Life Movements in Plants

The power of conduction varies widely in different plants. In the petiole of Mimosa pudica the velocity may be as high as 30 mm. per second. In, the stem the velocity is considerably less, i.e., about 6 min. "per second in the longitudinal direction; but conduction across the stem is a very much slower process. In the petiole of Averrhoa the longitudinal velocity is of the order of 1 mm* per second. The record of the transmitted effect of stimulus is found to exhibit a remarkable preliminary variation. This was detected by my delicate recorders, which gave magnifica¬ tions from fifty to hundred times. I shall give a detailed account of a typical experiment carried out with Averrhoa carambola, which will bring 8ut clearly the characteristic effects of Indirect Stimulus,

Experiment 47 .—Stimulus of electric shock applied at a point on the long petiole of Averrhoa causes successive fall of pairs of' leaflets. In the experiment to be described one 52.—Effect of indirect Stimulus on leaflet of Averrhoa caramhula. Stimu¬ lus was applied at the short vertical line. Successive dots at intervals of ■one second. Note the positive response preceding the negative. of the leaflets of the plant was attached to the recorder. Stimulus was applied at a distance of 50 mm. The success¬ ive dots in the record are at intervals of a second. It will be noticed that two distinct impulses—a positive and a negative —were generated by the action of Indirect Sti¬ mulus. The positive impulse reached the responding organ after 1*5 second and caused an erectile movement. The velocity of the positive impulse in the present case is 33 mm. per second. The normal excitatory negative impulse reached the motile organ 44 seconds after the application of stimulus, and caused a very rapid fall of the leaflet, the fall being far more pronounced than the positive movement of erection (Fig* 52). In this and in all subsequent records, the positive and negative responses offer a great contrast. The movement in response to positive reaction is slow, whereas that due to negative reaction is very abrupt, almost 4 explosive,* the successive dots being now very wide apart. As regards the velocity of impulse the relation is reversed, the positive being the quicker of the two. In the present case, the velocity of the excitatory negative impulse is 1*1 mm. per second, as against 33 mm. of the positive impulse.

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

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

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

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

From what has been said it will be understood that the exhibition of positive response is favoured by the con¬ ditions, that the transmitting tissue should be semi-conduct¬ ing, and the stimulus feeble. It is thus easier to ex¬ hibit the positive effect with the feebly conducting petiole of Averrhoa than with the better conducting petiole of Mimosa. It is, however, possible to obtain positive response in the Mimosa by application of indirect stimulus to the stem in which conduction is less rapid than in the petioles.

From the results given in course of the Paper we are able to formulate the following laws about the effects of Direct and Indirect Stimulus on pulvinated organs :— 1. Effect of all forms of Direct stimulus is a diminution of 3. Prolonged application of indirect stimulus OF MODERATE IN¬ 4. IF THE INTERVENING TISSUE BE HIGHLY CONDUCTING, THE TRANS¬ The laws of - Effects of Direct and Indirect stimulus hold good not merely in the case of sensitive plants, but universally for all plants. This aspect of the subject will be treated in fuller detail in later Papers of this series.

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

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

A muscle contracts under stimulus ; this ‘is assumed to be due to some explosive chemical change which leaves the tissue in a condition less capable of functioning, or in a condition below par. Herring designates this as a process of dissimilation. The excitability of the muscle is restored' after suitable periods of rest, by the opposite metabolic change of assimilation . “ Assimilation and Dis¬ similation must be conceived as two closely interwoven pro¬ cesses, which constitute the metabolism (unknown to us in its intrinsic nature) of the living substance. Excitabi¬ lity diminishes in proportion with the duration of D-stimu- lus, or, as it is usually expressed, the substance fatigues itself. It is perfectly intelligible that a progressive fatigue and decrement of the magnitude of contraction must ensue. The only point that is difficult to elucidate is the initial staircase increment of the ’ twitches, more especially in excised, bloodless muscle, which seems in direct contradic¬ tion with.. . the previous theory.”*

