152 BOTANY. 



a fluid, but a layer of cells is formed, lining the walls, and this thickens 

 until the whole space, except what is occupied by the small embryo, is 

 filled with them. These are called the "endosperm cells," but differ 

 from the endosperm cells of the gymnosperms, in the fact that they are 

 not developed until after fertilization, and can hardly, therefore, be 

 regarded as representing the prothallium of the gymnosperms and pteri- 

 dophytes. These cells finally form a firm tissue, whose cells are filled with 

 starch that forms a reserve supply of food for the embryo plant when the 

 seed germinates. The embryo (Fig. 81, 7f, era., Fig. 82, </), even when 

 the seed is ripe, remains very small, and shows scarcely any differentia- 

 tion. It is a small, pear-shaped mass of cells, the smaller end directed 

 toward the upper end of the embryo sac. 



The integuments grow with the embryo sac, and become 

 brown and hard, forming the shell of the seed. The stalk of 

 the ovule also enlarges, and finally forms the peculiar, spongy 

 appendage of the seeds already noticed (Fig. 81, J, K). 



