1921] BLISS—VESSEL IN SEED PLANTS 315 
safranin. This difference separates very distinctly the side and 
end walls when viewed under the microscope. An interesting con- 
dition is seen in fig. 1, where the end wall at the center is perforated 
by a series of bordered pits which gradually grade into scalariform 
pits and perforations at the lower end of the figure. Fig. 2 shows 
an end wall which is clearly reticulated, a stage intermediate 
between the typical scalariform perforation and the more unusual 
pitted type. It is interesting to note in this connection that in the 
higher groups of the Filicales (Osmundaceae and Ophioglossaceae) 
the side wall of the vessel is likewise pitted and not scalariform as 
in most ferns. These observations as seen in figs. 1 and 2 give 
weight to the view that the end wall is the more specialized portion 
of the vessel and more advanced in its organization, and that in 
the pitted end wall of Pteris we have the forerunner of the pitted 
type of perforation characteristic of the lateral walls of the element 
in the higher groups. 
Gnetales 
Turning now to the vessels of the seed plants, we may consider 
first the vessels of Gnetales. The vessels in the young twig of 
Ephedra (fig. 5) show perforations which are clearly bordered pits. 
Although the torus has entirely disappeared and the border is very 
much reduced in nearly all of the pits, the torus is present in two 
of the pits in the upper portion of the vessel at the right. Fig. 6 
shows larger vessels of an older twig, the vessel at the right being 
more advanced than the vessel at the left. Here the perforation 
consists of a series of bordered pits in which the membrane has 
entirely disappeared. Since the end wall of the vessel is the most 
specialized, it would naturally show the greatest advance in develop- 
ment, and this is true in the higher types of Gnetales. In radial 
sections of the stem of the Gnetum Gnemon type (figs. 7, 8) all 
stages are found, from the clustered bordered pits of the Ephedra 
type to the open pore of the higher forms, and transitional fusion 
perforations are clearly seen (fig. 7). The mature stem of the 
Gnetum Gnemon type is illustrated in fig. 9, where the vessels are 
characterized by the typical porous perforations of the higher 
forms. In contrast with the Gnetum Gnemon type, Gnetum 
scandens, an advanced vine species of the genus, is interesting. 
