GNETALES 



369 



The anatomy of the Gnetales is of particular importance at 

 the present time when they have come to the front once more, 

 either as a cognate stock with the dicotyledons or as their actual 

 ancestors. The study of the internal organization of the group 

 in comparison with the dicotyledonous angiosperms reveals many 

 features of marked 

 resemblance. Both 

 are provided with 

 large rays which 

 are clearly fusion 

 products; and in 

 both the wood 

 shows conducting 

 elements belonging 

 to the category of 

 vessels. The rays 

 apparently supply 

 a very cogent argu- 

 ment for the close 

 affinity of the 

 Gnetales and the 

 angiosperms. I n 

 the case of the 

 vascular struc- 

 tures, however, it 

 is not so clear that 

 a morphological 

 identity of the ele- 

 ments present in 

 the two groups can be successfully maintained. In the dicotyledons 

 the pits present on the terminal walls of the vascular elements are 

 not larger than those which appear laterally, and the perforation 

 of the vessel takes place as the result of fusions of opposite pits, as 

 has been shown in an earlier chapter. A very different situation 

 manifests itself, as indicated above, in the genera of the Gnetales. 

 Here the vascular elements have exceedingly large pits on their 

 terminal walls, and in Ephedra these are usually without membranes, 



FIG. 265. Transverse section of a leaf bundle in 

 Welwitschia mirabilis. 



