Researches on Groivth and Movement in Plants. 



395 



response was in both cases positively phototropic, i.e., a movement towards 

 the source of illumination. This effect was subsequently reversed to strong 

 negative by transverse conduction (fig. 17). Had a delicate means of record 



Fig. 17. — Photonastic response : a, effect}"of light applied ibelow on Mimosa, leaflet ; 

 preliminary down or positive movement followed by strong negative up movement. 

 5, effect of light applied above on Averrhoa leaflet ; preliminary up or' positive 

 movement followed by pronounced down or negative movement. It is to be noted 

 that movement towards source of illumination is positive. Later and more 

 pronounced negative movement is due to transmitted excitation to the more excitable 

 half of the puhanule. (Up-movement represented by up-curve and vice versa.) 



not been available, the gradual transition from positive to negative photo- 

 tropic curvature would have passed unnoticed. A continuity is thus 

 established between tropic and nastic reactions, rendering the assumption of 

 specific sensibility for each class of phenomena quite unnecessary. 



The following laws express the , movements of plants imder external 

 stimuli and are of universal application : — 



1. All forms of stimuli induce contraction as their direct and expansion as; 

 their indirect effect. 



2. Unilateral stimulation causes positive curvature by contraction of the 

 proximal and expansion of the opposite side. 



3. Transverse conduction of excitation induces contraction of opposite side, 

 neutralising or reversing the positive responsive curvature. 



VOL, xc. — B. 2 I 



