54 THE EVOLUTION OF PLANTS 



The most constant distinction, however, is 

 that from which the names of the two classes are 

 taken the number of cotyledons in the embryo- 

 plant. 



The cotyledons are commonly called the "seed- 

 leaves," though they by no means always per- 

 form the functions of ordinary leaves. They 

 often serve only as storehouses of starch and 

 other reserves of food, as in the Broad Bean, or 

 as suckers to absorb food from the endosperm, 

 as in Wheat and other Grasses. But in very 

 many plants (as in the Wallflower, the Beech, 

 the Sycomore, and the Lily) they ultimately ex- 

 pand and become green, resembling true leaves; 

 this is probably their real nature in all cases. In 

 Monocotyledons there is very constantly a single 

 seed-leaf only; in Dicotyledons there are nearly 

 always two, though there are a few exceptions. 

 The difference is important, affecting the whole 

 structure and mode of development of the em- 

 bryo. It is remarkable that all Gymnosperms 

 have essentially dicotyledonous embryos, for al- 

 though in some of them, as in our common Fir- 

 trees, there are many more than two cotyledons, 

 it has been shown that the larger number arises 

 by the early division of two primary seed-leaves. 



Thus, in two characters of the greatest im- 

 portance, the structure of the stem and the 

 structure of the embryo, the Gymnosperms 



