraction.


CONTINUITY BETWEEN INCIPIENT AND ACTUAL CONTRACTION.

It will thus be seen, that external stimulus of electric shock
induces a reaction which is of opposite sign to the normal growth
elongation or expansion. We may conveniently describe this effect as
‘incipient’ contraction; for under increasing intensity of stimulus,
the contractile reaction, opposing growth elongation, becomes more and
more pronounced; at an intermediate stage this results in an arrest of
growth; at the further stage, it culminates in an actual shortening
of the organ. There is no break of continuity in all these stages.
I shall, therefore, use the term ‘contraction’ in a wider sense,
including the ‘incipient’ which finds expression in a retardation of
growth.

In Table XVI is given the results of certain typical experiments on the
effect of stimulus of increasing intensity and duration.

TABLE XVI.--EFFECT OF INTENSITY AND DURATION OF ELECTRIC STIMULUS ON
GROWTH.

    +--------------------------+------------+------------------+
    | Duration of Application. | Intensity. | Rate of growth.  |
    +--------------------------+------------+------------------+
    |                          |Normal      |0.35 µ per second.|
    |5 seconds                 |1 unit      |0.22 µ  "     "   |
    |     "                    |2 units     |0.07 µ  "     "   |
    |     "                    |4   "       |Arrest of growth. |
    +--------------------------+------------+------------------+
    |                          |Normal      |0.30 µ per second.|
    |Continuous stimulation    |0.5 unit    |0.20 µ  "     "   |
    |     "         "          |1     "     |0.09 µ  "     "   |
    |     "         "          |3     "     |Contraction.      |
    +--------------------------+------------+------------------+

With regard to the question of immediate and after-effect of stimulus,
I find great difficulty in drawing a line of demarcation. Owing to
physiological inertia there is a delay between the application of
stimulus and the initiation of responsive reaction (latent period);
owing to the same inertia, the physiological reaction is continued
even on the cessation of stimulus. All responsive reactions are thus
after-effects in reality. The latent period is shortened under strong
stimulus, but the contractile reaction becomes more persistent. When
the stimulus is moderate or feeble, the recovery from incipient
contraction takes place within a short time. Stimulus, under certain
circumstances, is found to improve the ‘tone’ of the tissue, and as we
shall presently see bring about, as the after-effect, an enhancement of
the rate of growth.

The effect of electric stimulus is thus an incipient or actual
contraction.


SUMMARY.

In normal conditions electric stimulus induces an incipient contraction
exhibited by the retardation of the rate of growth. Growth is often
affected by an electric stimulus which is below human perception.

Under increasing intensity of stimulus, the contractile reaction
opposing growth elongation becomes more and more pronounced. At a
critical intensity of stimulus growth becomes arrested. Under stronger
intensity of stimulus growing organ undergoes an actual shortening in
length.

There is continuity between the incipient contraction seen in
retardation, arrest of growth, and contraction of the organ under
stronger stimulus.

The latent period of responsive variation of growth is shortened under
stronger stimulus, but the period of recovery becomes protracted.




XV.--EFFECT OF MECHANICAL STIMULUS ON GROWTH

_By_

SIR J. C. BOSE.


Amongst the various stimuli which induce excitation in _Mimosa_ may be
mentioned the irritation caused by rough contact, by prick, or wound.
Friction causes moderate stimulation, from which the excited pulvinus
recovers within a short time. But a prick or a cut induces a far more
intense and persistent excitation; the recovery becomes protracted, and
the wounded pulvinus remains contracted for a long period.

I shall now describe the effect of mechanical irritation on growth. For
moderate stimulus, I employ rough contact or friction; more intense
stimulation is caused by a prick or a cut.


EFFECT OF MECHANICAL IRRITATION.

[Illustration: FIG. 74.--(_a_) N, normal rate of growth; F, retarded
rate immediately after friction; A, partial recovery after 15 minutes.

(_b_) N, normal; W, immediately after wound; C, an hour after.

(Successive dots at intervals of 5″.)]

_Experiment 76._--In this experiment, I took a peduncle of
_Zephyranthes_, which had a normal rate of growth of 0.18 µ per second.
I then caused mechanical irritation by rubbing the surface with a piece
of card-board. The mechanical stimulation was found to have caused a
retardation of growth, the depressed rate being 0.11 µ per second, or
three-fifths the normal rate. As this particular mode of stimulation
was very moderate, the normal of rate growth was found to be restored
after a short period of rest. After 15 minutes the rate became 0.14 µ
per second; after an hour the recovery was complete, the rate being now
0.18 µ per second, the normal rate before stimulation (Fig 74_a_). We
shall presently see that not only is the growth rate greatly depressed
under intense stimulation, but the period of recovery also becomes very
much protracted.

I have often been puzzled by the fact, that specimens apparently
vigorous exhibited little or no growth, after attachment to the
recorder. After waiting in vain for an hour, I had to discard them for
others with equally unsatisfactory results. One of these specimens
happened to be left attached to the recorder overnight, and I was
surprised to find that the specimen, which had shown no growth the
previous evening, was now exhibiting vigorous growth after being left
to itself for 12 hours. I then realised that the temporary abolition
of growth must have been due to the irritation of somewhat rough
handling during the process of mounting and attachment of the specimen
to the recorder.

In the matter of mechanical stimulation, some specimens are more
irritable than others. The persistence of after-effect of irritation
in retardation of growth will be demonstrated in the following
experiments, where the stimulus employed was more intense.


EFFECT OF WOUND.

A prick causes an intense excitation in _Mimosa_. I tried the effect of
this form of stimulation on responsive variation in growth.

_Experiment 77._--The specimen was the same as had been employed in the
last experiment. After moderate stimulation due to friction it had, in
the course of an hour, completely recovered its normal rate of growth
of 0.18 µ per second. I now applied the stimulus of pin prick; the
actual injury to the tissue due to this was relatively slight; but the
retardation of growth induced by this more intense mode of stimulation
was very great. With moderate mechanical friction the rate had fallen
from 0.18 µ to 0.11 µ per second, _i.e._, to three-fifths the normal
rate: in consequence of prick the depression was from 0.18 µ to 0.05 µ
per second, _i.e._, to less than a third of the normal rate. After 15
minutes the rate recovered from 0.05 µ to 0.07 µ per second. After
moderate friction the recovery was complete after an hour; but in
this case the recovery after an equal interval was only three-fourths
of the original, the rate being now 0.12 µ per second (Fig. 74b).
I next applied the more intense stimulus caused by a longitudinal
cut This caused a depression of growth rate to 0.04 µ per second. A
transverse cut, I find, gives rise to a more intense stimulation, than
a longitudinal slit.

TABLE XVII.--EFFECT OF MECHANICAL IRRITATION AND OF WOUND ON GROWTH.

                            (_Zephyranthes._)
  +-------------------+-----------------------------+---------------+
  |Nature of stimulus.|         Condition.          |Rate of growth.|
  +-------------------+---------------------------------------------+
  |Mechanical friction|Normal rate                  |0.18 µ per sec.|
  |                   |Immediately after stimulation|0.11 µ    "    |
  |                   |15 minutes after stimulation |0.14 µ    "    |
  |                   |60 minutes after stimulation |0.18 µ    "    |
  +-------------------+---------------------------------------------+
  |Prick with needle  |Normal rate                  |0.18 µ per sec.|
  |                   |Immediately after stimulation|0.05 µ    "    |
  |                   |15 minutes after stimulation |0.07 µ    "    |
  |                   |60 minutes after stimulation |0.12 µ    "    |
  +-------------------+-----------------------------+---------------+

The effect of mechanical stimulus on growth is thus similar to that
induced by electrical stimulus. Moderate stimulus of rough contact
induces an incipient contraction, seen in retardation of growth,
the recovery being complete in the course of an hour; but intense
stimulation, induced by wound, gives rise to greater and more,
persistent retardation of growth.


SUMMARY.

Mechanical stimulus induces incipient contraction or retardation of
rate of growth, the effect being similar to that induced by electric
stimulus.

Stimulus by contact or friction induces a retardation which is,
relatively speaking, moderate. On the cessation of stimulus the normal
rate of growth is restored within an hour.

Intense stimulation caused by the wound gives rise to greater and more
persistent retardation of growth.




