PLANT ASSOCIATIONS AND HABITATS. iL 
passed over Oklahoma and into southwest Kansas. This encroachment 
of mesquite is partially accounted for by its weed-like capacity for occu- 
pying new ground . . . and by the influence of cattle in scattering 
the beans.’’ Such extensive and rapid invasion as is here described has 
not been observed in Arizona, so far as the writer is aware, but the same 
characteristics are noted here—its preference for low, flat areas, with 
compact soils, but with a marked capacity for extending beyond these 
on to higher ground, its successful reproduction where it is really at home, 
and the tenacity with which it holds its place where once established. 
Its endurance of conditions to which various other species are less fitted 
is well seen in many places from Texas to California, and particularly 
in the Salton Basin, where great mounds of sand are blown about the 
mesquite, which keeps on growing, its branches extending beyond the 
rising heap or dune, until the latter has reached a height of several meters, 
the protruding branches still covering it with an apparently healthy and 
normal growth. » 
All in all, in the success with which its relatively rapid dissemination 
is accomplished, in its ready adjustment to widely varying conditions, 
its utilization of available water, and in other ways, this plant is to be 
considered one of the best adapted of our desert species, and its wide 
distribution and present successful invasion of new areas mark it, not as 
a decadent species, but as one in pristine vigor, from which apparently 
several others are in process of evolution. 
Associated with the mesquite are a number of characteristic species, 
some of which are confined to the flood-plain, while others extend with 
it beyond these limits. Of the latter Acacza constricta is a conspicuous 
representative. Its habits are essentially those of the mesquite as to 
water requirements, and it closely resembles this species in its xerophytic 
structure. The two grow side by side near the river and to the summit 
of Tumamoc Hill, in precisely the same situations, one being, apparently, 
the ecological equivalent of the other. Other species behave differently. 
Acacia greggw grows with the mesquite in its lower range, but not on the 
hill above, and the same is true of Condalia lycioides. Both of these 
exhibit distinctively xerophytic structures, but both are as yet adapted 
to a somewhat more restricted range of soil conditions than are the mes- 
quite and Acacia constricta. Sambucus mexicana and Fraxinus velutina, 
also of this association, are still more limited in their range, growing near 
irrigating ditches, hardly affecting even the edge of the mesa-like slopes, 
and structurally are to be thought of as essentially mesophytic. 
Passing through the list of plants belonging to this association, it is 
seen that the same relations are maintained—certain species are strictly 
confined to the flood-plain, while others occur rather widely beyond it; 
it is to be noted, however, that even those that range most freely exhibit 
their best development on the flood-plain and not on the hillsides. The 
