SEVENTH ANNUAL MEETING 63 



While there is a general agreement between the individual 

 and average curve representing soil moisture, several points of 

 variation may be accounted for by corresponding changes in 

 conditions. In figure 4 the variations noted about July 1st may 

 be ascribed to weather conditions, while in figure 5, the ripen- 

 ing and harvesting of the wheat may affect the changes noted 

 at this time since the corresponding evaporation was not high. 



Weather conditions may account for the decrease in soil 

 moistures about August 1st, since there is a corresponding in- 

 crease in evaporation. 



In figure 5 the wilting coefficient of two strata coincide. 

 This is probably due to deep cultivation rendering the soil more 

 or less homogeneous. 



As pointed out by Fuller 1 , the most significant comparisons 

 are those shown in table 1, under growth water. The mean 

 growth-water of the beech-maple forest is taken as the standard 

 and represented by 100. This furnishes a quantitative state- 

 ment of the relation of different habitats in their mesophytic 

 conditions. Another comparison of habitats may be shown by 

 the ratio between the mean weekly growth water and the 

 evaporation rate for the same time. While it may be claimed 

 that these units are not comparable, the ratio seems to give 

 a quantitative relation between the habitats which is thought 

 to be valuable. Since the growth water represents the available 

 supply for plant growth and the evaporating power of the air 

 represents the demand made by the plants on the available 

 water, it seems advisable that such a comparison be used in 

 this kind of work. 



SUMMARY 



The evaporation rate at the surface of cultivated fields is 

 somewhat parallel to that of the climax beech-maple forest 

 and shows corresponding variations due to changes in weather 

 conditions. After crops are harvested the evaporation rate is 

 greatly increased, due to the increased exposure. 



In the beech-maple forest the soil moisture is much higher 

 in the stratum due largely to the great amount of humus. In 

 this association the wilting coefficient was reached only once 

 during the season. 



In the wheat field the upper stratum shows a marked re- 

 sponse in soil moisture due to periodic rainfall. In this as- 

 sociation the soil moisture falls decidedly below the wilting 

 coefficient after the wheat is harvested. 