XVI.--ACTION OF LIGHT ON GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.


The next subject of inquiry is the _normal_ effect of light on
growth. I speak of the normal effect because, under certain definite
conditions, to be described in a later Paper, the response undergoes
a reversal. The Crescograph is so extremely sensitive that it records
the effect of even the slightest variation of light. Thus, as I have
already mentioned, the opening of the blinds of a moderately-lighted
room induces, within a short time, a marked change in the record of
the rate of growth. The conditions of the experiment would thus become
more precise if the growth-rate in the absence of light is taken as
the normal. The specimens are, therefore, kept for several hours in
darkness before the experiment. But this should not be carried to the
extent of lowering the healthy tone of the plant.

I shall, in the present Paper, determine the characteristic response to
light in variation of growth, the latent period of response, the effects
of light of increasing intensity and duration, and the effects of the
visible and invisible rays of the spectrum.


METHOD OF EXPERIMENT.

The plant was placed in a glass chamber kept in humid condition. The
sources of light employed were: (1) an arc-lamp with self-regulating
arrangement for securing steadiness of light, and (2) an incandescent
electric lamp. Two inclined mirrors were placed close behind the
specimen so that it should be acted on by light from all sides.


NORMAL EFFECT OF LIGHT.

[Illustration: FIG. 75.--Normal effect of light. N, normal; S, retarded
rate of growth in response to light; N, recovery on cessation of light.]

_Experiment 78._--I shall first give records obtained with _Kysoor_
on the action of light. The first series exhibits the normal rate of
growth in darkness; in the next the retarding effect of light is seen
in the shortening of spacings, as compared with the normal, between
successive dots. The light was next cut off and record taken once more
after half an hour. Growth is now seen to have recovered its normal
rate (Fig. 75). With regard to the after-effect of light I may say in
anticipation that there are two different results, which depend on
the physiological condition of the tissue. In a tissue whose tonic
condition is below par, the after-effect is an acceleration; but with
tissues in an optimum condition, the immediate after-effect is a
retardation of the rate of growth. This is specially the case when the
incident light is of strong intensity and of long duration.


DETERMINATION OF THE LATENT PERIOD.

[Illustration: FIG. 76.--Latent period and time-relations of response
to light, applied at thick line. Successive dots at intervals of 5 sec.]

There is a general impression that it takes from several minutes to
more than an hour for the light to react on the growing organ. This
underestimate must have been due to the want of sufficient delicate
means of observation. For my recorders indicate in some cases a
response within less than 2 seconds of the incidence of light. This was
found, for example, in the record of response given by a seedling of
_Cucurbita_, to a flash of ultra-violet light. In the majority of cases
the response is observed within 15 seconds of the incidence of light.

_Experiment 79._--For the determination of the latent period, a record
of the effect of arc light of 30 seconds’ duration was taken on a
moving plate. It will be noticed (Fig. 76) that a retardation of growth
was induced within 35 seconds of the incidence of light. The incipient
contraction induced by light is thus similar to that induced by any
other form of stimulus. Growth became restored to the normal value, 5
minutes after the cessation of stimulus.


EFFECT OF INTENSITY OF LIGHT.

[Illustration: FIG. 77.--Action of light of increasing intensities: 1:
2: 3 in retardation of growth.]

_Experiment 80._--I next studied the action of light, the intensity
of which was increased in arithmetical progression. The intensity of
white light given by a half-watt incandescent electric lamp of 200
candle power, placed at a distance of a metre, is taken as the unit.
Much feebler light would have been sufficient, but it would have
required much longer exposure. The intensity was increased by bringing
the lamp nearer the plant; marks were made on a horizontal scale so
that the intensity of incident light increased at the successive marks
of the scale as 1: 2: 3: and so on. The duration of exposure was
same in all cases, namely, 5 minutes. After each experiment suitable
periods of rest were allowed for the plant to recover its normal rate
of growth. Records in Fig. 77 show increasing retardation induced
by stronger intensities of light. Table XVIII gives the result of a
different experiment.

TABLE XVIII.--EFFECT OF LIGHT OF INCREASING INTENSITY ON THE RATE OF
GROWTH.

    +-----------------------------------------+
    | Intensity of light. |  Rate of growth.  |
    +---------------------+-------------------+
    |0 (Normal)           | 0.47 µ per sec.   |
    |1 Unit               | 0.28 µ    "       |
    |2   "                | 0.17 µ    "       |
    |3   "                | 0.10 µ    "       |
    |4   "                | Arrest of growth. |
    +---------------------+-------------------+


EFFECT OF CONTINUOUS LIGHT.

_Experiment 81._--The continued effect of light of moderate intensity
in bringing about increasing retardation of growth will be seen in
Fig. 78(_b_) side by side with the record of effect of continuous
electric stimulation (Fig. 78_a_) on growth. In both the cases the
effect of continuous stimulation is seen to be the same, namely, a
growing retardation, which in the given instances culminated in arrest
of growth. This is true of stimulus of moderate intensity. Under a
more intense stimulation the incipient contraction does not end in a
mere arrest of growth, but the responding organ undergoes an actual
shortening.

[Illustration: FIG. 78.--Effects of continuous (_a_) electric and (_b_)
photic stimulation of moderate intensity, taken on a moving plate.]


EFFECTS OF DIFFERENT RAYS OF THE SPECTRUM.

Different observers have found[V] that it is the more refrangible rays
which exercise the greatest influence upon growth and tropic curvature.
The relative effects of different lights will, however, become more
precise from the curves of response to the action of different rays.
For this purpose, I first employed monochromatic lights from different
parts of the spectrum, produced by prism of high dispersion. In
practice, the usual colour filters were found very convenient, as
they allowed the application of more intense light. A thick stratum
of bichromate of potash solution transmitted red rays, a thinner
stratum allowed the transmission of yellow in addition; ammoniated
copper sulphate solution allowed the blue and violet rays to pass
through. It should be borne in mind that certain complicating factors
are introduced by the incidence of light on the organ; there may be a
slight rise of the temperature. We have seen however that moderate rise
of temperature induces an acceleration of the rate of growth (p. 175).
I shall later describe other experiments which will demonstrate the
antagonistic effects of light and warmth on growth. Warmth again may
induce a certain amount of dessication, but this is reduced to a
minimum by maintaining the plant-chamber in a humid condition. The
heating effect of the red is, relatively speaking, much greater than
that of the blue rays. But in spite of this it is found that while red
rays are practically ineffective, the blue rays are most effective in
inducing responsive retardation of growth.

[V] PFEFFER--Physiology of Plants--Vol. II., p. 104 (English
Translation)

_Effect of red and yellow light._--These rays had little or no effect
in inducing variation of growth.

_Effect of blue light: Experiment 82._--The blue rays exerted a marked
retarding effect on growth. Light was applied for 34 seconds and
retardation was initiated within 14 seconds of the incidence of light,
and the retarded rate was two-fifths of the normal (Fig. 79B).

[Illustration: FIG. 79.--N, normal. B, effect of blue light, and V, of
ultra-violet light. The records are on a moving plate.]

_Effect of ultra-violet light: Experiment 83._--Ultra-violet light was
obtained from a quartz mercury vapour lamp. The effect of this light
in retardation of growth was very marked. Response was induced within
10 seconds, the maximum retardation being one-sixth of the normal rate
(Fig. 79V).

_Effect of infra-red rays: Experiment 84._--In passing from the
most refrangible ultra-violet to the less refrangible red rays, the
responsive retardation of growth undergoes a diminution and practical
abolition. Proceeding further in the infra-red region of thermal
rays, it is found that these latter rays become suddenly effective in
inducing retardation of growth.

A curve drawn with the wave length of light as abscissa, and
effectiveness of the ray as ordinate shows a fall towards zero as we
proceed from the ultra-violet wave towards the red; the curve, however,
shoots up as we proceed further in the region of the infra-red. In
connection with this it should be remembered that while the thermal
rays induce a retardation of growth, rise of temperature, up to an
optimum point, gives rise to the precisely opposite reaction of
acceleration of growth.

The relative effectiveness of various rays on growth will be seen more
strikingly demonstrated in records of photo-tropic curvature to be
given in a succeeding Paper.


SUMMARY.

The normal effect of light is incipient contraction or retardation of
the rate of growth.

