144 STRUCTURE OF FLORAL ORGANS ; FERTILIZATION 



the process in angiosperms is as follows. The egg cell (Fig. 



158, A) some time after fertilization forms a transverse partition 



and is thus divided into two 

 cells, one of which (Fig. 158, 

 B, s) is to form the slender 

 suspensor of the embryo 

 (which serves various pur- 

 poses, such as forcing the 

 embryo into the nutritive 

 tissue of the seed, absorb- 

 ing food from the wall of the 

 ovary, or storing food for 

 the growing embryo) and the 

 other (e) is to form the embryo 

 itself. These cells in turn 

 subdivide, as shown in C, D, 

 and E. The whole pear- 

 shaped body in parts B-E is 

 called the pro-embryo, and 

 this continues to grow and 

 its cells to subdivide until 

 its structure becomes highly 

 complex. Finally it con- 



i, inner coating of ovule; o, outer coating 



of ovule ; p, pollen tube proceeding from tains many sharply defined 



one of the pollen grains on the stigma; iong wMdl gra d u ally de- 



c, the place where the two coats of the 



ovule blend. (The kind of ovule here velop into the Several organs 



FIG. 157. Diagrammatic representation 

 of fertilization of an ovule 



along one side of the ovule.) a to e, em- 179. Number of pollen 



^ai/SfcrJ^SS grains to each ovule. Only 



nucleus of the embryo sac; e, nucleated one pollen grain is necessary 



cells, one of which, the egg cell, receives , . M . -, i i 



the male nucleus of the pollen tube ; /, f u- to fertilize each OVule, but SO 



nicuius or stalk of ovule ; m, micropyle or many pollen grains are lost 



opening into the ovule. After Luerssen . , , 



that plants produce many 



more of them than they do ovules. The ratio, however, varies 

 greatly. In the night-blooming cereus there are about 250,000 



