EXPERIMENTS WITH DUEUM WHEAT. 29 



The climate is classed generally as semiarid. The average annual 

 precipitation over most of this area varies from 14 to 20 inches. 

 Only in the extreme southern part and along the eastern border is 

 this quantity exceeded. In general; the annual rainfall decreases 

 from east to west across the Great Plains with the increase in alti- 

 tude. Figure 11 shows the distribution of precipitation in the Great 

 Plains area of the United States by means of lines drawn through 

 points having the same average precipitation. 



Crop production is governed largely by the quantity and distri- 

 bution of the annual rainfall. Half to two-thirds of it comes during 

 the growing season, the four months from April to July, inclusive. 

 Wind movement is fairly constant, and summer temperatures are 

 rather high. These factors cause rapid loss of water by evaporation. 

 Both hot winds and hail sometimes occur, though hot winds are not 

 known at the higher altitudes. Wet weather in midsummer is con- 

 ducive to rust infection, which frequently occurs locally and some- 

 times extends to a general epidemic throughout the area. 



The length of the growing season varies with the altitude and 

 latitude. The frost-free period decreases from about 130 days in 

 the lower portions to about 90 days at the higher altitudes. In 

 general, early varieties have an advantage in escaping hot winds, 

 drought, and frost. A brief popular classification and description 

 of the important groups of hard spring wheat and the leading varie- 

 ties in each group is already available (Ball and Clark, 1915 b). 

 Plants, spikes, and kernels of representative varieties of the chief 

 groups of wheat grown in this area are shown in figures 4, 5, 6, 7, 

 and 9. 



Experiments conducted at 15 stations in the Great Plains area 

 are discussed in the following pages. The name of each station, the 

 altitude in feet above sea level, the normal or average rainfall, and 

 the period of years on which it is based, together with the soil type 

 on which each is located, are all shown in Table II (p. 15). Figure 

 12 is an outline map of this area, showing the location of each sta- 

 tion and, by means of graduated columns, the average yield of the 

 best variety of durum wheat and of the best variety of each group 

 of common wheat during the years indicated. Thirteen of these 

 stations are in the United States and two are in the Dominion of 

 Canada. Durum wheat has been under test at the two Canadian 

 stations since 1895. None of the experiments in the United States 

 began earlier than 1902, as most of the stations were established 

 later than that date. For the most part they are substations of the 

 State agricultural experiment station or independent field stations 

 of the United States Department of Agriculture. 



