34 PLANT SUCCESSION AND CROP PRODUCTION 



est groups of factors limiting plant distribution can be discussed 

 in such a study. The climatic, the edaphic, and biotic factors are 

 given a brief treatment with respect to the endemic vegetation. 

 Later for the crop studies, a fourth group of factors is added. The 

 rainfall-evaporation ratio, (Fig. 2) successfully employed by Tran- 

 seau ('05) in delimiting the forest centers, has been used as a basis 

 for this study. This was secured by dividing the total rainfall for 

 a given station by the evaporation obtained from Russell's (1888) 

 data on evaporation. 



The centers of vegetation of North America are as strongly 

 differentiated in the United States by the common crop plants as by 

 the native plants of the forest centers. Timothy, spring wheat, rye, 

 buckwheat, and potatoes occupy the same region as is dominated by 

 white pine, spruce, hemlock, balsam fir and white birch — in other 

 words the northeastern evergreen forest. The region occupied by 

 corn, winter wheat, oats, red clover and beans is related to both 

 the central deciduous forest and the prairies. This emphasizes the 

 importance of edaphic factors in plant successions and the relation 

 of the crop to edaphic conditions. 



For although the grain belt extends eastward into Ohio, it 

 does not reach far from the edaphic prairies. Tobacco occupies 

 a median position between the central deciduous forest and the 

 southeastern mixed forest. Cotton, yams, cowpeas, and peanuts 

 center in the southeast- 



The criteria which have been applied in delimiting the forest 

 centers hold for the crop centers as well. There is first of all 

 dominance. The evidence that one is approaching the center for 

 a given crop is the number of farms upon which it is being grown. 

 In this connection it is interesting to note that while central Illinois 

 is for very special reasons, the center of production of corn, yet 

 in point of acreage devoted to it in proportion to cultivated lands, 

 com is the most important crop of southeastern Kentucky. No 

 hills seem too steep or too high for the farmers to attempt to grow 

 a corn crop. Compared with production in Illinois, the results on 

 these hills seem far from encouraging. The production and total 

 acreage can be used in determining this center. (Fig. 3) . 



In the natural vegetation next to dominance come maximum 

 size for the species, greatest differentiation of type and widest 

 range of habitat as criteria of the biological centers. These are all 

 corollaries of general well being, the result of the limiting factors 

 being fewer and less effective. In the crop plants we can see that 