The latent period may in some cases be as short as 2 seconds. In large
number of cases it is about 15 seconds. The latent period is shortened
under stronger intensity of light.

Increasing intensity of light induces increasing retardation and arrest
of growth. Under continued action of light of strong intensity the
growing organ may undergo an actual shortening.

In these reactions the action of stimulus of light resembles the
effects of electric and mechanical stimuli.

The ultra-violet rays induce the most intense reaction in retardation
of growth. The less refrangible yellow and red rays are practically
ineffective. But the infra-red rays induce a marked retardation of
growth.

The effects of light and warmth are antagonistic. The former induces a
retardation and the latter an acceleration of growth.




XVII.--EFFECT OF INDIRECT STIMULUS ON GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.


It has been shown that the direct application of stimulus gives rise
in different organs to contraction, diminution of turgor, fall of
motile leaf, electro-motive change of galvanometric negativity, and
retardation of the rate of growth. I shall now inquire whether Indirect
stimulus, that is to say, application of stimulus at some distance from
the responding organ, gives rise to an effect different from that of
direct application.


MECHANICAL AND ELECTRICAL RESPONSE TO INDIRECT STIMULUS.

I have already described the effect of Indirect stimulus on motile
organs (p. 136). A feeble stimulus applied at a distance was found
to induce an erectile movement or positive response of the leaf of
_Mimosa_ or of the leaflet of _Averrhoa_. This reaction is indicative
of increase of turgor, an effect which is diametrically opposite to
the diminution of turgor induced by the effect of Direct stimulus. It
was also shown that an increase in the intensity of Indirect stimulus
or a diminution of the intervening distance brought about a diphasic
response, positive followed by negative. Direct stimulus gave rise only
to a negative response.

_Electric response to Indirect stimulus._--I have already explained
how an identical reaction finds diverse expression in mechanical and
electrical response, or in responsive variation of the rate of growth.
It is of interest in this connection to state that my attention was
first directed to the characteristic difference between the effects
of Direct and Indirect stimulus from the study of electric response
of vegetable tissues. I found that while _Direct_ stimulus induced
negative electric response, _Indirect_ stimulus gave rise to a positive
response. The clue thus obtained led to the discovery of positive
mechanical response under Indirect stimulus.

[Illustration: FIG. 80.--Electric response of _Musa_ (_a_) Positive,
(_b_) diphasic, (_c_) negative.]

_Experiment 85._--The records given in Fig. 80, exhibit the electric
response given by vegetable tissues. On application of feeble
stimulus at a distance from the responding point, the response was by
galvanometric positivity. Under stronger stimulus the response became
diphasic, positive followed by negative. Direct stimulus induced a
negative response.


VARIATION OF GROWTH UNDER INDIRECT STIMULUS.

Since the responsive reactions of growing and non-growing organs are,
as we shall find later, fundamentally similar, I expected that Indirect
stimulus would give rise in a growing organ to an effect which would be
of opposite sign to that induced by Direct stimulus--an acceleration,
instead of retardation of growth; that would correspond to the positive
mechanical and electrical responses to Indirect stimulus given by
pulvinated organs and by ordinary vegetable tissues. The account of the
following typical experiment will show that my anticipations have been
fully verified.

_Experiment 86._--I took a growing bud of _Crinum_ and determined the
region of its growth activity; lower down a region was found where the
growth had attained its maximum and may, therefore, be regarded as
indifferent region. I applied two electrodes in this indifferent region
about 1 cm. below the region of growth. On application of moderate
electric stimulus of short duration the response was by an acceleration
of growth which persisted for nearly a minute, after which there was a
resumption of the normal rate of growth. In this particular case the
interval of time between the application of stimulus and the responsive
acceleration of growth was 12 seconds. The interval varies in different
cases from one second to 20 seconds or more, depending on the
intervening distance between the point of application of stimulus and
the responding region of growth. I give a record (Fig. 81) obtained
in a different experiment which shows in an identical specimen, (1) an
acceleration of growth under Indirect and (2) a retardation of growth
under Direct stimulus.

[Illustration: FIG. 81.--Effect of Indirect and Direct stimulus on
growth of _Crinum_, taken on a moving plate. Dotted arrow shows
application of Indirect stimulus with consequent acceleration of
growth. Direct application of stimulus at the second arrow induces
contraction and subsequent retardation of rate of growth. Successive
dots are intervals of 5″. (Magnification 2,000 times).]

TABLE XIX--ACCELERATING EFFECT OF INDIRECT STIMULUS ON GROWTH
(_Crinum_).

    +----------+--------------------------+--------------------+
    |Specimen. | Condition of experiment. | Rate of growth.    |
    +----------+--------------------------+--------------------+
    |I         | Normal                   | 0.21 µ per second. |
    |          | After Indirect stimulus  | 0.26 µ  "     "    |
    +----------+--------------------------+--------------------+
    |II        | Normal                   | 0.25 µ  "     "    |
    |          | After Indirect stimulus  | 0.30 µ  "     "    |
    +----------+--------------------------+--------------------+

It is thus seen that the effect of Indirect stimulus on
growth-variation is precisely parallel to that obtained with the
response of sensitive plant; that is to say, the effect induced by
a feeble stimulus applied at a distance from the growing region is
a positive variation or acceleration of growth. The effect becomes
converted into negative or retardation of growth when the stimulus is
Direct, _i.e._, when applied to the responding region of growth; under
intermediate conditions, the growth-variation I find to be diphasic, a
positive acceleration followed by a negative retardation. This is found
true not merely in the case of a particular form of stimulus but of
stimuli as different as mechanical, thermal, electric, and photic.

I shall in a subsequent paper formulate a generalised Law of Effects of
Direct and Indirect Stimulus. From the experiments already described it
is seen that:

  _Direct stimulus induces negative variation of turgor, contraction,
       fall of leaf of_ Mimosa, _electric change of galvanometric
       negativity, and retardation of the rate of growth._

  _Indirect stimulus induces positive variation of turgor, expansion,
       erection of leaf of_ Mimosa, _electrical change of galvanometric
       positivity, and acceleration of the rate of growth._

It is seen that Indirect stimulus gives rise to dual reactions, seen
in positive and negative responses; of these the negative is the more
intense. When the intervening distance is reduced, the resulting
response becomes negative; this is due not to the absence of the
positive, but to its being masked by the predominant negative. From
the principle of continuity, this will also hold good in the limiting
case, where by the reduction of the intervening distance to zero,
the stimulus becomes Direct. In other words, Direct stimulus should
also give rise to both positive and negative reactions. Of these the
positive is masked by the predominant negative.

So much for theory; is it possible to unmask the contained positive in
the resulting negative response under Direct stimulus? This important
aspect of the subject will be dealt with in the following Paper.


SUMMARY.

The application of Direct stimulus gives rise to an electric response
of galvanometric negativity. The application of stimulus at a distance
from the responding point, _i.e._, Indirect stimulus, gives rise to
positive electric response.

The mechanical responses of sensitive plants also exhibit similar
effects, _i.e._, a negative response under Direct, and positive
response under Indirect stimulus.

In the responsive variation of growth, Direct stimulus induces a
retardation, and Indirect stimulus an acceleration of the rate of
growth.

The effects of Direct and Indirect stimulus on vegetable organs in
general are as follows:

  Direct stimulus induces negative variation of turgor, contraction,
       fall of leaf of _Mimosa_, electric change of galvanometric
       negativity, and retardation of the rate of growth.

  Indirect stimulus induces positive variation of turgor, expansion,
       erection of leaf of _Mimosa_, electrical change of galvanometric
       positivity and acceleration of the rate of growth.




XVIII.--RESPONSE OF GROWING ORGANS IN STATE OF SUB-TONICITY

_By_

SIR J. C. BOSE.


The normal response of a growing organ to Direct stimulus is
_negative_, that is to say, a retardation of the rate of growth. This
is the case under forms of stimuli as diverse as those of mechanical
and electric shocks, and of the stimulus of light.


ABNORMAL ACCELERATION OF GROWTH UNDER STIMULUS.

After my investigations on the normal retarding effect of light on
growth, I was considerably surprised to find the responses occasionally
becoming _positive_, an acceleration instead of retardation of growth.
I shall first give accounts of such positive responses and then explain
the cause of the abnormality.

