So far, however, it would seem that the variations in the temperature of the soil at that depth are considerable. For example, one day in the spring the variation at the depth of 30 cm. was 3° F. At the depth of 15 cm. it was 11° F., while at the depth of 2.5 cm. it was 40° F. The greatest difference in maximum temperature at any moment was on April 15, when there was a difference of 23.5° F. between the upper two levels.
From what has been said regarding the soil temperatures it will appear at once that at any moment during the daytime the roots of the desert plants are subject to a very large temperature stress. Those roots which penetrate most deeply, where probably moisture is the greatest, are in the coldest soil, while such roots as lie near the surface of the soil, where the moisture conditions are least favorable, are in the warmest soil. We therefore have the interesting paradox that roots placed where there is the most water are not so advantageously placed, physiologically speaking, as those roots where there is least water, for the reason that low temperature retards absorption. This is probably of considerable importance to perennials whose root-systems live throughout the year, but its exact effect has not been studied.
While speaking of the temperature of the soil, it may be interesting to glance briefly at the effect on the development of the root-systems of desert annuals which is brought about by a variation in the relation of the temperatures of the soil and of the air. Briefly stated, the case is as follows: In the "Root Habits of Desert Plants" the root-systems of the winter annuals are described as being easily distinguished from the root-systems of the summer annuals, because among other features the former have a more prominently developed tap root, and a poor development of laterals which are generally filamentous, or at least extremely slender. The summer annuals, on the other hand, have root-systems which resemble the generalized type, above described, of certain perennials: that is, the laterals are developed well, they are frequently rather coarse and the main root is often forked, thus the absorbing surface of the summer annuals is apparently greater than those of winter. The apparent reason for this difference is as follows: When the rains of summer come, the air temperatures fall disproportionately to the decrease in temperature of the soil, so that the soils are moist and relatively warm while the air is moist and relatively cool. In winter, on the other hand, the soils are always cooler than the air, which sometimes may be very warm. Under the first conditions the root absorption is favored, but under the latter conditions root absorption is not favored—conditions which lead to a strikingly different development in the two types of plants.
- W. A. Cannon, Publication No. 131, Carnegie Institution of Washington, 1911.