236 



A TEXT-BOOK OF BOTANY 



pollination ( 125), is effected in many Angiosperms by 



insects, and how this is brought about will be described 



later. 



The pollen grains that reach the stigma, the specially 



prepared surface for receiving them, begin to put out pollen 

 tubes. These tubes grow through the 

 stigma and enter the style; grow down 

 the style and enter the cavity of the 

 ovary; reach the ovules and enter their 

 micropyles; and finally penetrate the 

 ovule to the egg (Fig. 227). Through- 

 out this progress of the tube the male 

 cells are in its tip, and when the egg 

 is reached they are discharged from 

 the tube and one of them fuses with 

 the egg. This is the act of fertiliza- 

 tion, and through it the egg becomes 

 an oospore. 



An important difference between 

 Gymnosperms and Angiosperms should 

 be noted here. In Gymnosperms the 

 pollen reaches the ovules, for they are 

 exposed; but in Angiosperms the pol- 

 len reaches only the surface (stigma) 



FIG. 227.-Diagrammatic f the P lstil that encloses the OVUleS. 



representation of pollen 141. Embryo. The oospore, lying 



tubes penetrating the , . , , , , 



style; one of them en- in the midst of the ovule, at once be- 

 tering the ovary cavity, ~ ms to nr erm i na t e , and forms a young 



passing down its wall, . 



and reaching the female plant or embryo. While the embryo 



is forming, the ovule develops a hard 

 coat outside, and a seed is the result 

 (Fig. 228). The general structure of the seed and how the 

 young plant escapes from the seed have been described in 

 Chapter V. 



The two great divisions of Angiosperms are named 