_Abnormal acceleration under stimulus of light: Experiment 87._--A
rather weak specimen of _Kysoor_ was exposed to the action of light of
5 minutes’ duration. This induced an abnormal acceleration in the rate
of growth from 0.30 µ to 0.40 µ per second. But continuous exposure to
light for half an hour brought about the normal effect of retardation.
In trying to account for this abnormality in response I found that
while specimens of _Kysoor_ in a vigorous state of growth of about
0.8 µ per second exhibit normal retardation of growth under light, the
particular specimen which exhibited the abnormal positive response had
a much feebler rate of growth of 0.30 µ per second. As activity of
growth in a plant is an index of its healthy tone, a feeble rate of
growth must be indicative of tonicity below par. The fact that plants
in sub-tonic condition exhibit abnormal acceleration of growth under
stimulus will be seen further demonstrated in the next experiment.

In the parallel phenomenon of the response of pulvinated organs we
found that under condition of sub-tonicity, the response becomes
positive and that this abnormal positive is converted into normal
negative in consequence of repeated stimulation. In growth, response
likewise the abnormal acceleration of growth under light in the
sub-tonic specimen of _Kysoor_ was converted into normal retardation
after continuous stimulation for half an hour. From the facts given
above, we are justified in drawing the following conclusions:

(1) That while light induces a _retardation_ of growth in a tissue
whose tonic condition is normal or above par, it brings about an
_acceleration_ in a tissue whose condition is below par.

(2) That by the action of the stimulus of light itself a sub-tonic
tissue is raised to a condition at par, with the concomitant
restoration of normal mode of response by retardation of growth.

Another important question arises in this connection: Is the
restoration of normal response due to light as a form of stimulus, or
to its photo-synthetic action? An answer to this is to be found from
the results of an inquiry, whether a very different form of stimulus
which exerts no photo-synthetic action, such as tetanising electric
shocks, also induces a similar acceleration of growth in a sub-tonic
tissue.

The normal retarding effect of electric stimulus on specimens in active
state of growth was demonstrated in record given in Fig. 72, where the
normal rate was found greatly reduced after stimulation.

_Abnormal acceleration of growth under electric stimulus: Experiment
88._--For my present purpose I took a sub-tonic specimen of seedling
of wheat, its rate of growth being as low as 0.05 µ per second. After
electric stimulation the rate was found enhanced to 0.12 µ per second,
or about two and-a-half times. I give (Fig. 82) two records obtained
with two different specimens. In the first, the record was taken on a
stationary plate (Fig. 82). The closeness of successive dots in N show
the feeble rate of growth of the sub-tonic specimen, the wider spacing
after stimulation, S, exhibit the induced enhancement of growth.

[Illustration: FIG. 82.--Enhancement of rate of growth in sub-tonic
specimens of wheat seedling. First series of record on stationary,
second series (_b_) on moving plate N, record before stimulation. S,
after stimulation.]

In the second experiment the records (Fig. 82_b_) were taken on a
moving plate. The specimen was so extremely sub-tonic, that its normal
record N appears almost horizontal. The greater erection of the curve,
S, after stimulation demonstrates the induced acceleration of growth.

TABLE XX.--ACCELERATION OF GROWTH BY STIMULUS IN SUB-TONIC SPECIMENS.

    +---------------+---------------------------+---------------+
    |   Specimen.   |        Stimulus.          |Rate of growth.|
    +---------------+---------------------------+---------------+
    |Wheat seedling |Normal                     |0.05 µ per sec.|
    |               |After electric stimulation |0.12 µ  "   "  |
    +---------------+---------------------------+---------------+
    |_Kysoor_       |Normal                     |0.30 µ per sec.|
    |               |After 5' exposure to light |0.40 µ  "   "  |
    |               |  "  30'     "    "        |0.27 µ  "   "  |
    +---------------+---------------------------+---------------+

In my previous Paper on the ‘Modifying Influence of Tonic Condition’
I showed that while the response of the primary pulvinus of _Mimosa_
in normal condition is _negative_, i.e., by contraction, diminution of
turgor, and fall of the leaf, the response of a sub-tonic specimen is
_positive_, that is to say, by expansion, enhancement of turgor, and
erection of the leaf. I have shown further that in a sub-tonic specimen
the action of stimulus itself raises the tissue from below par to
normal or even above par, with the conversion of abnormal positive to
normal negative response.

I have in the present Paper shown that a parallel series of reactions
is seen in the response of growing organs. In vigorously growing
specimens the action of stimulus is _negative_, i.e., incipient
contraction, diminution of turgor, and retardation of the rate of
growth. But in sub-tonic specimens, with enfeebled rate of growth, the
effect of stimulus is _positive_, i.e., by expansion, enhancement of
turgor, and acceleration of the rate of growth. Continuous stimulation
also raises the sub-tonic growing tissue to a condition at par,
converting the response from abnormal positive to normal negative.

It was also explained that every stimulus gave rise to dual reactions,
positive and negative, and that in a highly excitable tissue the
positive is masked by the predominant negative. The positive, or
A-effect, is generally described as a “building up” process. By
choosing a sub-tonic specimen, I have been able to unmask the positive,
A. In the case of sub-tonic growing organs the positive, A, is
literally a building up process, giving rise to an acceleration of
growth.

From these facts and others given previously it will be seen that
the abnormal response of acceleration of growth under stimulus is by
no means accidental or fortuitous but is a definite expression of an
universal reaction, characteristically exhibited by all tissues in a
condition of sub-tonicity.


CONTINUITY BETWEEN ABNORMAL AND NORMAL RESPONSES.

A given plant-tissue may exist in widely different conditions of
tonicity. Let us take two extreme conditions, the optimum and the
minimum. The tonic level will be at its lowest at the minimum,
where growth will be at a standstill. The range between the optimum
and minimum will be very extended; hence strong and long continued
stimulation will be necessary to raise the tissue from the tonic
minimum to the optimum level. There are innumerable grades of
tonicity between the optimum and minimum. Within this wide range
the characteristic response will be the abnormal positive. As we
approach the optimum, the range for positive response will become
circumscribed, and the intensity and duration of stimulus necessary to
convert the positive to negative will be feebler and shorter. It will
be very seldom that a plant is likely to be found at the optimum. Hence
plants in general may be expected to give a feeble positive response
under sub-minimal stimulus.

These considerations led me to look for the positive response under
sub-minimal stimulation; the tracings which I have obtained with
my highly sensitive Crescograph and other recorders show that my
anticipations have been justified.

[Illustration: FIG. 83.--Acceleration of growth under sub-minimal light
stimulus. Record on moving plate; stimulus applied at 5th dot, and
subsequent erection of curve exhibits acceleration of growth. Last part
of curve shows recovery of normal growth on cessation of stimulus.]

_Positive response under sub-minimal stimulus: Experiment 89._--In
normal specimens, light of strong intensity induces a retardation of
growth. When the source of light is placed at a distance, the intensity
of light undergoes great diminution. Under the action of such feeble
stimulus I obtained an acceleration of growth even in specimens which
may be regarded as moderately vigorous (Fig. 83). Similar acceleration
of growth was also obtained under feeble electric stimulation. The
response is reversed to normal negative by increasing the intensity or
duration of stimulus. Very feeble stimulus thus induces an acceleration
and strong stimulus a retardation of growth. I have frequently obtained
positive mechanical and electrical responses under sub-minimal
stimulation. As chemical substances often act as stimulating agents,
the opposite effects of the same drug in small and large doses may
perhaps prove to be a parallel phenomenon.

It has been shown that stimulus induces simultaneously both A- and
D-effects, with the attendant positive and negative responsive
reactions, alike in pulvinated and in growing organs. A tissue, in an
optimum condition, exhibits only the resultant negative response; the
comparatively feeble positive is imperceptible, being masked by the
predominant negative; but with the decline of its tone excitability
diminishes, with it the D-effect, and we get the A-effect unmasked,
resulting response then becomes diphasic. In extreme sub-tonic
condition, it exhibits only the positive. The sequence is reversed when
we begin with a tissue in a state of extreme sub-tonicity, which first
exhibits only the positive. Successive stimulations continually exalt
the tonic condition, the subsequent responses becoming, diphasic, and,
with the attainment of optimum tone, a resultant negative response.
As a further verification of the simultaneous existence of both A-
and D-effects, it has been shown that in ordinary tonic condition a
_sub-minimal_ stimulus gives rise only to positive response; this
becomes converted into normal negative under moderate stimulation.

