198 



PLANT RELATIONS. 



evident that such a position avoids exposure of the leaf 

 surface to the noon rays, but obtains for these same sur- 

 faces the morning and evening rays. If these plants are 

 developed in the shade, the "compass" habit does not 



FIG. 165. Two compass plants. The two figures to the left represent the same plant 

 (Silphium) viewed from the east and from the south. The two figures to the right 

 represent the same relative positions of the leaves of Lactuca. After KEENER. 



appear (see 15). The profile position is a very common 

 one for the leaves of Australian plants, a fact which gives 

 much of the vegetation a peculiar appearance. All these 

 positions are serviceable in diminishing the loss of water, 

 which would occur with exposure to more intense light. 

 146. Motile leaves. Although in most plants the mature 



