PLANT ASSOCIATIONS AND HABITATS. YW 
uals of the slopes compared with those of the flood-plain and washes. 
Its mode of branching and of reproduction from the root is also advan- 
tageous. Whole systems of branches die, and thus permit their share of 
water from the soil to go to others, and if, under extraordinary stress, 
the whole top perishes, fresh shoots from the root come up upon the 
return of rain, and vigorous growth again takes place. Fitted in so 
many ways, and probably in others less obvious, to cope with the vicis- 
situdes of its habitat, the Larrea holds complete sway in this zone, and 
from Texas to California the broad belt of creosote-bush, covering’ the 
long slopes that form the approach to the mountains at different altitudes, 
presents a most striking and characteristic feature of the landscape. 
A few other species are of occasional occurrence in this special habitat 
of Larrea. ‘Thus on the slope at the foot of Tumamoc Hill a very few 
individuals of Fouquierra splendens are established, and in places Opuntia 
julgida is frequent; none of these, however, approach the creosote-bush 
as the universal and distinctively characteristic species of this zone. 
As the slope advances upward with the long continuance of disinte- 
gration and erosion, the creosote-bush advances with it, and the presence 
of this plant, both as the successful pioneer and the final possessor of 
the soil, may be seen almost everywhere throughout its range. Thus 
we have the interesting case of a well-marked habitat, with a single spe- 
cies forming in it, at least in places, a perpetually renewed close asso- 
ciation of its own. 
The description just given applies to the slope as seen near the foot 
of Tumamoc Hill and in situations such as the “alluvial fans,’’ where 
- the conditions of slope and soil characters are essentially similar. There 
are wide areas commonly included under the term “mesa”? upon which a 
greater variety of conditions, with corresponding differences of vegetation, 
prevails. An example of this is the ground included in the Laboratory 
domain lying to the west of the wash, presently to be described. 
(b) FRANSERIA ASSOCIATION. 
Beyond the wash, on the Tucson slope to the west of the laboratory, 
and elsewhere on the domain, areas of considerable extent occur where 
there is a soil of coarse texture, approaching gravel, on which /’ranseria 
deltoidea and Opuntia fulgida constitute the greater part of the vegeta- 
tion. ‘he former is often very numerously represented (plate 8), and in 
many places is not accompanied by the cholla, but where the two attain 
their best development they are likely to occur together. I*requently 
Krameria canescens and a few other species are associated with them. 
As far as appears from present evidence, this association is one that is 
determined first of all by soil relations. ‘These are described in a later 
paragraph. 
