May 24,1924 Significance of the Southwestern Desert Vegetation 
767 
Less Frequent or Rare 
Atriplex hymenelyira (Torr.) S. Wats. Parosela arborescens (Torr.) Heller. 
Chilopsis linearis (Cav.) Sweet. Petalonyx thurberi A. Gray. 
Isomeris arborea Nutt. Lepidospartum squamatum A. Gray. 
ANNUAL AND BIENNIAL SPECIES OF THE MESQUITE AND CHAMISO 
Common or Frequent 
Anogra trichocalyx (Nutt.) Small. Eremocarya micrantha (Torr.) Greene. 
Chylisma scapoidea (Nutt.) Small. Palafoxia linearis Lag. 
Dithyrea californica Harv. Achyronychia cooperi Torr. & Gray. 
Sphaerostigma veitchianum (Hook.) Eriogonum thomasii Torr. 
Small. Eremalche exilis (A. Gray) Greene. 
Cryptanthe sp. 
Less Frequent or Rare 
Baileya panciradiata Hary. & Gray. Sophia pinnata (Walt.) Howell. 
Calyptridium monandrum Nutt. Stillingia annua Muell. Arg. 
Croton californicus Muell. Arg. Thelypodium lasiophyllum (Hook. & 
Plantago erecta Morris. Arn.) Greene. 
APPEARANCE 
The chamiso is greener than the desert-sage and is considerably taller. In 
early spring, just after the new growth has come on, the plants are very green. 
In the latter part of summer most of the leaves have dropped (PI. 11, B) but 
large quantities of fruits cover the smaller branches. In favorable years the 
fruit is so abundant that the branches bend under its weight. 
The mesquite trees growing on the soil occupied by the mesquite and chamiso 
association have a distinct form. On the drier heavier soils occupied by the 
desert-sage, or the wetter saline soils occupied by the saltbush and arrowweed, 
the mesquite grows as a tree with a single large trunk often more than 13 inches 
in diameter. On the sand hills, however, the mesquite is shrubby, sprawling over 
large spaces of ground and forming small thickets. On some of the newer dunes 
these are partly buried under the sand so that only the smaller branches are seen 
above the surface (PI. 10, B). The mesquite begins growth only after warm 
weather sets in, as the leaves do not appear until March. Up to that time they 
are leafless and reddish-brown, due to the color of the bark of the branches and 
twigs. From March on, throughout the long hot summer, they retain a heavy 
foliage. 
PHYSICAL CONDITIONS INDICATED 
Soil Moisture 
The soil is practically a pure sand on the hills with a little silt in the depressions. 
It is very readily permeable, with practically no run-off, and has a very low mois¬ 
ture holding capacity, as is shown in Table XXI. Immediately after the winter 
rains there is water available to a depth of 4 feet. The surface foot is dried out 
rapidly by the annuals and by evaporation. By summer there is no available 
water to a depth of 4 feet. However, in cases where the hills are bare and the 
water has not been used by the plants there is available water in the third and 
fourth feet even as late as September (sample 16, Table XXI). 
