92 VEGETATION OF A DESERT MOUNTAIN RANGE. 
there is no work known to the writer which proves or disproves this 
view. The light factors have been but little investigated and may 
prove to occupy a réle of great importance. In spite of the funda- 
mental physiological réle of light, it is more than probable, however, 
that this factor plays a less important part in influencing the distri- 
bution of plants than do moisture and temperature. 
MOISTURE FACTORS. 
In the case of a mountain which arises from an arid region to a 
considerable height, the moisture factors are of critical importance in 
controlling the vertical distribution of plants. This group of factors 
may be- defined as those which have to do with the maintenance of a 
close degree of equality between the daily intake and outgo of water 
through the plant. The description of rainfall and soil moisture con- 
ditions in the Santa Catalinas has indicated the nature of the water 
supply for plants, and the data on atmospheric evaporation have shown 
the collective force of the chief of those ultimate external factors 
which determine the water loss of plants. During the humid seasons 
the ratio of water available to water lost is such as to make conditions 
favorable for all plants. During the most acute periods of aridity the 
value of this ratio becomes an item of the first moment. 
The soil moisture data given in an earlier section are from too slight 
a depth to indicate the possible supplies for trees and the largest shrubs. 
They are nevertheless from a depth which is freely exploited by the 
roots of perennial plants, and it is more than likely that they bear a 
rather definite ratio to the moisture at greater depths. 
Since the arid fore-summer is the portion of the year in which the 
maintenance of an equilibrium between intake and outgo of water is 
most difficult, it is instructive to determine their relation for this season 
at the different altitudes. This may be done by determining the ratio 
of evaporation (in terms of cubic centimeters per day) to soil moisture 
(in percentage of dry weight), using the average daily evaporation of 
the arid fore-summer, and the average soil moisture of the arid fore- 
summer at 15cm. These ratios are exhibited in table 20. The approx- 
imate ranges of the ratios are 1 to 25 for the Forest, 20 to 35 for the 
Encinal, and 35 to 50 for the Desert. If evaporation data had been 
secured at 9,000 feet the value of the ratio for that elevation would 
have been less than unity. 
The values of the ratio of evaporation to soil moisture afford a 
concise expression of the major conditions which affect the water 
relations of plants, and they demonstrate the wide divergence of these 
conditions in the desert valleys and on the forested mountain summits 
during the arid fore-summer. The average daily evaporation rate 
has been shown (fig. 12) to fall during the humid mid-summer to half 
the amount during the arid fore-summer. The soil moisture is like- 
