[Reprinted from the American Journal of Botany, X: 187-202, April, 1923.] 



EVOLUTION AND GEOGRAPHICAL DISTRIBUTION OF THE 

 GENUS VERNONIA IN NORTH AMERICA 



Henry Allan Gleason 

 (Received for publication July 13, 1922) 



The genus Yernonia, 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 Yernonia into its many 

 sections and subsections, relate chie.'y 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 

 nati : rlichen 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. anthelmintic a (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 



187 



