Chapter IX: Part II: Growth and Its Responsive Variations (2)

Text size

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.

_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.

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.

_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.

_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.

_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.

(_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.

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.

_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.

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.

_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.

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).

_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.

_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.

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.

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.

_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.

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.

_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.

Previous Next

Comments

Life Movements in Plants, Volume IChapter IX: Part II: Growth and Its Responsive Variations (2)

0%37 min left in chapter