316 BOTANICAL GAZETTE [APRIL 
While the vessels of this type in the mature secondary wood of 
the stem are also characterized by the large bordered open pores, 
a consideration of the primitive regions, such as the primary wood 
and the wood of the leaf trace, shows that the open pore has been 
derived from pit fusions. Fig. ro illustrates a radial section of the 
stem of Gnetum scandens cut directly through the primary wood. 
A protoxylem element with its spiral markings is visible just at 
the left of the vessel. At the lower part of the figure the pits are 
quite distinct, suggesting the Ephedra type, but in the center the 
fusion is practically complete. In the leaf trace of Gnetum scandens 
(figs. 11, 13) is a condition similar to the perforation found in the 
older twig of Ephedra, showing the scalariform pits obviously the . 
result of the fusion of bordered pits. A higher magnification of a 
similar perforation is shown in fig. 12, bringing out more clearly 
the borders of the pits. Thus in Gnetum scandens we do not find 
always the fusion of pits haphazardly arranged as THOMPSON (5) 
describes for Gnetum, but fusion of pits side by side (figs. 12, 13), 4 
condition which, as shown later, is paralleled in the fusion of pits 
in the vessels of Angiosperms. 
Welwitschia shows the open bordered pore type of perforation 
in the vessel of the mature wood (fig. 14). The border of the pit 
is clearly visible in the vessel to the left, while in the vessel to the 
right the edge of the perforation faces the observer. The vessel 
of the leaf trace in Welwitschia (fig. 15) shows the more primitive 
type of vessel as found in Ephedra. Thus the monotypic genus 
Welwiischia illustrates the same general principles shown for 
Gnetum, namely, that in the vessel wall of primitive regions is 
found the Ephedra-like type of vessel with pitted perforation. It 
may be added that Welwitschia and Gnetum are usually distin- 
guished from Ephedra by absence of the torus in the bordered pit. 
Angiosperms 
Liriodendron and Magnolia may be considered as representatives 
of a possibly primitive group of Angiosperms, the Magnoliaceae. 
In the side wall of the vessel is the same general principle illustrated 
in the end wall of the Gnetales. Fig. 16 shows vessels in the mature 
secondary wood of a small twig of Liriodendron Tulipifera. Here 
