SECT. II 



PHANEROGAM I A 



507 



;i wlioii of from two to several coi'yledons, and the plumule or bud of the 

 future shoot. 



fX 



I 



Fig. 407. — Upper end of the pollen-tube of Zamia 

 floriJana, showing the vegetative prothallial 

 cell (r), the sterile sister-cell (s), and the two 

 spermatozoids. a, Before movement of the 

 spermatozoids has commenced ; h, 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.) 



Fio. iGS.— Zamia florldawi. Mature, free- 

 swimming spermatozoid. (x 150. After 

 H. J. Webber.) 



The development of the sexual generation iu the Cycadaceae and Ginkgo ('-') 

 differs in a number of respects from the Coni- 

 ferae. The differences indicate that the group 

 is more closely connected with the Pteri- 

 dophyta, and 'their nature Avill be evident 

 from a description of Zamia floridana, which 

 lias been fully investigated by Webber. 



The female cones of Zamia bear numerous 

 sporophylls, the hexagonal shield-shaped ter- 

 minal expansions of which fit closely together. 

 Each sporophyll bears a pair of macro- 

 sporangia. The macrosporangium consists 

 of the nucellus and an integument (cf. Fig. 

 460). The micropyle forms an open canal 

 above the tip of 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. 465, p) 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 stiekymass which 

 fills the micropylar canal and forms a drop at 

 its entrance. The pollen-grains reach this drop 

 and, with the gradual drying up of the fluid, are drawn through the micro2)ylar 

 canal into the pollen-chamber. During the development of the pollen-tube (Fig. 



Fio. 469. — Zamia floridana. 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.) 



