432 A TEXTBOOK OF GENERAL BOTANY 
migrate. The pollen tube grows down through the style and 
enters the female prothallus, usually by way of the micropyle 
(Fig. 4). The two male nuclei are discharged in the embryo 
sac. One of them fertilizes the egg by fusing with its nucleus 
(Fig. 487). The fertilized egg germinates and produces an em- 
bryo. The second male nucleus and the two polar nuclei fuse 
to form an endosperm nucleus (Fig. 487). 
Endosperm. The endosperm nucleus undergoes a series of 
rapid divisions which result in the formation of endosperm tissue 
around the developing embryo, filling the embryo sac. The 
nuclei of the cells of this tissue contain a triple, or 3 2, number 
of chromosomes, because the primary endosperm nucleus was 
formed by the fusion of three nuclei. The endosperm furnishes 
nourishment for the developing embryo. Usually it completely 
absorbs the nucellus before the seed is mature. Frequently the 
embryo absorbs all of the endosperm during the development 
of the seed; in such cases there is no endosperm in the ripe 
seed (Fig. 268). In many cases the endosperm persists in the 
mature seed and is absorbed only during germination (Fig. 269). 
Seed. When the endosperm is absorbed during the develop- 
ment of the seed, the ripe seed consists of an embryo surrounded 
by one or two integuments which form the seed coats. If the 
endosperm persists in the seed, the embryo is surrounded by the 
endosperm, and this by the seed coats. 
