DESERT REGIONS OF NORTH AMERICA. 31 
2 feet. At the station Hesperia the juniper ends and the creosote-bush 
(Covillea tridentata) begins. As still lower elevations are reached, the 
creosote-bush becomes, except in the washes, the prevailing bush, and 
continues throughout the long waste of desert to the Colorado River. 
(See F. V. Coville, Botany of the Death Valley Expedition, Cont. U. S. 
Nat. Herb., 4, 1893, for a fuller account of the vegetation.) 
DEATH VALLEY. 
In 1891 Mr. Frederick V. Coville made a botanical examination of the 
Mohave Desert and of the Death Valley regions in southern California. 
The work was designed to be both systematic and comprehensive. It 
embraced a delineation of the principal vegetative conditions to be met 
with in deserts, some investigations of the relations of the chief environ- 
mental factors to the characteristic plants, and an examination of the 
more important adaptations of a large number of species. One of the 
features of this contribution of great importance was the recognition 
of the major problems to be encountered and an outline of further 
researches needed upon the subject. The region included in this survey 
consists, in large part, of mesas in which Covillea and Gaertneria are 
the prevailing plants. The surface layers of the soil consist of gravel, 
sand, and boulders. An average of the data obtained by the ten Weather 
Bureau stations nearest the region showed a rainfall of about 5 inches 
annually, and a precipitation amounting to 1.54 inches was observed in 
the region itself from January to June, inclusive, in 1891. The extreme 
dryness of the atmosphere is illustrated by the fact that the relative 
humidity at 5 p.m., taken daily during the five months mentioned, was 
15.6 percent. On the 4th and 5th of August of the same season a mini- 
mum of 5 per cent was recorded. A maximum temperature of 122° F. was 
recorded five times during the summer season of 1891 and a minimum of 
30° was reached in January and February of the same year. Vegetation 
in this district was seen to exhibit its greatest activity during the period 
of maximum precipitation, with medium temperatures from February 
to May; a quiescent condition during the season of maximum temperature 
and dryness during June to November; and a condition of slow growth 
during the low temperatures of December and January. 
Perhaps the most noteworthy result of this study of the flora consisted 
in the discovery that the vegetation was composed almost wholly of 
perennial shrubs and annual herbs, and but few structures for storage 
of water were found. The tendency to form fleshy fruits was almost 
lacking, and even the fruits of Opunita were comparatively dry and hard. 
The root-systems of a number of plants were examined and the mesquite 
(Prosopis) was found to have roots more than 50 feet long. Growth or 
increase in length and thickness was found to be extremely slow in the 
perennials, though very rapid in the annuals which carry out their entire 
