10 



PLANT RELATIONS. 



14. Motile leaves. — There are leaves, however, which 

 have no fixed light position, but are so constructed that 

 they can shift their position as the direction of the light 

 changes. Such leaves are not in the same position in the 



afternoon as in the 

 forenoon, and their 

 night position may be 

 very different from 

 either (see Figs. 2, da, 

 3b, 4). Some of the 

 common house plants 

 show this power. In 

 the case of the com- 

 mon Oxalis the night 

 position of the leaves 

 is remarkably different 



Fig. 3a. 



The day position of the leaves of redbud 

 {Cercis). — After Arthur. 



from the position in light. 

 If such a plant is exposed 

 to the light in a window and 

 the positions of the leaves 

 noted, and then turned 

 half way around, so as to 

 bring the other side to the 

 light, the leaves may be 

 observed to adjust them- 

 selves gradually to the 

 changed light-relations. 



15. Compass plants. — A 

 striking illustration of a 



special light position is found in the so-called "compass 

 plants." The best known of these plants is the rosin-weed 

 of the prairie region. Growing in situations exposed to 

 intense light, the leaves are turned edgewise, the flat faces 

 being turned away from the intense rays of midday, and 

 directed towards the rays of less intensity ; that is, those of 



Fig. 36. The night position of the leaves 

 of redbud (.Cercis).— After Arthur. 



