CHAPTER XI. 



THE LEAVES. 



The leaves are very important nutritive organs in most 

 green plants. They are adapted to catch the sunlight; 

 therefore their form, structure, and position are largely con- 

 trolled by this relation to light. (See ^^ 190, 191.) 



119. Primary and secondary leaves. — Leaves are dis- 

 tinguishable as primary and secondary. The primary leaves 

 are those first developed, usually in the youngest stage, the 

 embryo. In fernworts the primary leaf can be traced back in 

 its development even to the egg. In seed plants they are 

 usually formed before the young plant (embryo) enters its 

 resting state as the seed becomes ripe. 



The primary leaves of seed plants are called cotyledons 

 (figs. 85, 86). They are usually transient, and not rarely so 

 distorted by acting as storage places for reserve food that they 

 do not serve 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. 



Secondary leaves are generally numerous and much more 

 conspicuous. It is these which are usually meant by 

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



120. Development. — If the apex of the shoot be ex- 

 amined, 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 



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