214 HOOKER ON ABCTIC PLANTS. 



" Boreali-Americana," Ledebour's " Flora Rossica," which in- 

 cludes the Sitcha plants ; the American floras of Nuttall, Pursh, 

 Torrey, Gray, &c., and to the collections of Drs. Lyall and Wood 

 formed in Vancouver's Island and British Columbia : for the 

 Californian, Mexican, and Cordillera floras generally, to the 

 Herbarium at Kew, the works above mentioned, and the various 

 memoirs of Torrey and of Gray oh the plants of the American 

 Surveying Expedition. 



4. Arctic East America (exclusive of Greenland). — 



This tract of land is analogous to the Arctic- Asiatic in many 

 respects, of position and climate, but is very much richer in 

 species. It extends from the estuary of the Mackenzie River to 

 Baffin's Bay, and its flora diflers from that of the western part of 

 the continent, both in the characters mentioned in the notice of 

 that province, and in possessing more East- American species. 

 The western boundary of this province is an artificial one ; the 

 eastern is very natural, both botanically and geographically, for 

 Baffin's Bay and Davis' Strait (unlike Behring Strait) have very 

 deep water and different floras on their opposite shores. The 

 Arctic Circle is crossed in the longitude of the Mackenzie River- 

 by the isotherm of 12°, which thence trends south-eastward to the 

 middle of Hudson's Bay; and in longitude of Davis' Strait it is 

 crossed by the isotherm of 18|°. The June isotherm of 41° 

 descends obliquely from the shores of the Arctic Sea, near the 

 mouths of the Mackenzie, to the northern parts of Hudson's Bay, 

 south of the Arctic Circle, and the September isothenn of 41° 

 is everywhere south of the circle. Hence the western parts of 

 this province are very much warmer than the eastern, so much so, 

 that the whole west coast and islands of Baffin's Bay lie north of 

 a southern inflection of the June isotherm of 32°, which passes 

 north of all the other polar islands. The Parry Islands have an 

 analogous temperature of 40°. The warmth of the western portion 

 of this tract is no doubt mainly due to the influence of the Pacific 

 Ocean being felt across the continent of West America, though 

 possibly also to the presence of a comparatively warm polar 

 ocean, or to Atlantic currents crossing the Pole between Nova 

 Zembla and Spitzbergen, of which nothing certain is known.* Be 

 this as it may, the comparative luxuriance of the flora of Melville 

 Island is a well-known fact, and one inexplicable by considerations 

 of temperature, if unaccompanied by a humid atmosphere. The 

 whole region is of course far north of the isotherm of 32°, which, 

 in the longitude of its middle district, descends to Lake Winnipeg 

 in lat. 52°. 



That portion of this province which is richest in plants is the 

 tract which intervenes between the Copper mine and Mackenzie 

 River ; east of this vegetation rapidly diminishes, as also to the 

 northwards. The flora of the Boothian Peninsula, surrounded 

 as it is with glacial straits, and placed centrically among the 



* It is a well-known fact that the temperature always rises rapidly with the 

 north (as well as other) winds over all this Arctic-American area. 



