THE VESSEL IN SEED PLANTS’ 
Mary C. Biiss 
(WITH PLATES XVI-Xx) 
In view of the recent discussion of the evolution of the vessel 
in Gnetales and Angiosperms by THOMPSON (5), it was suggested 
to the writer that a comparative study of the vessel in seed plants 
would be of interest in bringing forward more evidence which 
might have weight for or against the theory that the presence of 
vessels in these two groups is an argument for genetic relation- 
ship. The discussion of scalariform pitting in the secondary wood 
of Angiosperms by Brown (2) also suggested the larger question of 
the origin of the vessel in seed plants, that is, has it been derived 
from the pitted tracheids of the more primitive plants, or is the 
scalariform vessel the more primitive, from which the pitted vessel 
has been derived ? 
By way of introduction I wish to discuss briefly the vessel of 
Pteris aquilina as an example of the lower vascular plants. The 
side wall, usually at a definite angle with the end wall, is marked 
with scalariform bordered pits (fig. 1, right). The borders are 
very clearly evident in the profile view of the vessel (fig. 4). The 
typical end wall of the vessel is characterized by scalariform 
perforations in which the border has wholly disappeared at the 
center, but is still visible at both ends of the perforation (fig. 3). 
The difference between the openings in the side and end walls is 
again very clearly seen in the profile view of the vessel (fig. 4). 
The border has practically disappeared from the perforation, 
although occasionally it is evident near the juncture of the end wall 
with the side wall. Another striking difference in the perforations _ 
of side and end walls is the reaction to stains. The end wall gives 
a definite cellulose reaction with haematoxylin, while in the side 
wall blue is present only in the middle lamella and the pit membrane, 
the border of the perforation taking a definite lignin stain with 
« Contribution from the Laboratories of Plant Morphology of Harvard University. 
Botanical Gazette, vol. 71] [314 
