24. BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
furnish the more highly specialized structures, while the Leguminose con- 
tribute the greater number of the woody trees and shrubs. 
One of the striking features of this region is the extreme localization, 
or strictness of colonization, exhibited by many species which are 
found to cover an area of a few square yards, the face of a slope, the crest 
of a cliff, or the floor of a barranca, with no outliers and with the nearest 
colony perhaps many miles away. 
The Cactacez are more abundant here than in any other part of the 
world yet visited, several of the species being massive forms, which con- 
stitute very prominent features of the landscape. 
Cereus geometrizans has a short stem with branches reaching a height 
as great as 15 feet, and is to be found in great abundance in the valleys 
and canyons that come down into the valley from the west. Cephalo- 
cereus macrocephalus (plate 15) is a tall species of the massiveness of the 
sahuaro and like it having a central shaft bearing numbers of branches 
which are more closely appressed. It was seen only along the cliff near 
the Rancho San Diego, along the eastern edge of the valley. Pulocereus 
fulviceps (plate 16), of more general distribution on slopes, has a series of 
branches, in many instances 40 or 50 in number, densely clustered and 
arising from a short trunk, which barely rises from the ground before it 
branches. P. chrysocantha (plate 17) has more slender branches and 
frequents the slopes to the northward. Opuntias were much in evidence 
as inclosures for small plots around dwellings, embracing several vari- © 
eties and furnishing an edible fruit. Echinocactus was represented by a 
half dozen species, of which one, E. grande (plate 17), is undoubtedly the 
most massive of all the genus, being as much as 8 or g feet in height and 
30 or even 36 inches in thickness, which, with the many convolutions of 
its surface, makes it a very grotesque feature of the scenery. The young 
of this species are characterized by very striking cross-stripes which dis- 
appear with age. Upon testing the pith to compare the watery content 
with the northern species, it was found that so much calcium had been 
taken up and stored in the form of calcium oxalate or carbonate that the 
tissue was unpleasantly gritty when chewed, and that its crispness made 
it difficult to express the juice. FE. flavescens (plate 18) forms small 
heads in clusters, while in E. robusta colonies 10 or 15 feet across, making 
mounds 2 or 3 feet high, include hundreds of heads. 
No systematic account of any desert is to be found in which the stor- 
age function appears so highly developed and by so many species. Of 
course all of the cacti exhibit this feature in a very marked degree, anda 
single plant of Pilocereus fulviceps may retain several hundred gallons of 
water. The large stems of Yucca, which is a prominent member of the 
flora of the slopes, function to this purpose to some extent, while the 
fleshy leaves of Agave marmorata and other species, and of Hectia, are 
essentially storage organs for reserve food and surplus water. Here is 
