1921] BLISS—VESSEL IN SEED PLANTS 317 
the vessels, both the lateral wall and the perforations, are obviously 
scalariform. This situation has recently been emphasized by 
Brown (2) as showing the derivation of the vessel of Angiosperms 
from the scalariform tracheid. The condition found in the 
primitive regions of Liriodendron, however, particularly the primary 
wood of the root, by no means seems to justify this conclusion. 
Fig. 17 shows a radial section in the region of the primary wood of 
the root of Liriodendron Tulipifera. All three vessels are char- 
acterized by pitted lateral walls and not by scalariform sculpture. 
A higher magnification of a portion of the vessel seen at the extreme 
right is shown in fig. 18. The center of the vessel exhibits scalari- 
form perforations. It is equally clear that this scalariform perfora- 
tion passes gradually into pits both above and below. Fig. 19 
shows the perforation above and the lateral wall of the vessel below, 
with the gradual transitions from pits to perforations particularly 
clear. The perforations show the so-called ‘‘ghosts” of former 
pits, which become more and more pitlike as one passes downward, 
until the typical bordered pits become clearly recognizable. Still 
another illustration of the same phenomenon is seen in fig. 20. It 
is clear from these illustrations that in Liriodendron Tulipifera 
there is a gradual transition from pits to scalariform perforations 
in the vessels of the primary wood. 
It is evident that the perforations of the vessels in Filicales and 
Gnetales represent the most advanced condition of the vessel wall. 
The obvious interpretation, as seen in Liriodendron, is that the side 
wall represents the primitive pitted condition, while the perforation 
has been derived from the fusion of pits precisely as in Gnetum and 
Welwitschia. The seriation of events cannot possibly be regarded 
as reversible, for if that were the case, we should have to regard 
the perforation as representing the primitive condition of the vessel 
wall, a position which is untenable. 
Magnolia Frazeri has a greater tendency to scalariform perfora- 
tions and lateral pitting than the other species of the genus which 
have been observed. As in Liriodendron, the most interesting 
condition here is found in the region of the primary wood of the 
root. Fig. 21 shows a radial section through this region. The 
perforation in the center gradually grades into pits above and below, 
