MALE GAMETOPHVTE AND FEirilLlZATlON 



465 



The pollen tube lives as a parasite on the structures tlu-ough 

 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). 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 cjuite universal among Angiosperm. In 



Fig. 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 sljown 

 fusing mth the two 

 polar nuclei (p). 



