120 Plant Physiology 



season, a relatively small precipitation say 25 to 30 

 inches may be sufficient for crop production. Early 

 maturing grains and other grasses require as little, perhaps, 

 as any other type of vegetation affording an equal yield. 



A chief cause of the annual variation in yield of many 

 staple crops is to be found in the variation in rainfall. 

 Smith has prepared charts showing a remarkable agreement 

 between yield of corn and precipitation in the corn belt 

 of the United States for the chief growing months June, 

 July, and August. In Figure 38 the dotted line gives 

 the average rainfall for the months mentioned, covering a 

 period of fifteen years, and the full line gives yield of grain 

 per acre for the same time. The data are taken from 

 Ohio, Indiana, Illinois, Iowa, Nebraska, Kansas, Missouri, 

 and Kentucky. 



The chart (Fig. 37) shows a rainfall map of the United 

 States for a period including, in the main, the growing 

 season. From this, it is apparent that the rainfall west of 

 the hundredth meridian practically to the Coast Range 

 valleys of the Pacific is less than the usual requirement, 

 and so the number of crops which may be grown in this 

 general region without irrigation is extremely limited. In 

 fact, throughout a very large portion of western North 

 America, eastern and southern Europe, northern Africa, 

 and a large part of Asia and Australia, crop production is 

 limited much more by insufficient or ill-distributed rain- 

 fall than by all other factors combined. 



It is believed that the great agricultural countries of 

 the world must be, in time, those of great area, such as 

 Australia, Brazil, China, India, Russia, and the United 

 States. In three of these, however (China, India, and 



