Seed-Plants 143 



in many regions both of the Old and New World, 

 where it would be quite impossible for them now to 

 exist. Great changes must have taken place in the 

 climate since the time when these trees were com- 

 mon over much of the Northern Hemisphere, where 

 now they maintain only a very precarious existence 

 in a few places where unusually favorable condi- 

 tions have permitted them to survive. 



GNETALES 



Distributed through the Tropics of both the Old 

 and New World, there are found about a dozen 

 species of trees and woody climbers belonging to the 

 genus Gnetum. Some of these are lianas, climbing 

 to the tops of lofty trees. The opposite, oval, 

 pointed leaves are net-veined like those of the 

 typical dicotyledons, and the flowers, borne in 

 catkin-like spikes, may be compared to those of a 

 poplar or willow. The plant, however, is " gym- 

 nospermous " ; that is, the seeds are not contained 

 in an ovary, and in this respect they agree with the 

 conifers and cycads; but otherwise Gnetum has lit- 

 tle in common with the other gymnosperms, nor is 

 its relationship to the other genera, Ephedra and 

 Welwitschia, which are associated with it to form 

 the order, Gnetales, at all certain. Gnetum is some- 

 times held to be intermediate between gymnosperms 

 and angiosperms, but the evidence for this is by 

 no means decisive. 



