15 



at either of these two stations than at the foot of the gully slope, but 

 further examine our data. What is really important in soil-moisture 

 comparisons is the relative amount of growth water, or the differ- 

 ence between the actual water content of the soil and the water con- 

 tent not available for growth in the plant. The average percent- 

 ages of moisture at a depth of 7.5 cm. for broad ravine, narrow ra- 

 vine, and gully slope were, in the order named, 28.3, 24.1, and 17.7. 

 The respective wilting coefficients for the same were 18, 15.1, and 

 10.2. This indicates that the average percentage of growth water in 

 the broad ravine was 10.3, in the narrow ravine 9, and for the foot 

 of the gully slope 7.5. Thus, from the standpoint of soil moisture, 

 supplies were slightly more abundant in the broad than in the narrow 

 ravine, and least abundant at the foot of the gully slope. 



If the wilting coefficient may be regarded as indicating a condi- 

 tion in which the water supply of plants is reduced to such a degree 

 as to cause permanent wilting, a recovery from which is only possible 

 by an increase of the soil moisture, the graphs clearly show that at 

 no time did the plants wilt permanently from the lack of water in 

 the soil. One of the plants that wilted regularly during midday on 

 sunshiny days was the jewel weed. This must not be attributed to 

 the lack of water in the soil, but to a more rapid rate of evaporation 

 from the portion of the plant above ground — a more rapid rate than 

 that at which the water was taken in by the root of the plant. Such 

 wilting would not be permanent wilting, which would eventually 

 mean the death of the plant, but a temporary wilting, which would 

 mean the restoration of the turgidity of the plant with a decrease of 

 evaporation such as is caused by the coming of nightfall. 



4. These data show in general so generous a supply of soil mois- 

 ture at the various stations within the ravine that it may be safely 

 assumed that the soil moisture is a factor contributing to the rapid 

 advance of succession in such situations, and should have consider- 

 ation in explaining the early attainment of advanced mesophytism. 



5. These data also show that while the variations in soil mois- 

 ture are not great at the different stations, the differences that do 

 exist correspond in a general way to those of evaporation and may 

 indicate a contributing factor in causing plant succession in a ravine. 

 In order to make these soil-moisture studies more convincing, weekly 

 examinations should have been made at the other fourteen stations. 



Conclusion 



The close relationship of evaporation and soil moisture to plant 

 succession in a ravine is clearly evident from this study. It would 

 have been more convincing if it had extended over a longer period; 



