The facts that have been given above prove that infra-red radiation is as effective a mode of stimulation as the more refrangible rays of the spectrum. Phototropic and radio-thermotropic reactions would therefore prove to be essentially similar. The following experiments fully confirm the similarity of the two reactions.

POSITIVE RADIO-THERMOTROPISM.

_Experiment 157._--I shall now describe the normal reaction of a growing organ to the unilateral stimulus of thermal radiation. Figure 151 gives a record of response of the stem of _Dregea_ to stimulus of short duration; the induced curvature is positive or towards the source of heat. On the cessation of stimulus, there is a recovery which is practically complete, and which takes place at a slower rate than the excitatory positive curvature. Repet.i.tion of stimulus gives rise to responses similar to the first. _Successive stimuli of moderate intensity thus give rise to repeated responses of growth curvature._ An arbitrary distinction has been made between the responses of pulvinated and of growing organs. The former is distinguished as a movement of variation, with its supposed characteristic of repeated response. But the experiment described shows that this is also met with in the response by growth curvature. It is only under long continued stimulation that the curvature is fixed by growth.

[Ill.u.s.tration: FIG. 151.--Positive response to short exposure to thermal radiation. Successive dots at intervals of 5 seconds.

(_Dregea volubilis._)]

DIA-RADIO-THERMOTROPISM.

The positive curvature is induced by r.e.t.a.r.dation of growth at the proximal side, and enhancement of growth at the distal side. This latter effect is, as we have seen, brought about by the effect of indirect stimulation.

But under long continued action of stimulus, the negative or excitatory impulse reaches the distal side, inducing diminution of turgor and r.e.t.a.r.dation of the rate of growth. This leads to neutralisation, the organ placing itself at right angles to the orienting stimulus.

[Ill.u.s.tration: FIG. 152.--Record of positive, neutral and reversed negative curvature under continued action of thermal radiation. The negative response went off the plate. Successive dots at intervals of 5 seconds. (_Dregea volubilis_).]

_Experiment 158._--This neutralisation is seen in the record given in figure 152, where under continuous unilateral stimulation, the growing organ exhibited its maximum positive curvature, after which the movement became arrested by the arrival of the excitatory impulse at the distal side, on account of which the first positive curvature became neutralised. Further continuation of stimulus caused a reversal into negative in the course of 7 minutes. It will thus be seen that in inducing phototropic curvature, the heat rays in sunlight play as important a part as the more refrangible rays of the spectrum.

SUMMARY.

The effects of rise of temperature and of radiation are antagonistic to each other.

Under unilateral action of thermal radiation a positive curvature is induced by the r.e.t.a.r.dation of growth at the proximal, and acceleration of growth at the distal side of the organ.

There is a complete recovery on the cessation of stimulus of moderate intensity and short duration. Repeated responses may thus be obtained similar to repeated responses in pulvinated organs. In certain tissues the power of conduction in a transverse direction is wanting; excitation remains localised at the proximal side, and the responsive curvature remains positive.

In other cases, there is a slow conduction of excitation to the distal side. The result of this under different circ.u.mstances is dia-radio-thermotropic neutralization, or a reversed negative curvature.

In inducing phototropic curvature, the heat rays in sunlight play as important a part as the more refrangible rays of the spectrum.

x.x.xVIII.--RESPONSE OF PLANTS TO WIRELESS STIMULATION

_By_

SIR J. C. BOSE,

_a.s.sisted by_

GURUPRASANNA DAS.

A growing plant bends towards light, and this is true not only of the main stem but also of its branches and attached leaves and leaflets.

Light affects growth, the effect being modified by the intensity of radiation. Strong stimulus of light causes a diminution of the rate of growth, but very feeble stimulus induces an acceleration. The tropic effect is very strong in the ultra-violet region of the spectrum with its extremely short wave length, but the effect declines practically to zero as we move towards the less refrangible rays--the yellow and the red with their comparatively long wave length. As we proceed beyond the infra-red region, we come across the vast range of electric radiation, the wave lengths of which vary from 06 cm., the shortest wave I have been able to produce, to others which may be miles in length. There thus arises the very interesting question, whether plants perceive and respond to the long ether waves including those employed in signalling through s.p.a.ce.

At first sight this would appear to be very unlikely; for the most effective rays are in the ultra-violet region with wave length as short as 20 10^{-6} cm.; but with electric waves used in wireless signalling we have to deal with waves 50 million times as long. The perceptive power of our retina is confined within the very narrow range of a single octave, the wave lengths of which lie between 70 10^{-6} cm.

and 35 10^{-6} cm. It is difficult to imagine that plants could perceive radiations so widely separated from each other as the visible light and the invisible electric radiation.

But the subject a.s.sumes a different aspect, when we take into consideration the _total_ effect of radiation on the plant. Light induces two different effects which may broadly be distinguished as external and internal. The former gives rise to movement; the latter finds no outward manifestation, but consists of an 'up' or a.s.similatory chemical change, with concomitant increase of potential energy. Of the two reactions then, one is dynamic attended by dissimilatory 'down'

change; the other is potential, a.s.sociated with the opposite 'up'

change. In reality the two effects take place simultaneously; but one of these becomes predominant under definite conditions.

