INDEPENDENT EVOLUTION OF VESSELS IN GNETALES 
AND ANGIOSPERMS 
W. P. THOMPSON 
(WITH ELEVEN FIGURES) 
The possession of vessels by both angiosperms and Gnetales is 
perhaps the strongest argument, both of those botanists who 
believe that the angiosperms have been derived from Gnetales, 
and of those who maintain that the two groups have descended 
from a common ancestor. It has therefore received much emphasis 
in all discussions of the origin of angiosperms and of the affinities 
of the Gnetales. The emphasis which it has received, however, is 
out of all proportion to the actual study of the vessels themselves. 
In a systematic study of the anatomy of the Gnetales (4) which 
the writer is carrying on, overwhelming evidence has accumulated 
that, although the completed vessels of the two groups bear a 
remarkable resemblance to each other, nevertheless their mode of 
development and their actual origin have been quite distinct in the 
two groups. In other words, we have in the case of these vessels 
another of the baffling examples of parallel development. 
Evolution of Gnetalean vessel 
The typical vessel of Ephedra, the most primitive of the Gne- 
tales, is characterized by the occurrence on its end wall of several or 
many large bordered pits which lack the middle lamella and in which 
the bordering area is narrow. The end of such a vessel is shown in 
radial section in fig. x and in tangential section in fig. 2. The 
figures show that this type of vessel differs from the familiar 
angiospermic type in having several small bordered perforations in 
place of the single large one of the higher type. Boopre and 
WorsbeELt (1), and the writer (4), have shown how this Ephedra 
type of vessel has been evolved from the ordinary tracheid of the 
coniferous type. The changes involve (1) the enlargement of the 
whole element, (2) the enlargement of several of the bordered pits 
83] [Botanical Gazette, vol. 65 — 
