RESPONSIVE GROWTH-CURVATURES IN PLANTS 73 I 



shoot, the tips of both organs behaving alike. The one 

 universal law which applies in every case is, that the direct 

 effect of stimulus is to induce contraction, and its indirect 

 effect to cause expansion. 



On taking a general survey of the responsive movements 

 which are induced by the unilateral action of stimulus, we 

 find that moderate stimulation of the growing region induces 

 a positive movement. Or negative movement, again, may 

 be induced in either of two ways — that is to say, by moderate 

 stimulation of the tip, or by very strong stimulation of the 

 growing region. 



Responsive action under stimulus of gravity.— I have 

 shown that the unilateral application of pressure of particles 

 is efficient to cause responsive contraction. An experiment 

 was described in which it was shown that the unilateral 

 pressure of magnetically attracted particles would induce 

 concavity of the side acted on (p. 497). The weight of 

 statolithic particles may thus be the efficient cause of 

 stimulation by gravity. It is to be borne in mind, however, 

 that stimulation caused by such means as the weight-effect 

 of these minute particles can only be moderate. We have 

 therefore in the case of geotropic stimulation to deal only 

 with the direct and indirect effects of unilateral stimulus of 

 moderate intensity. In the case of the stem the growing 

 region is directly stimulated. A horizontally laid stem 

 thus curves upwards to meet the lines of force, or rays of 

 gravity, just as it would bend upwards under heliotropic 

 action to meet the rays of incident light. It is supposed 

 that the curvature of the stem under gravity is mainly due 

 to an active growth of the convex side ; but I have shown 

 that it is due, on the contrary, to an excitatory response, 

 which consists, like all other forms of response to external 

 stimulus, of a contraction. The active element in the in- 

 duced responsive curvature is thus the contraction of the 

 upper side of the organ, aided subsidiarily by that expansion 

 of the under side which is brought about by the indirect 

 effect of stimulus on the distal. That this is the case is sec n 



