rye BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
now brings into activity the succulent, spinose and xerophytic types 
which especially characterize the season. This dry foresummer may be 
said to comprise April, May, and June, with a total average precipitation 
of 0.67 inch, while the maximum temperatures range from 95 to 112° F. 
The evaporation of course exceeds the precipitation in an enormous ratio. 
The succulents comprise two general types, one of which is represented 
by the cacti with atrophied foliar organs and storage stems, and the other 
by the yuccas and agaves with the thickened leaves and short stems 
functioning as reservoirs for the water which was accumulated during the 
latter part of the winter wet season. 
The greatest activity among the cacti is displayed by the cereuses and 
opuntias. The earliest of these in the vicinity of Tucson is generally 
Echinocereus fendlert, in which a few brilliant crimson flowers are displayed 
from the clumps of short, thickened, cylindrical stems late in March, and 
continue for a month, to be accompanied and followed by equally noticeable 
bloom of two or three other small species. Chief of the group, however, 
is the great sahuaro, the flower-buds of which develop as dense clusters 
on the portions of the apices of the stems most exposed to the sun, and | 
have been seen to open on March 25. The whitish flowers each remain 
open but a short time and apparently are pollinated by insects. A 
succession of them ensues, and although practically finished during May 
or June, yet belated buds open at various times, one having been seen 
as late as the middle of November. The seedy fruits mature in great 
quantity in midsummer, and are much prized by the Papagoes, who make 
much use of them in various ways (plates 54, 55). 
The prickly-pears, or opuntias with flat stems, begin to make some 
growth of new joints and to push out flower-buds in March, and late in 
that month or early in April bloom in great profusion, the fruits maturing 
early and dropping to the ground. Fifteen or twenty species are native 
to the Tucson region, but the greatest confusion prevails as to their 
identity. Of the various desert plants, this group has been the subject 
of the most inquiry as to its possible economic utilization. After a con- 
sideration of the various practical questions connected with open cattle 
ranges, it has been found that the best use of them for forage is made by 
growing or allowing to grow spinose species, from which the spines are 
burned when they are to be consumed by animals. This is now done 
with the plants growing in various places. Unarmed forms are subject 
to the attacks of so many animals that it is practically impossible to 
secure a crop without protecting fences. A few species are known in 
which the spines are very sparse. One of these, Opuntia levis, occurs in 
the canyons of the Santa Catalina Mountains, but chiefly on rocks or in 
places inaccessible to grazing animals (plate 56). 
The cylindrical opuntias include many forms with a central stem and 
well-developed system of branches which give them the form and impos- 