With reference to Herring’s theory given above, Bayiiss in his “ Principles of General Physiology” (1915); page 377 says, u In the phenomenon of metabolism, two processes must be distinguished, the building up of a complex system or substance of high potential energy, 6 anabolism,’ and the breaking down of such a system ‘catabolism,’ giving off energy in other forms. The tendency of much recent work, however, is to throw doubt on the universality of this opposition of anabolism and catabolism as explana¬ tory of physiological activity in general”

The results obtained with the response of plants to stimu¬ lus may perhaps throw some light on the obscurities that surround the subject. They show that the two pro¬ cesses may be present simultaneously, and that the 4 down ’ change induced by stimulus may, in certain instances, be more than compensated by the 4 up 5 change.* I shall, for convenience, designate the physico-chemical modification, as¬ sociated' with the excitatory negative mechanical and elec¬ trical response of plants, as the “D” change; this is attend¬ ed by run down of energy. The positive mechanical and electrical response must therefore connote opposite physico- chemical change, with increase of potential energy. This I shall designate as the “ A ” change, which by increasing the. latent energy, enhances the functional activity of the tissue. That stimulus may give rise simultaneously to both 4,. and D, effects, finds strong support in the dual reactions ex¬ hibited, in plant-response. Under indirect stimulus, the two responses are seen separately, the. more intense negative following the feeble, positive. When by the reduction of the .intervening distance, stimulus is made direct, the result¬ ant response,, as previously stated, is negative; and this, .is due .not to the total absence of. the positive but to its being masked by the predominant negative. Let' us next

. ^ lu the .response of inorganic matter *. have obtained records of positive, diphasic and negative responses. It. would perhaps be advisable to refer the A and D effects, to physico-chemical change. The simultaneous double reaction, combination and decomposition, is of frequent occurrence in many chemical changes. ■ consider the 'question of unmasking this positive element in the resultant negative response. Under favourable conditions of the environment, the ex¬ citability of the organs is at its maximum. A given stimulus will bring about an intense excitation, and the ‘down 5 D-change will therefore be very much greater than the A-change. Let us now consider the case at the opposite extreme where, owing to unfavourable condition, the excita¬ bility is at its lowest. Under stimulus the excitatory D- change will now be relatively feeble compared to the A- change, by which the potential energy of the system .becomes increased. In such a case successive stimuli will increase the functional activity of the tissue, and bring about staircase response. Biedermann mentions the staircase response of excised bloodless muscle as offering difficulty of explanation. It is obvious that the physiological condition of the excised muscle must have fallen below par. The staircase response in. such a tissue is thus explained from considerations that have just been adduced.

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

■ But the plant in a sub-tonic condition will exhibit feeble dr no ^excitation. ■ The B-change will be absent while the A-change will take place under the action of stimulus. This, by increasing the potential energy,, will enhance the functional activity of the tissue. Fig. 53.—Record showing the effect of stimulus modify¬ ing tonicity and producing staircase effect. ( Ja imosa .) in Mimosa : Experiment 48 .—T$ie theoretical considerations will be found experimentally verified in the record obtained with a specimen of Mimosa in a sub-tonic condition (Fig. 53), Owing to the lack of favourable 4 tone ? the leaf was relax¬ ing as seen in the first part of the curve. The stimulus of electric shock, applied at the thick dot in the curve slanting downwards, gave no response but raised the tone of the tissue by arresting- the growing relaxation. Subsequent stimuli gave rise to staircase responses. Stimulus has, through the A-effect, raised the functional activity of the tissue to a maximum.

