64 BOTANY: PRINCIPLES AND PROBLEMS 



External Structure (Figs. 31 and 32). — Externally, the typical 

 leaf consists of a broad, flat, and thin portion, the blade, which 

 is the seat of its important activities. This is green in color and 

 provided with a system of ribs or vei7is of stouter texture than the 

 rest of the tissue. The blade may sometimes be attached directly 

 to the stem, but is usually supported by a leaf-stalk or petiole, 

 which holds it out in a place favorable for the performance of 



Fig. 32. — Simple and compound leaves. A, compound leaf of Mountain Ash, 

 with eleven leaflets. B, simple leaf of Apple. 



its functions and serves as a highway for transportation of water 

 and food between blade and stem. At the base of the petiole 

 are often two small appendages, the stipules, the function of 

 which is in many cases obscure. 



Leaves vary widely in size, shape, texture, margin, venation, 

 and other characters. In its outline the blade may be even, or 

 it may be lobed or sometimes actually divided into separate 

 portions, the leaflets, in which state the leaf is said to be com- 

 pound (Fig. 32). The margin is sometimes quite smooth, but 



