ENVIRONMENTAL AND HISTORICAL FACTORS. 99 
MEASUREMENTS OF EVAPORATION. 
Recent work by Livingston (1907, 1908) at the Desert Laboratory on 
evaporation and plant development and habitats, in connection with 
observations at stations representing widely different climatic conditions 
in different parts of the United States, indicate the great importance 
of this line of research. 
It is shown by Dr. Livingston in the papers referred to that the evapo- 
rating power of the air depends upon three factors—humidity, tempera- 
ture, and wind velocity—and that measurement of evaporation, which 
is simpler than the measurement of any one of these, gives the directly 
controlling factor in water-loss from plants. For a given soil-moisture 
content, there is a maximum rate of water-supply to any plant through 
root-absorption, and when the evaporating power of the air is great 
enough to cause the rate of loss to surpass that of supply, the plant wilts 
and suffers injury, and death even may ensue. If the rate of transpiration 
approaches or equals that of supply, little or no growth can take place. 
It appears, therefore, that evaporation must be considered as capable 
of affecting the plant directly, as well as through its indirect effect on 
soil-moisture conditions. In arid regions it seems that this effect is par- 
ticularly marked, and that here evaporation actually inhibits the growth 
of many plants, even though the soil-moisture content may be rather 
high. The conclusion necessarily follows that evaporation may exercise 
a determining influence on the distribution of plants in different habitats, 
even in relatively close proximity. 
The methods of experimentation, with full descriptions of the apparatus 
employed, are given by the author in the papers referred to, and it is 
only necessary here to refer briefly to the results, which indicate, as already 
stated, that in any habitat, but especially in arid regions, the evaporating 
power of the air is one of the most important among the conditions which 
determine local distribution, and that the measure of evaporation for a 
given locality also includes some evidence in regard to soil-moisture, which 
is of even greater importance to plant life than the atmospheric conditions. 
Turning for the moment from desert conditions, some comparisons 
of rate of evaporation at certain mountain stations in the neighborhood 
of Tucson and at points in the eastern United States are of great interest. 
The highest instrument on the Santa Catalina Mountains was set up at 
an altitude of 8,000 feet, and showed, for the latter half of May, 1907, a 
weekly rate of evaporation equal to 133 c.c. During the same period 
the instruments at Orono, Maine, and Burlington, Vermont, indicated 
a weekly rate of evaporation of 123 c.c., and 112 c.c., respectively. Such 
correspondence of rate is the more striking in view of the fact stated by 
Livingston (1908, p. 8) that ‘‘the vegetation about the highest instrument 
in the Santa Catalina Mountains possesses the same ecological characters 
