170 



CHAP. XV. PHOTONASTIC PHENOMENA 



A continuity is thus established between phototropic and 

 photonastic reactions, rendering the assumption of specific 

 sensibihty for each class of phenomena quite unnecessary. 



The effects described are equally true of photic, thermal, 

 and electric stimulations. From these results of observation 

 and experiment on the movements of plants, the following 

 laws, which are of universal application, can be deduced : 



1. All forms of moderate or strong stimulation of organs 



in normal tonic condition induce contraction as 

 their direct, and expansion as their indirect, effect. 



2. The response to unilateral stimulation is a positive 



curvature, effected by contraction of the proximal 

 and expansion of the distal side. 



3. Transverse conduction of excitation induces con- 



traction of the opposite side, consequently neu- 

 tralising or reversing the positive responsive 

 curvature. 



4. These effects are accentuated by the differential 



excitability of the two halves of an anisotropic 

 organ. 



The fundamental reactions of pulvinated and growing 

 organs to direct and indirect stimulation are summarised 

 in the following table : 



Table XVIII. — Mechanical and Electric Responses in 

 Motile and in Growing Organs. 



I give a classification of some of the principal types of 

 response to light that are met with in practice. In anisotropic 

 organs, stimulus is supposed to be applied on the less 

 excitable side. 



