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 
Fiu. 227.—Diagrammatic 
representation of pollen 
tubes penetrating the 
style; one of them en- 
tering the ovary cavity, 
passing down its wall, 
and reaching the female 
gametophyte by way of 
the micropyle. 
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) 
of the pistil that encloses the ovules. 
141. Embryo.—The odspore, lying 
in the midst of the ovule, at once be- 
gins to germinate, and forms a young 
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 arc named 
