86 VEGETATION OF A DESERT MOUNTAIN RANGE. 
highest individuals of every species—other than those of aquatic or 
streamside habitat—on or near the summits of ridges, and their invari- 
able absence from the bottoms of cafions at these higher elevations, 
are to be attributed to the absence of cold-air drainage from the ridges 
and higher slopes, together with the influences grouped in the ‘‘factor”’ 
of slope exposure. 
SOIL TEMPERATURE. 
In the autumn of 1913 instruments were placed at the 6,000-and 8,000- 
foot stations to secure the absolute winter minimum temperature of 
the soil at a depth of 3 cm., and the thermometers were maintained in 
TaBLE 19.—Minimum temperatures of the soil and of the air at 3 elevations in the 
Santa Catalina Mountains for irregular periods. 
* Air Soil i 
Station. temperature. | temperature. Difference. 
At 6,000 feet: 
Sept. 23 to Sept. 27, 1913,...... 42 47 +5 
Sept. 28, 1913, to May 16, 1914 18 28 +10 
May 17 to 19................. 53 51 -2 
May 20 to July 22 44 45 +1 
July 23 to 27... 0... cee eee eee 58 60 + 2 
July 28 to Oct. 10............. 40 42 + 2 
At 8,000 feet: 
Sept. 25, 1913 ................ 34.5 41 + 6.5 
Sept. 26, 1918................ 30.5 37 + 6.5 
Sept. 27, 1913, to May 17, 1914 5 30 +25.0 
May 18 266.3 sa as cannaaeeasanss 39.5 43 + 3.5 
May 19 to July 24............ 33.5 41 + 7.6 
AUS! 2D ia ce ons -ecb- a a.e.p npataccransannees 50.5 69 +18.5 
UG: 2 ie, seicas aise ce ca Sites Siena 48.5 58 + 9.6 
DUNS DT sistee tin alga ie hes aide Reaasiee% 51.5 57 + 5.5 
July 28 to Oct. 11............. 29.5 39 + 9.5 
At 7,600 feet: 
Sept. 25, 1918................ 33.5 39 +9 
place and read at irregular intervals during the summer of 1914. The 
object in placing the thermometers at so slight a depth was to obtain 
a measure of the activity of terrestrial radiation by a comparison of the 
superficial minima of the soil and the atmospheric minima. The ordi- 
nary type of Six’s thermometer was used, buried in a wooden box and 
covered with earth. The readings secured in this manner and the 
readings of atmospheric minima for the corresponding periods at the 
same stations are given in table 19. 
At the 6,000-foot station the soil minima are higher than the air 
minima in every case except one, the over-winter difference being 10°. 
At the 8,000-foot station all of the 9 readings secured show a higher 
minimum for the soil. The over-winter period shows a difference of 
25°, and the night of July 25 shows a difference of 18.5°. The readings 
