SPERMATOPHYTES 



143 



a group of three naked cells, one of which is the egg. Three 

 other nuclei become placed at the other end of the sac, and 

 may remain as free nuclei or become 

 organized into a group of three cells. 

 The two remaining nuclei behave in 

 a remarkable way, for they come to- 

 gether and fuse to form a single large 

 nucleus, which is called the endosperm 

 nucleus because it produces the endo- 

 sperm, a tissue developed within the 

 embryo-sac to nourish the embryo. A 

 female gametophyte ready for fertiliza- 

 tion, therefore, consists of seven cells 

 or nuclei (Fig. 114) : a group of three 

 at the end of the sac towards the mi- 

 cropyle, one of which is the egg ; 

 another group of three at the other 

 end of the sac ; and the large endo- 

 sperm nucleus (two nuclei fused) lying 

 between. 



86. Fertilization. The act of fer- 

 tilization must be preceded by pollina- FlG> 115 . _ Diagram of 

 tion, which is a notable feature of 

 Angiosperms. Among Angiosperms 

 there is a good deal of wind-pollination, 

 but in addition to this there is a re- 

 markable development of insect-pollin- 

 ation. So important and elaborate is 

 this relation between the flowers of 

 Angiosperms and insects that it will be 

 discussed in the following chapter. At 

 this point, all that is necessary to state 

 is that through the agency of wind or of 

 insects the pollen is carried from the stamens to the 

 stigmatic surfaces of pistils, with of course much loss 



pollen tubes penetrating 

 the style (grains can 

 be seen lying on the 

 stigma) ; one of the 

 tubes has passed 

 through the style, en- 

 tered the ovary cavity, 

 passed along the wall 

 of the ovary, entered 

 the micropyle of the 

 ovule, penetrated the 

 tip of the nucellus, and 

 discharged its two 

 sperms into the embryo- 

 sac ; one of the sperms 

 fuses with the egg, the 

 other with the endo- 

 sperm nucleus. 



