EVOLUTION AND GEOGRAPHICAL DISTRIBUTION OF THE 
GENUS VERNONIA IN NORTH AMERICA 
Henry Allan Gleason 
(Received for publication July 13, 1922) 
The genus Vernonia, with its vast assemblage of over 500 species, ranges 
through the western hemisphere from Argentina to Manitoba, occupying 
a region of great climatic variation and habitats of great ecological diversity. 
In preparing manuscript for the treatment of the genus in the “ North 
American Flora,” 123 species have been recognized north of Colombia and 
Trinidad. Within this number a few stand comparatively isolated from all 
the others, while many are so closely related in form and structure and so 
similar in distribution that they must be closely akin genetically. Over 
30 species-groups may be distinguished in this way. Within these minor 
groups evidences of specific evolution correlated with geographic distribu¬ 
tion are frequently seen, while most of the groups, considered each as a 
whole, present strong evidence in favor of their relation to, and probable 
origin from, each other. It is therefore possible to build up a general 
scheme of evolution and migration within the genus in which the two lines 
of evidence, structural and geographic, complement and support each other. 
There can be little doubt that the ancestral home of the genus, as far 
as North American species are concerned, is tropical South America. This 
is shown by the presence there of a large number of species of greater struc¬ 
tural diversity than exist on the North American continent, and also by the 
fact that many South American species are of a structure which clearly 
indicates their primitive nature. 
Within the genus as a whole, the more fundamental structural differ¬ 
ences, which have been used in the division of Vernonia into its many 
sections and subsections, relate chiefly to the structure of the achenes, the 
pappus, and the involucral scales. Nothing can now be said concerning 
the possible evolution of these groups. Of those distinguished by Bentham 
and Hooker in “Genera Plantarum ” and accepted by Hofman in “Die 
natiirlichen Pflanzenfamilien, ” four have reached North America. 
1. The section Stengelia, of the East Indies, with veiny, foliaceous in¬ 
volucral scales, is represented by a single species, V. anthelmintica (L.) 
Willd., sparingly introduced into a few islands of the West Indies. Certain 
Mexican species which bear a superficial resemblance to this section appear 
to belong rather to the section Lepidaploa, and their similarity to Stengelia 
is better explained by convergent evolution. 
2. The section Tephrodes, of the paleotropical region, with terete achenes, 
is represented by a single species, V. cinerea (L.) Less., widely introduced 
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