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. 



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. 



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 \vith 

 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 



