CHAPTER IX 



THE EVOLUTION OF STRUCTURES 



Nearly every feature of the cycads shows enough 

 range in development to make it worth a study from the 

 standpoint of evolution, but we shall consider only a few 

 which seem to be particularly suggestive. 



THE EVOLUTION OF THE CONE 



Even the living cycads afford an excellent opportunity 

 to study the evolution of the cone; but, taken together 

 with an encouraging amount of information with regard 

 to the Paleozoic Cycadofilicales and the Mesozoic Ben- 

 nettitales, it is possible to state with considerable con- 

 fidence how the most compact cone of the cycads has 

 been evolved from a group of fernlike leaves bearing 

 scarcely any resemblance to a cone. 



The ferns of the Paleozoic, like those of today, bore 

 their sporangia on the back or on the margin of the leaf. 

 In some cases the leaves bearing reproductive structures 

 looked just like the ordinary foliage leaves, and their 

 vegetative functions were probably not very much cur- 

 tailed; in other cases the leaves with sporangia were 

 considerably smaller than the fohage leaves and probably 

 died soon after the spores had been shed. Both types 

 are common in the living ferns. The unmodified foliage 

 type is undoubtedly the more primitive and has given 

 rise to the other, which, through various modifications, 

 has given rise to the cone. 



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