CH. Ill, 8] FORMS OF FOLIAGE LEAVES 



61 



ing is oftentimes associated with exposure to strong winds, 

 as in Palms, where the compound-leaved forms tower high 

 over the forests, or grow along wind-beaten strands, while 

 the simple-leaved forms are confined perforce to shelter (Fig. 

 30) ; and it is probable that the compound leaves of the 

 Tree Ferns (Fig. 31) originated in this way. Compounding, 



FIG. 31. Alsophila oligocarpa, a tropical Tree Fern, showing the much- 

 compounded leaves. (From Bailey.) 



however, has also other associations. Thus, in the Pulse 

 Family, it seems clearly connected with the " sleep," or 

 drooping at night of the leaves. In submersed water plants, 

 where it is common, the compounding, by its exposure of 

 more surface, facilitates the absorption of the carbon dioxide 

 dissolved in the water (Fig. 32). 



While leaf size seems thus largely adaptational, it is 

 sometimes as clearly structural or hereditary. Thus the 



