198 



PLANT EELATIONS. 



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

 {Silphium) viewed from the east »nd from the south. The two figures to the right 

 represent the same relative positions of the leaves of Lactuca. — After Kerner. 



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. 

 149. Motile leaves. — Although in most plants the mature 



