WINTER TEMPERATURES AND DISTRIBUTION OF PLANTS I99 
not only by the forest cover but by the surface litter of the soil, and 
by the greater humus content and moisture content of the soil. 
The marked character of the temperature inversions has made it 
necessary, in determining the temperature gradients of the Santa 
Catalina Mountains, to compare only the stations which are in topo- 
graphically similar situations. It also makes it necessary to compare 
separately the gradients of the desert and those of the forested altitudes. 
The absolute minimum temperatures of the winter of 1912-13 at 
several mountain stations showed that there was a successive fall of 
minimum from 17° at the Desert Laboratory (2,663 ft-) to 13° at 4,000 
2663 2328 3000 4000 5000 6O0O 7000 8000 
Fig. I. A vertically exaggerated section from the Desert Laboratory to the 
summit of the Santa Catalina mountains, to show the elevation and topographic 
location of temperature stations, together with the absolute minima of the winter of 
1912-13. 
ft., and — 6° at 6,000 ft. There was then a rise of minimum to .5° 
at 7,000 ft., the first station in the forest, and a fall to — 2° at 7,725 
ft. (see Fig. i). Similar figures were secured in the winter of 1911-12, 
