GENERAL CONCLUSIONS. 111 
The principal departures of the vegetation from the ideal gradient 
that would be found on a geometrical cone are expressed in the irregu- 
larity of the upper or lower limits of vegetations or of individual species 
as observed in different habitats. The chief departure is that due to 
slope exposure, by virtue of which the vegetation of north-facing and 
south-facing slopes at the same elevation shows striking differences. 
A second departure is that due to the influence of streams and the 
high moisture content of the soil of arroyos and flood-plains, by reason 
of which the plants of all altitudes are carried below their normal 
lowest occurrences on slopes. Another departure is due to the influence 
of ridges, on which the plants of all elevations (and particularly those 
of the Desert) find their highest occurrences. These departures seldom 
result in the occurrence of distinctive plant communities, but are 
operative rather in the carrying of the usual and widespread communi- 
ties into elevations at which they are exceptional. The effect of slope 
exposure is to carry the normal vegetation of a given elevation both 
up and down the mountain, so that its lowest occurrences are on north 
slopes and its highest on south slopes. The effect of streamways is 
to carry either the normal or the streamside vegetation down the moun- 
tain, so that the extreme lowest occurrences of almost all Encinal and 
Forest plants may be sought along the streamways. The effect of 
ridges is to carry the vegetation (or more particularly individual 
species and small groups of species) up the mountain, so that all highest 
occurrences of Desert and Encinal species are to be found on narrow 
ridges—the highest occurrences of Forest plants are not reached on 
the Santa Catalina Mountains, and they are controlled by a very 
dissimilar group of factors. 
It is impossible to study the distribution of vegetation in a region 
where pronounced differences may be found within short distances 
without being impressed with the independence which each species 
exhibits in its allocation. Plants which are associated on the Lower 
Desert Slopes, for example, range to very different maximum altitudes, 
and plants which are associated in the Upper Encinal are found to be 
in part at the upper edges of their ranges, in part at the lower edges, 
and also in part rather closely restricted to that region. It is nowhere 
possible to pick out a group of plants which may be thought of as 
associates without being able to find other localities in which the asso- 
ciation has been dissolved. Certain plants may be thought of as having 
closely identical physical requirements because of their associated 
occurrence in the same spot. Nevertheless, the fact that the vertical 
ranges and habitat characteristics of these species will reveal more 
or less pronounced differences goes to show that each of them has 
survived in a particular section of the climatic gradient. It is true in 
the Santa Catalina Mountains, as it is true in all other places, that the 
associated members of a plant community are not able to follow each 
