6o 



REPORTS ON INVESTIGATIONS AND PROJECTS. 



tors. During the principal drought period — the arid fore-summer — the soil- 

 moisture falls to as low a figure in the desert and chaparral belts as in the 

 desert plain itself, while in the forested belt the surface moistures are nearly 

 as low. The vertical gradient of evaporation indicates that it falls to ap- 

 proximately one-third as much at the forested altitudes as on the desert, a 

 ratio which holds equally for the high evaporation rates of the arid fore- 

 summer and the lower ones of the humid mid-summer. 



The ratio of soil-moisture supply (SM) to the evaporating power of the 

 air (H) is a measure of the conditions which determine the maintenance of 

 the absorption-transpiration balance of the plants. It has been possible, 

 from the data in hand, to determine the ratio of these factors for the several 

 altitudes of the Santa Catalinas. The conditions indicated by these ratios 

 are operative for a length of time (T) which diminishes with altitude, a 

 fact which may be incorporated in the ratios by using as a factor the number 

 of weeks from the opening of the growing season to the close of the arid 

 fore-summer. The sets of ratios thus obtained are shown in the table 

 herewith. 



The set of ratios which involve the time factor are 

 a graphic index of the relative aridity of the several 

 altitudes and indicate that the desert conditions are 

 nearly 30 times as severe as those of the forested 

 mountains at 8,000 feet. A comparison of these ratios 

 as worked out for north-facing and south-facing slopes 

 at the six stations indicates that the conditions of which 

 the ratios are an expression are chiefly responsible for the differences in the 

 vertical limits of desert, chaparral, and forest on opposite slopes. The ratio 

 of soil-moisture to evaporation appears, therefore, to be the chief compound 

 factor detemiining the extension of forest and chaparral and their limitation 

 at the edges of the desert. 



Water-relations of Plants, by B. E. Livingston. 



Prof. B. E. Livingston has continued his studies on the physics of the 

 water-relations of plants and the relations which obtain between organism 

 and environment in this regard. Studies of the aerial water-relations of 

 plants have, during the last year, added several new and apparently impor- 

 tant points in this field, as well as several refinements and improvements in 

 the methods of measurement and integration of the evaporating power of 

 the air. These methods — depending largely upon atmometry — have now 

 been generally adopted and are proving themselves valuable, not only in 

 physiological and ecological researches with plants, but also with animals. 



The preliminary investigation of the conditions of soil and air which de- 

 termine the wilting of plants, carried out in the summer of 1910, with the 

 assistance of Dr. William H. Brown, has now been published by Dr. Brown, 

 and the same line of study was carried much farther in the experimental 



