506 



BOTANY 



PART II 



e 







m 





l^^^i 



^^) 









Fio. 464. — Formation of spermatozoids in Zamia floridana. a, Mature pollen-grain (x 800); 

 V, vegetative prothallial cell— tlie dark streak at its base indicates the position of another 

 cruslied cell ; k, nucleus of the pollen-tube ; s^i, sperniatogenous cell, b, c, d, stages in the 

 development of the antheridium (h, c x 400 ; d x 200) ; v, persisting vegetative cell growing 

 into the sterile sister-cell of the antheridium (s) ; m, mother-cell of the antheridium, i.e. mother- 

 cell of the spermatozoids; t^exine. In the motlier-cell the large hi epharoplasts(W) which form the 

 cilia are visible ; in h and c they are star-shaped, while in d they are composed of small granules, 

 which will form the cilium-forming spiral band. Starch-grains are present in the pollen-tube, 

 and in c they are appearing in the vegetative cell and the sister-cell, both of which in d are 

 packed with starch. In d the two spermatozoids (sp) derived from the mother-cell are seen 

 divided from one another by a wall. (After H. J. Webber.) 



In other genera a separation of the fonr rows 

 of cells takes place, and each bears a young embryo. 

 As a rule, however, only a single embryo continues 

 its development in each macrospore although 

 several archegonia may have been fertilised. The 

 embryo consists of a main root or radicle directed 

 «f.-^..-. • •■ ■• •■.• '■■■■:-j!\ towards the micropyle, an axis, the hypocotyl. 



Ki. 40.5. — Longitudinal section of 

 a young macrosporangium of 

 Ginkgo hiloba. irt, Micrciijyle ; i, 

 integument ; fi, poUen-cliamber ; 

 e, embryo-sac ; w, outgrowth of 

 sporoj)hyll. The possession of a 

 well - developed pollen - cliamber 

 points to the relationshij) with 

 the Cycadaceae. (x 35. After 

 Coulter and C'hamheri.ain.) 



Fio. 4G0. — Ginkgo hihiba. Apex of the nncell 

 pollen-tubes (p) projecting into the archegon 

 a, archegonia. (x 27. After IIikasic.) 



'- a 



us with the 

 ial chamber. 



