ROOT GROWTH AND DEVELOPMENT 
At the Desert Laboratory there have been kept for several years 
three thermographic series giving the temperature of the soil. These 
series relate to three depths, namely, 15 cm., 30 cm., and 2.6 m.^ 
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Fig. 2. Mean maxima soil temperatures, for different depths, at the Desert 
Laboratory, 19 10. 15cm, ( ); 30 cm. ( ); ca. 2.6 m. ( ). 
Since the soil temperatures obtained from the records have been dis- 
cussed by several writers dealing with the botanical conditions of the 
region, only so much of them as will be necessary for illustrating the 
points brought out in this paper will be utilized. For this purpose 
the records covering a representative year, 1910, have been selected. 
We will consider, in the first place, the temperature of the soil at 
a depth of 15 cm. In 1910 the mean maxima at this depth ran from 
It will be seen that in the 24 minutes' duration of the experiment the energy, 
as measured by the black bulb thermometer, changed little. By assuming the read- 
ings of the 90° angle as a basis, we see that the relative values are as follows: 90°, 
100 per cent.; 60°, 81.3 per cent.; 30°, 50 per cent., which are close to the theoret- 
ical percentages as given in figure i. 
2 The thermographic records for the depth of 2.6 m. are of the constant tempera- 
ture chamber, but it is assumed that they represent, sufficiently well for the present 
needs, the temperature of the soil at the depth given. 
