PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 637 



positive heliotropic and negative heliotropic response. This 

 will be seen still more clearly in the following statement, 

 applying to the two extreme cases of radial and of dorsi- 

 ventral organs, the plagiotropic constituting an intermediate 

 link between the two. 



General view of responsive curvatures induced in 

 different organs by unilateral application of light 



1. Radial organs : {a) Positive response : stimulus localised, 

 the proximal side more excited and concave ; e.g. Sinapis 

 under moderate unilateral light (p. 609). 



ip) Ititermediate or neutral response : stimulus internally 

 diffused, causing equal excitation of proximal and distal sides ; 

 the organ takes up a neutral (so-called dia-heliotropic) posi- 

 tion, at right angles to light ; e.g. Sinapis under moderately 

 strong unilateral light (p. 609) ; certain tendrils apparently 

 insensitive. 



(c) Negative response : strong stimulus internally diffused, 

 which also induces anisotropy, the distal being then the more 

 excitable ; concavity of the more excited distal ; e.g. seedling of 

 Sinapis, and tendril of Vitis under strong unilateral sunlight 

 (pp. 609, 611). 



2. Dorsi-ventral organs : (a) Positive response : stimulus 

 localised by relative non-conductivity of organ : the more 

 excited proximal side concave ; e.g. positive movement of 

 terminal leaflet of Desmodium ; the diurnal sleep movements 

 of Robinia, Erythrina indiea, and Clitoria tematea (p. 629). 



(b) Intermediate or neutral response : stimulus internally 

 diffused ; upper and lower halves equally excited ; e.g. 

 terminal leaflet of Desmodium, under several hours of 

 vertically acting sunlight (p. 604). 



ic) Negative response : strong stimulus acting from above 

 on highly conducting organ, of which lower half is con- 

 siderably more excitable ; concavity of the more excitable 

 lower half ; e.g. fall of Mimosa leaf under strong illumination 

 from above ; diurnal sleep movements of Oxalis, Biophytum, 

 and Averrhoa, 



