CHAPTER XVI 



LEAVES AND FOLIAGE 



201. Leaves may be studied from two points of view 

 — with reference to their function, or what they do; and 

 with reference to their form, or their shapes and kinds. 



202. Function. — Leaves, as we have seen, make organic 

 matter from carbon dioxid. Almost any part of the plant, 

 however, may bear chlorophyll and perform the function 

 of leaves. The general form and structure of leaves is 

 intimately associated with their function: they are thin and 

 much expanded bodies, thereby exposing the greatest pos- 

 sible surface to light and air. The position of the leaves 

 usually has relation to light, as we have seen (Chapter 

 VIII). Leaves usually hang in such a way that one casts 

 the least shade on the other; those that have the least 

 favorable positions die and fall. 



203. Parts. — Leaves are simple or un- 

 branched (Fig. 138), and compound or 

 branched (Fig. 139). The method of 

 compounding or branching follows the 

 style of veining. The veining, or venation, 

 is of two general kinds: in most plants 

 the main veins diverge, and there is a 

 conspicuous network of smaller veins: 

 such leaves are netted-veined. In other 

 plants the main veins are parallel, or 

 nearly so, and there is no conspicuous 

 network: these are parallel-veined leaves 

 simple loaf. One (Fig. 150). The venation of netted- 

 veined leaves is pinnate or feather-like, 

 (95) 



of the cupatoriums or 

 bonesets. 



