THE EVOLUTION OF STRUCTURES 145 



probable that some missing links would have been found. 

 As it is, the female cone of those Bennettitales which 

 have been described could not have given rise to the 

 cones of any of the living cycads. As far as the living 

 cycads are concerned, the Bennettitales of the later 

 Mesozoic must be regarded as a Hne which ends blindly, 

 becoming extinct without leaving any progeny. 



However, the Bennettitales illustrate some features 

 in the evolution of the cone. The male and female 

 sporophylls are borne upon the same plant and are close 

 together upon the same axis, a condition which is 

 undoubtedly more primitive than that shown by the 

 living cycads, in which the male and female structures 

 are borne upon different plants. The male sporophylls 

 of the Bennettitales doubtless represent a stage which 

 existed in the ancestors of the living cycads. 



The female cone. — The female cone of the living 

 cycads is easily derived from the condition kn(!)wn to 

 exist in some of the Cycadofilicales and represented very 

 diagrammatically in Fig. 79. This condition differs 

 little from that shown by the female sporophylls of 

 Cycas, and it is as certain as anything can be in matters 

 of phylogeny that Cycas has persistently retained this 

 primitive type of female sporophyll which characterized 

 the most ancient seed plants. 



In Cycas the female sporophylls are really a crown 

 of modified leaves bearing seeds on their margins, 

 usually only on the margins of the lower part of the 

 sporophylls. The apex of the stem does not become 

 converted into sporophylls, but grows, producing crowns 

 of foliage leaves and sometimes crowns of sporophylls. 

 The general appearance of this crown of sporophylls 



