BIOLOGICAL PAPERS 



135 



large percentage in ^lay, June and October, and the plentiful and 

 constant supply during the critical weeks of midsummer. Only 

 once does the supply fall below the wilting point — namely, during 

 September, at the 25 cm. level (Fig. 4). The mesophytism of 

 the soil may possibly best be expressed by noting the average 

 amount by which the actual water content exceeds the wilting 

 coefficient during the critical ten weeks beginning with July ist. 

 At the 7.5 cm. level there will be found to be an average of 18.3 

 per cent, and at 25 cm. 7.8 per cent of the dry weight of the soil 

 in water available for growth during these midsummer weeks. 

 At station three, the smaller amount of humus present in the 



Oak-hickory forest 7. 5crT1. 



25cm, 



Figure 5. Graphs representing the range of soil moisture at 7.5 cm. 

 (A) and at 25 cm. (C) below the surface in the oak-hickor>' forest, and 

 the wilting coefficients (B, D) at these depths. 



