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193 



evident that such a position avoids exposure of the leaf 

 surface to tlie noon rays, hut obtains for these same sur- 

 faces the morning and evening rays. If tliese plants are 

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





Fic:. 170. Two compass plants. The two figarestothe kift represent the same i)lant 

 {Silphiu?n) 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 loaves of Australian plants, a fact which gives 

 much of the vegetation a peculiar appearance. All these 

 ijositions ai'c servicealjle in diminishing the loss of water, 

 which would occur with exposure to more intense light. 

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



