ARID PORTIONS OF SOUTH AUSTRALIA. 



29 



(40.5° C.) for the entire month of July." Even higher temperatures 

 of superficial soils have been reported (Coville and MacDougal, 

 q. v., p. 41). Therefore, at one and the same moment the roots of spe- 

 cies of deeply penetrating root-habit may, near Tucson, be subject to a 

 temperature stress of 33.1° F. (18 3° C.) or over, or more than the total 

 maximum yearly variation at a depth of 30 cm. 



Owing to the large number of factors which determine the tempera- 

 ture of the soil, it is impossible, in the absence of actual temperature- 

 measurements, to satisfactorily adjudge this important feature of the 

 physical environment of plants. Hann (1903 : 43) states that the daily 

 variations in temperature hardly extend one meter into the ground, and 

 that one observation daily at greater depths suffices to give good means. 

 Conversely, all things being equal, it should be possible to roughly 

 evaluate the mean annual temperature of average soils at a depth of 

 one meter from the air means of the latitude. At the middle and 

 higher latitudes, however, Hann states that the soil at a depth of one 

 meter has an annual mean about 1° C. above that of the air. Taking 

 the annual average temperature of the air for Australian regions along 

 135° east longitude, as given by Taylor (1918- : 4), we have, therefore, 

 an estimate of the mean annual temperatures of the soil at a depth 

 of one meter and at different latitudes. These are given in table 8, 

 adapted from Taylor. It will be seen that at the depth given and 



Fig. 96. — Mean temperature of Australia for July, after Hunt. 



