1921] MACDUFFIE—VESSELS 439 
torus. In the center of the same vessel, passing upward from the 
lower end, is one pit which retains its border but has lost the torus 
and membrane. The center of this vessel shows as many as five 
perforations which have gone a long way in evolution and have 
lost both membrane and torus. It is to be concluded that these 
are perforations highly specialized to carry on the vascular function. 
The vessel to the right has a precisely similar condition at either 
_ end, namely enlarged tracheary pits. Toward the center of this 
vessel from top or bottom are three pits in the first case, and two 
in the second which retain border and torus, but have reached 
distinctly the size characteristic of vessels. The eight more 
central pits retain a definite border, but each shows an absence of 
membrane and torus. This vessel is obviously less specialized, and 
therefore less efficient than the one previously considered; never- 
theless it shows marked progress in the line of development. To the 
extreme right are typical tracheids, once again proving that these 
two vessels occur side by side with the elements from which they 
originate. 
In Gnetum, the highest genus of Gnetales, conditions are espe- 
cially interesting. Fig. 2 shows a longitudinal section, in rather low 
magnification, taken from the medullary region. The first vessel 
to the left has pits of the tracheary type at its top. These are 
many, crowded, and retain border and membrane, there being 
usually no torus in Guetum. Toward the center the pits are some- 
what enlarged, although still retaining the border. In the region 
of the perforation of the vessel there is a group of seven pits. 
These are distinctly larger than those on the side wall of the vessel 
and larger than those of most tracheids, and their borders are still 
distinct. The most interesting feature of this group, however, is 
found in the two lowest pits whose adjacent borders have broken 
down and show well defined fusion. This is the first example of 
pit fusion to be described, and is clear and convincing. Moreover, 
this is obviously a step higher than the conditions found in Ephedra, 
and foreshadows the more extensive fusion of pits to form a single 
large opening, which is the natural outcome from such conditions. 
In the same figure to the right are two large vessels in series. The 
higher vessel shows a large perforation resulting from pit fusion. 
