RESPONSIVE GROWTH-CURVATURES IN PLANTS 737 



leaflets, of such leaves as those of Oxalis, Biophytum, and 

 Averrhocx. In the heliotropic responses of ordinary leaves, 

 again, we have exactly similar classes of phenomena. In 

 these cases it has been shown that there is no specific dia- 

 heliotropic sensitiveness, such as that by which the upper 

 surface of the leaf was supposed to place itself at right angles 

 to the light, for the purpose of absorbing the largest possible 

 amount of stimulus. It was shown, moreover, that with 

 regard to that response by which the ultimate position of the 

 leaf is determined, the lamina was not the perceptive organ. 

 In ordinary, as in pulvinated, leaves we find response to be 

 of two extreme types, connected by innumerable gradations. 

 First, we have leaves like that of Mangifera indica, in which 

 on account of the feeble conductivity of the pulvinoid 

 vertical illumination induces a positive response, or move- 

 ment upwards ; and as an example of the second type we 

 saw that negative response, or movement downwards, was 

 given by the leaf of A rtocarpus under vertical light. These 

 responsive movements induced by light, although, generally 

 speaking, predominant, are modified by the presence of 

 other subsidiary factors, which all contribute in various 

 degrees to bring about the variety of attitudes ultimately 

 assumed by the leaves. These subsidiary factors were 

 enumerated as: (1) the epinastic or hyponastic tendency; 

 (2) the general turgescent condition of the plant ; (3) the 

 characteristic limits of flexibility of the motile organ ; and 

 (4) the differential geotropic excitability of the organ. When 

 a petiole is acted on laterally by light a torsion is induced, 

 by which the upper surface of the leaf is made to face the 

 incident stimulus. It has been shown that this movement is 

 not due to any specific dia-heliotropic sensibility ; for any 

 form of lateral stimulation, say chemical or thermal, will 

 induce a similar response by torsion, the result being always 

 that the less excitable surface is made to face the stimulus. 

 Similar effects are also observed in compound strips made of 

 such unequally contractile substances as ebonite and india- 

 rubber. 



3 B 



