Apr., 1923] GLEASOX VERXOXIA IN XORTH AMERICA 197 



it is shared by virtually all the groups hitherto discussed. Here the scales 

 are relatively few in number and poorly imbricated. The inner scales are 

 progressively more exposed than the middle and outer ones, contrasting 

 plainly with the numerous regularly imbricated scales of most other species 

 of the United States. Of the four species in the group, V. texana (A. Gray) 

 Small, is best known and occurs in Texas, Louisiana, and Arkansas. 



From the area of the Texanae, migration accompanied by specific 

 evolution has proceeded in two directions, northward through the prairie 

 region and eastward along the coastal plain. In each direction one or more 

 of the primitive structures have been lost, until in Michigan and Massachu- 

 setts they have disappeared completely. 



In northern Texas occurs the group Lindheimerianae of three species, 

 two of which are suspected to be hybrids. V. Lindheimeriana Gray & 

 Engelm., which is undoubtedly a good species, retains the primitive invo- 

 lucre and narrow outer bristles of the Texanae, from which it seems to be 

 derived, and differs chiefly in its tomentose leaves and scales. 



The Fasciculatae, extending from Texas and New Mexico northward 

 and eastward to Manitoba and Ohio, retain the pitted leaves and present 

 to a still greater degree the narrow, undifferentiated outer bristles of the 

 pappus. They have lost the primitive involucre and have developed long 

 heads with numerous scales imbricated with great regularity. It is note- 

 worthy that the more southern species, as V. marginata (Torr.) Raf., still 

 show a tendency toward acumination of the scales, as in Texanae, which 

 character is lost to a large extent in V. fasciculata Michx., ranging from 

 Nebraska to Ohio, and completely in V. corymbosa Schw., distributed along 

 the Red River of the North in Minnesota, the Dakotas, and Manitoba, 

 where it marks the extreme northern limit of the genus. 



The peculiar local species, V. Lettermanni Engelm., of Arkansas and 

 adjacent Oklahoma, bears a strong superficial resemblance to V. fasciculata 

 and is possibly an evolutionary development from it. It retains the gla- 

 brous leaves with pitted lower surfaces and the congested heads with closely 

 imbricated scales, like the latter species, but has broader, well differentiated 

 outer pappus bristles. 



The group Interiores takes its name from V. interior Small, which is 

 undoubtedly the basic species. Common in central and northern Texas, 

 where it overlaps the range of the ancestral Texanae, it extends north to 

 Nebraska, and thence east to the Mississippi River. The involucre in this 

 species has only partially lost its primitive structure; the outer pappus is 

 narrow but nevertheless plainly differentiated; the leaves are broad, without 

 pits, and characterized by multilocular hairs forming a more or less tomen- 

 tose pubescence. V. Baldwini Torr. is an Ozarkian derivative, with broader 

 outer pappus bristles, and with the acuminate involucra! scales recurved 

 at the tip and pubescent on the inner face. The species is probably of 

 recent origin, and specimens from the overlapping ranges of V. Baldi^uu 



