278 ESSAYS. 
under present opportunities ; and there is reason for thinking 
that all the prairies east of the Mississippi, and of the Mis- 
souri up to Minnesota, have been either greatly extended or 
were even made treeless under Indian occupation and annual 
burnings. These prairies are flowery with a good number of 
characteristic plants, many of them evidently derived from the 
plains farther west. At this season, the predominant vege- 
tation is of Composite, especially of Asters and Solidagoes, 
and of Sunflowers, Silphiums, and other Helianthoid Com- 
posite. 
The drier and barer plains beyond, clothed with the short 
Buffalo-grasses, probably never bore trees in their present 
state, except as now some Cottonwoods (i. e. Poplars) on the 
margins of the long rivers which traverse them in their course 
from the Rocky Mountains to the Mississippi. Westward, the 
plains grow more and more saline; and Wormwoods and Che- 
nopodiacee of various sorts form the dominant vegetation, 
some of them swi generis or at least peculiar to the country, 
others identical or congenerie with those of the steppes of 
central Asia. Along with this common campestrine vegeta- 
tion there is a large infusion of peculiar American types, 
which I suppose came from the southward, and to which I will 
again refer. 
Then come the Rocky Mountains, traversing the whole con- 
tinent from north to south ; their flanks wooded, but not richly 
so, —chiefly with Pines and Firs of very few species, and with 
a single ubiquitous Poplar, their higher crests bearing a well- 
developed alpine flora. This is the arctic flora prolonged south- 
ward upon the mountains of sufficient elevation, with a certain 
admixture in the lower latitudes of types pertaining to the 
lower vicinity. 
There are almost 200 alpine Phenogamous species now 
known on the Rocky Mountains ; fully three-quarters of which 
are arctic, including Alaskan and Greenlandian ; and about 
half of them are known in Europe. Several others are north 
Asian but not European. Even in that northern portion of 
the Rocky Mountains which the Association is invited to 
visit, several alpine species novel to European botany may be 
