POLAR PLANTS. 281 



the prairies of Oregon, and those of the Missouri and Sas- 

 katchewan, than there is between the latter and the eastern 

 parts of the United States and Canada. In still more 

 southern latitudes the case may be different ; and Ehrenbero- 

 has found a totally different group of Infusoria in California 

 from that which exists on the east side of the continent ; 

 the Rocky Mountains, in his opinion, proving a complete 

 barrier to these organisms. 



The families of Polar plants which are most rich in species 

 are the Cruciferce, Graminece, Saxifragece, Caryophyllece, 

 and Composite. Of these the Saxifragece are most cha- 

 racteristic of extreme northern vegetation. All of them 

 that inhabit the 74th parallel in America are found also in 

 Spitzbergen, Lapland, or Siberia ; and even the polar 

 species are twice as numerous as those which exist in the 

 wide district which Gray's " Flora of the Northern States " 

 comprehends. If we reckon all that enter the arctic circle, 

 we shall find them to be four times as many as those which 

 Dr. Gray enumerates; and we may add that the plant 

 which Humboldt traced highest on the Andes was a saxi- 

 frage. The Caryophyllece and Cruciferce, which vie with 

 the saxifrages in number on the 74th parallel, include many 

 of the doubtful local species above alluded to. Of the most 

 northern Graminece, about one half are, as far as we yet 

 know, exclusively American ; the few species which the 

 other families contain have as extensive a lateral ran^e as 

 the saxifrages. 



Arctic zone. — On descending to the main land from the 

 71st parallel down to the arctic circle, including a zone of 

 four degrees of latitude, we find that the species have in- 

 creased eight-fold in number, and there is a large addition 

 of generic forms, as might be expected on entering within 

 the limits of the forest. 



