Life Movements in Plants

The action of dilate vapour of ammonia is a prelimin¬ ary enhancement followed by depression of growth. Ether vapour depresses the rate of growth. On the re¬ moval of the vapour there is a recovery of the ..normal The effect of carbonic acid is a great enhancement of the rate of growth; after this preliminary action, growth undergoes a decline. The effect described takes place equally in light or in darkness. Oral gas induces a depression of the rate of growth from which there is a recovery after the removal of the gas. The action of sulphuretted hydrogen is far more toxic, the after-effect being very persistent.

Solution of ammonium sulphide induces increasing re¬ tardation of growth, with the strength- of the solution. Copper sulphate solution acts as a toxic agent, retarding the rate of growth and ultimately billing the plant.. The movements of leaves of sensitive plants are caused by variation of turgor in the pulvinus induced by stimulus. The down movement or negative response of Mimosa is caused by a diminution or negative variation of turgor, while the erection or positive response is brought about by an increase, or positive variation of turgor.

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

In experimenting with Mimosa the plant was subjected to the condition of drought, water being withheld for a day. On supplying water, the leaf, after a short period, exhibited a positive or erectile movement ( Expt. 12). The delay was evidently due to the time taken by the water absorbed-by root t® reach the responding organ. Method of Irrigation: Experiment 70 .—In order to investigate' the effdct 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 wasi ^Q^awurely 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. Tho specimen was then subjected to a condition of drought, water e 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

I record (H) was obtained after irrigation with tepid water. It will be seen tnat the spaces between successive dots, representing magni¬ fied growth at intervals of ten seconds, are very different. While a given elongation cook place under drought in 19 x 10 seconds, a similar lengthening took place, after irri¬ gation with cold water, in 1*1x10 seconds, arid after irrigation with warm water in 3x10 seconds. Irrigation with warm water is thus seen to increase the rate of growth more than six times.

F,G. 70.—Effect of irrigation: D, record of growth under 'drought; 0. acceleration after, irrigation with cold water; H, enhanced acceleration on irrigation with warm water. (S. Kyvoor.) The enhancement of the rate of growth on irrigation with coid 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 wav 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 he too sudden ; ifc should commence with tepid, and end with w&rm water* Snd-den application of hot water brings about certain 'complications due to excitatory effect As regards the per¬ sistence 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 Zephymnthes the growth under partial drought was found to be 0*04 jt£ per second ; application of warm water increased the growth rate to 0*20 /x per second* After 15 minutes the gro%vth rate fell to 0*13 /* per second ; and after jan hour to 0*03 p per second. It will be noted that even then the race was twice the initial rate before irrigation.

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 pres- Bare enhances the rate of growth till a critical point, is reached, beyond which there is a depression. This critical point varies m different plants.

TABLE XIII.—EFFECT OF INCREASED INTERNAL HYDROSTATIC PRESSURE I shall now describe the influence of induced diminution of turgor on the rate of growth. Method of phis moly sis : Experiment 72 ,—Being desirous of demonstrating the responsive growth variations of opposite signs in an iden¬ tical specimen under alternate increase. and diminution of turgor, I continued the experiment with the same peduncle of Zsphyranthes in which the growth acceleration was induced by irrigation with warm water. In that experiment the growth * rate of 0*04 p per second was enhanced to 0*20 p per. second after irrigation. A strong solution of KN0 3 was now applied at the root;

Eir 71_Effect of alternate increase and diminution of surgor on the same specimen*: X," normal rate under drought; H, enhanced rate unper irrigation'with warm water; X', normal permanent rate after irrigation; P, dimmumed rate after plasmo- and the growth-rate fell almost immediately to 0*03 p per second, or nearly to one-third the previous rate, the depres¬ sion induced being thus greater than under condition of drought (Fig. 71). TABLE XIV.— EFFECT OF ALTERNATE VARIATION OF TURGOR ON GROWTH

From the senes 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 en¬ hanced by an increase of turgor. A diminution or nega¬ tive 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 varia¬ tion of , turgor itself is also determined by that' activity. ■

the leaf of Mimosa and the movement due to growth, which is summarized as follows: (1) An increase or positive variation of turgor in¬ duces 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 retarda¬ tion of the rate of growth. 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 »rams. The table given above shows how growth-rate in¬ creases with the tension till a limit is reached, after which there is a retardation.

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 glowing region, from the root. Thl 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.

Thexe 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 fail 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. - . In plant physiology, the word 1 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 sn animal physiology. An induction shock, a condenser dis¬ charge, 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.

