Apr., 1923] GLEASOX VERNONIA IN NORTH AMERICA 201 



tendency toward the congestion of the inflorescence by repeated branching or 

 toward its reduction by shortening the cymes. The former is most apparent 

 in the species-group Arborescentes, the latter in the Racemosae and Acu- 

 minatae. There is a great reduction in the number of flowers in the outlying 

 members of the Acuminatae, Sagraeanae, and Racemosae. Specialization 

 of the involucre by the development of broad, blunt scales occurs in the 

 Schiedeanae, and of narrow, prolonged scales in the Sagraeanae, while the 

 Buxifoliae have increased the number and the regularity of imbrication 

 of the scales. Leaves have shown a tendency to become broad and blunt 

 from the Arborescentes through the Longifoliae and into the Bahamenses, 

 or narrow, one-nerved, and revolute from the Arborescentes into the Race- 

 mosae. Montane species have been developed in the Acuminatae and the 

 Buxifoliae, and in both cases are characterized by crowded, few-flowered 

 heads and by small and broad leaves. 



Among the 63 species with bractless cymes there is less diversification 

 in structure, except in the inflorescence, which has already been discussed. 

 In the Stellares there is a gradual progression from narrow, acuminate 

 involucral scales to short and blunt ones. A similar tendency occurs among 

 the Interiores and Altissimae, and reaches a climax in the series from the 

 Umbelliformes to the Alamanianae and Mexicanae, with their highly spe- 

 cialized, broad or veiny scales. On the other hand, there is a notable ten- 

 dency toward prolongation of the scales in V. crinita and in the Glaucae, 

 Pulchellae, and Xoveboracenses. Excepting the Stellares, all these groups 

 show likewise a tendency to larger heads, reaching a maximum in the Mexi- 

 canae, Alamanianae, and V. crinila. Forms with unusually small heads 

 rarely occur, and are most characteristic of the single species^ V. gigantea. 

 Two groups only have developed montane forms, the Alamanianae and Mex- 

 icanae, and in their unusually large, many-flowered heads differ remarkably 

 from the montane forms of the West Indies. 



Neither is there any correlation between structure and habitat. The 

 variation between the montane species of the West Indies and Mexico has 

 already been mentioned. The relatively arid conditions of the Bahamas 

 are reflected in the thick, firm leaves of the Bahamenses and V. insularis, 

 but those of the former, with broad-leaved ancestors, are broad and blunt, 

 while the latter, originating from the Angustifoliae of the Gulf States, 

 preserves the linear leaves. The Racemosae, of arid situations in Cuba and 

 Hispaniola, and V. texana, likewise a xerophyte, have narrow leaves, but 

 the hydrophytic V. fasciculata has narrow leaves also, while the xerophytic 

 V. Baldwini has broad leaves. The xerophytic Bahamenses have assumed 

 the form of bushy shrubs, while V. texana has remained an herb, although 

 growing in a region where the shrub form is common. 



Three processes seem to have been concerned in the general history of the 

 genus, by which it has reached its present distribution and differentiation. 

 Physiological evolution, scarcely indicated by structure, has enabled the 



