LEAVES 285 



there is a tiny bud, called the plumule; this gives rise to 

 the stem and true leaves of the seedling. The cotyledons 

 are at one end of a minute, embryo stem, called the 

 hypocotyl (from hypo, "under," and cotyledon). That end 

 of the hypocotyl which does not bear the plumule sends 

 out the roots of the seedling. 



308. Other Seeds. The germination of other seeds is slightly 

 different from that of the bean. In many seeds the food of the embryo 

 is not stored in the cotyledons, but in a space surrounding the embryo; 

 food so stored is called the endosperm. In the pea the cotyledons 

 remain within the* testa, supplying the nutrition required by the 

 growing embryo and seedling. In the castor bean the cotyledons es- 

 cape, but they differ from those of the garden bean in that they become 

 large and green, and do the work of true leaves. 



A large class of plants has only one cotyledon. These are called 

 monocotyls, while beans, peas, etc., are called dicotyts. Corn (maize) 

 is a monocotyl. In corn the embryo is at one side, just beneath the 

 testa. When the testa breaks open, the embryo sends its plumule 

 upward, and the root end of the hypocotyl downward. The cotyledon 

 and the endosperm remain inside the testa. 



309. Leaves. The expanded part of the leaf is called 

 the blade. In some plants the blade is attached directly 

 to the plant stem; in other plants there is a short stalk, 

 called the petiole. Sometimes the plant produces tiny 

 blades, called stipules, at the place where the petiole 

 joins the stem. 



The blade is marked by a network of veins. If there is 

 a prominent central vein, this is called the midrib. Veins 

 are arranged in three general ways (Fig. 233). The 

 first leaf (on the left of the figure) is palm-veined; the sec- 

 ond, feather-veined; the third, parallel-veined. A leaf 



