The Relationships of the Different Types of 
Angiospermic Vessels. 
BY 
W. P. THOMPSON, Ph.D. 
With eleven Figures in the Text. 
I T was recently demonstrated that in some angiosperms the simple 
or ‘ porous ’ perforation of the vessel has been evolved by the loss 
of bars from the scalariform type. At the same time it was pointed out that 
the scalariform type itself may either represent a retention and slight modi¬ 
fication of the primitive scalariform pitting of the older gymnosperms and 
pteridophytes or may have been secondarily evolved by the fusion of 
circular bordered pits. Subsequently Miss Bliss ( 1 ) has called attention to 
the fact that in some cases the simple perforation seems to have been formed 
by the haphazard fusion of circular perforations derived from bordered pits 
after the manner which I had described as occurring in Gnetales ( 5 ). She 
considers that in some cases circular bordered pits have fused in horizontal 
rows to form scalariform pits which by a slight modification have become 
scalariform perforations, and these by a further fusion have become simple, 
while in other cases the circular bordered pits have fused haphazardly 
to produce simple perforations. The two methods are simply variations of 
the same fusion process. Brown ( 2 ), on the other hand, maintains that 
scalariform pits and perforations are not secondarily derived by pit fusions, 
but are retentions of the primitive scalariform pitting, and that the circular 
pits of angiosperms, as of lower plants, have been formed by the breaking 
up of scalariform pits through a reticulate condition. 
It was therefore desirable that a comprehensive survey of angiospermic 
vessels be made to determine how the vessel had been evolved in each 
group, how commonly haphazard fusions had occurred, their significance in 
the general evolution of the angiospermic vessel, and their relationship to 
the process described for Gnetales. In this survey it became clear that the 
existence of certain other types of perforations required emphasis. 
[Annals of Botany, Vol. XXXVII. No. CXLVI. April, 1923.] 
