36 PLANT SUCCESSION AND CROP PRODUCTION 



in the centers even the rough, hilly land produces a fair yield. 

 Then it will be noticed that more varieties of a given plant are 

 known. For example it is at the Cornell Station that work in select- 

 ing and improving timothy can best be carried on, because of the 

 ready access to timothy variations. At the Minnesota station the 

 collection of wheat varieties and improved strains of wheat is very 

 large. The same is true for corn at the Ohio, Nebraska, and Illi- 

 nois Stations. 



There is another analogy still to be drawn. In the endemic 

 vegetation the center of distribution, that is, where a species domi- 

 nates, is not necessarily the center of dispersal, the place from 

 which the species is spreading. Rather it is implied that in the 

 center the limiting factors are fewest, and least effective. As one 

 recedes from a center the limiting factors increase in number and 

 in intensity. In the crop plants it can be shown that the regions of 

 greatest yield per acre — which correspond to the centers of dis- 

 persal of the vegetation — are frequently separated from the regions 

 of maximum production. For instance in the case of wheat, the 

 yields are higher in the humid section where the rainfall is eight- 

 tenths or more of the evaporation called for. Yet the maximum 

 production of wheat centers in the region where the rainfall is only 

 six-tenths or less of the evaporation called for. An attempt to cor- 

 relate crop centers and vegetation centers considering only the 

 climatic and edaphic factors would present many strange anomalies. 

 These explanations alone would not support the facts of the dis- 

 tribution. It is necessary to go outside the field of physiology to 

 discover the basis of crop distribution. 



The fundamental difference in the occurrence of plants belong- 

 ing to the natural vegetation and the crop plants lies in culture. 

 In the vegetation invaders from another center are frequently re- 

 stricted to one habitat, often a poor one. Arbor vitae in central 

 Ohio is confined to the rock terraces of stream gorges and to bogs. 

 The white pine and the scrub pine meet on rock cliffs east of 

 the glacial boundary. On Lake Michigan sand dunes the jack Pine 

 and the cactus meet. Yucca extends northward along the sand plains 

 of the Atlantic seaboard. In the better habitats competition with 

 the plants belonging to the center would be too keen for the in- 

 vaders. With the crop plants the case is almost the reverse. When 

 they are beyond their climatic optima they are generally to be 

 found on the richest lands of the farms. In New York and eastern 



