THE EVOLUTION OF STRUCTURES 143 



predecessors of the cycads have advanced in the evolu- 

 tion of the cone. 



The Mesozoic Bennettitales, commonly called '' fossil 

 cycads," illustrate a stage in our series. A somewhat 

 diagrammatic figure, borrowed from Professor Wieland, 

 will illustrate the general condition of the Bennettitales 



Fig. 79.— Idealistic view of a primitive seed plant such as may have 

 given rise to both the fossil and the living cycads. 



of the later half of the Mesozoic (Fig. 80). The micro- 

 sporophylls, one of which is shown expanded and the 

 other recurved in the bud, are considerably modified, 

 but they still show the pinnate character of the fohage 

 leaf, which was many times the length of the much- 

 reduced sporophyll. The microsporangia are borne on 

 the under side of the leaflets of the sporophyll. This is 



