136 
INTRODUCTION TO BOTANY 
supposed that the pollen tube is nourished by the cells that 
are broken down in the path of the tube. 
The time required 
for the pollen tube to reach the ovule varies in different plants, 
| 
il ip 
iN 
GG ye he [ 
SH) 
“in 
HI i 
Fig. 11), Diagram to illustrate course 
of the pollen tube during fertilization 
p, pollen grains; ¢, pollen tube; n, nucel- 
lus, or body of the ovule ; a, antipodul 
cells of embryo sac; en, endosperm nu- 
cleus of embryo sac; egg, the egg ap- 
paratus, consisting of the egg cell and 
two coédperating cells; m, the nicropyle, 
or small opening, through which, in most 
ordinary flowering plants, the pollen 
tube makes its way to the egg at the tip 
of the embryo sac 
in the higher ones. 
ranging from a few hours to 
more than a year. Usually 
the tube finds its way into 
the ovule through a minute 
opening known as the miecro- 
pyle (fig. 119, m), but in 
some plants it grows directly 
through the substance of the 
ovule. 
127. Fertilization. On en- 
tering the ovule, one of the 
male cells unites with the egg 
nucleus of the embryo sac, 
which is within the ovule, and 
the other male cell in some 
cases, or perhaps usually, 
unites with the central nu- 
cleus of the embryo sac to 
form the endosperm nucleus 
(fig. 119, en). The nature 
and function of the latter 
union are not as yet per- 
fectly understood. The fusion 
of egg nucleus and male cell 
is, however, a very common 
and most important phenom- 
enon, occurring in many of 
the simpler plants as well as 
In general it may be said that fertilization 
consists in the union of the nucleus of a mate cell with the nucleus 
of an egg cell. Other illustrations of this will be given in 
Chapters XV-XVIII. After fertilization the egg rapidly 
grows into the embryo of a new plant. The ovule also grows 
