eae DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
where the water-supply is intermittent, and second, the high rate of 
evaporation in connection with the inconstant water-supplv. . 
Associations of the flood-plain.—No associations are more characteristic 
than the two which belong to the flood-plain of the Santa Cruz and 
other rivers of the same region, nor could there well be a case in which 
the delimitation of the smaller area occupied by one of these associa- 
tions from the larger one in which it lies is more clearly referable to 
special conditions. In this case accumulations of alkali salts, commonly 
in areas of defective drainage, constitute the essential peculiarity of the 
habitat of the salt-bushes. 
Outside of these areas the flood-plain is the habitat of an association 
composed of two groups of plants of very different biological require- 
ments. These are, on the one hand, the mesquite and Acacias, whose 
long roots extend to depths where a sufficient water-supply is assured, 
and on the other, the Bzgelowia and various other plants of low growth, 
the roots of which occupy more superficial soil-layers and are, as far as 
soil relations are concerned, subjected to more distinctively xerophytic 
conditions. These differences of ecological requirements in plants closely 
associated on the same ground are of fundamental importance as regards 
competition. 
Associations of the slopes——To this group belong the creosote-bush 
association, the palo verde-catclaw association of the wash, and the asso- 
ciation of F’ranserva and cholla. The different behavior of certain species 
of these associations as regards strict maintenance of habitat choice is 
especially worthy of note. While the catclaw, for example, hardly passes 
- beyond the limits of the wash, the creosote-bush not only invades this, 
but grows there far more luxuriantly than within its own special habitat. 
The relations are complicated, but it is plain that this behavior of the 
creosote-bush is the result of a greater capacity of adjustment on its part 
to differences in amount of soil-water than is possessed by such plants 
as the catclaw and palo verde. 
Associations of the hill.—Up to this point it is manifest, even upon 
casual observation, that soil relations have had a preponderating influ- 
ence in determining the different associations of plants and their limits. 
On coming to the hill, however, it is apparent that another factor, namely, 
aspect, or direction of slope, has also exerted a marked influence in deter- 
mining the composition and place of several of the associations. ‘This 
is seen with special clearness in the case of the Lippia association and 
that of Cereus giganteus and Encelia farinosa, the former occurring here 
exclusively on north exposures, the latter well represented only on the 
east, west, and especially on the south sides of the hill. 
The association of annuals, belonging to superficial soil-layers, though 
well represented on the hill, is not confined to it. Temperature and 
soil-water are obviously the factors which determine the appearance in 
