ROOT DEVELOPMENT OF CROP-PLANT ECADS. 



141 



The evaporating power of the air, while not measured in the crop 

 plats, (where indeed the crops in each plat would probably have caused 

 a different evaporation rate) was recorded in adjoining areas of upland 

 and lowland prairie (p. 37). Figure 12 shows that evaporation on the 

 upland, due largely to greater wind movement, is constantly higher. 

 During the driest period of the summer (July 7 to August 18), the 

 evaporating power of the air on the upland was 64 per cent greater. 

 This difference in evaporation, which must greatly accelerate transpi- 

 ration, together with a somewhat lower water-content of the subsoil, go 

 far towards explaining these differences in crop development. An in- 

 spection of table 19 shows that, while no constant difference exists in 



1 The soil had been no drier at any time since May 5. 



2 Surface soil had deep cracks, no rain had fallen for about two weeks. 



relative wetness or dryness of the soils in the first 3 feet, below this 

 depth the subsoil at the lowland station has a constantly higher water- 

 content. The samples were taken, as usual, with a Briggs soil- tube, in 

 the plats of spring wheat, timothy, and bluegrass which grew poorly 

 (p. 126). Care was taken to secure samples for the different determi- 

 nations from similar plats in the two sites. In the plats with normal 

 plant-growth it is not improbable that the water-content was lower and 

 thus differences in the water relation accentuated. Judging relative 

 temperatures in the two areas from data obtained from adjacent low- 

 land and upland sites (p. 38, and plates 4, b, and 6) it is believed that only 

 small differences in either soil or air temperature occurred. We may 

 conclude then, at least tentatively (for work must be carried on for 

 several years before generalization may be made), that differences in 

 root development in the two areas are due to the water-content of soil 



