24 



THE AMERICAN NATURALIST 



[Vol. XLY 



main locations furnish the chief facilities for these some- 

 what extensive experiments, which may be briefly charac- 

 terized as follows. The domain of the Desert Laboratory 

 has a subtropical arid climate, with one cool moist sea- 

 son, one warm wet season, two intervening dry seasons, 

 the vegetation being chiefly composed of spinose xero- 

 phytic shrubs and woody plants, with a large number of 

 the more advanced types of desert plants, which carry an 

 immense water balance, such as the cacti and other suc- 

 culents. The total rainfall is 12 inches, relative humid- 

 ity falls as low as 5 per cent, for extended periods and 

 the soil moisture remains below 10 per cent, for weeks, 

 and the altitude is 2,300 feet ; maximum temperatures of 

 112°-114°, minima of about 16° F., with a total exposure 

 below the freezing point of from 12 to 80 hours per 

 annum are encountered. 



The xero-montane plantation lies at 5,400 feet on the 

 near-by slopes of the Santa Catalina Mountains at the 

 extreme upper edge of the characteristic desert flora in 

 the oak belt of vegetation with a rainfall of 16 to 18 

 inches per year, minima a few degrees lower than those 

 of the Desert Laboratory, with such an extension of cold 

 nights as to make temperature a distinct limiting factor; 

 relative humidity is extremely low, soil moisture quite as 

 low as that of the base plantation, and the activity of 

 vegetation of the winter wet season which is such a 

 marked feature of the lower plantation is entirely lack- 

 ing. The meteoric and other agencies carry a constant 

 stream of seeds from this locality into the region of the 

 laboratory. 



The montane plantation lies at an elevation of 8,000 

 feet in a forest of pine, spruce and aspen, with a climate 

 equivalent to that of northern Michigan, the growing 

 season being about 110 days, the winter being character- 

 ized by a heavy snowfall and temperatures as much as 

 20°-25° below zero Fahrenheit. The spring and autumn 

 are dry, and midsummer has the usual manifestation of 

 heavy thunder-storms, in which the precipitation is 



