CHAPTER X 

 LINES OF EVOLUTION 



A prolonged comparative study of any group would 

 probably bring an investigator to some conclusions with 

 regard to the evolution of structures and relationships. 

 In the cycads, where a structure like the sporophyll can 

 be- traced not only through the living group but through 

 Mesozoic and Paleozoic predecessors, mistakes in judg- 

 ment are not so likely to occur as in the case of groups 

 known only through their living representatives. The 

 real tendency of evolution is most reliably recognized 

 when the development of a structure within a living 

 group can be compared with the same structure in fossil 

 groups, and where the same organ can be recognized in 

 related plants of various geological horizons, a reliable 

 interpretation of the organ in living forms becomes quite 

 possible. 



One of the most satisfactory evolutionary series was 

 described in the beginning of chapter v, deahng with the 

 evolution of the compact cone from a loose crown of 

 sporophylls. The most primitive sporophylls most 

 nearly resemble the foliage leaves, and from this point 

 there is a shortening of the sporophyll, a reduction of 

 the leaflets, and a thickening of the midrib region, until 

 the series closes with a sporophyll which is little more 

 than a thickened, short-stalked expansion bearing two 

 ovules. With respect to this single character it is not 

 hard to arrange the genera in order, and taxonomic 

 keys are based largely upon this series. Cycas is first, 



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