I have described the action of stimulus on tissues in which, on account
of sub-tonicity, growth has become enfeebled. I shall next take up the
question of effect of stimulus on tissues in which growth, on account
of extreme sub-tonicity, has been brought to a state of standstill.


SUMMARY.

The modifying influence of tonic condition on response is similar in
pulvinated and growing organs.

The motile organ of _Mimosa_ in a condition of sub-tonicity, exhibits
a _positive_ response, by expansion, increase of turgor, and erection
of the leaf. Continuous stimulation converts the abnormal _positive_ to
normal _negative_.

In sub-tonic growing organs stimulus likewise induces a _positive_
response, by expansion, increase of turgor and acceleration of the rate
of growth. Continuous stimulation converts the abnormal acceleration to
normal retardation.

Sub-minimal stimulus tends to induce even in normal tissues, an
acceleration of rate of growth. Stimulus of moderate intensity induces
in the same tissue the normal retardation of growth.




XIX.--RESUMPTION OF AUTONOMOUS PULSATION AND OF GROWTH UNDER STIMULUS

_By_

SIR J. C. BOSE.


The autonomous activity of growth is ultimately derived from energy
supplied by the environment. The internal activity may fall below par
with consequent diminution or even arrest of growth; this condition of
the tissue I have designated as sub-tonic. The inert plant can only be
stirred up to a state of activity by stimulus from outside; and we saw
that under the action of stimulus the rate of growth of a sub-tonic
tissue was enhanced.

As the general question of depression of autonomous activity and
its restoration by the action of stimulus is of much theoretical
importance, I shall describe experiments carried out on a different
form of autonomous activity, seen in spontaneous pulsation of the
lateral leaflets of _Desmodium gyrans_. Under favourable conditions
of light and warmth these leaflets execute vigorous movements, the
period of a single pulse varying from one to two minutes. As the energy
for this activity is ultimately derived from the environment, it is
clear that isolation from the action of favourable environment will
bring about a gradual depletion of energy with concomitant decline
and ultimate cessation of spontaneous movement. For this we may keep
the plant in semi-darkness; we may further hasten the rundown process
by isolating the leaflet from the parent plant. A leaflet immersed in
water was kept in a dimly lighted room; it was attached by a cocoon
thread to the recording lever of an Oscillating Recorder to be fully
described in the next Paper. The pulsation continued even in this
isolated condition for about 48 hours, after which the spontaneous
movement came to a stop. Further experiments showed that the arrest
of pulsation was not indicative of mortality but of ‘latent life’ in
a state of suspense, to be stirred up again by shock stimulus into
throbbing activity.


REVIVAL OF AUTONOMOUS PULSATION UNDER STIMULUS.

_Experiment 90._--In figure 84, is a seen record of the action of light
on the sub-tonic _Desmodium_ leaflet at standstill. A narrow pencil of
light from electric arc was first thrown on the lamina in which the
presence of chlorophyll rendered photo-synthetic action possible. This
had no effect on the renewal of pulsation. But the autonomous activity
was revived by the action of light on the pulvinule. This preferential
effect on pulvinule showed that the renewal of activity was due not to
photo-synthesis but to the stimulating action of light. The pulsation
was also restored by chemical stimulants, such as dilute ether, and
solution of ammonium carbonate.

[Illustration: FIG. 84.--Renewal of autonomous activity in _Desmodium
gyrans_ at standstill by action of light. Up-curve represents
up-movement. The horizontal lines below represent durations of exposure
to light.]

As regards the action of light, the pulsation continued for a time,
even on the cessation of light. This persistence of autonomous
activity increases with the intensity and duration of incident
stimulus, that is to say with the amount of incident energy. In the
present case a duration of five minutes’ exposure gave rise to a
single pulsation, after which the movement of the leaflet came to a
stop. The next application lasted for ten minutes and this gave rise
to four pulsations, two during application, and two after cessation
of light. The next application was for forty-five minutes, and the
pulsation persisted for nearly an hour after the cessation of light.
The experiments on sub-tonic specimens show clearly that the energy
supplied by the environment becomes as it were latent in the plant,
increasing its potentiality for work.

The renewal of autonomous activity in a sub-tonic tissue by the action
of external stimulus, will be found in every way parallel to the
renewal of growth in a sub-tonic organ.


REVIVAL OF GROWTH UNDER STIMULUS.

[Illustration: FIG. 85.--Record of responses of a mature style in which
growth had come to a stop. Up-curve shows contraction under stimulus.
Renewal of growth at sixth response, after which growth-elongation is
shown by the trend of the base-line downwards.]

_Renewal of growth under stimulus: Experiment 91._--I find that
application of electric stimulus renews growth in specimens where,
on account of extreme sub-tonicity growth has come to a state of
standstill. The resumption of growth in grass haulms under the stimulus
of gravity is a phenomenon probably connected with the above. The
causes which bring about cessation of growth in a mature organ are
unknown; that there is a potentiality of growth even in a fully grown
grass haulm is evidenced by the fact of its renewed growth under fresh
stimulation. That this is not an exceptional phenomenon appears from
the record which I obtained with a fully grown style of _Datura alba_.
I subjected it to periodic stimulation, and obtained from it a series
of contractile responses. After recovery from stimulus it regained its
normal length which remained constant for some time as seen in the
horizontal base-line. But as a result of successive stimulations, the
mature style resumed its growth with increasing acceleration. This is
seen in the recovery overshooting its former horizontal limit (Fig. 85).

From the investigations that have been described in this and in the
previous Papers an insight is obtained into the complexity of response
arising from various factors. It has been shown that the sign of
response is modified by the intensity of stimulus, by its point of
application, and by the tonic condition of the responding tissue. The
fundamental reactions have been found to be essentially the same in
pulvinated, in growing and non-growing organs. The results described
enable us to enunciate general Laws of Effects of Direct and Indirect
stimulus on tissues in normal and in sub-tonic condition.


LAWS OF EFFECTS OF DIRECT AND INDIRECT STIMULUS.

1. THE EFFECT OF DIRECT STIMULUS IS NEGATIVE VARIATION OF TURGOR,
NEGATIVE MECHANICAL AND ELECTRICAL RESPONSE, NEGATIVE VARIATION, OR
RETARDATION OF RATE OF GROWTH.

  _a_. SUB-MINIMAL STIMULUS GIVES POSITIVE RESPONSE.

  _b_. POSITIVE RESPONSE IS ALSO GIVEN BY A TISSUE IN A SUB-TONIC
       CONDITION: CONTINUOUS STIMULATION CONVERTS THE ABNORMAL POSITIVE
       TO NORMAL NEGATIVE RESPONSE.

  _c_. AUTONOMOUS ACTIVITY IN A STATE OF STANDSTILL, MAY BE REVIVED BY
       STIMULUS.

  _d_. THE EFFECTS OF STIMULUS AND WARMTH ARE ANTAGONISTIC.

2. THE EFFECT OF INDIRECT STIMULUS IS POSITIVE VARIATION OF TURGOR,
POSITIVE MECHANICAL AND ELECTRICAL RESPONSE AND POSITIVE VARIATION OR
ACCELERATION OF RATE OF GROWTH.

I have referred to the fact previously demonstrated, that while Direct
stimulus induces contraction and retardation of growth, moderate
rise of temperature induces the opposite effect of expansion and
acceleration of growth. Further demonstration of the antagonistic
effects of stimulus and warmth will be given in the next Paper.


SUMMARY.

The autonomous activity of pulsating leaflet of _Desmodium gyrans_
comes to a stop under depletion of internal energy. A cut leaf isolated
from the plant maintains the rhythmic activity of its leaflets for
about 48 hours, after which there is an arrest of movement.

In this state of sub-tonicity the arrested autonomous activity is
revived under the action of various stimuli. Thus the incidence of
light on the pulvinule initiates pulsatory movements, which persists
for a time even on the cessation of stimulus. This persistence of
autonomous activity increases with the intensity and duration of
stimulus to which the leaflet had been subjected.

