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FORREST SHREVE 



shows the two gradients thus obtained. A slight and indetermin- 

 able allowance for the greater operation of the cold air drainage 

 depression at the Acclimatisation Laboratory, on account of the 

 great size of the Santa Cruz drainage basin, would bring the two 

 gradients more nearly into parallel. The rates of fall per 1000 

 feet of rise between the several stations are: Desert Laboratory 

 to 4000 feet, 5.2°F. (0.95°C. per 100 m.) ; 4000 to 6000 feet, 2.8 °F. 

 (0.52°C. per 100 m.) ; 6000 to 7000 feet, 3.6°F. (0.67°C. per 100 



Fig. 1 Curves to show the fall of temperature with increase of altitude in 

 the Santa Catalina mountains. Solid line, curve for ridge stations; broken 

 line, curve for valley stations. 



m.); Acclimatisation Laboratory to 8000 feet, 2.2°F. (0.40°C. per 

 100 m.). 



A vivid realisation of the importance of cold air drainage in 

 relation to the vertical distribution of plants may be had by 

 selecting a temperature on the ridge curve and then finding the 

 elevation at which the same temperature occurs on the canon 

 curve. For example the mean minimum on the ridge at 7000 feet 

 is 50.6°; by following in the abscissal direction to the canon curve 

 we find the same temperature at an elevation of 4650 feet. In 

 other words the minimum temperature conditions of a ridge at 

 7000 feet will not be found in a canon until we have descended to 

 4650 feet. A difference of topographic site at the same altitude 

 is, then, the equivalent of 2350 feet (716 m.) of difference in alti- 



