322 BOTANICAL GAZETTE [APRIL 
pore. Fig. 39 shows vessels from the leaf trace of Vztzs in its course 
in the stem. On the left in the region of the primary wood two 
vessels show scalariform perforations; to the right is the single open 
pore typical of the vessels of the group. An interesting reversion 
to the primitive type of perforation in this genus is seen in the radial 
section at the end of the annual ring (fig. 40). The spring vessels 
show the open pore characteristic of the mature wood, but in the 
summer wood, which is in some respects a primitive region, we have 
the reversion to the scalariform type of perforation characteristic 
of the regions of the primary wood and of the leaf trace. From 
_ the evidence given it seems clear that in the Dicotyledons as in 
the Gnetales the scalariform perforations have arisen by the fusion 
of pits, and that this fusion may be haphazard or serial. 
Conclusions 
This investigation covers the origin of the vessel in Pieris, in 
Gnetales, and in dicotyledonous Angiosperms. In Pieris the 
primitive condition of the vessel is scalariform, with the obvious 
tendency to the development of pitted sculpture in the end wall of 
the element. It is of interest to note that in the Osmundaceae and 
Ophioglossaceae the pitted as contrasted with the scalariform 
sculpture appears in the side walls of the vascular elements. 
In Gnetales the vessels have obviously been derived from pitted 
and not from scalariform tracheids. Their evolution is connected 
with the appearance of particularly large pits in the end walls of 
the vascular elements, which first lose their membranes and sub- 
sequently undergo fusions, either transversely or irregularly. As 
in Pteris, it is the perforation region of the vessel which shows the 
greatest advance and specialization. In the Dicotyledons two 
general types of vessels are found, those with scalariform perfora- 
tions and those with porous perforations. Investigation of the 
first type from the Magnoliaceae and Betulaceae appears to 
establish the fact that the perforations have arisen by pit fusions 
precisely as in the higher Gnetales. In the second type the vessel 
with porous perforation, often with bordered margins as in Gnetum, 
in most cases in the Angiosperms has had its immediate origin from 
the vessel with scalariform perforations. These in turn, as illus- 