The modifying condition is the _quality_ of light; with reference to this I quote the following from Pfeffer: "So far as is at present known, the action of different rays of the spectrum gives similar curves in regard to heliotropic and phototactic movements, to protoplasmic streaming and movements of the chloroplastids as well as the photonastic movements produced by growth or by changes of turgor. On the other hand, it is the less refrangible rays which are most active in photo-synthesis."[26] The dynamic and potential manifestations are thus seen to be complementary to each other, the rays which induce photo-synthesis being relatively ineffective for tropic reaction and _vice versa_.

[26] Pfeffer--Vol. II, p. 104.

Returning to the action of electric waves, since they exert no photo-synthetic action they might conceivably induce the complementary tropic effect. These considerations led me to the investigation of the subject fourteen years ago, and my results showed that very short electric waves induce a r.e.t.a.r.dation of rate of growth; they also produce responsive movements of the leaf of _Mimosa_, when the plant was in a highly sensitive condition.[27] The energy of the short electric waves is very feeble, and undergoes great diminution at a distance; hence the necessity of employment of a specimen of plant in a highly sensitive condition.

[27] "Plant Response"--p. 618 (1905).

I resumed my investigations on the subject at the beginning of this year. I wished to find out whether plants in general perceived and responded to the long ether waves which reached it from a distance. The perception of the wireless stimulation was to be tested not merely by the responsive movement of sensitive plants, but by diverse modes of response given by all kinds of plants.

Stimulus induces, as we have seen, three different types of response in plants. It causes excitation in sensitive plants like _Mimosa_, in consequence of which the leaf undergoes a fall; this is the mechanical response to stimulus. Stimulus also induces electric response in plants, both sensitive and ordinary, the excited tissue undergoing an electric change of galvanometric negativity. Finally, the effect of stimulus on growing plants is a variation in the rate of growth, an acceleration under feeble, and a r.e.t.a.r.dation under strong stimulus. I undertook to investigate the effect of electric waves on plants by the methods of mechanical and of electrical responses, and also by that of induced variation of growth.

[Ill.u.s.tration: FIG. 153.--Diagrammatic representation of method employed for obtaining response to wireless stimulation. Transmitting apparatus seen to the right. Receiving aerial connected to upper part of plant, the lower part of the plant or the flower-pot being connected with the earth.]

THE WIRELESS SYSTEM.

For sending wireless signals, I had to improvise the following arrangement, more powerful means not being available. The secondary terminals of a moderate sized Ruhmkorff's coil were connected with two cylinders of bra.s.s, each 20 cm. in length; the sparking took place between two small spheres of steel attached to the cylinders. One of the two cylinders was earthed, and the other connected with the aerial 10 meters in height. The receiving aerial was also 10 meters in height and its lower terminal led to the laboratory, and connected by means of a thin wire to the experimental plant growing in a pot; this latter was put in electric connection with the earth (Fig. 153). The distance between the transmitting and receiving aerial was about 200 meters, the maximum length permitted by the grounds of the Inst.i.tute.

MECHANICAL RESPONSE OF _Mimosa_.

_Experiment 159._--One of the leaves of _Mimosa_ was connected with the aerial by means of a thin tinsel of loose wire, which did not interfere with the free movement of the leaf. This latter was attached to the recording lever. Wireless signals induced a responsive fall of the leaf (Fig. 154) which was gradual as under action of light, and not so abrupt as under a mechanical blow.

[Ill.u.s.tration: FIG. 154.--Mechanical response of leaf of _Mimosa_ to electric wave.]

[Ill.u.s.tration: FIG. 155.--Electric response of _Mimosa pudica_ to wireless stimulation.]

THE ELECTRIC RESPONSE.

_Experiment 160._--The leaf of _Mimosa_ was in this experiment held securely, and two electrical connections made, one with the less excitable upper and the other with the more excitable lower half of the pulvinus. The incident ether-wave induced an electric response in the pulvinus, the more excitable lower half exhibiting galvanometric negativity. On the cessation of stimulus there was a recovery (Fig.

155).

It is not at all necessary to employ the sensitive _Mimosa_ for exhibition of electric response; for this is universally exhibited by all plants. The only condition for electric response is that the points of electric contacts should be made with two unequally excitable areas in the plant. This may be secured by artificial means as by causing 'injury' to one point of contact.[28] It is however much better to take advantage of the natural difference of excitability of two different areas in the organ as in the pulvinus of _Mimosa_. This difference of excitability is also found between the inner and outer sides of a hollow tubular organ as in the peduncles of various lilies. I was thus able to secure specimens which were far more sensitive to the action of electric waves than the pulvinus of _Mimosa_.

[28] "Comparative Electro-Physiology"--p. 149.

EFFECT OF WIRELESS STIMULATION ON GROWTH.