CHAPTER XVI 



COMPARATIVE ANATOMY OF THE GYMNOSPERMS AND 

 THEIR ALLIES * 



The skeletal structure of vascular plants has in the past 

 been used for phylogenetic purposes to a much smaller extent 

 than that of the higher animals. During recent years, however, 

 important advances in our knowledge of the anatomy of fossil 

 plants have made it apparent that the primary fihrovascular 

 skeleton of the vascular plants is even more conservative than 

 their reproductive organs, and consequently of great impor- 

 tance in arriving at the relationships of the larger groups. The 

 most extreme ecological conditions, acting for long periods, seem 

 to have little effect in modifying the essential features of the 

 primary fihrovascular framework, so that, for example, the 

 extremely xerophytie cactus and the hydrophytic water-lily 

 have exactly the same type of skeleton from the standpoint of 

 comparative anatomy. It sometimes happens, however, that 

 the woody framework is extremely complex in the adult. Re- 

 cent investigations which cover the whole field of living vascular 

 plants make it clear that the study of the development of the 

 sporeling or seedling provides a satisfactory key to the inter- 

 pretation of the most intricate skeletal structures of maturity. 



A brief account of certain general results of recent anatom- 

 ical and developmental research in the case of the vascular 

 plants is accordingly necessary for an understanding of those 

 skeletal features of the Gymnosperms and their allies which 

 are of phylogenetic importance. 



PTERIDOrilYTES 



The simplest type of stem in the Pteridophytes is that in 

 which there is present a single pithless fihrovascular conductive 



♦Contributed by Professor Edward 0. Jeffrey, of Harvard University. 

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