No.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 27 



tacece (chiefly Opuntiw and Mamillariw), increasing southward ; a thick- 

 rooted perennial Gucurhita (perennis), with some relatives southwest- 

 ward; the species of Machwranthera, or biennial Asters; jlplopappus 

 spinulosus and some other species ; Bigelovia and Qutierrezia in charac- 

 teristic forms which are shared with the ultramontane arid district, 

 and a great development of Senecionoid Compositae, perhaps not exceed- 

 ing the other parts of the United States, yet more conspicuous to the 

 eye; the two species of Solanum with prickly calyx closed over the fruit; 

 Pentstenion in species equaled only by California; Hedeoma and Mo- 

 narda; Leucoerinum, which, however, extends westward. 



Besides those variously mentioned, a goodly number of genera are 

 peculiar to this and the more western districts, which we need not here 

 enumerate. Of absolutely peculiar genera, there is Selenia, in Oruciferae ; 

 Gristatella, in Capparidaceae ; Museniwn, Polytcenia, and Trepocarpus, in 

 Umbelliferse ; Thelesperma (except for a Buenos Ayrean species), Engel- 

 mannia, Bradburia, Diaperia, &c., among Compositae; Steplianomeria, 

 Lygodesma and Troximon are very characteristic Cichoraceous genera, 

 which also abound far westward. 



The characteristics of the Eocky Mountain flora — whether taken as a 

 broad whole or in its constituent geographical i)arts — are in no small 

 degree negative. What this flora lacks is perhaps more remarkable 

 than what it possesses. This will appear on a comparison of the vege- 

 tation of the three great regions : the Atlantic naturally wooded region ; 

 the Central region, woodless except on mountains ; the Pacific region, 

 largely but not wholly wooded. 



II. 



COMPARISON OF THE " ATLANTIC, PACIFIC, AND ROCKY MOUNTAIN 



EEG-ION FLORAS. 



A full and critical comparison would require a tabulation of the gen- 

 era and species of the iNTorth American flora, and of their geographical 

 distribution, and this would be a large and difficult undertaking. 



Even the sketch of the principal or salient features, which we may 

 here present, it is best to confine to the central belt, along which the 

 three regions are particularly well defined, namely, to the United States 

 north of the peninsula of Florida (which has considerable tropical vege- 

 tation) and of Texas, leaving out of view the Texano-Arizoniau region, 

 which, with the adjacent parts of Mexico, has in general a vegetation 

 of its own, and is not very distinctly separable into wooded and wood- 

 less, or even into eastern, middle, and western, districts. The same is 

 ti e case, in a different way, in the country north of the United States 

 boundary, as has been already explained. 



The comparison attempted is, therefore, that of the flora of the 

 Atlantic States between the Gulf of St. Lawrence and the Gulf of 

 Mexico, on the one hand, with that of California and Oregon nnd with 



