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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 and 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. 
138. Motile leaves, Although in most plants the mature 
