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W. A. CANNON 
ation of the results as factors in the development of root types and in 
the distribution of the species. 
Temperature of the Soil 
The soil acts as a reservoir of heat due to the circumstance that 
there is usually less loss by radiation during the night than accumulates 
in the soil by day. It is, thus, a great temperature stabilizer. It is 
owing to these two facts that, to a large degree, soil temperatures are 
so important to the growth of the roots of plants, and, also, to plant 
growth in general. 
As is well known the temperature of the soil varies considerably, 
especially with the depth beneath the surface. In the colder seasons, 
however, the superficial soil may register a lower temperature than 
the deeper soil. The amplitude of the daily and the seasonal vari- 
ations in temperature, also, varies inversely as the depth. From the 
last condition it happens therefore that such root-systems as extend 
close to the surface are subject to maximum temperature changes, and, 
in this particular, their temperature relations form a marked contrast 
to the relations of the roots which lie deeply. 
Among the factors which directly affect the soil temperature are 
its color, moisture content, and slant of the surface with respect to 
the position of the sun. Of these, the most important is the moisture 
content, for the reason that water has a specific heat about five times 
greater than the specific heat of the solid constituents of the soil.^ In 
a later paragraph will be given a striking example of the lowering of 
soil temperature as a result of moistening by the summer rains. (See 
also, fig. 2.) 
Although no extended studies have been made in the vicinity of 
the Desert Laboratory on exposure as a direct cause of temperature 
variation in soils it is clear that this factor is an important one and 
its importance is emphasized by the fact that the soil of the desert is 
poorly protected by vegetation. That the principle is of much sig- 
nificance can be easily shown. For example, having given a shaft of 
heat rays of a certain total energy E, and of a certain area A, the 
amount of energy impinging upon the surface of the soil will amount 
to E if the surface is of A extent, and this occurs when the surface lies 
at right angles to the incident rays. If the surface is at another angle 
than 90° the area is greater than A, and, for that reason, the amount 
1 Free, E. E., Studies in soil physics. Plant World 14: 188. 191 1. 
