[Chap. LIU THE VEGETATION OF NORTH AMERICA 763 



The southeastern pine forest has been the source of much kimber and 

 many resin products. More recently processes have been discovered by 

 which the wood of the hard pines may be used in the manufacture of 

 paper and wall board. This region is the "cotton belt," the principal 

 source not only of cotton fiber but also of many secondary cellulose 

 products, as well as cottonseed oil and its by-products. Other important 

 crops are sugar cane, certain varieties of tobacco, sovbeans, sweet 

 potatoes, peanuts, rice, early vegetables and fruits, and pecans. 



Many of both the wild and the cultivated species are limited north- 

 ward by the occurrence of extreme winter temperatures, by the length 

 of the growing season, and by the prevailing temperatures during the 

 period of growth. Many of these plants when planted farther north fail 

 to grow well or to mature their fruits and seeds. Some trees continue to 

 live northward as shrubs, the shoots of which freeze to the ground almost 

 every winter. Other wild species have been cultivated successfullv more 

 than a hundred miles north of their natural boundary. 



Grasslands (Figs. 389-394). Between the eastern forests and the Rocky 

 Mountains from Alberta to the Gulf of Mexico is a \'ast area of rolling 

 or flat land originally dominated by grasses, among which numerous 

 species of legumes, composites, and other herbs were conspicuous at the 

 time of their flowering. The rivers and streams are more or less en- 

 trenched in the plains which rise from an elevation of less than 500 feet 

 near the Mississippi to four or five thousand feet near the mountain 

 front. In presettlement days this was the grazing land of several million 

 buffaloes and antelopes; of rabbits, jack rabbits, prairie dogs, pocket 

 gophers, ground squirrels, and smaller rodents; as well as of several 

 hundred species of grasshoppers and other plant-consuming insects. 



The eastern prairie border extended as a wedge into the deciduous 

 forest in the region now called the "corn belt." Here the average annual 

 precipitation is about 35 to 40 inches; northwestward it drops to about 

 20 inches, and southwestward to 30 inches in Texas. In general, the 

 precipitation in the grasslands is more irregular than in the forested 

 regions eastward and northward. Westward the precipitation gradually 

 declines to 10 to 12 inches near the mountains. Rains are most frequent 

 in the spring months and are usually in the form of showers. Drizzling 

 rains are rare. Droughts are characteristic of late summer, and the aver- 

 age winter snowfall is light. Blizzards and deep snows, however, do occur 

 northward during occasional winters. 



Late summer droughts are often intensified by the hot winds from 



