638 plant response 



Summary 



The responsive action of growing organs being not 

 essentially different from that of pulvinated organs, a 

 similar explanation must be applicable to the two classes of 

 phenomena. 



Owing to long-continued unilateral excitation by light, the 

 upper side of a plagiotropic stem is rendered the relatively 

 less excitable. Hence such organs may be regarded as 

 equivalent to diffuse pulvinoids, of which the upper side is 

 the less excitable. 



In both these cases, of pulvini and diffuse pulvinoids, the 

 local application of moderate stimulus on either side induces 

 normal positive response. 



But with long-continued application of strong stimulus 

 two types of response may be obtained, according as the 

 organ is characterised by a feeble or high power of trans- 

 verse conductivity. In the former of these cases the stimu- 

 lus will remain localised, and the response will be positive. 

 In the second, the stimulus will become internally diffused, 

 and if the lower side be the more excitable, stimulation of 

 the upper will give rise to the concavity of the lower, con- 

 stituting negative response. 



As examples of the first of these classes may be 

 mentioned the diurnal sleep movements of Robinia, Erythrina 

 indica, and Clitoria teniatea, in which vertical illumination 

 induces upward folding of the leaflets. 



As representing the second class, we have both plagio- 

 tropic and pulvinated organs. 



The plagiotropic stems of Mimosa, Ipomcca, and Cucur- 

 bita exhibit increasing concavity of the lower side with the 

 duration of the day's illumination. 



A periodic downward movement is thus induced in 

 them, which reaches its maximum at the end of the day. 

 The reverse movement, of gradual erection, occurs during the 

 night. 



In pulvinated organs, negative heliotropic response, with 



