SEED PLANTS 173 



THE GNETACE^E 



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The last order of the Gymnosperms, the Gnetaceae, 

 are forms familiar only to the botanist, the only exam- 

 ples occurring in the United States being a few species 

 of Ephedra in the deserts of the Southwest. The other 

 two genera are strictly tropical. It is a question how 

 closely the three genera are related, as they differ very 

 much from one another, as well as from the other Gym- 

 nosperms. Some of them show certain analogies with 

 the Dicotyledons, and they are sometimes regarded as 

 forms connecting the Gymnosperms with the latter. 

 Their development is not known with sufficient com- 

 pleteness, however, to make this at all certain, and the 

 few fossil remains attributed to this order are much too 

 imperfect to throw much light upon their geological 

 history. 



FOSSIL CONIFERS 



Most of the living genera of Conifers are also found 

 fossil, and some of them which are now restricted to a 

 very limited area were evidently much more widespread 

 in earlier geological times. None of the living genera 

 can be traced with certainty further back than the earlier 

 Mesozoic rocks, although a number of fossils from the 

 coal measures have been doubtfully assigned to existing 

 genera. In the later Mesozoic and early Tertiary rocks, 

 however, there are abundant evidences of the existence 

 of many living genera, or, in a few instances, even spe- 

 cies. A notable case is that of the genus Taxodium, with 

 two existing species in the southeastern United States 

 and Mexico. Of these, the common bald cypress of the 



