XVII.—EFFECT OF INDIRECT STIMULUS ON GROWTH

By

Sir J. C. Bose,

Assisted by

Guruprasanna Das.

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 gal­vano­metric 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 responding organ, gives rise to an effect different from that of direct application.

MECHANICAL AND ELECTRICAL RESPONSE TO INDIRECT STIMULUS.

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 positive 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 expression in mechanical and electrical response, or in responsive 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.