180 GUIDEBOOK OF THE WESTERN UNITED STATES 
A woodpecker with a red head (male) and a black and white ladder- 
striped back so greatly prefers the branching cacti for its nest that it is 
called the cactus woodpecker (Dryobates scalaris cactophilus). 
The sahuaro is a huge reservoir of water, made up of rods about an 
inch in diameter connected by plant tissue so that it has considerable 
strength to withstand the wind and also great capacity for rapid 
expansion when rain brings a supply of water. When dead the sahuaro 
loses its pulp, leaving a spectral skeleton of a bunch of tough wooden 
rods that burn with a bright flame; they are much used by the Indians 
for sheathing huts and making inclosures. The sahuaro prefers south- 
ward-facing slopes of rocky character where its roots can penetrate the 
soil between the rock fragments; basalt and tuff are favorable, but 
caliche appears not to be. 
Members of the cactus family have remarkable ability to store 
water, because the roots extend widely, for the most part only a 
few inches below the surface, so as to absorb a quantity of water 
from the soil very quickly after a rain. It has been found by investi- 
gations by MacDougal, Spalding, and others at the Desert Botanical 
Laboratory at Tucson that once stored in the plant tissues, this 
water is retained with great tenacity as a provision for long, dry 
intervals. Water absorbed by plants is expended continuously in 
the process of living, mostly by evaporation through their green 
surface. While most of the cacti have leaves, as a rule these are 
minute or even microscopic (the conspicuous parts are botanically 
stems, not leaves), and the structure of their cells is such as to hinder 
tr: tion and conserve the water stored. The slowness of 
chemical reactions in these and most other desert plants aid in the 
conservation of moisture. In the walls of the cells of the cacti are 
thin sievelike places which permit the easy passage of water from 
one cell to another throughout the interior. A barrel cactus was 
found to contain 96 per cent of water. A large sahuaro contains 
many gallons of water, sufficient to maintain it for a year or more. 
The water in some cacti is palatable, but that in others is very bitter, 
and it is interesting to note that those of the latter class are less pro- 
tected by spines than those whose juice is acceptable. The spines 
of the cactus are straight or curved, hairy or feathery, and grouped 
in starry clusters or in rows. They have been used for fishhooks, 
needles, combs, and in various other ways by the primitive tribes. 
The flowers of the cactus vary in form, and most of them are beautiful 
_in brilliant tints of purple, yellow, orange, and rose. (See pl. 31, A.) 
Some open by day; others, such as the night-blooming cereus, by 
ey night. Many of the species bear edible fruits, several of them deli- 
_ ious, and some of them yield seeds used by the Indians for food. 
In Arizona the cacti and some other desert. plants are legally pro- 
from removal, with a ~~ of $50 to $300 for each offense. 
