230 POMOLOGY 



eastern margin of the Great Plains, which roughly coin- 

 cides with the 20-inch annual rainfall line and also with the 

 100th meridian, to the Atlantic Ocean, and southward nearly 

 to the Gulf of Mexico. The strip bordering on the Gulf 

 may be set apart as a subordinate district, the Gulf province. 

 The Plains province includes the Great Plains proper, and 

 extends westward to the Rocky Mountains. Between the 

 Rocky Mountains and the Sierra Nevada-Cascade ranges 

 comes the Plateau province. The Pacific slope constitutes 

 a natural climatic region which may be called the Pacific 

 province. 



''The differences between north and south, resulting from 

 differences in latitude, suggest a further subdivision of the 

 Plains, Plateau, and Pacific provinces into northern and 

 southern sections. Similarly, the Gulf province occupies 

 the more southern latitudes of the Eastern province." 



200. The Eastern province. — This extensive area is 

 characterized by great uniformity in its climatic conditions 

 and weather types. Over most of it the seasons are strongly 

 contrasted. The summers are very warm and the winters 

 cold. The rainfall is abundant or at least sufficient for agri- 

 culture and nowhere in this district is there permanent ne- 

 cessity of irrigation. There are only relatively slight and 

 unimportant differences of topography, the whole area be- 

 ing freely open from Canada on the north to the Atlantic 

 Ocean on the east, and to the Gulf of Mexico on the south. 



In January, the isotherms over the eastern United States 

 are very closely crowded together. The temperature then 

 decreases northward at the rate of 2.7° F. in each degree of 

 latitude, both on the Atlantic coast and in the Mississippi Val- 

 ley, which is an extraordinarily rapid temperature-gradient. 

 There is, however, much less difference of temperature be- 

 tween South and North in summer. It becomes 1.1° along 

 the eastern coast and 0.7° in the Mississippi Valley. 



