28 



PLANT HABITS AND HABITATS IN THE 



In the preceding summary of temperature differences characteristic 

 of different depths of soil, actual temperatures only were considered; 

 but another point of view, which is instructive, can be held by a summa- 

 tion or integration of the temperatures, month by month, for different 

 depths. Reference is here made to a depth of 15 cm. and 30 cm. and, 

 in addition, that of 2.6 meters is included for comparison. As a 

 whole, it appears that there is a greater amount of heat at 30 cm. than 

 at 15 cm., although the latter has the higher maximum. And it ap- 

 pears that the rains of summer cause a sharp fall in the total heat, 

 but that in the rainless early autumn the total, if not the monthly 

 maximum, temperatures recover and the final drop in heat comes only 

 with mid-autumn. The relative amount of heat at a depth of 15 cm. 

 and at a depth of 30 cm. is surprisingly close. It is only with con- 

 siderably greater depth that a marked falling off in the total heat is to 

 be found. Finally, it appears that the total amount of heat is greatest 

 during January at a depth of 2.6 meters. 



It will be noted that the soil depths above used in the studies on 

 temperature were relatively great. Higher temperatures are known 

 to occur at less depths. Thus, Coville and MacDougal (1903:41) 

 report a temperature of 111° F. (43.89° C.) in volcanic sand and alluvial 

 deposit at a depth of 5 cm. and cite Toumey to the effect that "the 

 temperature of the soil at the depth of one inch near Tucson reaches 

 the temperature of 113° F. (45.0° C.) with a mean average of 104.9° F. 



Fio. 9o. — Mean temperature of Australia for January, after Hunt. 