The form of stimulus which is extensively used m physiological investigations, is the electric stimulus 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 demon¬ strated 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 la below the range of human perception,

■ Effect of Intensity: Experiment 74 .—I shall now de¬ scribe a typical experiment on the effect of intensity of Pio. 72.—Effects of electrical stimulus of increasing intensities: of 0*25 unit, 1 unit, and 3 units. Short daghos represent the moments of application of- stimulus^.-.' ' second. On the application of electric shock of unit iixten- sity for 5 seconds, the rat© became reduced’ to 0*22 p per second. When the stimulus was increased to 2 units, the retarded rate of growth was 0*07 p 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. Effecc of continuous stimulation: Experiment 75 .— The effect of continuous stimulation of increasing in¬ tensity will be seen in the record (Fig. 73), taken on a Fig 73.—Effect of continuous electric stimulation of increasing intensity. The last record exhibits the actual shorteaing of the growing organ under strong stimulus. moving plate. On application of continuous stimulus of increasing intensity an increased flexture 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. 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 mav -con¬ veniently describe this effect as 4 incipient 9 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 culmin- *ates 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 4 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 inten¬ sity and duration. TAJiLE XVI. — EFFECT OF INTENSITY AND DURATION OF ELECTRIC STIMULUS 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

In normal conditions electric stimulus induces an inci¬ pient contraction exhibited by the retardation of the, rate of growth. Growth is often affected by an electric stimu¬ lus which is below human perception. ' Under increasing intensity of stimulus, the contractile reaction opposing growth elongation becomes more and more pronounced. ‘At a ciitical intensity of stimulus growth becomes arrested. Under stronger intensity of stimulus growing organ undergoes an actual shortening in

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. 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 stimu¬ lation, from which the excitated pulvinus recovers within a short time. But a prick or a cut induces a far more in¬ tense and persistent excitation; the recovery becomes pro¬ tracted, 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 con¬ tact or friction ; more intense stimulation is caused by a prick or a cut. Experiment 76 .—In this experiment, I took a peduncle of Zephyranthes, which had a normal rate of growth of 0T8 p per second. I then caused mechanical irritation by rub¬ bing the surface with a piece of card-board. The mecha¬ nical stimulation was found to have caused a retardation of growth, the depressed rate being 0*11 p per second, or three- fifths the normal rate. As this particular mode of stimula¬ tion was very moderate, the normal of rate growth was found to be restored after a short period of rest. After 15

minute*.. the rate became 014 /* per second; after ar hon; the recovery was complete, the rate being now Oli jL per second, the normal rate before stimulation (Fia ./4a, We shall presently see that not only is the growth rate greatlv depressed under intense stimulation, Km the period of recovery also becomes very much pro¬ tracted. . 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 .hoar 'I had to discard them for others with equally un- satisfactory results. One of these specimens happened to be left attached to the recorder overnight, and I was surpnsed- to find that the specimen, which had shown no growth the

friction; A, partial recovery after 15 minutes. (6) N, normal; W, immediately after wound; (Successive clots at intervals of.5.) exhibiting vigorous growth after hours. 1 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 demon¬ strated in the following experiments, where the stimulus employed was more intense.

A prick causes an intense excitation in Mimosa. I tried the effect of this form of stimulation on responsive varia¬ tion in growth. Experiment 77.—The specimen was the same as had been employed in the last experiment. 'After moderate stimula¬ tion due to friction it had, in the course of an hour, com¬ pletely recovered its normal rate of growth of 0T8 /x 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 0T8 /x to 0*11 /x per second, i.e., to three-fifths the normal rate; in consequence of prick the depression was from 0T8 > to 0*05 /x per second, i.e., to less than a third of the normal rate. .After 15 minutes the rate recovered from 0*05 /x to 0*07 /x per second. After moderate friction the recovery was complete after an hour; but in iS this case the recovery after an equal interval was only three-fourths of the original, the. rate being now 0T2 /x per second (Fig. 33b). I next applied the more ( intense stimulus caused by a longitudinal cut

This caused a depression second. A transverse cut, intense stimulation, than a of growth rate to 0-04 y- per I find, gives rise to a more longitudinal slit. 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 mart persistent retardation of growth.

Mechanical stimulus induces incipient contraction or re¬ tardation 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. 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 condi¬ tions of the experiment would thus become more p-ecise 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

response to light in variation of growth, the latent period of response, .he effects of light of increasing intensity and duration,-and the effects of the visible and invisible rays of arc-lamp with self-regulating arrangement for securing steadi¬ ness of light, and (2) an incandescent electric • lamp* Two inclined mirrors were placed 'dose''behind the specimen so that it should be acted on by light from ail sides. 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 retard¬ ing effect of light is seen in the shortening of spacing®, as compared with, the normal, between successive dots. The light-’ was next cut off and record ■ taken