102 VEGETATION OF A DESERT MOUNTAIN RANGE. 
Neither the data for soil moisture nor those for evaporation show 
the exact alternation exhibited by the vegetation itself, by virtue of 
which a given north slope is similar in vegetation to a south slope 
about 1,000 feet above it, and a given south slope is similar to a north 
slope about 1,000 feet below it (with the exception of the highest 
altitudes). The conditions of evaporation found through the range 
of Pinus cembroides, which has been used as an example of the effects 
of slope exposure, are indicated in figure 18, where curves are given 
showing the seasonal march of evaporation on a north slope at 5,000 
feet, the average of the evaporation on north and south slopes at 6,000 
feet, and the amounts on the south slope at 7,000 feet. These curves 
follow a course which is parallel and indicate evaporation conditions 
which are remarkably ¢ 
similar for the lower, cen- 
tral, and upper individ- 
uals of this pine, except 
for the higher rate at 
5,000 feet during the arid 
fore-summer. 
The ratios of soil mois- 
ture to evaporation at 
different altitudes have «| 4 
been worked out sepa- oie a ee 
rately for the north and : = = 
south exposures at the 6 7%, 1-—Granbs showing scasonal marah of rte of evap 
stations (see table 20) i average for a north and a south slope at 6,000 feet (solid 
Since these ratios are an line), and for a south slope at 7,000 feet (broken line). 
expression of the conditions of the arid fore-summer they must be 
taken as elucidating only those phases of slope exposure which are 
themselves due to the climate of that season. Any subsidiary in- 
fluence of temperature in affecting the slope exposure phenomena of 
vegetation in the Santa Catalinas still awaits a full investigation. 
The comparative conditions of the lower, central, and upper habitats 
of Pinus cembroides may be again investigated in the light of the ratios, 
which are as follows: North slope at 5,000 feet 21.3, average of north 
and south slopes at 6,000 feet 24.5, south slope at 7,000 feet 24.1. These 
figures indicate a still more remarkable similarity of the water con- 
ditions in the three habitats than the evaporation figures do. To 
compare a plant of lower range we may take Agave schottii, which 
encounters conditions expressed by the following ratios: north slope at 
4,000 feet 33.1, average of north and south slopes at 5,000 feet 20.6, 
south slope at 6,000 feet 33.0. These figures fail to show as close 
agreement, but indicate a close similarity of the water conditions at 
the lowest and uppermost habitats, and a more favorable set of con- 
ditions in the central habitat. To make a similar comparison for a 
. 
