Introduction 9 



well-known forest species. Thus the range of white pine 

 as a commercial crop is practically limited to a region ex- 

 tending westward from the New England States to Min- 

 nesota, while the long-leaf yellow pine is restricted to the 

 sandy coastal region of the Southern Atlantic and certain 

 Gulf States. 



A similar relation of particular crops to one or more 

 factors of the environment is strikingly brought out by 

 those crops especially that are commonly associated with 

 southern climates. Cotton has a relatively long season of 

 growth, and it is restricted in the United States to a region 

 practically below the thirty-seventh parallel,- -a region 

 which is, for about seven or eight months of the year, free 

 from frost and with a high mean temperature. Citrus 

 fruits are accustomed to an almost continuous growing 

 season, where frosts are few and severe freezing practically 

 unknown. 



A large proportion of the varieties of rice, also restricted 

 to warm regions, may not be grown beyond those sections 

 in which irrigation is possible. Hard wheats gradually 

 lose the quality of " hardness ' (high per cent of gluten) 

 when grown in moist regions. The potato is grown from 

 Canada to Texas and from Scotland to Italy. It is inter- 

 esting to note, however, that in the United States, usually, 

 the yield diminishes toward the South ; and, except under 

 special conditions, the crop matures relatively early 

 throughout the United States. Under such conditions 

 average production is but about 85 bushels per acre. 

 With intensive culture 400 bushels is a maximum for some 

 of the most productive lands in the eastern United States, 

 although 1000 bushels have been reported under peculiar 



