192t] BLISS—VESSEL IN SEED PLANTS 323 
trated by Quercus and Cydonia, have been derived from the fusion 
of pitted perforations. It follows that the mode of origin of the vessel 
in the Dicotyledons and Gnetales is essentially similar, in both cases 
_ being the consequence of the fusion of open bordered pits, either in 
rows or irregularly. 
These remarks apply to the end wall of the vessel, particularly 
to the perforation. In the few cases among the Angiosperms where 
the lateral vascular walls are scalariform, it appears clear that the 
scalariform lateral pits have resulted from the horizontal fusion of 
circular or oval pits. It is further to be noted that the scalariform 
sculpture appears first in the terminal region of the vessel and may 
later appear in the side wall, always resulting from pit fusions. It 
follows that the vessel of the Angiosperms as of the Gnetales has 
been derived not from the scalariform but from the pitted tracheid. 
In this respect the vessel is in harmony with the other evolutionary 
developments in the wood, since according to the best established 
view both the mechanical fibers and the longitudinal parenchyma- 
tous elements of higher plants have been derived from the pitted 
tracheid (3). It would be surprising if the vessel, which is much 
later in geological times than longitudinal storage elements and 
mechanical elements of strength, should have originated from a 
type more primitive than the pitted tracheid. 
Summary: 
t. In Pteris the scalariform perforation of the vascular end wall 
often becomes pitted. 
2. In the Gnetum type of vessel the fusion of pits to form the 
porous perforation is haphazard, but in Gnetum scandens the 
fusion of pits is often more regular, resulting in a scalariform 
perforation. | 
3. Haphazard fusion of pits is also found in Paeonia, Cydonia, 
and Leea; while in Liriodendron, Magnolia, Paeonia, Betula, Alnus, 
Quercus, and Vitis the fusion is often serial, resulting in scalariform 
perforations. 
4. The evolution of.the perforations of the vessels in Gnetales 
and Dicotyledons is similar, and in both cases is the result of pit 
fusions. 