The arrested autonomous activity of growth may often be revived by
the action of stimulus. Thus the arrested growth in a mature style or
_Datura alba_ was renewed by electric stimulation.




XX.--ACTION OF LIGHT AND WARMTH ON AUTONOMOUS ACTIVITY

_By_

SIR J. C. BOSE.


In the preceding Paper I have shown the essential similarity of effect
of stimulus on autonomous activity of the _Desmodium_ leaflet, and of
the growing organ. It was shown how stimulus revived the pulsatory
activity of _Desmodium_ leaflet in a state of standstill, in the same
way as it renewed the arrested growth-activity.


THE OSCILLATING RECORDER.

The investigation of this subject was rendered possible by the
successful device of my Oscillating Recorder. A very light glass fibre
was used for the construction of the lever, which was supported on
jewel bearings. The short arm of the lever was 2 cm. in length, and the
long arm 8 cm. This gave a magnification of 4 times. But it is quite
easy to increase the magnification to 10 times or more.

The pull exerted by the pulsating leaflet is extremely slight, and the
relatively heavy lever made of steel wire used in the Resonant Recorder
is not well-suited for our purpose. The pulsation of the leaflet is
relatively slow, being once in two minutes or so. The intermittent
contact of ten times in a second, given by the Resonant Recorder, is
therefore too quick. In the Oscillating Recorder the intermittence
was, therefore, reduced to once in a second, or once in five seconds,
the recording plate itself being made to move to-and-fro at this rate.
The carrier of the plate-holder slides backwards and forwards on ball
bearings; a wheel in the clockwork connected with an eccentric is
released periodically, at intervals which may be varied between one
and five seconds. By the action of the eccentric, the plate carrier
approaches the writing lever with diminishing speed till the movement
is zero at the contact. This contrivance is essential, since any
sudden shock of the plate against the lever is apt to give rise to
after-vibrations of the writer. The plate carrier is quickly withdrawn
after the production of a dot on the smoked glass plate by contact with
the writing lever.

The clockwork is governed by a revolving fan which can be gradually
opened out by a regulating screw. The speed can thus be adjusted within
wide limits, and maintained constant and at any desired speed. A second
set of wheels connected with the clockwork moves the plate-holder in a
lateral direction. A series of records may thus be taken for fifteen
minutes, half an hour, or an hour.

The record obtained in this way is very perfect. Not only is the effect
of an external agent shown by variation in the amplitude and frequency
of pulsations, but the change of speed in any phase of the pulse
becomes automatically recorded.


RECORD OF PULSATION OF _DESMODIUM GYRANS_.

The whole plant can not be conveniently manipulated for different
investigations. It is, however, possible using the precautions
described below to use the detached petiole carrying the pulsating
leaflets. The terminal large leaf may also be removed. The necessary
amputation is often followed by an arrest of pulsation. But as in the
case of isolated heart in a state of standstill, the movement of the
leaflet may be revived by the application of internal hydrostatic
pressure. Under these conditions, the rhythmic pulsations may easily be
maintained uniform for many hours.

The petiole carrying the leaflet is mounted water-tight in the short
arm of an U-tube filled with water; for producing internal hydrostatic
pressure in the plant the height of water in the longer arm is
suitably raised. The U-tube holding the specimen may be adjusted up
and down, and laterally. A hinged support also allows the specimen to
be placed at any inclination. The movement of the leaflet, it is to
be remembered, does not always take place in a vertical direction.
The object of the mechanical adjustments is to place the specimen at
such an angle that its up and down movements when in a straight line
should be vertical, or have its long axis vertical when the movement
is elliptical. It is important that the specimen should be illuminated
equally from all sides; for one-sided illumination causes a bending
over of the leaflet towards light.

The pulvinule of the leaflet acts like the pulvinus of _Mimosa_, that
is to say, the leaflet undergoes a sudden fall to down position by the
contraction of the more effective lower half of the pulvinule; the ‘up’
position denotes recovery and expansion of the more effective half. The
up-and-down movements of the leaflet correspond to the diastolic and
systolic movements of the animal heart. There is, indeed, as I have
shown elsewhere[W] a very close resemblance between the activities of
rhythmic tissue in the plant and in the animal.

[W] BOSE--Irritability of Plants--p. 295.


EFFECT OF DIFFUSE LIGHT ON PULSATION OF _DESMODIUM_.

_Experiment 92._--For the study of effect of light on _Desmodium_,
I first obtained record in darkness. A horizontal beam of divergent
light from an arc lamp placed at a distance of 200 cm. was made to act
diffusely on the leaf from all sides. This was done by means of three
inclined mirrors, the first throwing the light vertically downwards,
the second vertically upwards, and the third horizontally forward from
the side opposite the lantern. The effect of light is seen demonstrated
in Fig. 86.

[Illustration: FIG. 86.--Effect of light on pulsation of _Desmodium_
leaflet. Duration of application of light is represented by the
horizontal line. Up-curve represents diastolic expansion and down-curve
systolic contraction. Note contractile effect of light in diminution of
amplitude and reduction of diastolic limit of pulsation.]

Light was applied at the second pulsation. It will be seen that light
retards or arrests the autonomous activity. On the cessation of light
the normal activity was found to be gradually restored. It is of much
interest to note here the similarity of action of light on autonomous
activity of the leaflet of _Desmodium_ and of a growing organ. In both,
we find that while in the sub-tonic condition of the tissue the effect
of light is to enhance or renew the autonomous activity of growth and
pulsation, in normal condition the effect is to retard it.

Inspection of the record exhibits another very interesting
characteristic. We saw that light retarded growth by inducing an
incipient contraction. In the _Desmodium_ leaflet the contractile
reaction of light is exhibited by the characteristic modification of
its pulsations. The duration of application of light is represented
by the horizontal line. In Fig. 86 the up-curve represents up-movement
of diastolic expansion, and the down-curve of systolic contraction.
The contractile reaction of light is seen to counteract the normal
expansion, with diminution of diastolic limit of pulsation.


EFFECT OF RISE OF TEMPERATURE ON PULSATION.

It has been shown that while rise of temperature up to an optimum
enhanced the rate of growth, the effect of light was to retard it.
Hence the effects of light and warmth are antagonistic.

[Illustration: FIG. 87.--Effect of rise of temperature on pulsation of
leaflet of _Desmodium gyrans_. Horizontal line represents the duration
of gradual rise of temperature from 30°C. to 35°C. Note the expansive
effect of rise of temperature in reduction of systolic limit of
pulsation.]

_Effect of rise of temperature on pulse-record: Experiment 93._--In
studying the effect of rise of temperature on the pulsation of leaflets
of _Desmodium_, we discover similar antagonistic reactions of light
and warmth. The leaflet was placed in a plant-chamber with an electric
arrangement for gradual rise of temperature. The first two records
were taken in the normal temperature of the room, which was 30°C.
The temperature was gradually raised to 35°C, the record being taken
all the time. It will be seen (Fig. 87) that the effect of warmth
is diametrically opposite to that of light. The record in Fig. 86
exhibited _the contractile_ effect of light by reducing the diastolic
limit of expansion. In the present case the _expansive_ reaction of
warmth is exhibited by the reduction of systolic limit of contraction.
The temperature of the plant chamber was now allowed to return to
30°C., and we observe the gradual restoration of normal systolic limit
of contraction.


SUMMARY.

Two different effects are found in the action of the stimulus of light
alike on the autonomous activity of leaflet of _Desmodium gyrans_ and
of growing organs. In condition of sub-tonicity light renews pulsation
of _Desmodium_ and enhances the activity of growth. In normal tonic
condition the effect induced is the very opposite, light causing an
arrest of pulsation and retardation or arrest of growth.

The contractile effect of light is seen not only in the retardation of
growth, but also by the characteristic modification of pulsation of
_Desmodium_ in the diminution of diastolic limit of expansion.

The antagonistic reactions of light and warmth are found not only in
growth but also in the rhythmic activity of _Desmodium gyrans_. In
the pulsation of _Desmodium_ the contractile effect of light induces
a rapid diminution of the diastolic limit of expansion, while the
expansive reaction of warmth brings about a marked reduction of the
systolic limit in successive pulsations.




XXI.--A COMPARISON OF RESPONSES IN GROWING AND NON-GROWING ORGANS

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.


