1918] THOMPSON—GNETALES AND ANGIOSPERMS 85 
border may disappear, however, in the old wood of large trees. 
In spite of the great differences between the typical vessels of 
Gneium and those of Ephedra, a comparative study 
of the conservative regions of many species of 
the former has shown that the Gnetum type 
undoubtedly has been derived from the Ephedra 
type and has revealed the course of its evolution. 
In such regions of Gnetum the Ephedra type of 
vessel is of common occurrence, as has been noted 
by Durutr (2) and the writer (5). Such a vessel 
from the young root of a seedling of G. Gnemon is 
shown in fig. 6. Another vessel is shown in tan- 
Fic. 5.—Typical 
vessel of Gnetum. 
gential view in fig. 7. In this vessel even the 
il 
relationship to bordered pits is shown. While 
Gnetum naturally does not show the transitions to tracheids as 
well as Ephedra, nevertheless intermediate conditions may easily 
OO "i 
OS || 2 
O) 
8 
Fics. 6, 7.—Fig. 6, vessel 
from root of seedling of Gnetum 
Gnemon: note that it is of the 
type characteristic of E phedra; 
fig. 7, tangential section of 
vessel similar to that shown in 
fig. 6. 
be found. In some species of Gnetum 
the type of vessel characteristic of 
Ephedra is much more common than in 
others, and within the same species it is 
more common in certain conservative 
regions and certain individual specimens 
than in others. 
The way in which the Gnetum type 
of vessel has been evolved from the 
Ephedra type is easily observed in such 
regions and is illustrated in fig. 8. The 
changes involve the further enlargement 
of the individual perforations and the 
disappearance of the portions of the wall 
between them. In this way the several 
perforations fuse in a single large one. 
In fig. 8a three of the perforations near 
the top have fused into a common open- 
ing, although parts of their original out- 
lines are still distinct. Near the bottom 
two pits have fused in similar fashion. At two points (one near 
the top and the other near the middle) it may be observed that 
