2l6 
W. A. CANNON 
8.1° C, in January, to 34.0° C, in July, and the mean minima temper- 
atures for the same months were 3.9° C. and 30.8° C, respectively. 
The mean maxima for 1910, at a depth of 30 cm., range between 
12.2° C, in January, and 33.0° C, in July. During June, July, 
August and September, the mean maxima at this depth did not fall 
under 32.2° C. The mean minima temperatures, of midwinter and 
midsummer, were 10.0° C, and 32.2° C. 
At a depth of 2.6 m. the mean maxima temperatures, in 1910, 
ranged between 16.6° C, in January, and 27.2° C, in July. 
When we compare the mean maxima temperatures for the three 
depths we notice that, from April to August, inclusive, the shallowest 
soil is also the w^armest. We see, also, that in September the 30 cm. 
horizon has the highest mean temperature, and, finally, that during 
the winter months the highest temperature occurred in the deepest soil. 
The relation of the mean maxima temperatures for the three depths 
is given, month by month, in figure 2. 
In the preceding paragraphs the actual soil temperatures have been 
considered, but another viewpoint, also instructive, can be had by 
integrating the temperatures of the records used above. 
The results of such temperature summation, of all soil temperatures 
above 10° C, are presented graphically in figure 3. It will appear at 
once that the amount of heat in the soil, at a depth of 30 cm. beginning 
with April, was greater than the amount at the 15 cm. horizon, although 
the latter provided the higher maxima. The maximum amount of 
heat, at the 15-30 cm. depths, was received in June, while the maximum 
amount at a depth of 2.6 m. was not received until August. The 
sudden drop in temperature at the shallower depths seen in July is 
associated with the occurrence of the summer rains which began at 
that time. However, the penetration of the moisture did not serve 
wholly to arrest the upward temperature movement at the depth of 
2.6 m. until the following month. 
In figure 3 is given, also, as horizontal lines, integrations of tem- 
peratures averaging 20.0° C. and 30.0° C. They show, among other 
things, what portion of the entire year has soil temperatures averaging 
more than 20.0° C. and 30.0° C, and at what depths. In a general 
way, also, the two lines delimit the seasons of root growth, and to a 
degree shoot growth, also, of the shallowly rooted and the deeply 
rooted plants. Thus, vegetative activity can be seen in Prosopis 
velutina Wooton in March-April, while shoot growth in Fouquieria 