I have in the preceding series of Papers demonstrated the effects of
various forms of stimuli on growth. I have also given accounts of
numerous reactions which are extraordinarily similar, in growing and
non-growing organs. In fact certain characteristic reactions observed
in motile pulvinus of _Mimosa_ and other ‘sensitive’ plants led to
the discovery of the corresponding phenomena in growing organs. For
fully realising the essential similarity of responses given by all
plant-organs, growing and non-growing, I shall give here a short review
of the striking character of the parallelism.

1. The incipient contraction of a growing organ under stimulus
culminates in a marked shortening of the organ.

2. The similarity of contractile responses in growing and pulvinated
organs.

3. Similar modification of both under condition of sub-tonicity.

4. The opposite effects of Direct and Indirect stimulus, both in motile
and in growing organs.

5. The exhibition by all plant-organs of _negative_ electric response
under Direct, and _positive_ electric response under Indirect stimulus.

6. Similar modification of autonomous activity in _Desmodium gyrans_
and in growing organs under parallel conditions.

7. Similar excitatory effects of various stimuli on pulvinated and
growing organs.

8. Similar discriminative effects of different rays of light in
excitation of motile and growing organs.


CONTRACTILE RESPONSE OF GROWING AND NON-GROWING ORGANS.

I have shown (page 198) that a growing organ under stimulus, undergoes
an incipient contraction as shown in the responsive retardation of its
rate of growth; that this retardation increases with the intensity of
the incident stimulus till growth becomes arrested. Above this critical
intensity the induced contraction causes an actual shortening of the
organ. There is no breach of continuity in the increasing contractile
reaction, which at various stages appears as a retardation, an arrest
of growth or a marked shortening of length of the organ.


CONTRACTILE RESPONSE OF PULVINATED AND GROWING ORGANS.

[Illustration: FIG. 88.--Contractile response of growing organ under
electric shock. Successive dots at intervals of 4″. Vertical lines
below represent intervals of one minute. (Magnification 1,000 times.)]

_Experiment 94._--In order to show the striking similarity between the
response of ‘sensitive’ _Mimosa_ and that of a growing organ, I give
a record (Fig. 88) obtained with a growing bud of _Crinum_ under the
stimulus of electric shock above the critical intensity. The recorder
gave a magnification of a thousand times. In Fig. 88, the normal
growth elongation is represented as a down-curve. On the application
of stimulus the normal expansion was suddenly reversed to excitatory
contraction, the latent period of reaction was one second and the
period of the attainment of maximum contraction (apex-time) was 4
minutes. The organ recovered its original length after a further period
of seven minutes and then continued its natural growth elongation.
Repetition of stimuli gave rise to successive contractile responses
which are in every way similar to the mechanical responses of _Mimosa
pudica_. The essential similarity of response of pulvinated and
growing organs will be seen in the following tabular statement:

TABLE XXI.--TIME RELATIONS OF MECHANICAL RESPONSE OF PULVINATED AND
GROWING ORGANS.

  +-------------------------+----------+------------+--------------+
  |                         |  Latent  | Apex-time. |    Period    |
  |        Specimen.        |  period. |            | of recovery. |
  +-------------------------+----------+------------+--------------+
  |Motile pulvinus of       |          |            |              |
  |  _Mimosa pudica_.       | 0.1 sec. |   3 secs.  | 16 minutes.  |
  |Motile pulvinus of       |          |            |              |
  |  _Neptunia oleracea_.   | 0.6  "   | 180   "    | 60    "      |
  |Growing bud of _Crinum_. | 1.0  "   | 240   "    |  7    "      |
  +-------------------------+----------+------------+--------------+

The contraction in growing organs under stimulus is sometimes
considerable. Thus in the filamentous corona of _Passiflora
quadrangularis_ the contraction may be as much as 15 per cent. of
the original length. This is not very different from the excitatory
reaction of the typically sensitive stamens of the _Cynereæ_, which
exhibits a contraction from 8 to 22 per cent.


MODIFICATION OF RESPONSE BY CONDITION OF SUB-TONICITY.

In _Mimosa_ the normal response to direct stimulus is _negative_, the
leaf undergoing a fall. But sub-tonic specimens exhibit a _positive_
response with erection of the leaf. The action of the stimulus itself
improves the tonic condition, and the abnormal positive is thus
converted into normal negative, through diphasic response (p. 147).
Similarly in growing organs, while the normal effect of stimulus is
incipient contraction and retardation of growth under condition of
sub-tonicity the response is by acceleration of growth. Continuous
stimulation converts this abnormal acceleration into normal
retardation of growth (p. 225).


EFFECTS OF DIRECT AND INDIRECT STIMULUS.

Direct stimulus induces in _Mimosa_ and other ‘sensitive’ plants a
_negative_ response. There is a diminution of turgor and contraction
in the motile organ, resulting in the fall of leaf. Indirect stimulus,
on the other hand, gives rise to a _positive_ or erectile response,
indicative of increase of turgor and expansion (p. 138).

In growing organs Direct stimulus induces an incipient contraction
and retardation of rate of growth; the effect of Indirect stimulus is
expansion and acceleration of the rate of growth (p. 216).

The opposite reactions to Direct and Indirect stimulus are also found
in the electric response given by all plant organs. Thus while Direct
stimulus induces an electromotive change of galvanometric negativity,
Indirect stimulus induces the opposite change of galvanometric
positivity (p. 214).


MODIFICATION OF AUTONOMOUS ACTIVITY.

The autonomous activity of _Desmodium gyrans_ exhibited by the
pulsation of its leaflets come to a stop under condition of
sub-tonicity. The arrested movement is, however, revived by the action
of stimulus (p. 228). The depressed or arrested growth of a growing
organ is similarly accelerated or revived by the action of stimulus
(p. 230).

In vigorous specimens stimulus induces the opposite effect by retarding
or arresting the pulsatory activity or growth.

Warmth induces an effect which is antagonistic to that of stimulus. The
contractile effect of stimulus is seen in the pulsations of leaflet
_Desmodium_ by the reduction of their expansive or diastolic limit,
and in growing organs by the retardation of the rate of growth. The
expansive effect of warmth is seen in reduction of the systolic limit
of _Desmodium_ pulsation, and in the acceleration of rate of growth in
growing organs (p. 237).


EXCITATORY EFFECTS OF VARIOUS STIMULI ON PULVINATED AND GROWING ORGANS.

Certain agents induce excitation in living tissues, the excitatory
change being detected by contraction, or by electromotive variation,
or by change of electric resistance, and in growing organs by the
retardation of the rate of growth. In general, the various stimuli
which excite animal tissues also excite vegetable tissues.

It has been shown that _every form of stimuli, however diverse, also
induces incipient contraction and retardation of the rate of growth_.
Thus mechanical irritation, such as friction or wound, induces a
retardation of growth (p. 202); they also induce an excitatory
contraction in _Mimosa_, attended by the fall of the leaf. Different
modes of electric stimulation act similarly on both growing and
pulvinated organs. The action of light visible and invisible will
presently be seen to react on both alike. And in this connection
nothing could be more significant than the discriminative manner in
which both the pulvinated and the growing organs respond to certain
lights and not to others.

In contrast to the contractile effect of stimulus, certain agents
induce the antagonistic reaction of expansion. It has been shown that
while stimulus induces a retardation, rise of temperature up to an
optimum point, induces an acceleration of the rate of growth. I have
also referred to the fact that while the autonomous pulsations of
_Desmodium_ leaflet exhibit under stimulus a diminution of the extent
of the diastolic expansion, warmth on the other hand, induces the
opposite effect by diminishing the systolic contraction.


EFFECT OF LIGHT ON PULVINATED ORGANS.

I have referred to the well-known fact that it is the more refrangible
portions of the spectrum that are more effective in inducing excitatory
reactions and have already given records of the responsive reactions
of various lights on growing organs. I shall now give records of the
effect of various lights on the pulvinus of _Mimosa pudica_. The
amplitude and time relations of the curves of response will give a more
precise idea of the quantitative effects of various lights in inducing
excitation.

