THE EVOLUTION OF STRUCTURES 153 



gradual lessening of the number of sporangia and the 

 number of spores in the individual sporangium. There 

 are no sporophylls with leaflets Hke the microsporophylls 

 of the Mesozoic Bennettitales. 



The megasporophylls are so characteristic that an 

 artificial key based upon this single character will 

 identify the nine genera, but it would be difficult or 

 impossible to construct a usable key based upon the 

 microsporophylls. This emphasizes the fact that the 

 microsporophylls are comparatively conservative, re- 

 minding one of the rather uniform appearance of the 

 antheridia of liverworts or mosses. 



While it would be an evolutionary impossibility to 

 derive the female cone of the cycads from the female 

 portion of the cone of any known member of the Mesozoic 

 Bennettitales, it is perfectly easy to derive the male 

 cone of the cycads from such a loose crown of micro- 

 sporophylls as Wieland has described for Cycadeoidea, 

 the most completely known member of the upper 

 Mesozoic Bennettitales. 



THE EVOLUTION OF THE FEMALE GAMETOPHYTE 



The female gametophyte presents considerable uni- 

 formity throughout the group, and at present we do not 

 see any evolutionary tendencies which would prompt 

 us to claim that one genus is more primitive than another 

 in this feature, except that Microcycas has advanced far 

 beyond all the other genera. 



In all cases there is a well-developed megaspore mem- 

 brane, a survival of the thick spore coat of some extinct 

 fern ancestor which gave rise to the less ancient Cycado- 

 fihcales of the Paleozoic. The persistent retention of 



