124 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



Table 11. — Precipitation at the several statioiis. 



1 March reading missing. 



2 February reading missing. 



study. Nor is a knowledge of the annual precipitation for the year 

 during which the crops were grown of great worth. The precipitation 

 for the period of January to July 1919, together with the variations 

 from the normal, are of greater interest. However, as pointed out by 

 Chilcott and Cole (1917), the uncertainty of the distribution of rainfall 

 rather than the total average quantity received is the factor that makes 

 crop production over the Great Plains hazardous. Shantz (1911:29) 

 emphasizes the fact that the average annual rainfall alone gives almost 

 no idea of the conditions favorable or unfavorable for crop production, 

 for crop failures sometimes occur during the years with the greatest 

 annual rainfall. The great amount of run-off and the high evaporation 

 rates in short-grass plains are factors to be kept constantly in mind in 

 evaluating efficient rainfall. That water-content and not soil fertility 

 is the chief limiting factor in crop production in this grassland form- 

 ation should again be emphasized. In fact, because of the fertility 

 of the soil, cultivated crops on short-grass land usually make such 

 an excellent growth in spring and early summer that the very rapidity 

 and luxuriance of growth exhaust the water-supply all the more 

 rapidly during the drier intervals of summer, and thus accentuate 

 the drought. 



In order to compare the relative development of cereal crop plants 

 somewhat more directly, the averages of growth of tops and roots are 



