CROP INDICATORS. 259 



gracilis are the dominants. Cotton reaches its maximum in a well-marked 

 region which corresponds with the southern forest, except in central Texas 

 and Oklahoma. Under irrigation it promises to develop a secondary center 

 for long-staple varieties in the desert scrub climax of the Southwest. Of 

 the other types of crops, vegetables are more or less evenly distributed over 

 the eastern half of the country, with marked regional differentiation for cer- 

 tain kinds and many local foci. Fruits and nuts show a similar uniform dis- 

 tribution in the East, but they are almost wholly confined to the forested 

 region and its extension into the southeastern prairies. This correlation is 

 wholly to be expected on the basis of similarity in life-form. The most im- 

 portant fruit districts of the West he in the sagebrush and chaparral climaxes 

 and depend upon irrigation, as the difference in the life-forms indicates. 



Climatic indicators of kinds of crops.— The correlation of the kind of crop 

 with indicator communities has already been touched upon. It is often less 

 definite than with types of crops, but there are a number of correspondences 

 of much interest and value. These are perhaps best shown by the three kinds 

 of wheat, namely, winter, spring, and durum. Winter wheat has its center 

 m the true prairies of Kansas and Nebraska, in which Andropogon plays an 

 important part. Spring wheat and durum reach their best development in 

 the mixed prairies or in the northern portion of the true prairies, where Stipa 

 spartea and Agropyrum glaucum are especially important. They are more or 

 less equal m value in the eastern portion of the true prairies, but durum shows 

 an increasing advantage to the west, and is superior to spring wheat practi- 

 cally throughout the mixed prairies. In the bunch-grass prairies of the North- 

 west the advantage is reversed, and spring wheat outyields durum. The 

 general use of summer tillage in connection with the winter precipitation favors 

 winter wheat because of its earlier period of growth. 



The legion of the maximum production of barley comprises the northern 

 half of the true prairies, while that of oats includes the major portion of both 

 the subclimax and true prairies. Flax finds its maximum in the transition 

 from the true to the mixed prairies, but it is extending more and more into 

 the latter in western North Dakota. While there is a marked correlation 

 between the sorghums as a group and the short-grass plains and their transi- 

 on to the tall-grasses, the various kinds of sorghums show no clear correla- 

 tions with indicator communities. This is perhaps due in some measure to 

 the relatively short period of trial, but probably results chiefly from the fact 

 that qualities of earliness and dwarfness are more significant than the group 

 differences (Ball, 1911; Ball and Rothgeb, 1918:88). In contrast with the 

 gram-sorghums, the sorgos show an increasing correlation with the tall- 

 grasses, and in western Nebraska and the Dakotas are to be related to the 

 mixed prairies. 



Climatic indicators of varieties.— The significant correlation of indicator 

 plants with varieties is naturally more difficult and less satisfactory than in 



e case of types and kinds of crops. This is largely because the differences 

 between varieties can be modified or reversed by seasonal variations or cul- 

 tural methods, as well as by the complex of local conditions. It is also due 

 i part to the fact that the minor differences in indicator communities arising 

 rom varying grouping of the dominants and from changes in the subdomi- 



