CHAPTER XXV 



THE METAGYMNOSPERMAE: GNETALES 



The aggregation of forms included in the present chapter is 

 very small, but is of great importance from the phylogenetic 

 standpoint. The Gnetales are represented in the existing flora 

 of our earth by three genera. Of these the genus Ephedra occurs 

 throughout the Northern Hemisphere, being somewhat abundant 

 in the American and Asiatic continents and rare in Europe. Gnetum 

 is a characteristically vinelike form occurring in the eastern and 

 western tropics. The third genus, Welwitschia, is monotypic 

 and is confined to the southwestern region of the African continent. 

 Of the three generic types enumerated, Ephedra must be consid- 

 ered on account of both its reproductive and its vegetative features 

 as on the whole the most primitive, although naturally its desert 

 habitat has exerted more or less influence on its internal organiza- 

 tion. Gnetum in general represents the highest stage of develop- 

 ment attained in the group, and this statement is particularly 

 true of the features presented by the organization of its gameto- 

 phytes. Anatomically, however, Gnetum seems to occupy a 

 somewhat less specialized position than does the extremely xero- 

 phytic Welwitschia. The South African genus just named in its 

 gametophytic organization is in a general way intermediate 

 between Ephedra and Gnetum. The consideration of the repro- 

 ductive structures proper, using that designation to cover both 

 the floral organization and the gametophytes, need not be cov- 

 ered more than incidentally in the present volume, since the 

 morphology of the parts related to sex has recently been fully 

 discussed in Coulter and Chamberlain's Morphology of Gymno- 

 sperms. 



A transverse section of a stem of Ephedra (Fig. 255) reveals an 

 anatomical situation not found in any of the Coniferales. Here the 

 woody cylinder is characterized by rays which become very broad in 

 the outer region of the wood but are quite narrow in the vicinity 



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