68 



BIOLOGY AND HUMAN LIFE 



The fleshy leaves are called seed leaves, or cotyledons. The 

 part below the meeting point is the hypocotyl ("below the coty- 

 ledon"). The part above is called the plumule or first bud, or 

 the epicotyl ("above the cotyledon") (Fig. S3)- 



Although the cotyledons do really correspond to leaves, there 

 are many plants in which they never come out of the ground, as 

 in the pea. In other plants the cotyledons do get above the 

 earth but do not spread out or become green, as in the bean. 



H 



12 3 



Fig. S3- Embryos of plants 



7, diagram showing relative positions of the parts of the'fembryo: 2, embryo of pea- 

 nut; 3, embryo of pea; 4, embryo of pine; C,C, cotyledons; E, epicotyl; H, hypocotyl 



When we compare the embryo of a grain, such as the corn, with the 

 other embryos that have been mentioned, we find one great difference in 

 the structure. The grain has but a single cotyledon. This is rather large, 

 though not fleshy, and remains in contact with the endosperm and serves 

 as an absorbing organ, withdrawing food material from endosperm and 

 transferring it to the growing plant (see 4, Fig. 34). 



Many plants besides the grains have but one cotyledon in the seed. 

 This fact may not be of any great importance by itself, but it is con- 

 nected with so many other characters (such as the veins in the leaves, 

 the structure of the stem, the structure of the flower, and general habits 

 of hfe) that we sometimes designate one of the main divisions of seed- 

 bearing plants as the monocotyls, meaning the "one-cotyls," and another 

 as the dicotyls, or ''two-cotyledon plants." The latter includes most 

 common weeds and cultivated plants except the grains. 



Seeds of the plants in the pine family (fir, spruce, hemlock,, etc.) have 

 usually several cotyledons, and this family is accordingly called poly- 

 cotyls, meaning ''plants having many cotyledons" (see 4, Fig. 2>3)- 



