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 Mu- 

 ni i n a tion. 

 This oan 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 helioiropism, a turning influenced 

 by the sun or sun- 

 light. 



196. Diaheliot- 

 ropism. — Horizon- 

 ;al leaves and 

 ihoots are diahe- 



* ' ' ' Dark chamber with opening at one side to show heliotropism. 



iiageotropic. The < After Schieichert.) 



general direction which leaves assume under this influence is 



hat of placing them with the upper surface perpendicular to 



