86 OUTLINE OF PLANT GEOGRAPHY 



extensive steppes or deserts, really part of the great Mexican 

 plateau, and in climate and vegetation perhaps rather sub-tropical 

 than temperate. 



As a whole the climate of temperate North America is decidedly 

 continental in character, the range of temperature being large, 

 especially in the dry plains of the interior. In Montana, the follow- 

 ing extreme temperatures have been recorded, 1 viz: — 65° and 

 117° F., a range of 182°! 



The temperature of the sea-board, both Atlantic and Pacific, 

 is naturally influenced by the proximity of the ocean, but this is 

 much more marked on the Pacific coast, where the prevailing 

 westerly winds traverse the ocean, whose temperature varies but 

 little, and the high mountains to the east protect the coastal belt 

 from the effects of extreme temperature changes of the interior 

 regions. 



The whole Pacific coast has a remarkably equable climate, mild 

 winters and cool summers. The January isotherm of 0°C, which 

 on the Atlantic coast is in the neighborhood of New York and 

 Philadelphia, (lat. 40°), on the Pacific is pushed north as far as 

 Sitka, in Alaska (lat. 58°). In San Francisco there is only a differ- 

 ence cf ten degrees Fahrenheit, between the coldest and warmest 

 months (50°-60°) ; in Washington, nearly in the same latitude, 

 the difference is more than four times as great (32°-78°). The 

 differences are even greater in the interior of the country. 



In general, the climate of the eastern third of the LTnited States 

 is one of hot humid summers and cold winters. Except for a small 

 part of southern Florida, no part of the eastern states is immune 

 from occasional killing frosts; while over the greater part of the 

 area the winter is a season of absolute cessation of all plant ac- 

 tivity, and in the interior more than half the year is a dead season 

 for pretty much all vegetation. 



In the northern tier of states vegetation rarely starts before 

 April, of course becoming earlier as one proceeds southward, and 

 the coastal region has a decidedly earlier spring and more pro- 

 tracted autumn than inland stations in the same latitude. 



At the south, spring is somewhat gradual, but toward the north 

 the transition from winter to summer is much more abrupt, and 



1 Kirkwood, J. E., Forest Distribution in the Northern Rocky Mountains, Univer- 

 sity of Montana Studies, p. 49, 1922. 



