SPERMATOPHYTES 



147 



other hand, a Monocotyledon is restricted to one cotyledon 

 because it is a terminal structure, but it must be remembered 

 that an embryo with one cotyledon may belong to a Dicoty- 

 ledon. 



88. The seed. The features of a seed have been described 

 under Gymnosperms ( 76, p. 127), and they differ in no es- 

 sential way among the Angiosperms. The embryo develops 

 to a certain stage, varying widely in different plants, and then 

 passes into the dormant stage, probably due in large measure 

 to the cutting off of the water supply. Dur- 

 ing the development of the embryo the hard 



testa develops, protecting the delicate struct- 

 ures within against the exposures of an 

 unfavorable season, as the cold of winter, 

 dryness, etc. (Fig. 117). 



The seeds of Angiosperms differ widely as 

 to the amount of endosperm left by the em- 

 bryo when it becomes dormant. The endo- 

 sperm in a seed, therefore, may vary from 

 a great deal (Fig. 117) to none at all. For 

 example, in the seeds (" grains ") of cereals 

 (wheat, corn, rice, etc.) a great amount of 

 endosperm is left, and the world gets much 

 of its food from this source ; while in peas 

 and beans no endosperm is left, but the coty- 

 ledons have stored up the food supply taken 

 from the destroyed endosperm and have become bulky, so 

 that in this case we use the cotyledons for food instead 

 of the endosperm directly. 



89. The fruit. While the seeds of Angiosperms are 

 ripening, changes take place also in structures outside the 

 seed. For example, the ovary wall may change and become 

 a hard or parchment-like seed-vessel, as in peas and beans, 

 whose seed-vessels are called pods (Figs. 118 and 119). In 

 other cases, the whole ovary may become a thin-skinned 



FIG. 119. Pod of 

 iris ("three- 

 celled") burst- 

 ing open. 

 After GRAY. 



