ISO 



TEXTBOOK OF BOTANY 



sperm is used as food for the embryo about as rapidly as 

 the endosperm itself develops ; and finally most of the food 

 that had been stored in the ovule is transferred to the two 

 seed leaves of the embryo. The ripe seed thus contains no 

 endosperm, and the thick seed leaves fill much the greater 

 part of the space inside the seed coats. In this respect the 

 bean seed is like the seed of the squash. 



The body of the embryo (consisting of radicle and plumule) 

 is much smaller than the seed leaves. It lies between the 



seed leaves close to the 

 edge by which the seed 

 •was attached to the pod. 

 The end of the radicle 

 projects from between 

 the seed leaves and is 

 turned toward the micro- 

 pyle. The plumule is 

 curved and bears two or 

 three very small second- 

 ary leaves. The food in 

 the seed leaves is chiefly 

 starch and proteins. The 

 bean, like some other 

 plants of the same family, such as the pea and the lentil, 

 stores a comparatively large proportion of proteins in its 

 seed leaves. This is one reason why the seeds of these 

 plants are so valuable to us as food. Little or nothing of 

 the body of the ovule is left in the ripe seed . The integu- 

 ments develop into thin, hard seed coats. 



The parts of the bean seed thus correspond closely to those 

 of the pine seed, except that such endosperm as was formed,' 

 although it resembled the endosperm of the pine seed, arose 

 in a quite different way and disappeared before the seed 

 was mature. In the ripe bean seed (Fig. 91) the scar that 

 marks the point at which the seed was attached to the pod 



Fig. 91. — A bean seed: A, as seen 

 from the side; B, an edge view, showing 

 the scar by which the seed was attached, 

 and, just above, the micropyle ; C, with 

 one-half of the seed coat and one seed leaf 

 removed, showing the plumule, radicle, 

 and remaining seed leaf. 



