562 



BOTANY 



PART II 



(a) Cyeadeae 



In the Cyeadeae and in Ginkgo these male cells still have the 

 form of spermatozoids, and thus connect directly with the hetero- 

 p sporous Archegoniatae. Their mode of development 

 is shown for Zamia in Fig. 536. The description 

 of the figure deals with the details. As is further 

 shown in Fig. 537 (a) the two spermatozoids remain 

 for a time back to back attached to the sister cell of 

 FIG 535.-Poiien.grain the ant heridium ; after their separation (b) they round 



of Ginkgo biloba still . . .,,/ J . ,, 



within the micro- " the anterior end being provided with a spirally- 

 sporangium. ( x arranged crown of cilia by means of which they 

 are capable of independent movement (Fig. 538). 



The female cones of Zamia bear numerous sporo- 

 phylls, the hexagonal shield-shaped terminal expansions (Fig. 583) of 

 which fit closely together. Each sporophyll bears a pair of macro- 



300. After STRAS- 



BURGER.) 



FIG. 536. Formation of spermatozoids in Zamia floridaiia. A, Mature pollen -grain (x 800); 

 v, vegetative prothallial cell the dark streak at its base indicates the position of another 

 crushed cell ; fc, nucleus of the pollen-tube ; sp, spertnatogenous cell. B, C, D, Stages in the 

 development of the antheridium (B, 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; e,exine. Inthemother cell the large blepharoplasts (W) which form the 

 cilia are visible ; in B 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.) 



sporangia. The macrosporangium consists of the nucellus and an 

 integument. The micropyle forms an open canal above the tip of 



