1921] BLISS—VESSEL IN SEED PLANTS 319 
that ‘“‘since the Gnetalean vessel usually has only two rows of 
circular pits, no matter how the fusions take place no scalariform 
bars can result.” It has already been seen that in Gnetwm scandens 
(figs. 12, 13) the scalariform perforations are very evidently the 
result of fusion of opposite circular pits. Further evidence of 
such progression in the evolution of the perforation is seen in the 
Betulaceae. The typical end wall of the vessel in this group is 
scalariform. Such an end wall is seen in the stem wood of Betula 
alba in radial aspect in fig. 28. In this type of scalariform end 
wall there is usually no indication of the derivation of the scalari- 
form perforation from fusion of pits, although instances of this con- 
dition are not infrequent even in the mature wood. It is the vessels 
which lie near the primary wood which are of greatest interest from 
an evolutionary standpoint. Fig. 26 shows three such vessels, 
the spiral elements of the protoxylem lying to the left of them. The 
vessel nearest the protoxylem has a perforation which is inter- 
mediate between scalariform and pitted, this condition being most 
clearly recognizable at the middle of the figure. In the vessel next 
to the right the transitions appear only at the top and bottom of 
the perforation, while in the third vessel the fusion of pits is prac- 
tically complete. Haphazard fusion is evident in the lower portion 
of the perforation. Fig. 27 illustrates another vessel taken from 
a different preparation, in which the perforation is much less 
extensive than in the preceding figure, and the derivation of the 
scalariform perforation from pit fusions at the top and bottom is 
particularly clear. 
The same conditions which have been noted for Betula are found 
also in Alnus. The typical end wall of the vessel is scalariform, as 
seen in fig. 31. Practically no indication of pit fusions is seen, but 
fig. 29 is evidence that such fusion has taken place. In the lower 
part of the figure more or less haphazard fusion is evident, while in 
the upper region two elongated pits are still distinct. A higher 
magnification of the same end wall is seen in fig. 30. It is clear 
for the Betulaceae as exemplified by Betula and Alnus, as for the 
Magnoliaceae, that the scalariform perforation as shown by primi- 
tive regions is derived from pit fusions and does not represent the 
persistence of a primitive scalariform condition of the vessel wall. 
