16 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 1 



feature do foliage leaves vary little and that is the thickness, 

 or rather the thinness, of their green tissue, which is nearly 

 the same no matter what their sizes and shapes. 



The thin flat expanse of green tissue, called the blade, 

 is always the essential, and often the only, part of the leaf. 

 In many kinds, however, the blade is provided with a slender, 

 cylindrical stalk, called the petiole, various in length even 



up to several feet ; and upon it the 

 blade is adjusted to the light, and 

 has free play in the wind. In addi- 

 tion, some kinds possess a pair of 

 small appendages, one on each side 

 of the base of the petiole, called 

 stipules, which, though usually 

 green like the blade, are very diverse 

 in form. Blade, petiole, and stipules 

 are parts of a complete leaf, of which 

 a typical example is pictured here- 

 with (Fig. 1). 



In some kinds of leaves, es- 

 pecially large ones, the blade is 

 not all one piece, but is cleft more 

 or less into divisions, as familiar in 

 Oak or Maple. The same process 

 continued much farther results in 

 the formation of separate leaflets, 

 each with a stalk of its own, as in Rose or Strawberry 

 (Fig. 37), while the leaflets also may become themselves 

 subdivided, even more than once, as in some kinds of Ferns. 

 Such leaves are called compound, in distinction from simple, 

 the two being distinguishable by the fact that the leaflets 

 of a compound leaf always stand in one flat plane, while 

 sample leaves are distributed around a stem, at least at their 

 bases. Further, leaflets have no buds in their axils, but 

 leaves, whether simple or compound, always do. 



While typical leaves, the kinds designated foliage, are 



Fig. 1. — A leaf of the 

 Quince, showing blade, petiole, 

 and stipules ; reduced. (After 

 Gray's Structural Botany.) 



