358 BOTANICAL GAZETTE [DECEMBER 
only the heaviest rains ever penetrate the foliage sufficiently to wet 
more than the surface of the ground. A large proportion of the 
soil surface is bare, puddling ‘easily, and thus facilitating direct 
evaporation. These, and possibly other unnoted factors, can 
account for the condition here. 
EVAPORATING POWER OF AIR (fig. 3).—The effect of the climatic 
factors that influenced soil moisture conditions is plainly evident 
in the atmometric data also. There was more rainy and cloudy 
weather during May than during June, and the rates of evaporation 
show a corresponding decline during the latter month. Then, with 
the beginning of the hot, dry weather of July the rates increased 
suddenly and enormously. There was a sudden drop after the on- 
coming of the early August rainy period. Throughout August the 
daily temperatures were markedly lower than those for July (fig. 4), 
with much cloudy weather, although actual precipitation was con- 
fined to brief local showers; with this was correlated a reduction in 
the transpiration curves to one-half or less of their July heights. 
A short heated period at the beginning of September brought an 
increase in the rate, but the ensuing cool, humid days that closed 
the period of observation brought the curve down, and the season 
came to an end with a period of very low transpiration rates. 
A feature of some interest in the set of atmometric curves is 
presented by the deep ‘‘dip” during the month of June. It was 
natural that evaporation rates should be higher during July, as 
already pointed out, but the sharp decrease from the May rates 
seems at first somewhat anomalous. One possible reason suggests 
itself in the comparative amounts of general exposure due to the 
foliation of the trees. The spring of 1921 was late and cool, and 
during May the trees, especially the oaks, were still nearly naked. 
As the foliation increased, changes in three of the four main factors 
controlling evaporation might naturally be expected; direct insola- 
tion and air movement would be reduced, and relative humidity 
probably increased. All of these changes would be in favor of 
reduced evaporation. The fourth main factor, temperature, in- 
creased somewhat during this time. This would have worked in the 
opposite direction, but presumably the operation of the other three 
had the greater effect. 
