CHAPTER X. 



THE LEAVES. 



145. Primary leaves. — Leaves are distinguishable into 

 primary and secondary. The primary leaves arise directly 

 from the first cells produced by division of the egg. In the 

 fernworts two of the octants into which the egg divides 

 produce the primary leaf. This is entirely unlike the secondary 

 leaves, which arise upon the sides of the stem. In seed 

 plants, one, two, or more leaves develop as members of the 

 embryo, only a few plants (and those probably degenerate in 

 this respect) not forming leaves before the embryo enters its 

 resting stage. 



The primary leaves of seed plants are called cotyledons 

 (figs. 134, 135). They are usually transient, and not rarely 

 so distorted by acting as storage places for reserve food that 

 they do not function as foliage leaves at all. In extreme 

 cases of this kind they remain in the seed coats when the 

 embryo resumes its growth, as in pea and oak. 



146. Secondary leaves are generally numerous and much 

 more conspicuous. It is these which are usually meant by 

 '■' leaves," unless primary leaves are specially named. 



147. Development. — If the apex of the shoot is examined, 

 its progressive differentiation into stem and leaves can be 

 observed. Upon the sides of the growing point swellings of 

 various size appear, the smallest being nearest the apex (fig. 

 95). These swellings are the fundaments of the leaves, into 

 which they become transformed by further development. 

 Similar swellings appear later just above the leaf fundaments, 



117 



