[18] "Plant Response"—p. 293, etc.
[19] "It has been argued by James that the feeling does not cause, but is caused by the bodily expression.... Münsterberg concludes that the feeling of agreeableness is the mental accompaniment and outcome of reflexly produced movements of extension, and disagreeableness of the movement of flexion." Schāfer—Text Book of Physiology, Vol. II, p. 975 (1900).
XXXIII.—THE TRANSMITTED EFFECT OF PHOTIC
STIMULATION
By
Sir J. C. Bose,
Assisted by
Jyotiprakash Sircar, M.B.
Plant organs exhibit, as we have already seen, a heliotropic curvature under direct stimulation. Still more interesting is the transmitted effect of light giving rise to a curvature. Thus if the tip of the seedling of wheat be exposed to light, the excitation is transmitted lower down into the region which acts as the responding organ. Growth is very active in this particular zone, and the change of growth, induced by the transmitted effect of stimulus, brings about a curvature by which the tip of the seedling bends towards light. The seedling thus appears to be differentiated into three physiological zones subserving three different functions. The tip is the perceptive zone, the intervening distance between the tip and the growing region is the zone of conduction, and the growing region is the responsive zone. These differentiations are shown in a striking manner by certain Paniceae, Setaria for example. In this seedling the tapering sheathing leaf or cotyledon is about 5 mm. in length, and it is the upper part of the cotyledon that is most sensitive to light. Below the sheathing leaf is a narrow length which will be distinguished as the hypocotyl, and where growth is very active. The apex of the leaf perceives the stimulus, and the effect is transmitted to the hypocotyl, which responds by becoming curved so that the seedling bends towards light.
It is necessary here to make special reference to the confusion that arises from want of precision in the use of the term stimulus, used indifferently to denote both the cause and the resulting effect. An external agent, say light, causes certain excitatory change in the tissue, and we refer to the agent which induces it, as the stimulus. Thus in the instance cited above, light is the stimulus, and it is the stimulus-effect that is transmitted to a distance. But in physiological literature no distinction is made between the stimulus and its effect, hence arises frequent use of the phrase 'transmission of stimulus'. It is obvious that it is not light but its effect that is transmitted.
Such want of precision in the use of the term stimulus would not have seriously affected the truth about the description of facts, had the transmitted effect been only of one kind. In a nerve-and-muscle preparation, the velocity of transmission of excitation is so great, that it completely masks the positive impulse (assuming the existence of such an impulse). The effect of indirect stimulation is, therefore, the same as that of direct stimulation. Any indefiniteness in the use of the term stimulus for its transmitted effect does not, in animal physiology, seriously militate against the observed facts. But lack of precision in the employment of the term in plant physiology leads to hopeless confusion. For owing to the semi-conducting nature of vegetable tissue, the transmitted effect is not of a definite sign, but may be positive or negative; in the first case, the response is by expansion, in the latter, by contraction. Thus the transmitted effect will be very different in the two cases, according as the intervening tissue is a good or a bad conductor. These facts accentuate the urgent necessity of revision of our existing terminology.
I have shown that the effects of other forms of stimuli are also transmitted from the perceptive to the responding region along the intervening path of conduction. Thus the petiole of Mimosa perceive any form of stimulus applied to it, and the induced excitation is conducted to the distant pulvinus to evoke the familiar responsive fall of the leaf. The pulvinus, moreover, perceives and responds to direct stimulation. In a nerve-and-muscle preparation the responding muscle is alike perceptive and responsive.