CHAPTEE IV 



GNETALES 



THIS group includes three genera which differ remarkably 

 in habit and habitat. Certain angiospermous characters which 

 they display have suggested that they may have given rise to 

 Angiosperms, but such a theory seems to have been abandoned 

 by most morphologists. The two characters of the group which 

 have been regarded as distinguishing it from other Gymno- 

 sperms are (1) the occurrence of "true vessels" in the sec- 

 ondary wood, and (2) the presence of a " perianth." A mere 

 statement of these characters is apt to lead to an exaggerated 

 conception concerning their importance. The " true vessels " 

 are usually intermediate in character between the tracheids of 

 ordinary Gymnosperms and the tracheae of Angiosperms, and 

 are as easily associated with the former as with the latter. The 

 " perianth " is a doubtful structure, and is probably no more the 

 equivalent of the perianth of Angiosperms than 'are the bracts 

 closely related to the stamens and ovules in other Gymnosperms. 



In addition to the two distinguishing characters given 

 above, the group has the four following characters in common, 

 but not peculiar to it : ( 1 ) opposite leaves ( a cyclic character 

 also displayed by the Cupresseae); (2) dicotyledonous embryos 

 (found also in Cycads and in several groups of Conifers) ; (3) 

 cauline ovules (true also of Taxaceae and perhaps of all Coni- 

 fers) ; and (4) no resin ducts (true also of Taxus). To these 

 characters perhaps should be added the remarkable prolonga- 

 tion of the integument of the ovule to form a long tubular 

 micropyle. 



The genera are Ephedra, with about thirty species, from the 

 arid regions of both hemispheres; the monotypic Tumboa 

 (Welwitschia), from the extremely arid regions of western 

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