76 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
part of the yellowish note in the landscape is due to the masses of Bigelovia 
of two species which occur in abundance over the mesas and are plentifully 
supplied with resinous secretions. The pods of the leguminous trees, 
including the acacias and the mesquites, ripen during this season and offer 
abundant food to the larger grazing animals. 
SUMMER ANNUALS. 
Several euphorbias spread their small dense mats of thin stems and 
minute leaves on the surface of the ground, and are rich in a latex or 
milky juice containing resins, starch, and some caoutchouc, which is 
reputed by the Indians of various tribes to furnish an antidote for the 
venom of the rattlesnake—a supposition not confirmed by experimental 
evidence, however. Another group of species which forms green mats 
on the surface is comprised within the genus Tribulus. Some of these 
cover an area of nearly a square yard with a dense mass of green com- 
pound leaves offering a background contrasting with the bright yellow 
and reddish colors of the flowers, which show interesting opening and 
closing movements. The thin yellowish, almost leafless, stems of the 
parasitic Cuscuta, or dodder, make a rapid growth during the humid 
season and quickly twine round the stems and sink their haustoria deep 
into the bodies of many host-plants, being capable of attacking success- 
fully some forms with an extremely indurated bark or epidermis. Dur- 
ing all of this season the humid air, especially after the sudden rains, 
becomes laden with the pungent odors of the creosote-bush and of the 
various volatile substances produced by many of the desert forms. 
THE DRY AFTER-SUMMER. 
The latter part of the moist midsummer has witnessed the beginning 
of growth of a number of grasses of the genera Iriodia, Bouteloua, and 
Aristida, which ripen their seeds and persist as tufted bunches of dry 
haulms and leaves during the rainless season of October and November, 
being eagerly eaten by grazing animals. During this season an almost 
total cessation of vegetative activity ensues, and continues until the 
double stimulation of the moisture of the winter rains and of the increas- 
ing heat of the sun after the winter solstice is received. Then the seasonal 
succession of forms ensues as described, with various modifications due 
to the wide departures from the normal or averageconditions. Anexam- 
ple of this diversity is suggested by the records of precipitation, which 
may vary from 5 to 25 inches per year. 
