CLIMATIC FACTORS 59 



reduce vitality.^ For example, one section might have 

 sufficient average rainfall for a normal crop, but if this 

 rainfall so fluctuated that at one season it was excessive and 

 at another deficient, the normal crop might be reduced one- 

 half or more; while another region with no more total 

 rainfall but a better distribution would have a normal 

 crop. In the same way, a single frost out of season or a 

 hot wind might do great damage, although the average 

 temperature might appear favorable. Average annual 

 rainfall, temperature, and sunshine are not a safe guide, 

 unless the fluctuation of these factors during the growing 

 season is also known. 



47. Length of the growing season. — Corn differs 

 somewhat from other cereals in being able to adjust 

 itself to the growing season. Wheat, oats, and barley 

 grown in northern regions yield as well as when moved 

 farther south, or even better. They have a somewhat 

 longer growing season when taken south, but do not oc- 

 cupy the available period as does corn. Some North 

 Dakota varieties of corn will mature in 80 days, while 

 Gulf States varieties often take 200 days. There are 

 large corn regions with a growing season of more than 

 200 days, but it does not appear that corn has been 

 able in any region to utilize to advantage a longer 

 growing period. 



As the tropics are approached, while frosts cease to 

 limit the crop-growing season, at the same time there is 

 usually a dry period which serves as a limit. In Mexico 

 the growing season is limited in this way. 



All other factors being favorable, we may assume that 



' The effect of fluctuation of rainfall on crop production is discussed in 

 Bui. 130, Bur. Plant Indus., U. S. Dept. Agr., 41-49, in an article on 

 " Cost of Crop Production under Humid and Dry Conditions." 



