222 
PRACTICAL COURSE IN BOTANY 
251. Fertilization. — When a pollen tube has penetrated 
to the ovary, it next enters one of the ovules, usually through 
Fria. 330.— Diagram of a simple 
flower, showing course of the pollen 
tube: a, transverse section of an 
anther before its dehiscence; 6, an 
anther dehiscing longitudinally, with 
pollen; c, filament; d, base of floral 
leaves; e, nectaries; f, wall of carpels; 
g, style; h, stigma; 7, germinating 
pollen grains; m, a pollen tube which 
has reached and entered the micropyle 
of theovule; n, stalk of ovule; 0, base 
of the inverted ovule; p, outer integu- 
ment or testa; g, inner integument; 
s, rudimentary ovule; t, cavity of the 
embryo sac; wu, its basal portion; », 
endosperm ; z, odsphere. 
the micropyle (Fig. 330, m). 
There it penetrates the wall of 
a baglike inclosure called the 
embryo sac (Fig. 330, wu, ¢t, 2), 
where one of the generative 
cells emitted by the pollen tube 
fuses with a large cell contained 
in the embryo sac, known as 
the germ cell, or egg cell (Fig. 
330, z). The fusion of these 
two bodies is what constitutes 
fertilization. The cell formed 
by their union finally develops 
into the embryo, and the other 
contents of the sac into the 
endosperm, and the ripened 
ovules become seeds. 
252. Stability of the process 
of fertilization. The phe- 
nomena that characterize the 
functions of fertilization and 
reproduction are the most uni- 
form and stable of all the life 
processes, varying little not 
only in different species and 
orders, but throughout the whole vegetable kingdom. And 
since these functions furnish a more reliable standard for 
judging of the real affinities of the different groups than do 
mere external resemblances, which are more liable to varia- 
tion and may often be accidental, they have been chosen 
by botanists as the ultimate basis for the classification of 
plants. 
253. Embryology. — The study of the developing plantlet, 
known as embryology, is a comparatively recent branch of 
