46 A Study of the Vegetation of 
quite satisfactorily summed up by measuring the evaporating 
power of the air, the differences of temperature and humidity 
shown in the two habitats need be discussed only briefly. Al- 
though the plant community reacts directly upon temperature, 
humidity, and wind, and these react upon each other, the response 
of the plant is controlled by humidity. 
The temperature of the air at a height of 8 cm. above the soil 
surface as recorded by thermographs and checked by a large 
number of thermometer readings shows a mean daily range vary- 
ing from about 25° F. in April and May, to 38° F. in July and 
August. The daily maximum air temperature is reached some- 
what earlier on the north than on the south slope, and is from 
2°-3° F. lower. 
The steep hillsides are often in the shadow long before the 
setting of the sun. For long periods during the winter months 
the sun does not strike the steep north slopes, as is evidenced by 
the presence of frost, which may soon be melted elsewhere. 
A continuous record of humidity kept for more than two com- 
plete growing seasons shows that the air is often 5-10 per cent. 
drier on the exposed than on the sheltered slopes. It is not un- 
common on dry slopes and during late afternoons for the humidity 
to fall to 15-20 per cent., while during the night it may rise again 
to 75 per cent. or even to 95 per cent. 
These data on aérial physical factors show conclusively that 
the plants on south slopes are subjected to much more severe 
conditions as concerns water loss than those on sheltered hillsides. 
Likewise the water supply of the former is usually less and this 
supply is depleted much earlier in the growing season. Before 
taking up the subject of soil moisture, however, other important 
differences in the soil occupying the two slopes should be noted. 
The wind has drifted much soil and humus materials from the 
exposed south slopes and deposited it on the steeper north and 
northeast leeward slopes. The difference in the mechanical com- 
position of the first foot of prairie soils on the two slopes re- 
spectively, is shown in Table IT. 
The humus content of the soils of north hillsides is greater at 
all depths to 5 feet, in some cases more than 12 per cent. greater, 
46 
