144 BOTANICAL GAZETTE [AUGUST 
and temperature are less effective, for as the leafing out of the 
trees proceeds, and the increased undergrowth also becomes effect- 
ive in shade and interference with air currents, the retention of the 
moisture in the air decreases the evaporation rate and the relative 
humidity is raised. It would be instructive to follow in more detail 
the effect of the various meteorological factors on evaporation. 
This effect can very well be seen if the more important factors 
like temperature, intensity and duration of light, precipitation, 
wind, soil, and vegetation are referred to individually. But the 
results are uncertain and suggest the desirability of preliminary 
investigations in artificially maintained conditions by laboratory 
methods. In a general way, however, the data show that the 
inner temperatures of the maple-alder zone are lowered and the 
temperature extremes moderated, but the extremes in summer 
temperature much more so than those of the winter. The range in 
temperature is therefore more affected than the absolute tempera- 
tures. The importance of shade producers does not consist alone 
in their effectiveness to reduce transpiration, but also in their 
inverse influence upon meteorological factors. 
The foregoing table also shows that the greater saturation 
deficiency was recorded for the station on the university campus. 
The relative evaporation in the three stations is according to the 
totals 1349.2 cc., 933.8 cc., and 690.8 cc., the corresponding 
ratios are 100, 69.2, and 51.2. These differences for the three 
stations remained fairly constant throughout. The fact that the 
evaporation rate for the central zone with its numerous xerophytes 
should be less than that for an area which supports mesophytic 
forest trees seems anomalous and surprising. Thus for the vege- 
tation on the university campus the furtherance of transpiration 
by the evaporating power of the air is during some periods approxi- 
mately two times greater than that on Cranberry Island. This 
clearly shows that the evaporating power of the air, though furnish- 
ing a very valuable criterion for the differentiation between great 
centers of plant distribution and for the differentiation of habitats, 
is not an important factor in controlling bog vegetation or deter- 
mining the character of it. 
With the data on hand, it is not difficult to see that the chief 
