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 canons 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 autumnof 191 3 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. 



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 



