Reproduction in Flowering Plants 207 



grow down through the style as there are ovules in the ovulary 

 below. 



The seed. The seed is the final product of polUnation and 

 fertilization. Its complete development ends the role of the 

 flower. The essential part of the seed is the embryo, for it 

 is the embryo that will later produce the seedling; and in 

 order to supply the embryo with nourishment during the 

 early stages of growth, food from the parent plant is ac- 

 cumulated in the seed. This may either be stored inside the 

 embryo itself, as in the bean ; or it may be stored in the tissue 

 surrounding the embryo, as in the castor bean and corn. The 

 food-containing tissue surrounding the embryo in many seeds 

 is called the endosperm. Since the seed carries the plant over 

 the winter or through an imfavorable season, the embryo with 

 its food supply is protected by the seed coats, one of which is 

 usually hard and resistant. 



The three constituents of a seed, then, are (i) the embryo 

 or young plant, (2) the food supply, either inside the embryo 

 or in the endosperm, and (3) the seed coats or protective cover- 

 ing. 



The structure of seeds. Although seeds vary as much in 

 form as do other plant organs, the different arrangements of 

 the three essential parts may be illustrated by a castor bean, 

 a bean, and a grain of corn. 



In the castor bean the seed coats consist of a hard outer 

 layer and a thin inner membrane. These inclose an endo- 

 sperm, which is a mass of cells containing food in the form of 

 starch, oil, and protein. Within the endosperm lies the 

 embryo, ready to grow when favorable conditions for germi- 

 nation come. The embryo consists of the hypocotyl and two 

 cotyledons, with a small bud between the cotyledons, called 



