MOVEMENT IN PLANTS DUE TO IRRITABILITY. Ill 



slender than those grown in the light. Light then has a retard- 

 ing influence on the elongation of the 

 stem. 



195. Influence of light on direction of 

 growth, While we 

 are growing seed- 

 lings, the pots or 

 boxes of some of 

 them should 

 be placed 

 so that the 

 plants will 

 have a one- 

 sided illu- 



Seedling of castor-oil bean, before and 



after a one-sided illumination. m j 



This can be 



done by placing them near an open win- 

 dow, in a room with a one-sided illumi- 

 nation, or they may be placed in a box 

 closed on all sides but one which is facing 

 the window or light. In 1 2-24 hours, or 

 even in a much shorter time in some cases, 

 the stems of the seedlings will be directed 

 toward the source of light. This influence 

 exerted by the rays of light is heltolropism, a turning influenced 

 by the sun or sun- 

 light. 



196. Diaheliot- 



ropism. Horizon- 

 tal leaves and 

 shoots are diahe- 

 liotropic as well as _. 



Uark chamber with opening at one side to show heliotropism. 

 did geo tropic. The (After Schleichert.) 



general direction which leaves assume under this influence is 

 that of placing them with the upper surface perpendicular to 