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

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

Experiment 49 .—The staircase response of Mimosa given in., figure 53 was obtained with the stimulus of induction shock. In order to establish a wider generalisa¬ tion I now used the stimulus of light given by an diurnal movement of Mimosa; the leaf, generally speaking, has u movement in a downward direction from morning till noon, after which there is a comparative state of rest. It is better to choose the time of noon for experi¬ ment. In any case the response to stimulus is very

ment A horizontal pencil of light was thrown upwards by means of a small mirror and made to fall on the lower half of a pulvinus of the Mimosa leaf. The excita¬ tory down movement is followed by recovery on the my first series of experiments a specimen that had been isolated for si£ hours. Stimulation was caused by suc¬ cessive applications of light for 25 seconds at intervals of 3 minutes. Figure 54 shows how the functional activity of the sub-tonic specimen is enhanced by stimulus, the successive responses thus exhibiting the staircase effect.

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

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

The excitability of a plant is found to be modified by its tonic condition. A sub-tonic specimen of Mimosa, like an excised blood¬ less muscle, shows a preliminary staircase response. Stimu¬ lus" induces simultaneously both “A” and “D” effect?, with their attendant positive and negative reactions. A tissue in optimum condition exhibits only the result¬ ant negative response, the ^comparatively feeble positive be¬ ing masked by the predominant negative. With decline of tone, the “D” effect diminishes and we get “A” effect unmasked;

In .extreme sub-tonic specimen, we get first only the o a.” effect,; with its positive response. Successive stimu¬ lation converts the pure positive into diphasic and ulti¬ mately into normal negative response. In discussing the difficulties connected with investiga¬ tions relating to longitudinal growth and its variations, special stress must be laid on the importance of maintaining external conditions absolutely constant. This constancy can only be maintained in practice for a short time. Lengthy periods of observation, moreover, introduce the uncertainty of complication arising from spontaneous variation of growth. The possibility of accurate investigation, therefore lies in reducing the period of the experiment to a few minutes during which we have to determine the normal rate of growth and its variation under a given changed condition. This’ would necessitate the devising of a method of very high magnification for record of the rate of growth.*

With auxanometers now in use, which give a magnifiea ticn of about twenty times, it takes nearly four hours to determine the influence of changed condition in inducing * A short account of my researches with the High Magnification Crescograph has been published in the Proceedings of the Koyal Soc^ty. I shall in the following Papers give a detailed account of my investigations on growth and on allied phenomena. variation of growth. It will be seen that if we succeeded in enhancing magnification from twenty to ten thousand times, the necessary period for experiment would be reduced from four hours to thirty seconds. The importance of secur-' ing a magnification of this order is sufficiently obvious.

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

I have, for the past six years, been working with a different method, which has now been brought to a great state of perfection. The problem to be solved was the devising of a direct method of high magnification and the automatic record of the magnified rate of growth. The magnification in my Creseograph is obtained by a compound system of two levers. The growing plant is attached to the short arm of a lever, the long arm of which is attached to the short arm of the second lever. If the magnification by the first lever be m, and that by the second, «, the resulting magnification would be mn.

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

Weight of the Lever .—As the first lever is to exert* a pull on the second, it has to be made rigid* The second lever selves as an index, and can therefore be made of fine glass fibre. The securing of rigidity of the first lever entails large cross section and consequent weight, which exerts considerable tension on the plant. Excessive tension greatly modifies growth ; even the weight of 'the index used in self-recording auxanometers is found to modify the nor¬ mal rate” of growth. The weight of the levers introduces an additional difficulty in the increased friction at the fulcrums, on account of which there is an obstruction of the free movement of the recording arm of the lever. The conditions essential for overcoming these difficulties there¬ fore are : (1) construction of a very light lever possessing sufficient rigidity, and (2) arranging the levers in such a way that the tension on the plant may be reduced to any extent, or even eliminated,

I found in navaldum , an alloy of aluminium, a light material possessing sufficient rigidity. The first lever is con¬ structed out of a thin narrow sheet 25 cm. in length; it has, as, ; explained before, to. be fairly rigid in order to exert a pull on the second without undergoing any bend¬ ing; this rigidity is secured by giving the thin- narrow plate of the : lever a . T-shape. The first lower balances, to a . certain extent, the second, Finer adjustments are macte by means of an adjustable counterpoise B, at the end of the levers. By this means the tension on tjie plant can