Gymnosperms : The Cycads 



505 



the ovulate sporophylls are divided, resembhng greatly reduced 

 leaves ; in the most specialized genus (Zamia) the sporophylls 

 are scale-like. The ovulate cone thus varies from loosely aggre- 

 gated, leaf-like sporophylls each with several ovules, to tight 

 cones of scale leaves each bearing two ovules. 



The gametophyte generation. The mature microspore, or 

 pollen grain, consists of three cells, one of which forms the 

 sperms. The pollen is carried by the wind, and by chance 

 some reaches the open end and pollen chamber of the ovule. 

 There the pollen germinates, developing pollen tubes that grow 

 into the nucellus and absorb food from the adjacent cells 

 (Fig. 3 14) . After several months the pollen chamber has been en- 

 larged by the breaking down of the cells of the nucellus, and the 



C. J. Chamberlain 



Fig. 313. Large cycads (Diodn) in southern Mexico, showing staminate cones on plants 

 at left and carpellate cone at right. 



