30 THE ECOLOGICAL RELATIONS OF ROOTS. 



The profound effect of a growth of hazel upon modifying prairie con- 

 ditions is well illustrated by a series of evaporation and soil-moisture 

 readings obtained on a flat hilltop in the edge of a hazel thicket and a 

 few meters beyond in the prairie. The evaporating power of the air 

 measured during the growing season of 1916 and 1917 was about 50 

 per cent less in the Corylus thicket. On the other hand, the water- 

 content of the soil, as in the case of the other shrub communities, was 

 found to be somewhat higher. 



During 1917 another series of evaporation readings and soil-moisture 

 determinations was made at Peru in the loess soil. The atmometers 

 gave an average daily loss of 20.9 c.c. in a prairie on a southeast slope, 

 while in a Conjlus thicket on a northwest slope the loss was only 14.4 

 c.c. daily. This ratio was maintained almost throughout the summer 

 and at no time were the losses in the shrub as great as those in the 

 prairie. That the water-content of the soil in the shrub was usually 

 much higher than that in the prairie may readily be seen by an exami- 

 nation of tables 4 and 5 on pages 21 and 22. 



