Rainfall and the Crops 41 



Critical Periods Of Growth 



If the rhythmical changes in rainfall are to give the 

 clue to the changes in the yield of the crops, the varia- 

 tions in the rainfall must be closely related with the 

 variations in the yield of the crops. But different crops 

 have different times of planting and of harvesting, 

 different periods of growth, and different requirements 

 of moisture at the various stages of growth. The direct 

 way to find whether the course of rainfall determines 

 the course of the varying yield of the crops is first to 

 ascertain the critical season for every crop ; and then to 

 compare the course of the yield of each crop with the 

 course of the rainfall of its critical season. 



The method of discovering the critical period of a 

 crop may be illustrated in the treatment of corn. In 

 Table II of the Appendix to this chapter, the mean * 

 monthly rainfall for Illinois is tabulated for seven 

 months, March, April, May, June, July, August, and 

 September. Table I of the Appendix records the data 



Bureau of Statistics of the United States Department of Agriculture 

 and from recent Yearbooks of the United States Department of 

 Agriculture. 



x The raw data were taken from Bulletin W of the Weather 

 Bureau of the United States and from the Annual Reports of the 

 Chief of the Weather Bureau. The stations used in computing the 

 mean monthly rainfall were, in Northern Illinois: Aurora, Cam- 

 bridge, Chicago, Tiskilwa, Galva, Kishwaukee, Ottawa, Winnebago 

 and Henry. In Central Illinois: Charleston, Carlinville, Coatsburg, 

 Decatur, Griggsville, Knoxville, Havana, LaHarpe, Pana, Peoria 

 and Springfield. In Southern Illinois: Cairo, Cobden, Carlyle, 

 Golconda, Flora, Greenville, McLeansboro, Mascoutah, Mt. 

 Carmel and Palestine. 



