256 



TROPIC MOVEMENTS 



showed that centrifugal force acted in the same way as gravity upon leaves. 

 Under normal conditions, however, the influence of light preponderates, so 

 that dorsiventral leaves when illuminated from beneath may bend so as 

 to face downwards. These movements take place independently of the 

 epinastic tendency, which the stimulus of light is in fact able to overcome. 

 The plagio-geotropism of the leaf is also able to overcome its epinasty, so 

 that a leaf which has attained its plagio-geotropic position usually needs 

 only to move slightly in order to become plagio-heliotropically oriented. 



FIG. 47. Coleus sp. A. Plant in normal position. B. After a day's rotation on a klinostat. 



In addition, the curvature of the stem is usually such as to aid in the 

 assumption of the proper position by the leaves. The movements of the 

 latter are usually performed by the petiole or in sessile leaves by the 

 lamina, and in most cases the power of movement is lost when growth 

 ceases. The latter, however, often persists for a long time in certain regions 

 of the leaf, so that a leaf may remain capable of orienting movements long 

 after it is fully adult. Leaves which possess motile pulvini usually retain 

 this power until death. 



De Vries and also Wiesner have assumed that the plagiotropic orientation of 

 leaves is due to negative geotropism and autogenic epinasty, whereas Frank, Darwin, 



