54 MORPHOLOGY OF LBATES AS FOLIAGE. [lESSON 8. 



LESSON VIII. 



morphology of leaves as foliage. 



130. Hating in the last Lesson glanced at some of the special 

 or extraordinary forms and uses of leaves, we now return to leaves 

 in tlieir ordinary condition, namely, as foliage. We regard this as 

 the natural state of leaves. For although they may be turned to 

 account in other and very various ways, as we have just seen, 

 still their proper office in vegetation is to serve as foliage. In this 

 view we may regard 



131. Leaves as a Contrivance for Increasing tlie Surface of that large 



part of the plant which is exposed to the light and the air. This is 

 shown by their expanded form, and ordinarily slight thickness in 

 comparison with their length and breath. While a Melon-Cactus 

 (115, Fig. 76) is a striking example of a plant with the least pos- 

 sible amount of surface for its bulk, a repeatedly branching leafy 

 herb or tree presents the largest possible extent of surface to the 

 air. The actual amount of surface presented by a tree in full leaf 

 is much larger than one would be apt to suppose. Thus, the Wash- 

 ington Elm at Cambridge — a tree of no extraordinary size — was 

 some years ago estimated to produce a crop of seven millions of 

 leaves, exposing a surface of 200,000 square feet, or about five 

 acres, of foliage. 



132. What is done by the foliage we shall have to explain in 

 another place. Under the present head we are to consider ordinaiy 

 leaves as to their parts and their shapes. 



133. Tlie Parts of tlie leaf. The principal part of a leaf is the 

 blade, or expanded portion, one face of which naturally looks toward 

 the sky, the other towards the earth. The blade is often raised on 

 a stalk of its own, and on each side of the stalk at its base there is 

 sometimes an appendage called a stipule. A complete leaf, there- 

 fore consists of a blade (Fig. 83, h), a foot-stalk or leaf-stal^, called 

 the petiole (p), and a pair of stipules (st). See also Fig. 136. 



134. It is the blade which we are now to describe. This, as 

 being the essential and conspicuous part, we generally regard as the 

 leaf: and it is only when we have to particularize, that we speak of 

 the blade, or lamina, of the leaf. 



