iVo. l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 27 



facta- (chiefly Opuntia and McmiUariw), increasing southward ; a thick- 



rooted perennial Cttcurhita (pcrcnnis), with some relatives sont li west- 

 ward : the species o\' Macharanthera, or biennial Asters; Aplopappw 

 spinulostts and some other species; Bigelovia and Gutierrezia in charac- 

 teristic forms which are shared with the ultramontane arid district, 



and a great development of Senecionoid Oomposit®, perhaps not exceed- 

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

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

 Pentstemon in speeies equaled only by California; Hedeoma and Mo- 

 narda ; Leucoorinum, 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, inCruciferae; 

 Cristdtella. in Capparidaceai ; Musenium, Polytcenia, and Trepocarpus^in 

 UmbelliteraB ; Thclesperma (except for a Buenos Ayrean species), Engel- 

 SMNMtto, Bradburia, Diaperia, &c, among Composite; Stephanomeria, 

 Lygodesnid 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 parts — 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 



REGION FLORAS. 



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

 era and species of the North 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 \ 

 tation) and of Texas. Leaving out of view the Texaiio-Arizonian region, 

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

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

 <>r even into eastern, middle, and western, districts. The same LB 

 . in a ditVerent way. in the country north of the United States 



boundary, as has been already explained. 

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



Atlantic State- between the dull of St. Lawrence and the Gulf of 

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



