32 



BIOLOGY FOR BEGINNERS 



TYPICAL SEED 



rut INTtHHKL iTRUCTUXC 



Embryo. If endosperm be present, the embryo may be poorly 

 developed, even showing no sign of its usual parts, as in the 

 orchids. On the other hand, the embryo may be highly developed 

 and show well-defined stem and leaves, as in the bean; for since 

 there is no endosperm in the bean, the plantlet must seek its own 

 nourishment very early. The embryo, or miniature plant, consists 

 of three parts: the cotyledons, plumule, and hypocotyl. 



Cotyledons. These are the seed leaves or the first leaves of the 

 plant and, though often not resembling ordinary leaves either in 



appearance or use, still 

 play a very important 

 part in the early growth 

 of the seedling. They 

 may be really leaf-like 

 and come up when the 

 plant begins to grow, 

 forming true green leaves, 

 as in the squash. In this 

 case they are thin and 

 have little stored food, 

 because they get all they 

 need as soon as they rise 

 above the soil. On the 

 other hand the cotyle- 

 dons may be so well sup- 

 plied with food that they cannot act as leaves at all, merely coming 

 above ground, giving over their stored food to the growing seedling, 

 and then withering and dropping off, as is the case with most beans. 

 In other cases, such as the pea, the cotyledons are so greatly en- 

 larged .with food, that they cannot be lifted from the soil at all, 

 and so supply the plant from their place in the ground below. 

 In cases where the food is stored outside the embryo as the endo- 

 sperm, the cotyledon often remains in contact with it to digest and 

 transfer food from endosperm to embryo, as is the case in corn. 

 Not only do the cotyledons vary in size and use (function), 

 but also in number, there being only one in many plants such 



FIG. 4. The internal structure of 

 a typical seed. 



