264 BOTANICAL GAZETTE [APRIL 
III. Woody or partly herbaceous plants, exhibiting peculiar 
modifications of distinctly xerophytic types. Fouquieria, Hola- 
cantha, Koerberlinia, Zizyphus, Atriplex. 
IV. Plants of the most pronounced xerophytic character. Opun- 
tia, Cereus, and other cacti. 
V. Plants adapted by habit, rather than structure, to desert 
conditions. Sphaeralcea and many other half-shrubby or more or 
less herbaceous forms. 
Of the species employed in the experiments, Celtis pallida is a 
shrub, growing commonly to the height of one to one and one-half 
meters on the laboratory hill, where it is rather abundant. It holds 
its foliage so well that it might be ranked as an evergreen, though 
it suffers to some extent from the effects of frost. Its leaves are 
rough-hairy, thin but firm in texture, and conforming in general 
to the generic type. Covillea tridentata, the well-known creosote 
bush, is the most abundant woody species of this region. Its small 
coriaceous leaves, presented more or less edgewise to the sun and 
covered with waxy varnish, are well protected against excessive 
transpiration. Lycium Berlandieri is a small shrub, more than a 
meter in height, of frequent occurrence on rocky exposures. These 
species, of the three genera named, while well adapted to their 
habitat, exhibit characters far less conspicuously xerophytic than 
those of many of the plants with which they are associated. 
Coming to the second group, Parkinsonia Torreyana attains 
the dimensions of a small tree, and is conspicuous by reason of its 
green bark, from which it has the common name of palo verde. 
Though a denizen of the desert, it is not a dry ground form, but 
frequents low places, where more water is available than on the 
mesa or even on the adobe soil of the hills, where Parkinsonia micro- 
phylla, a related species, does well. Prosopis velutina, the mes- 
quite, grows chiefly in low ground, within reach of abundant water, 
but it also occurs, though scattering and undersized, on the adobe 
soil of rocky hills. Like the palo verde and many other legumi- 
nous plants, the leaves of the mesquite exhibit in their structure 
and position excellent adaptations for the prevention of excessive 
transpiration. Acacia constricta, of similar distribution, occurs 
on the mesa and also on rocky upland. It is a vigorous shrub, 
