Life Movements in Plants

I once more after half an hour. Growth is mow 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.

Fig. 75.—Normal effect of light. N, normal 5 S, retarded rate of growth in response to light j N, recovery on cessation of light. There is a general impression that it takes from several minutes to more than an. hour for the light to react on 1 * to the want of sufficient delicate means of observa- For my recorders indicate in some cases a response less 'than 2 seconds of the incidence ci light. S „ tend, for example, in the record of response eiven by a seedling of OncurUta, to a Sash of nltra-viole ^”t In the majority of oases the response is oneerved Within 15 seconds of the incidence of light.

Experiment 79.—For the determination of the latent to/a record of the effect of arc light of 30 seconds Cttn - tlen on a moving plate. It wili be noticed (Kg Id) that a retardation of growth was mdneed within 35 second, of the incidence «^ cipient contraction mduced “ ° of stimu lus. Growth 80 _x next studied the action of " which was increased in ar 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. Alter 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.

Fig-. 77.—Action of light of", increasing intensities; 1:2:3 in retardation of growth. Experiment 8L —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. 78a) on growth. In both the cases the effect of continuous stimula¬ tion is seen to be the s.ame, 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 hot end in a mere arrest of growth, but the responding organ undergoes an actual shortening

F, S 78 -Effects of continuous (a) electric and (b) photic stimulation of moderate intensity, taken on a moving plate. Different observers have found* that it is the more refran sible 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 re¬ sponse to the action of different rays. For this P ur P ose ’ the spectrum, produced by prism of high dispersion. In practice, the usual colour filters were found very con¬ venient, as they allowed the application of more intense

transmitted red rays, a thinner stratum allowed the trans¬ mission of yellow in addition; ammoniated copper sul¬ phate solution allowed the blue and violet rays to pass through. It should be borne in mind that certain compli¬ cating 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). 1 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 ravs are practically ineffective, the blue rays are most effective in inducing responsive retardation of growth.

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 exert¬ ed 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). Fa*. 7 *),—N, normal. B, effect of blue light, and V, of ultra-violet light. The records are on a moving plate.

' Effect of ultra-violet light: Experiment 83 ,—l/Jffra-violet light was obtained from a' quarts 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. 79Y). 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 retardaiion 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. The normal effect of light is incipient contraction or retardation of the rate of growth. The latent period may ■ in some cases he as short as 2 seconds."' In large number of cases it Is about 15 seconds. The latent period is shortened under stronger intensity, of Incasing in.™** of light induces increasing retarda¬ tion and arrest of growth. Under continued action of ligh of strong intensity the growing organ may undergo an actual shortening.

In these reactions the action of stimulus of light resem¬ bles 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 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 galvano- metric 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 respond¬ ing organ, gives rise to an effect different from that of direct application.

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 posi¬ tive 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 ex¬ pression in mechanical and electrical response, or in respon¬ sive 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.

Fig. SO.—Electric response of Musa (a) Positive, (6) diphasic, (c) negative. the electric response given by vegetable tissues. On applica¬ tion 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. 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 giowing organ to an effect which would be of oppo¬ site 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 ra’te of growth. In this particular case the inter¬ val 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

Fig. 81.—Effect of Indirect and Direct stimulus on growth of (!rinnm. t taken on a moving plate. Dotted arrow shows application of Ir.direct stimulus with consequent acceleration of srrowth. Direct application of stimulus at the second arrow induces contraction and subsequent retardation of rate of growth. Successive dots are intervals of 5". (Magnification 2,000 timen). 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

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, /.<?., 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 Direct stimulus induces negative variation of turgor 7 contraction , fall of leaf of Mimosa, electric change of galvanometric negativity , and retardation of the rate of growth . Indirect stimulus induces fwsitive variation of turgor, expansion, erection of leaf of Mimosa, ; electrical change of galvanometric positivity , and accelera¬ tion of the rate of growth.

It is seen that Indirect stimulus gives rise to dual reac¬ tions, 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, -wheve, 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 nega-

tive reactions. Of these* the positive is masked by the predominant negative. Bo 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. The application of Direct stimulus gives rise to an electric response of galvanometric negativity. The applica¬ tion of stimulus at a distance from the responding point, Indirect stimulus, gives rise to positive electric response.

The mechanical responses of sensitive plants also exhi¬ bit similar effects, i.e. y 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 vege¬ table 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 retarda¬ tion of the .rate' of growth.

Indirect stimulus induces positive variation of turgor ? expansion, erection, of leaf of Mimosa, electrical change of galvanometric po-sitivity and accelera¬ tion of the rate ".'of growth. The ; uormal 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. After my investigations on the normal retarding effect of light on growth, I was considerably surprised to find the responses occasionally becoming posttive, an acceleration instead of retardation of growth. I shall first give accounts of such positive responses and then explain ■ the cause of the abnormality.

to the action of light'of o minutes’ duration. This induced an abnormal acceleration in the rate of growth from 0-30 ix. to 0-40 i*. per second. But continuous exposure to light for half an hour brought about the normal effect of retar¬ dation. In trying .to account fdr this abnormality in response I found that while specimens of Kysoor in a vigorous state of growth of about 0*8 y 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 ji per second. As activity, of giowth 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 pul- vinated organs we found that under condition of sub¬ tonicity, the response becomes positive and that this abnor¬ mal positive is converted into normal negative in con¬ sequence of repeated stimulation. In growth, response likewise the abnormal acceleration of growth under light in the sub-tonic specimen of Kysuor 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 condi¬ tion 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 nofmal 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-synthetie action, such as tentanising 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-topic specimen of seedling ot* wheat, its rate of growth being as low as 0*05 /x per second. After electric stimula¬ tion the rate was found enhanced to 0*12 /x per second, or about two and-a-half times. I give (Fig. 82) two records

Fig.* 82.--Enhancement; of rate, of ■ growth in sab-tonic specimens of wheat seedling. ■ First series of record on stationary, second series (b) on 'moving plate* Wj record before, stimulation. S, after stimulation. 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 . widfer spacing after stimulation, 3, exhibit the', induced'' enhancement of

In the second experiment the records (Fig. 825) were taken on a moving plate* The specimen was so extremely sub-r tonic, that its normal record N appears almost horizontal. The greater erection of the curve, S, after stimulation 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 lo 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,