CHAPTER X. 



THE LEAVES. 



145. Primary leaves. — Leaves are distinguishable into 

 primary and secondary. The primary lea\"es arise directly 

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

 fernworts t-\\'0 of the octants into which the egg divides 

 produce the primary leaf This is entirely unlike the secondarv 

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

 plants, one, t^^■o, or more leaves de\'elop as members of the 

 embryo, onl)' a tew plants (and those probably degenerate in 

 this respect) not forming leaves before the embryo enters its 

 resting stage. 



The ]irimary 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 

 thev 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 generallv numerous and much 

 more conspicuous. It is these which are usually meant bv 

 " leaves," unless primary leaves are specially named. 



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

 its progressi\e differentiation into stem and leaves can be 

 observed. L^pon the sides of the growing point swellings of 

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

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

 which they become transformed bv further de\'elopment. 

 Similar swellings appear later just above the leaf fundaments. 



