398 BOTANICAL GAZETTE [JUNE 
fixion thorn (Zizyphus lycioides), representing the shrubs; and the 
cat claw (Acacia Greggii), the mesquite (Prosopis velutina), and two 
species of palo verde (Parkinsonia microphylla and P. Torreyana), 
representing the trees. 
These plants grow in habitats which are very unlike in appearance, 
and which presumably are dissimilar also in such fundamentals 
as soil characteristics and water supply. Speaking now of the vicin- 
ity of the Laboratory only, for a radius of about ten miles the habitats 
may be classed as (1) mesa, (2) river bottoms, (3) the rocky slopes 
of the lower mountains, and (4) the “draws” of these mountains. 
As regards the amount of water in the soil available to the plants, 
the river bottoms should be classed first, after which should be placed 
the draws; the rocky slopes and the mesa are frequently very dry. 
The mesa is peculiarly desertic, because a subsoil, the so-called 
“‘calliche,” which is practically impervious to water, reaches within 
a few inches of the surface of the ground. The top soil, the adobe, 
is a clay. In the river bottoms the top soil is several feet in depth. 
The mesquite and the cat claw are found especially on the bottom 
lands, where they may attain the size of forest trees, especially the 
former. Both of these trees adapt themselves to the drier habitats, 
however; but in such places they are of much smaller size and are 
gnarled and much changed in outward appearance. Celtis and 
Zizyphus also grow on the river bottoms; Fouquieria seems to be 
confined to the rocky slopes. Parkinsonia microphylla also is mostly 
found on rocky or well-drained slopes, while the other species of palo 
verde (P. Torreyana) is to be found in the draws at a lower level. 
Finally, the creosote bush, which is the characteristic shrub of the 
mesa, grows on the bottoms as well as on the mesa, and is there 
large and most vigorous. The natural distribution of these plants, 
and also the modification in form which they assume in the various 
habitats, point to the conclusion that the optimum conditions for 
them are attained where, other conditions being equal, water is 
to be had in abundance.? 
That the greater vigor of the plants on the river bottom, as opposed 
to those on the mesa, for instance, is not due mainly, if at all, to 
2 Compare “The creosote bush (Covillea tridentata) in its relation to water supply,” 
V. M. SPALDING, Bot. GazETTE 38:124. 1904. 
