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- 

 tains many sharply denned 

 regions which gradually de- 

 velop into the several organs 

 of the full-grown embryo. 



179. Number of pollen 

 grains to each ovule. Only 

 one pollen grain is necessary 

 to fertilize each ovule, but so 

 many pollen grains are lost 

 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 





FIG. 157. Diagrammatic representation 

 of fertilization of an ovule 



i, inner coating of ovule ; o, outer coating 

 of ovule ; p, pollen tube proceeding from 

 one of the pollen grains on the stigma ; 

 c, the place where the two coats of the 

 ovule blend. (The kind of ovule here 

 shown is inverted, its opening m being 

 at the bottom, and the stalk / adhering 

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

 bryo sac, full of protoplasm ; a, so-called 

 antipodal cells of embryo sac ; n, central 

 nucleus of the embryo sac ; e, nucleated 

 cells, one of which, the egg cell, receives 

 the male nucleus of the pollen tube ; /, fu- 

 niculus or stalk of ovule ; m, micropyle or 

 opening into the ovule. After Luerssen 



