THE ROOT-SYSTEM OF DESERT PLANTS 97 



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 sur- 

 face 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 " Boot Habits of Desert Plants " 1 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 sur- 

 face 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 de- 

 crease in . temperature of the soil, so that the soils are moist and rela- 

 tively warm while the air is moist and relatively cool. In winter, on 

 the other hand, the soils are always cooler than the air, which some- 

 times may be very warm. Under the first conditions the root absorp- 

 tion is favored, but under the latter conditions root absorption 

 is not favored — conditions which lead to a strikingly different develop- 

 ment in the two types of plants. 



*W. A. Cannon, Publication Xo. 131, Carnegie Institution of Washington, 

 1911. 



VOL. LXXXI. — 7. 



