So 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 application of stimulus at a distance from the responding point, _i.e._, Indirect stimulus, gives rise to positive electric response.
The mechanical responses of sensitive plants also exhibit similar effects, _i.e._, 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 vegetable 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 retardation of the rate of growth.
Indirect stimulus induces positive variation of turgor, expansion, erection of leaf of _Mimosa_, electrical change of galvanometric positivity and acceleration of the rate of growth.
XVIII.--RESPONSE OF GROWING ORGANS IN STATE OF SUB-TONICITY
The normal 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.
ABNORMAL ACCELERATION OF GROWTH UNDER STIMULUS.
After my investigations on the normal retarding effect of light on growth, I was considerably surprised to find the responses occasionally becoming _positive_, an acceleration instead of retardation of growth. I shall first give accounts of such positive responses and then explain the cause of the abnormality.
_Abnormal acceleration under stimulus of light: Experiment 87._--A rather weak specimen of _Kysoor_ was exposed to the action of light of 5 minutes’ duration. This induced an abnormal acceleration in the rate of growth from 0.30 µ to 0.40 µ per second. But continuous exposure to light for half an hour brought about the normal effect of retardation. In trying to account for this abnormality in response I found that while specimens of _Kysoor_ in a vigorous state of growth of about 0.8 µ 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 µ per second. As activity of growth 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.