The effect of feeble stimulus transmitted longitudinally is thus found always to induce convexity, a negative curvature and movement away from stimulus. I have obtained similar responsive movement of negative sign with various plant organs, and under various forms of stimuli. Thus in the stem of Dregea volubilis the longitudinally transmitted effect of light of moderate intensity was a negative curvature; direct application of light on the growing region gave, on the other hand, a positive curvature and movement towards light.
Thus while the effect of direct unilateral stimulation is a positive curvature, the effect of indirect stimulation is a negative curvature. The following table gives a summary of results of tropic effects under unilateral application of indirect stimulus.
TABLE XXIII.—SHOWING TROPIC EFFECT OF UNILATERAL APPLICATION OF INDIRECT STIMULUS.
| Stimulus. | Character of intervening tissue. | Sign of tropic response. |
| Moderate | Conducting | Positive curvature. |
| " | Semi-conducting | Negative followed by positive. |
| Feeble | Semi-conducting | Negative Curvature. |
| Moderate | Non-conducting | " " |
| Direct application of unilateral stimulus induces a positive curvature. | ||
SUMMARY.
In sensitive plants stimulus applied at a distance induces in the responding region an expansion indicative of increase of turgor.
The effect of indirect stimulation is also exhibited by an electric change of galvanometric positivity, indicative of enhancement of turgor and expansion.
Indirect stimulus induces in growing organs an enhancement of rate of growth.
Unilateral application of stimulus causes an expansion higher up on the same side to which the stimulus is applied; the result is an induced convexity, a movement away from the stimulus, i.e., a negative curvature. Direct stimulus applied unilaterally at the responding region induces, on the other hand, a positive curvature.