44 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.Xl. 



is here restricted to the Atlantic flora. This divides Gelsemiumv^ith East- 

 <erii Asia, Spigelia and Polypremutn with tropical America, Mitreola with 

 both. 



Gentianace^. — The Gentians, generally most numerous in mount- 

 ain districts, preponderate in our western floras ; yet the Atlantic States 

 do not lack species. The amj^higsean genus Urythrea is finely repre- 

 sented in the Pacific flora and in the Texano-Arizonian region, sparingly 

 in the Eocky Mountain region, while in the Atlantic States there is 

 probably no indigenous species north and east of Arkansas. Microcala 

 has probably reached California from South America. Menyantlies tri- 

 foliata is all around the northern part of the temperate zone. M. crista- 

 galli is one of the few plants which the Pacific flora shares with that of 

 Jai)an. The two species of Limnantliemum are strictly Atlantic, and are 

 connected with tropical species of the eastern side of the continent. 

 Salenia is at the same time a high-northern and an Andean genus. 

 Swertia is absent from the Atlantic side of the continent. As to the 

 peculiarly American genera, the finest is Frasera, with one Atlantic spe- 

 cies, and a few others both of the Eocky Mountains and of the Pacific 

 side of the continent. JEustoma reaches from the Texano-Arizonian 

 region just within our border over to the eastern border of the iDlains. 

 Sahhatia, of thirteen species, Bartonia, and Oholaria are wholly peculiar 

 to the Atlantic flora. 



POLEMONIACEJE. — Although not wholly absent from Europe and 

 IsTorthern Asia, compose a truly characteristic American order, and, al- 

 though half the genera are Mexican and South American, at least nine- 

 tenths of the species must belong to the United States. Of these, an 

 equally large proportion adorn the western regions, whether the mount- 

 ains, the valleys, or the x)lains, under the various forms of Gilia, Gol- 

 lomia, and Phlox. Yet, to the Atlantic States belong the herbaceous 

 l^erenuial species of the latter genus, which have longest been known to 

 botanists. 



Hydrophyllace^.— This is more strictly an American, and even 

 more predominantly a Western ]!forth American, order than the pre- 

 ceding. A very large part of our species and forms inhabit the Eocky 

 Mountain region, chiefly its plains and valleys, and fewest the At- 

 lantic region. Ko genus is restricted to the latter, though hito it only 

 extends the southern Hydrolea; into the lower parts of the intermediate 

 regions extends the mainly Texano -Mexican genus JSfama ; to it belongs 

 Conanthus, Tricardia, and essentiallj Lemmonia ; to it and to the Pacific 

 flora belong Emmcnantlie, Hesperocliiron^ Eriodictyon ; to the Pacific 

 flora alone belong Draperia and Roinanzoffia. 



BoEEAGiNACEJE. — This is a larger order, and is found all over the 

 world. The tribes or suborders other than the Borragece hardly come 

 at all within our limits, excepting two or three species of Heliotropium^ 

 one of which is very characteristic of the plains east of the Eocky Mount- 

 ains (\iz, S. convolvulaGeum, the Euplnca of ynttall. and it ^^^s- also 



