Part 18 (1/2)

There now remains the very interesting question as to whether the effect of long ether waves induce any variation of growth. The results given below show that growing plants not only perceive but respond to the stimulus of electric waves. The effects to be presently described are exhibited by all plants.

I shall, however, content myself in describing a typical experiment carried with the seedling of wheat. The specimen was mounted on the Balanced Crescograph, and the growth exactly balanced. This gives a horizontal record; an acceleration of growth above the normal is, in the following records, represented by a down curve, and a r.e.t.a.r.dation by an up-curve.

_Effect of feeble stimulus: Experiment 161._--I first studied the effect of feeble stimulus. This was secured by decreasing the energy of sparks of the radiator. The response was an acceleration of rate of growth as seen in figure 156a. The a.n.a.logy of this with the accelerating effect of sub-minimal intensity of light (p. 224) is very remarkable.

[Ill.u.s.tration: FIG. 156.--Record of responses to electric wave by the Balanced Crescograph (_a_) response to feeble stimulus by acceleration of growth, (_b_) response to strong stimulus by r.e.t.a.r.dation, (_c_) responses to medium stimulation--r.e.t.a.r.dation followed by recovery.

Down-curve represents acceleration, and up-curve r.e.t.a.r.dation of growth: (Seedling of wheat.)]

_Effect of strong stimulus: Experiment 162._--The maximum energy radiated by my transmitter, as stated before, was only moderate. In spite of this its effect on plants was exhibited in a very striking manner. The balance was immediately upset, indicating a r.e.t.a.r.dation of the rate of growth. The latent period, _i.e._, the interval between the incident wave and the response, was only a few seconds (Fig. 156b). The record given in the figure was obtained with the moderate magnification of 2,000 times only. But with my Magnetic Crescograph, the magnification can easily be raised ten million times; and the response of plant to the s.p.a.ce signalling can be exalted in the same proportion.

Under an intensity of stimulus slightly above the sub-minimal, the responses exhibit r.e.t.a.r.dation of growth followed by quick recovery, as seen in the series of records given in Fig. 156c.

A remarkable peculiarity in the response was noticed during the course of the experiments. Strong stimulation by ether waves gives rise, as we have seen, to a very marked r.e.t.a.r.dation of the rate of growth. Repeated stimulation induces fatigue, and temporary insensitiveness of the organ.

Under moderate fatigue the effect is a prolongation of the latent period. Thus in a particular experiment the plant failed to give any response to a short signal. But after an interval of five minutes a marked response occurred to the wireless stimulus that had been received previously. The plant had perceived the stimulus but on account of fatigue the latent period was prolonged, from the normal 5 seconds to as many minutes.

SUMMARY.

Plants not only perceive, but also respond to long ether waves employed in signalling through s.p.a.ce.

Mechanical response to wireless stimulation is exhibited by the leaf of _Mimosa pudica_.

All plants give electric response to the stimulus of long ether waves.

Growing plants exhibit response to electric waves by modification of rate of growth. Feeble stimulus induces an acceleration, while strong stimulus causes a r.e.t.a.r.dation of the rate of growth.

The perceptive range of the plant is far greater than ours; it not only perceives but responds to the different rays of the vast ethereal spectrum.

x.x.xIX.--GEOTROPISM

_By_

SIR J. C. BOSE.

No phenomenon of tropic movement appears so inexplicable as that of geotropism. There are two diametrically opposite effects induced by the same stimulus of gravity, in the root a movement downwards, and in the shoot a movement upwards. The seeming impossibility of explaining effects so divergent by the fundamental reaction of stimulus, has led to the a.s.sumption that the irritability of stem and root are of opposite character. I shall, however, be able to show that this a.s.sumption is unnecessary.

The difficulty of relating geotropic curvature to a definite reaction to stimulus is accentuated by the fact that the direction of the incident stimulus, and the side which responds effectively to it are not clearly understood; nor is it known, whether the reaction to this stimulus is a contraction, or its very opposite, an expansion.

Taking the simple case of a horizontally laid shoot, the geotropic up-curvature is evidently due to differential effect of the stimulus on upper and lower sides of the organ. The up-curvature may be explained by one or the other of two suppositions: (1) that the stimulus of gravity induces contraction of the upper side; or (2) that the fundamental reaction is not a contraction but an expansion and this of the lower side. The second of these two a.s.sumptions has found a more general acceptance.

Tropic curvatures in general are brought about by the differential effect of stimulus on two sides of the organ. Thus light falling on one side of a shoot induces local contraction, the rays being cut off from acting on the further side by the opacity of the intervening tissue. But there is no opaque screen to cut off the vertical lines of gravity,[29]

which enter the upper side of a horizontally laid shoot and leave it by the lower side. Though lines of force of gravity are transmitted without hindrance, yet a differential action is found to take place, for the upper side, where the lines of force enter, becomes concave, while the lower side where they emerge becomes convex. Why should there be this difference?

[29] I shall in what follows take the _direction_ of vertical lines of gravity as that of movement of falling bodies, from above towards the centre of the earth.

For the removal of various obscurities connected with geotropism it is therefore necessary to elucidate the following: