MALE GAMETOPHYTE AND FERTILIZATION 465 
The pollen tube lives as a parasite on the structures through 
which it passes, using their tissues as food for growth and mak- 
ing a passageway for itself at the same time. The growth of the 
pollen tube is directed by the tube nucleus-which maintains a 
position near the end of the tube. Soon after the pollen tube is 
well started, the generative nucleus passes from the pollen grain 
into the tube and later divides, forming two 
sperms which are carried along with the con- 
tents of the tube to the embryo sac. The 
male gametophyte, consisting of tube nucleus 
and two sperms, is now complete. In some 
plants, however, the formation of the sperms 
occurs before the development of the tube is 
begun. 
When the tube reaches the embryo sac and 
comes in contact with its contents, the mem- 
brane enclosing the tube is destroyed, and the 
tube nucleus, sperms, and other contents of 
the tube flow into the embryo sac. The con- 
tents of the embryo sac apparently destroy 
the tube nucleus, for it soon disappears, while 
the sperms apparently thrive. Since there 
are no cell walls in the embryo sac, the sperms 
are free to move about. As to how they are 
moved is not known, for they have no cilia, 
but one very soon reaches the nucleus of the 
egg and the other the polar nuclei or the 
primary endosperm nucleus, with which they 
come in contact and fuse. Since there are two 
fusions, one with the egg nucleus and the 
other with the polar nuclei or the primary 
endosperm nucleus, there are two fertilizations 
or double fertilization, and this also is a notable 
feature of Angiosperms (Fig. 411). 
Ficag. 411. — An 
embryo sac of a 
Lily, showing 
double fertilization. 
At the upper end of 
the sac the egg (e) 
and a sperm (s) are 
shown fusing, and 
near the center of 
the sac the second 
sperm (s) is shown 
fusing with the two 
polar nuclei (p). 
Of course fertilization is 
difficult to follow and has been seen in only a comparatively few 
Angiosperms. It is therefore possible that many times the 
second sperm does not fuse with the polars or the primary 
endosperm nucleus, but double fertilization has been found so 
generally in the Angiosperms whose fertilization has been studied 
that it is believed to be quite universal among Angiosperm. In 