[Illustration: FIG. 89.--Effect of white light on the pulvinus of
_Mimosa_. Successive dots in this and in the following records are at
intervals of 10″. (Magnification 100 times)]

_Action of white light: Experiment 95._--The source of light was an arc
lamp; a pencil of parallel light is made to pass through a trough of
alum solution. This process of excluding thermal rays is adopted for
the visible rays of the spectrum. Colour filters were also used for
obtaining red, yellow and blue lights. The pencil of light is thrown
upwards by an inclined mirror on the lower half of the pulvinus. The
response is taken by an Oscillating recorder, giving successive dots at
intervals of 10 seconds, the magnification employed being 100 times.
The pulvinus being subjected to light for 10 seconds gave response by
a fall of the leaf (Fig. 89). The response to light is thus found to
be essentially similar to that induced by electric stimulus, the only
difference being in the relative sluggishness of the reply. Electric
shock passes instantaneously through the mass of the pulvinus, stirring
up the active tissues to responsive contraction. The latent period
is, therefore, as short as 0.1 second and the maximum contraction is
effected in about 3 seconds. In the case of the stimulus of light the
shock-effect is not so great; excitation, moreover, has to pass slowly
from the surface of the pulvinus inwards. Hence the latent period is
twelve seconds, and the period of maximum contraction is as long as
90 seconds. As the stimulation is moderate, the recovery is effected
in 11 minutes, instead of 16 minutes, which is the usual period for
_Mimosa_ to recover from an electric shock. The important conclusion to
be derived from this experiment is, that light is a mode of stimulation
and that it induces a responsive contraction, similar to that caused
by other forms of stimuli. This contractile response under light is
exhibited not merely by the motile pulvinus of _Mimosa_, but by other
pulvini as well, such as those of _Erythrina indica_, and of the
ordinary bean plant.

_Action of red and yellow lights._--The pulvinus gave little or
practically no response to these lights.

_Action of blue light: Experiment 96._--Light was applied for 10
seconds and the amplitude of response was similar to that induced by
white light (Fig. 90).

[Illustration: FIG. 90.--Effect of blue light on pulvinus of _Mimosa_.]

_Action of Ultra-violet rays: Experiment 97._--The source of light was
a quartz mercury-vapour lamp. The effect was so intense that, to keep
the record within the plate, I had to reduce the period of exposure to
half, _i.e._, to five seconds. The responsive movement was initiated
within six seconds of the application of light. The intensity and the
rapidity of reaction is independently evidenced by the more erect curve
of response (Fig. 91).

[Illustration: FIG. 91.--Effect of ultra-violet rays on the pulvinus of
_Mimosa_.]

_Action of Infra-red rays: Experiment 98._--The obscure thermal rays
also caused a strong excitatory reaction (Fig. 92). Attention is here
drawn once more to the antagonistic reactions of temperature and
radiation effects of heat.

It has been shown that the rays which cause the most intense
excitations in _Mimosa_ also induce the greatest retardation in the
rate of growth. Thus ultra-violet is not only the most effective in
causing excitation in _Mimosa_ but also in retardation of growth.
Next in order comes the blue rays: the yellow and red are practically
ineffective in both the cases. Infra-red rays are, however, very
effective in exciting the sensitive _Mimosa_ and in retarding the rate
of growth.

[Illustration: FIG. 92.--Effect of infra-red rays on the pulvinus of
_Mimosa_.]


DIVERSE MODES OF RESPONSE TO STIMULUS.

In _Mimosa_ excitation is followed by the striking manifestation of
the fall of the leaf. But in rigid trees contraction under excitation
cannot find expression in movements. I have shown elsewhere that even
in the absence of realised movement, the state of excitation can be
detected by the induced electromotive change. I have shown that not
only every plant but every organ of every plant is sensitive and reacts
to stimulus by electric response of galvanometric negativity.[X]

[X] BOSE--Friday Evening Discourse--Royal Institution of Great Britain,
May 1901.

There is an additional electric method by which the excitatory
change may be recorded. I find that excitation induces a variation
of the electrical resistance of a vegetable tissue.[Y] Thus the same
excitatory reaction finds diverse concomitant manifestations, in
diminution of turgor, in movement, in variation of growth, and in
electrical change. The correspondence in the different phases of
response in pulvinated, ordinary, and growing organs may be stated
as follows: Excitation induces diminution of turgor, contraction and
fall of the leaf of _Mimosa_; it induces an incipient contraction or
retardation of rate of growth in a growing organ; it gives rise in
all plant organs to an electric response of galvanometric negativity
and of changed resistance. All these excitatory manifestations will,
for convenience, be designated as the _negative_ response. There is
a responsive reaction which is opposite to the excitatory change
described above. In _Mimosa_ the fall of leaf under excitation is
due to a sudden diminution of turgor; the erection of the leaf is
brought about by natural or artificial restoration of turgor. Rise
of temperature induces an expansive reaction which is antagonistic
to that induced by stimulus. Warmth also enhances the rate of growth
and induces an electric change of galvanometric positivity.[Z] The
restoration of normal turgor or enhancement of turgor is associated
with expansion, erection of the leaf of _Mimosa_, enhancement of rate
of growth in a growing organ, electric response of galvanometric
positivity, and contrasted change of electric resistance. All these
will be distinguished as _positive_ response.

[Y] This variation is sometimes positive, and at other times negative,
according to the condition of the tissue.

[Z] BOSE--“Comparative Electro-physiology”--p. 75.

There are thus several independent means of detecting the excitatory
change or its opposite reaction in vegetable tissues. It will be seen
that the employment of these different methods has greatly extended our
power of investigation on the phenomenon of irritability of plants.

We have seen how essentially similar are the responsive reactions
in pulvinated and in growing organs. It is therefore rational to
seek for an explanation of a particular movement in a growing organ
from ascertained facts relating to the corresponding movement in a
pulvinated organ. The investigations on motile and growing organs that
have been described fully establish the two important facts that,
Direct stimulus induces contraction and Indirect stimulus induces
the opposite expansive reaction. These facts will be found to offer
full explanation of various tropic curvatures to be described in the
subsequent series of Papers.


SUMMARY.

There is no breach of continuity in the increasing contractile reaction
in a growing organ under increasing intensity of stimulus; the
incipient contraction seen in retardation of rate of growth culminates
in a marked shortening of the length of the organ.

Time relations of response, the latent period, the apex time, and the
period of recovery are of similar order in pulvinated and in growing
organs.

In condition of sub-tonicity the pulvinus of _Mimosa_ responds to
stimulus by an abnormal _positive_ or erectile response. Under
continued stimulation the abnormal positive is converted into normal
_negative_. Growing organs in sub-tonic condition responds to stimulus
by abnormal acceleration of rate of growth, which is converted into
normal retardation under continuous stimulation.

Direct stimulus induces in _Mimosa_ a _negative_ response, with the
fall of leaf. But Indirect stimulus induces the _positive_ or erectile
response. Similarly, Direct stimulus induces in a growing organ a
_negative_ variation, or retardation of rate of growth, and Indirect
stimulus a _positive_ variation or acceleration of rate of growth.

The electric response to Direct stimulus is by galvanometric
_negativity_, that to Indirect stimulus by galvanometric _positivity_.

Under condition of sub-tonicity the autonomous activity of leaflet of
_Desmodium gyrans_ and of growing organs comes to a stop. The arrested
activity in both is revived by the application of stimulus. Active
pulsation in _Desmodium_, and active growth in growing organs are,
however, retarded or arrested by stimulus.

The contractile effect of stimulus on pulsation of leaflets of
_Desmodium gyrans_ is seen by the reduction of the diastolic limit
of its pulsations; to this corresponds the incipient contraction and
retardation of rate of growth in a growing organ. The effect of warmth
is antagonistic to that of stimulus. The expansive effect of rise of
temperature is seen in _Desmodium_ by the reduction of the systolic
limit of its pulsation; in growth it is exhibited by an acceleration of
the rate of growth.

All stimuli which induce an excitatory contraction and fall of the leaf
of _Mimosa_ also induce incipient contraction and retardation of rate
of growth in a growing organ.

Excitatory effects of different rays of light on motile and growing
organs are similarly discriminative. Ultra-violet light exerts the most
intense reaction which reaches a minimum towards the less refrangible
red end of the spectrum. Beyond this, the infra-red or thermal
rays become suddenly effective in inducing excitatory movement and
retardation of growth.