PLANT ASSOCIATIONS AND HABITATS. 15 
of Tumamoc Hill. The catclaw deports itself in the same way. Ephedra 
trifurca (plate 6), another characteristic species, ranges still more widely, 
approaching the habits of the creosote-bush in attaining its best develop- 
ment where there is an abundant water-supply, but, like the latter, capable 
of maintaining itself successfully where the water content of the soil is 
lower, especially on light, more or less sandy soils. 
A few other woody species find in this habitat a congenial home. Con- 
dalia spathulata attains here its best development. Condalia lyciordes, 
a companion of the mesquite, advances with it up the washes, and Celtis 
pallida, here a plant of the cliffs, also occurs in the wash, absolutely 
avoiding the intervening slopes, thus presenting further evidence, if such 
were needed, that soil-water is a factor of prime importance in deter- 
mining the distribution of this association of plants. 
Five of the species that have been named as members of this association _ 
have been observed by Blumer (1908) to change their topographic loca- 
tion with altitude. These are Cercidiwm torreyanum, Acacia greggit, 
Prosopis velutina, Condalia lycioides, and Celtis pallida. At Tucson, at 
an altitude of 2,200 to 3,000 feet, all of these inhabit the washes, and 
with a single exception (Celtzs pallida) are hardly met with at all else- 
where; but at higher elevations, 3,500 to 4,500 feet, in the neighboring 
mountains of the Tucson Range, the Santa Catalinas, and the Rincons, 
they are found spreading out on gravelly and other upland soils, no longer 
confined to washes, and deporting themselves as ordinary members of 
the shrubby upland growth there prevalent. Taking these higher eleva- 
tions as the point of departure, it is found that even 1,000 feet lower all 
these species exhibit a marked tendency to confine themselves to water- 
courses or, at all events, to places where there are good conditions as 
regards soil-moisture and some degree of protection from the more extreme 
desert conditions prevailing at the lower levels. The single individual 
of Yucca elata, a plant of higher levels, that has been found on the Lab- 
oratory domain, is at the edge of the wash, protected by the higher 
vegetation around it. 
It should again be noted, as already suggested, that many of the char- 
acteristic species of the habitats thus far discussed, though exhibiting 
more or less plainly certain structural features distinctive of xerophytes, 
are semi-mesophytic or mesophytic, in some cases even hydrophytic, as 
regards soil relations. The willows and cottonwoods of the river-banks 
are, in their habits, what they are the world over. The mesquite of the 
flood-plain sends its roots down to the water-table, and elsewhere is 
restricted in its range to habitats in which a satisfactory water-supply 
is at hand, and the palo verde and catclaw of the washes evidently have 
much the same dependence on a sufficient amount of soil-water. 
The same thing appears to be true of various species of salt-bushes, 
which follow watercourses, or—in the salt-spots—occupy places of seep- 
