18 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. _ 
(7) THE HILL. 
Above the Larrea zone the hill is the habitat of a large number of con- 
spicuous and characteristic perennials, which, although they may be 
classed together as a single association, differ among themselves to such 
an extent in ecological traits and choice of habitat as to call for consid- 
eration under separate groups. 
(a) PLANTS OF GENERAL OCCURRENCE ON ALI, EXPOSURES; ASSOCIATION 
OF FOUQUIERIA AND PARKINSONIA MICROPHYLLA. 
The species of this group, more than any others, give its character to 
the vegetation of the hill as a whole. Growing as they do equally well 
on all exposures, to differences of which members of other groups show 
remarkable sensitiveness, they form at once the most widely spread and 
most typical representatives of the vegetation of Tumamoc Hill (plate 
9). Their adjustment to a wide range of physical conditions seems 
well-nigh perfect, yet it is exhibited in widely different ways. The habits 
of Fouguierva and Parkinsonia, to go no further, are very different, as 
regards the production and fall of foliage leaves, arrangements for pro- 
tection, and other ecological characters, but they grow side by side on all 
exposures and on various soils, so that, however limited some of them 
are as to geographical range, it is plain that within its limits they have 
attained a high degree of adaptation to widely varying soil and atmos- 
pheric conditions, and are to be reckoned as highly successful desert 
species. 
(b) PLANTS OCCURRING ON SOUTHERN EXPOSURES AND LARGELY WANT- 
ING ON NORTHERN ONES; ASSOCIATION OF CEREUS GIGANTEUS AND 
ENCELIA FARINOSA. 
The species just named are closely and widely associated and are both 
far more numerous on southern exposures, as well as eastern and western, 
than on northern ones. ‘This is strikingly shown on the two sides of 
the gulch adjacent to the Desert Laboratory on the southwest. On the 
right side of this gulch, with a southern and partly western aspect, there 
are upwards of 70 sahuaros and the ground is almost covered with a flour- 
ishing growth of Encelia, numbering thousands of individuals (plate 
9). The left bank, on the other hand, with its generally northern aspect, 
has less than a dozen sahuaros and only a few scattering groups of Encelia 
(plate 10). ; 
Observations of temperature and humidity and experiments with 
seedlings have been in progress for some time, with a view to determining 
as far as possible the factors to which this striking limitation as regards 
local habitat is due. (See p. 44 et seq.) 
