THE NATURE OF SEEDS 343 



principal characters which determines the division of 

 angiosperms into two great groups is the character of their 

 embryos as to the number of cotyledons. This is the 

 character which gives name to these great groups. One 

 is called the monocotyledons, the other the dicotyledons. 

 (See page 183.) The two cotyledons of dicotyledonous 

 plants are lateral organs, that is, they arise from the 

 sides of the embryo, below its apex, one on each side. 

 This becomes evident when the embryo enlarges. (See 

 Figure 150.) The one cotyledon of monocotyledonous 

 plants is, on the other hand, a terminal organ. It arises 

 at the apex of the embryo ; in fact, it is the apex of the 

 embryo, and, naturally enough, is but one in number. 



The embryos of monocotyledons are, as a rule, much 

 smaller than those of dicotyledons. Their cotyledons are 

 never enlarged and rich in food like those 

 of a bean. The embryo usually lies at 

 one end of the seed, while the much 

 larger endosperm fills the rest of it. 



,„, , „. N Fig. 151. —Diagram of 



(Study Figure 149.) lengthwise section of 



The embryos of all but one of the seed of pine. Note 



j. ... j. / o \ that the embryo has 



families of gymnosperms (see page 183) several cotyledons . It 



have tWO Cotyledons. Most of the mem- lies embedded in the 



bers of that family (Pinacece) have em- <? d ? s f?' Notethe 



truck tcstd. 



bryos with several cotyledons, and it 



happens that our commonest gymnosperms belong to that 



family. (See Figure 151.) 



G. Seeds and Foods. — In the chapter on leaves you 

 learned that foods, chemically considered, are. divided into 

 three great classes, the carbohydrates, the proteins, and the 

 fats. You learned that the products of photosynthesis are 



