98 COLLEGE BOTANY 



ing grown down through the style, enters the ovary, then through 

 the micropyle and into the imcellus and eventually into the 

 synergid end of the embryo sac. The two< sperm nuclei are dis- 

 charged into the embryo sac and one, which may be called the 

 male gamete, unites with the egg or female gamete. This union 

 of the two sex nuclei is the true fertilization and gives rise to 

 the embryo plant within the mature seed. The two polar nuclei 

 unite and form the endosperm nucleus, which gives rise to the 

 endosperm of the seed. In many cases, possibly in all, the sec- 

 ond sperm nucleus unites with the endosperm nucleus; when this 

 occurs it is known as double fertilization. The antipodal nuclei 

 usually disintegrate, but in some cases appear to be important 

 factors in the nourishment of the embryo. 



The Formation of the Seed. The further development in- 

 cludes a number of structures : 



(1) The embryo is at first a single cell formed by the union 

 of two nuclei, the sperm and the egg. This cell undergoes re- 

 peated divisions, the resulting cells undergoing more or less dif- 

 ferentiation to form the first organs of the embryo or young plant. 

 These organs include the cotyledons, the plumule, the radicle and 

 the hypocotyl, which are described in Chapter IX. 



(2) The endosperm (Fig. 74, d, e, 1} arises from the divi- 

 sions of the endosperm nucleus and together with the embryo, 

 more or less completely fills the embryo sac. It is very rich in 

 stored foods and is the first nutriment used' by the embryo when 

 the seed germinates. 



(3) The perisperm of the mature seed is developed from 

 the nucellus. It is well supplied with food, which is used by the 

 embryo during the period of germination. 



(4) The seed Coats (Fig. 73) of the mature seed are the 



