CHAPTER VIII 



GNETALES 



This group includes three genera which dififer remarkably in habit 

 and geographical distribution. Their great diversity and scattered 

 distribution suggest the existence of older and more widely distributed 

 forms, but no such history has been discovered. They are notable 

 among gymnosperms for the presence of certain angiospermous 

 features, and for this reason they have been regarded often as a tran- 

 sition group between gymnosperms and angiosperms. The two most 

 important resemblances to angiosperms are the occurrence of true 

 vessels in the secondary wood, and the elimination of archegonia in 

 Welwitschia^ and Gnetum. The presence of a "perianth" has also 

 been cited as an angiospermous character, but it is not to be dis- 

 tinguished from the bracts associated with ovules and stamens in 

 other gymnosperms, as in Torreya. 



The common characters that hold these three genera together in a 

 single group are the compound strobili (both staminate and o^allate) 

 resembling inflorescences in which the simple and axillary strobili 

 resemble flowers, the conspicuous micropylar tube, the opposite leaves, 

 the dicotyledonous embryo, the true vessels, and the absence of resin 

 canals. No one of these characters is peculiar to Gnetales among 

 gymnosperms, excepting the true vessels and the compound staminate 

 strobilus; but the combination of characters makes the group very 

 distinct. 



The genera are Ephedra, with about thirty species growing in 

 the arid regions about the Mediterranean, and in tropical to tem- 

 perate Asia, North America, and South America; the monotypic 

 Welwitschia {Tumboa), in an extremely arid and restricted region of 

 western South Africa; and Gnetum, with about fifteen species growing 

 in the tropical forests of America, Asia, and Africa. 



The earlier knowledge of the group was included in the accounts 

 published by Hooker (i), Eichler (2), Strasburger (3, 5), and 



I The Brussels Congress (May igio) included Welwitschia among the genera con- 

 senanda; therefore the name is used in this volume instead of the older Tumboa. 



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