CLIMATIC RELATIONS 271 



ably a more important reason is that the sum total of 

 heat during the period of growth, above the minimum 

 temperature requisite for cereal development, is greater 

 in proportion to the total for the year in the north than in 

 the south. In the case of winter cereals, the shorter 

 period toward the south is caused by both longer days in 

 the winter, and greater angle of the sun's rays with the 

 earth's surface, and also by the shorter winters, thus per- 

 mitting a greater number of days with temperatures above 

 the minimum required for growth. 



High altitudes also shorten the period of growth of 

 spring grains. Altitude increases the length of days 

 slightly, and the light intensity is not lessened, in fact 

 may be slightly greater the higher the altitude. Other 

 local and minor factors affect the period of growth to a 

 less degree, such as precipitation and soil conditions. 

 Cereals of the Great Plains mature quicker as a rule than 

 in humid localities in the same latitude. Sandy soils are 

 often said to be adapted to early crops (see 270). 



287. Seasonal variations in this country. An approx- 

 imate general idea of the varying lengths of cereal crop 

 seasons can now be obtained from an exhaustive collec- 

 tion of information recently made by Covert (1912) of 

 the Bureau of Crop Estimates covering that portion of 

 the United States westward from the Atlantic to and 

 including the States of the Plains. The information is 

 not exact, as the dates given are those of seeding and 

 harvesting and not the more definite ones of emergence 

 and ripening. Based upon the data contained, charts 

 are presented below showing the decrease in length of the 

 period in days between seeding and harvesting of spring 

 sown oats going northward, and the decrease in length 

 of the same period for winter wheat going southward 



