64 



THE ECOLOGICAL RELATIONS OF ROOTS. 



varies from 15 to 43 per cent, the greater run-off naturally occurring 

 during heavier rains. This combination of a meager amount of 

 precipitation with a relatively low degree of penetration would lead 

 one to think the soil must be quite dry. Weekly soil-moisture deter- 

 minations at various depths for the summer of 1918 are shown in table 

 15, which gives the available water-content during the summer of 

 1918. The minus sign indicates that no moisture was available for 

 plant growth. 



Table 14. 



Time. 



Jan. . 



Feb. 

 Mar, 

 Apr. 

 May 

 June 

 July, 



Amount. 



0.20 

 0.32 

 0.75 

 1.55 

 2.43 

 1.88 

 2.81 



Time. 



Aug 



Sept 



Oct 



Nov 



Dec 



Annual . . 



Amount. 



2.20 

 1.17 



0.67 

 0.35 

 0.25 



14.58 



An examination of these data shows that, aside from variations in 

 the surface foot, the soil to a depth of 7 feet was rather uniformly dry. 

 How much more deeply the rain may penetrate during a series of 

 wet years was not determined. Certainly, available moisture must 

 occur at least locally to depths reached by living roots of Psoralea^ 

 Argemone, and other species which penetrate to 10 or 12 feet. Only a 



Table 15. 



Date. 



Depth 

 to 6 

 inches. 

 Wilting 

 coeffi- 

 cient 7.9. 



Depth 

 6 to 12 

 inches. 

 Wilting 

 coeffi- 

 cient 8.4. 



Depth 

 1 to 2 

 feet. 

 Wilting 

 coeffi- 

 cient 4.9. 



Depth 

 2 to 3 

 feet. 

 Wilting 

 coeffi- 

 cient 5.1. 



Depth 

 3 to 4 

 feet. 

 Wilting 

 coeffi- 

 cient 6.5. 



Depth 

 4 to 5 

 feet. 

 Wilting 

 coeffi- 

 cient 6.8. 



Depth 

 5 to 6 

 feet. 

 Wilting 

 coeffi- 

 cient 8.7. 



Depth 

 6 to 7 

 feet. 

 Wilting 

 coeffi- 

 cient 7.0. 





0.6 

 -1.0 

 -0.5 

 -3.5 

 -2.8 



5.0 

 -2.0 

 -4.7 

 -4.0 

 -3.4 



-0.5 

 -0.5 

 -1.0 

 -3.3 

 -2.4 

 0.8 

 0.0 

 -3.2 

 -4.0 

 -3.0 



1.8 

 3.5 

 3.2 

 1.2 



-0.6 

 1.1 

 1.8 

 0.4 



-0.4 



0.3 

 3.9 



-0.8 

 2.0 









June 14 



0.7 



-0.4 



0.3 



June 26 



July 1 













July 8 













July 15 



2.5 



1.1 



1.6 



1.9 



4.4 



July 29 



Aug. 5 













Aug. 12 



-1.0 



-0.8 



-0.6 



0.5 



1.8 



Aug. 19 

















long series of soil-moisture determinations, coupled with a dynamic 

 study of root growth through at least one wet and one dry cycle, can 

 answer this question of moisture penetration and its correlation with 

 root development. 



In the short-grass community at Akron, Shantz found that for a 

 period of several years little or no water was available for plant growth 



