FEMALE GAMETOPHYTE 



449 



sporangium as in Selaginella, but a new feature of the Cycads is 

 that the megasporangium does not open to allow the megaspore 

 to be exposed, and therefore the female gametophyte remains 

 permanently enclosed in the sporangium. The developing female 

 gametophyte uses most of the nucellus for food and thereby 

 makes room for itself. When the gametophyte is mature the 



m 



FIG. 399. Section through a Cycad ovule containing a mature gameto- 

 phyte. /, female gametophyte with two archegonia (a) shown; m, micro- 

 spores developing tubes, and male gametophytes; n, nucellus; i, integument; 

 p, pollen chamber into which the micropyle shown just above opens. Re- 

 drawn from Webber. 



nucellus is so nearly used up that it is reduced to a thin layer, 

 except at the micropylar end where a beak-like portion remains. 

 A female gametophyte when fully formed consists of a large num- 

 ber of cells, most of which form a nutritive tissue for the devel- 

 oping sporophyte and are therefore spoken of as endosperm, 

 although the endosperm of Cycads is not the same in origin as the 

 endosperm of Angiosperms. The archegonia, usually several in 



