(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.