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 



P 



its structure becomes highly 



FIG. 157. Diagrammatic representation 

 of fertilization of an ovule 



t, inner coating of ovule ; o, outer coating complex. Finally it C011- 



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



one of the pollen grains on the stigma; i i i 11 j 

 c, the place where the two coats of the re S 10118 wMch gradually de- 

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



shown is inverted, its opening m beina: n -i e n 



at the bottom, and the stalk > adhering ot bhe Ml-grown embryo. 



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



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



antipodal cells of embryo sac; n, central S rams tO each OVUlC. Only 



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



cells, one of which, the egg cell, receives - ,.,. -, ^ ^ 



the male nucleus of the pollen tube ; f, fu- to fertilize each OVUle, but SO 



niculus 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 



