DIA-HELIOTROPISM AND DIA-GEOTROPISM 643 



subsidiary factors which combine with the heliotropic action 

 proper in determining the final position of the leaf. 



1. Epinasty and hyponasty. — These unequal growths of 

 one side or the other, De Vries believed to be brought about 

 by some spontaneous unknown cause in the plant itself. 

 Detmer, however, came to the conclusion that, as regarded 

 epinasty, it was not spontaneous, but induced by the action 

 of light. This conclusion was based on the observations 

 (1) that cotyledons of Cucurbita remained closed up, in 

 continuous darkness, but opened out when subjected to light ; 

 and (2) that the primordial leaves of P/iaseolus, kept in 

 darkness, remained folded, and only opened out on illumina- 

 tion. The investigations of Vines, however, though he 

 supports the contention of Detmer with regard to Phaseolus, 

 have led him to a different view in the case of Cucurbita, 

 where he finds the epinastic movement to be induced even 

 in the absence of light. He has therefore come to the 

 conclusion that the phenomena of epinasty and hyponasty 

 are spontaneous, and not directly due to the action of 

 illumination. 



These differences of opinion, however, have arisen from 

 the obscurity in which autonomous or spontaneous movement 

 has been involved, and they may be expected to disappear 

 when its true nature is clearly perceived. These alternate 

 movements of growth are in fact only another example of 

 multiple or autonomous response, the difference between 

 them and those other forms with which we are already 

 acquainted lying in the greater slowness of their period, and 

 in the relative fewness of the pulsations that can be exhibited. 

 In the case of autonomous pulsation or circumnutation of 

 stems, since the growth on which they depend is indefinitely 

 prolonged, we have an indefinite continuance of these pulsa- 

 tions. In the case of leaves, however, the organ usually 

 completes only half its swing, whether epinastic or hyponastic, 

 and by that time further pulsation is arrested, owing to the 

 cessation of growth ; yet in some cases there is more than 

 a semi-pulsation, as where hyponastic movement is followed 



