THE EVOLUTION OF STRUCTURES 



149 



leaves that their nature is established by the evidence 

 of comparative morphology rather than by their own 

 appearance (Fig. 84). 



In the series just described stress has been laid upon 

 the cone as a whole, and we believe that the evidence 

 shows conclusively that , -^ -^mpv « ■ ^^ \ i m 

 even the most compact '^^^^,fL»^^% I J^ 

 cone has been derived 

 from a crown of sporo- 

 phylls differing little, 

 or not at all, from the 

 foliage leaves. It 

 seems worth while to 

 present a series of 

 sporophylls, beginning 

 with those which show 

 unmistakable leaf char- 

 acters and ending with 

 those in which the 

 resemblance to leaves 

 is lost most completely. 

 Such a series, like most 

 evolutionary series in 

 the family, begins with 

 Cycas and ends with 

 Zamia. 



In Cycas revoluta the sporophyll shows clearly its 

 derivation from the foliage leaf (Fig. 85). It is shorter 

 than the foliage leaf, has fewer leaflets, and the midrib 

 region is broader and somewhat thickened, the appear- 

 ance suggesting that some of the broadening has been 

 due to a coalescence of the lower portions of the 



Fig. 84. — Zamia floridana: female cone 



