DIV. n 



SPERMATOPHYTA 



563 



the nucellus. At the period during which the male cones are shedding 



their pollen, the macrosporophylls become slightly separated from 



one another so that the wind- 



borne pollen -grains can readily 



enter. A more or less extensive 



cavity (POLLEN -CHAMBER, Fig. 



539) has by this time been formed 



at the apex of the nucellus, while 



the disintegrated cells, together 



perhaps with fluid excreted from 



the surrounding cells of the 



nucellus, have given rise to a 



sticky mass which fills the micro- 



pylar canal and forms a drop at FlG - ssr.-Upper end of the pollen-tube of z 



, ,, r . floridana, showing the vegetative proth 



its entrance. The pollen-grams 

 reach this drop and, with the 

 gradual drying up of the fluid, 

 are drawn through the micropylar 

 canal into the pollen-chamber. 



During the development of 

 the pollen-tube (Fig. 510) and the formation of the motile spermato- 

 zoids, the embryo-sac filled with the prothallial tissue is increasing in 

 size within the nucellus. As in the Coniferae the embryo-sac arises 

 by the tetrad division of an embryo-sac mother cell which usually 



floridana, showing the vegetative prothallial 

 cell (<), the sterile sister cell (s), and the two 

 spermatozoids. o, Before movement of the 

 spermatozoids has commenced ; b, after the 

 beginning of ciliary motion ; the prothallial cell 

 is broken down and the separation of the two 

 spermatozoids is taking place, (x circa 75. After 

 H. J. WEBBER.) 



FIG. 538. Zamia floridana. Mature, free- 

 swimming spermatozoid. (x 150. After 

 H. J. WEBBER.) 



FIG. 539. Longitudinal section of 

 a young macrosporangium of 

 Ginkgo biloba. m, Micropyle ; i, 

 integument ; p, pollen-chamber ; 

 e, embryo-sac ; w, outgrowth of 

 sporophyll. (x 35. After 

 COULTER and CHAMBERLAIN*.) 



crushes the other sporogenous cells, as in the case of the macro- 

 sporangium of Selaginella. As it crushes the tissue of the upper 

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



2oi 



