DTV. TI 



SPERMATOPHYTA 



565 



thallium, the archegonial chamber (Fig. 540), which in Dioon is about 

 1 mm. in depth and 2 mm. across. The pollen-tubes grow into this 

 depression and liberate their spermatozoids together with a drop of 

 watery fluid in which they swim. The spermatozoids require to 

 narrow considerably in order to pass through the space between 

 the neck-cells. The spermatozoid strips off the ciliated band on enter- 

 ing the protoplasm of the egg, and its nucleus fuses with that of the 

 latter. The nucleus of the fertilised ovum (Fig. 541) soon divides, 

 and the daughter nuclei continue to divide rapidly, until after the 

 eighth division there are about 256 free nuclei within the cell. These 



FIG. 541. Zamiafloridana. An ovum im- 

 mediately after the fusion of the nucleus 

 of a spermatozoid with the female 

 nucleus has taken place. The ciliary 

 band of the spermatozoid remains in 

 the upper portion of the protoplasm 

 of the ovum. A second spermatozoid 

 ' has attempted to enter the ovum, 

 (x 18. After H. J. WEBBER.) 



C 



FIG. 542. Two young pro-embryos 

 of Dioon edule showing their 

 relation to the archegonial 

 chamber. S, suspensor ; e, em- 

 bryo. (After CHAMBERLAIN.) 



are crowded towards the lower end of the fertilised egg, where cell 

 walls commence to form between them. 



The so-called PRO-EMBRYO is thus formed (Fig. 542), at the 

 growing end of which the embryo develops from relatively few cells. 

 The cells farther back elongate greatly and as a SUSPENSOR carry the 

 embryo into the prothallus. This in Spermatophytes is termed the 

 ENDOSPERM and serves as a nutritive tissue for the growing embryo. 

 The latter ultimately develops, at the end directed into the prothallus, 

 two large COTYLEDONS between which is the rudiment of the apical 

 bud or PLUMULE. The region of the stem below the cotyledons is 

 termed the HYPOCOTYL ; it passes gradually into the main root or 

 RADICLE, which is always directed towards the micropyle. 



202 



