13 



most uniformly greater near the top of the southeast- and south- 

 facing slopes than near the top of the opposite northwest- and north- 

 facing slopes ; usually greater in the oak uplands than in any meso- 

 phytic portion of the ravine ; and, finally, the greatest average daily 

 evaporation for any weekly period for any of the stations was in the 

 open and exposed upland, as noted for the strawberry bed. (PI. XVI, 

 Fig. 28.) 



If the differences in the evaporative power of the air at the dif- 

 ferent stations are taken as an index of the relative xerophytism and 

 mesophytism of the vegetation, the preceding summary permits the 

 following ranking of the stations, based upon their progressive xero- 

 phytism : the embryonic ravine, the narrow ravine, the foot of the 

 mesophytic slopes, near the top of the northwest- and north-facing 

 slopes, near the top of the southeast- and south-facing slopes, the 

 oak uplands, and, the most xerophytic of all, the open uplands, repre- 

 sented by the strawberry bed. In general the movement is from a 

 moderate rate of evaporation in the embryonic ravine to a lower rate 

 in the narrow portion, and then a reversion to a higher rate, which 

 gradually increases until it reaches its climax for the ravine near the 

 top of the southeast- and south-facing slopes, and the climax for all 

 of the stations in the open uplands. 



The average amount of evaporation for each of the stations, as 

 shown in Figure 28 (PI. XVI), strongly reinforces diagrammatic- 

 ally the preceding conclusion. 



The data clearly sliow that the differences in the rates of evapora- 

 tion at the various stations are sufficient to indicate that the atmos- 

 pheric conditions are effective factors in causing plant succession in a 

 ravine. 



Measurement of Soil, Moisture 



Samples of soil were taken weekly at depths of 7.5 cm. and 15 cm. 

 in the narrow ravine, station 2 ; at the foot of the most mesophytic 

 slope of the broad ravine, station 3 ; and at the foot of the gully slope, 

 station 13. Each sample was placed in an air-tight soil jar and 

 weighed when brought to the laboratory, and then placed in a drying 

 oven at a temperature of 100° C. for one week, or until it ceased to 

 lose weight. It was then weighed to determine the loss in weight 

 resulting from the removal of the soil moisture, and the amount of 

 such moisture was calculated in terms of percentage of the dry weight 

 of the soil. The results have been plotted with the weekly intervals 

 ^s abscissae and the percentages of moisture as ordinates (PI. XVI, 

 Fig. 29, and PI. XVII, Fig. 30 and 31). 



