CHAPTER VII. 
SUMMARY. 
The work thus far completed, with the most important results, may 
be summarized as follows: 
(1) Within the limits of the Desert Laboratory domain and that part 
of the Santa Cruz Valley immediately adjacent, chosen as a representa- 
tive area for the study of desert plants in southern Arizona, 12 plant 
associations have been recognized and defined. These fall naturally into 
A groups corresponding with the main topographical features of the area 
under observation. ‘The local distribution of certain species of these 
associations has been carefully mapped with special reference to topo- 
graphical and soil relations, by this means confirming conclusions based 
on previous observation. 
(2) Observations within this limited area, supplemented by compara- 
tive studies in the Gila Valley and elsewhere, have led to the conclusion 
that soil properties and aspect are of paramount importance in deter- 
mining the local distribution of desert plants. With regard to the former, 
evidence has been gained that soil-water exercises a controlling influence, 
but that, with certain species at least, aeration and percentage of alkali 
salts are also efficient factors. 
(3) These conclusions have been confirmed by investigations of the 
soils of the Laboratory domain conducted by Dr. B. E. Livingston. The 
correspondence between per cent of soil-moisture and the distribution of 
both the plant associations and their constituent species is especially 
striking and convincing. 
(4) The importance of aspect in determining distribution on closely 
adjacent areas has been shown by many different observations, but 
especially by floristic comparison of opposite sides of the gulch near the 
Laboratory, where, on equivalent areas, 2.5 times as many species are 
found on the northerly as on the southerly exposure, and the difference 
in number of individual plants is still more striking. 
Analysis of aspect preference, in connection with its observation at 
different altitudes and continued records of temperature, leads to the 
conclusion that it is correlated first of all with range of temperature, 
though other factors, in certain cases at least, are involved. It appears 
clear that whatever else is involved the lower temperatures of winter 
on northern exposures at this place have interfered with their occupa- 
tion, to any considerable extent, by the sahuaro and Fncelza, while the 
extreme heat of southern exposures in summer has, at this altitude, 
prevented their occupation by various species, of which Lippra wraghtir 
is a notable example. The behavior of this plant at different altitudes 
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