278 STRUCTURAL BOTANY 



protoplasm of the ovum itself (see Fig. 113, c). The 

 two generative cells are now at the growing end of 

 the pollen-tube. Their nuclei are shown in the figure. 

 The leading generative nucleus, accompanied by a 

 small amount of protoplasm, now passes through an 

 opening in the membrane at the end of the pollen-tube 

 (Fig. 114, A, sri). It traverses the intervening proto- 

 plasm and reaches the nucleus of the ovum, with which 

 it eventually unites 1 (Fig. 114, B). Fertilisation is 

 now effected. We see now that, different as many of 

 the preliminary processes have been, yet the act of 

 fertilisation in itself is essentially the same as in 

 Angiosperms. Fertilisation is, in fact, an identical 

 process in all plants in which it occurs at all. 2 



d. Development of the Embryo 



The fertilised nucleus, resulting from the fusion of 

 the male and female nuclei, passes into the lower end 

 of the ovum. It there undergoes two divisions, so 

 that there are now four nuclei, all of which lie in 

 the same horizontal plane (see Fig. 115, (7, where, 

 of course, only two of them are shown). Successive 

 divisions now take place in a direction at right angles 

 to the long axis of the ovum, until there are sixteen 

 nuclei altogether, lying in four tiers one above another. 

 Each nucleus of the lower three tiers is enclosed with 

 its surrounding protoplasm within a cell-wall; the 

 uppermost four are free nuclei (see Fig. 114, D, E). 

 It is to be noticed that only a part of the ovum is 

 concerned in these divisions ; much the greater portion 



1 For an account of the fertilisation of certain Gymnosperms by 

 means of spermatozoids or motile male cells, see Part II. (Flowerlcss 

 Plants}, p. 301. 



2 DouUe fertilisation, however, described above in the Lily (p. 186) 

 is, so far as we know, a phenomenon peculiar to Angiosperms. 



