Apr., 1923] GLEASON-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 Noveboracenses. Excepting the Stellares, all these groups 
show likewise a tendency to larger heads, reaching a maximum in the Mexi¬ 
canae, Alamanianae, and V. crinita. 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 
