11 



and south-facing slopes than on the northwest- and north-facing 

 slopes. Furthermore, this difference tends to explain the presence of 

 tender mesophytic forms on the north-facing slopes of a ravine dur- 

 ing the early part of spring, before the development of leaves on the 

 trees, and their absence on the south-facing slopes. 



4. The graphs in Figure 21 (PI. XII) show that the rate of 

 evaporation was greater throughout the season in the oak uplands 

 than near the top of the northwest- and of the southeast-facing 

 slopes, being 2. 11 c.c, or 31.6%, greater per day for the entire season 

 in the oak uplands than near the top of the northwest- facing slope, 

 and 1. 18 c.c, or 15.5%, greater than near the top of the southeast- 

 facing slope. The graphs in Figure 26 ( PI. XV) show similarly that 

 the rate of evaporation in the oak uplands was generally greater than 

 near the top of the north- and south- facing slopes, the average daily 

 evaporation' for the entire period being 2.78 c.c, or 46.3%, greater in 

 the oak uplands than near the top of the north-facing slope, and .85 

 c.c. per day, or about 10%, greater than near the top of the south- 

 facing slope. This indicates that the upper portions of the south- 

 facing ravine slopes have an evaporation very close to that of the oak 

 uplands. Indeed one might expect a greater evaporation near the top 

 of the south-facing slopes than in the oak uplands, due to the in- 

 fluence of slope exposure. Further, the differences in the average 

 amount of evaporation per day between the oak uplands and near the 

 floor of portions of the ravine were 4.03 c.c. and 5. 11 c.c. In summary. 

 all of the preceding statements show greater evaporation in the oak 

 uplands than in any part of the mesophytic portion of the ravine. 

 These differences may account for the more xerophytic aspect of the 

 vegetation in the oak uplands than of that in the ravine (see p. 

 5). The graphs in Figure 21 (PI. XII) and Figure 26 (PI. 

 XV) further show that the rate of evaporation in the open up- 

 lands, the strawberry bed. was nearly twice as great as in the oak up- 

 lands or in the most xeroph)^tic portion of the ravine, the average 

 daily rate for the entire period near the top of the northwest- south- 

 east-, north-, and south-facing slopes, the oak uplands, and the open 

 uplands being 6.67 c.c, 7.6 c.c, 6 c.c, 7.93 c.c, 8.78 c.c, and 16.39 c.c. 

 respectively. The high rate of evaporation in open uplands clearly 

 shows the effect of shade in retarding evaporation. This gives some 

 conception of the relatively high rate of transpiration of the plants 

 that constitute the various crops as compared with the vegetation that 

 springs up in the forests, it having been clearly shown by Livingston 

 ('10) that there is a definite relation between rate of evaporation of 

 water from a free surface and the rate of transpiration in a leaf. 



