508 



BOTANY 



PART II 



464) and the formation of the motile sperinatozoids, which have been found in all the 

 Cycads yet investigated and in Ginkgo (Figs. 467, 468), the embryo-sao filled with the 

 prothallial tissue is increasing in size within the nucellus. As it crushes the tissue 

 of the upper portion of the nucellus it approaches the base of the pollen-chamber. 

 At the apex of the embryo-sac are found the archegonia, usually four in nunibei', 

 and separated from one another by some layers of cells. Each archegonium has a 



neck, and ultimately cuts 

 oif a canal-cell. Thearche- 

 gonia are situated at the 

 base of a depression in the 

 prothallium, the arche- 

 gonial chamber (Fig. 466), 

 which is about 1 mm. in 

 depth and 2 mm. across. 

 The pollen - tubes grow 

 into this depression and 

 liberate their sperm ato- 

 zoids together with a drop 

 of watery fluid in which 

 they swim. Since the 

 bursting of the pollen- 

 tubes is brought about by 

 their contact with the pro- 

 jecting cells of the neck, 

 the spermatozoids cannot 

 easily lose their way to 

 the egg - cell. They re- 

 quire, however, to narrow 

 considerably in order to 

 pass through the space 

 between the neck -cells. 

 The nucleus of the fertil- 

 ised ovum (Fig. 469) soon 

 divides, and the daughter- 

 nuclei continue to divide 

 rajiidly, until after the 

 eighth division there are 

 about 2.56 free nuclei 

 within the cell. These 

 are crowded towards the 

 lower end of the pro- 

 embryo, where cell-forma- 

 tion commences around them. The embryo is forced into the endosperm by the 

 elongation of a suspensor. It ultimately possesses a pair of large cotyledons, a 

 well-developed plumule between these and a relatively short hypocotyl. 



The development of the sexual generation and the fertilisation in Ginhjo biloha, 

 the only known species of the genus, is very similar to what lias been described for 

 Zamia. The possession of a well-developed pollen-chamber indicates the relation- 

 ship to Cycads. The fertilised ovum becomes filled with a continuous tissue and 

 forms the embryo at the end farthest from the micropyle. This grows out and pene- 

 trates the endosperm, but there is no clear distinction into suspensor and embryo. 



Fig. 470. —Apex of the embryo-sac of Gnetum Ituinphianuin 

 shortly before the development of the female cells ; wk, female 

 nuclei ; ink, male nuclei ; PK, pollen-tube nuclei ; ps, pollen- 

 tube, (x 500.) 



