A large group of desert plants conserve water for use in periods of
drought by storing it in specialized tissues during the wet season. Some
of these “succulents,” principally the yuccas, eschevarias, and agaves,
have developed water-storage tissues in their leaves. A few, notably the
NIGHTBLOOMING CEREUS (see appendix for scientific names of plants), have
slender stems but an enormous, carrot-shaped root in which the
moisture-storage tissue is located. The GOURDS also have large, thick,
moisture-retaining roots, as does the WILD-CUCUMBER.
The cactuses store water in their stems and thus are called stem
succulents. In the course of its evolution the cactus has eliminated
leaves, and their function has been taken over by the green outer
covering of the stems. Thus the amount of transpiration (moisture loss
through plant breathing pores) has been greatly reduced.
[Illustration: Saguaro buds.]
The cactuses are thought to have evolved from relatives of the rose
family in the West Indies, beginning some 18,000 to 20,000 years ago.
From there they spread to most parts of the Western Hemisphere, but
particularly to the drier regions, changing their forms to meet new
conditions. One of the youngest of plant families, the cactuses are
still evolving rapidly. This doesn’t make the task of classifying them
easier for the taxonomists.
In varied forms, cactuses enliven the paloverde-saguaro community. In
size they range from tiny button and pincushion types, some of which
weigh only a few ounces, to the giant saguaro, the bulkiest of which may
weigh several tons.
Cactuses as a group are easily recognized, although many people
mistakenly believe that any desert plant with spines or thorns is a
cactus. Shreve describes their main characteristics thus:
Several structural features have served to give the cacti their
outstanding appearance, so unlike that of other plants. Most general
have been the loss of the leaf as a permanent organ, the enlargement
of the stem to accommodate water-storing tissue, and the development
of local spinebearing structures known as “areoles.” In several
genera, the stem is segmented into sections which are flat and
somewhat leaflike; in others the stem is round, much branched, and the
surface occupied by close-set tubercles. In a large group, including
massive erect forms, as well as slender climbing ones, the stem is
grooved or fluted and thus able readily to accommodate its surface to
great fluctuations in the water content of the tissues.
If you are trying to identify species, however, cactuses can be
annoying, since they often hybridize. You must expect to find some
individuals that don’t fit the book descriptions.
_The Saguaro—Monarch of the Monument_
Of all the species of cactus recorded in Saguaro National Monument, the
giant SAGUARO (pronounced sah-WAH-roe) holds the center of interest.
From the visitor’s standpoint, all other plants, no matter how bizarre
in appearance or peculiar in living habits, are merely stage scenery for
setting off the star of the desert drama. For size, this vegetable
mammoth tops all other succulents of this country; heights of more than
50 feet and weights of more than 5 tons have been reported. There have
been specimens with more than 50 arms, or branches. Although no accurate
method of determining saguaro age has been devised, it is estimated that
an occasional veteran may reach the two-century mark.
Structurally, the giant cactus is well adapted to meet the stern
requirements of its habitat. Its widespread root system, as much as 70
feet in diameter, lying close to the surface of the ground, anchors and
holds the heavy plant erect. The shallow root system quickly and
efficiently collects and channels to the main stem any moisture that may
penetrate the topsoil. The trunk and branches have a cylindrical
framework of long slender poles or ribs fused at the constricted base.
This woody skeleton supports the mass of pulpy tissue, the whole being
covered with tough, waxy, spine-bearing “skin.” Numerous vertical
ridges, like the pleats of a huge accordion, permit the stems and
branches to expand in girth as the tissues swell with water during wet
weather and to shrink during times of drought.
So efficient is the saguaro’s water-storage system that, even after
years of extreme drought, the plant retains enough moisture in reserve
to enable it to produce flower buds. The buds appear in vertical rows at
the tips of the main stem and branches, a few opening each evening over
a period of several weeks in May and June. The flowers, up to 4 inches
in diameter, have waxy white petals. This beautiful blossom is the State
flower of Arizona. The egg-shaped fruits mature in late June and July,
splitting open when ripe to reveal masses of juicy, deep-red pulp filled
with tiny black seeds. Pulp and seeds are consumed by several kinds of
birds, especially white-winged doves. Many fruits that fall to the
ground are promptly eaten by rodents and other animals.
Indians, too, eat the fruits. European explorers who followed Coronado’s
expedition into this region found peaceful Papagos and Opatas living
here, hunting animals and utilizing many native plants. Among the most
dependable of Papago food sources was the fruit of the giant cactus. So
important was this fruit harvest in their economy that they designated
this season as the start of the new year. Today, in some parts of the
desert, Pima and Papago Indians still harvest the fruits. The word
“saguaro” is believed to derive from a Spanish corruption of a Papago
word for the big cactus.
[Illustration: Spiny armor of the saguaro.]
[Illustration: Saguaro in full bloom.]
Saguaros provide not only food for man and beast, but homes for animals.
Walk through a giant cactus forest and you will be amazed at the number
of holes drilled in these plants. The holes are made by Gila woodpeckers
and related gilded flickers, which often relinquish them after one
nesting season. The next occupant may be any of a host of desert
dwellers, including screech and elf owls, purple martins, and
invertebrates. Some birds use the plant as a foundation for their homes.
White-winged doves, for instance, often build flimsy stick platforms on
the tips of saguaro arms; red-tailed hawks and horned owls construct
more substantial nests in the forks.
Although billions of saguaro seeds are produced yearly in the extensive
stands of the monument, only a very few find favorable locations for
germination and growth. Trees, rocks, dead limbs, pebbles—anything that
reduces evaporation in the immediate vicinity of the seed—improve the
chance of germination. These kinds of shelter not only provide the
necessary moisture conditions, but also hide the seed from armies of
ants, rodents, and other animals searching for food.
[Illustration: Saguaro blossoms.]
[Illustration: Saguaro fruit.]
Early growth is extremely slow. A 2-year-old saguaro may be only
one-quarter of an inch in diameter, and a 9-year-old plant may be 6
inches high. These years are the most hazardous. Insect larvae devour
the tiny cactuses. Woodrats and other rodents chew the succulent tissue
for its water, and ground squirrels uproot the young plants with their
digging. In later life, the saguaro must contend with uprooting wind and
human vandalism, as well as the earlier foes—drought, frost, erosion,
and animals.
[Illustration: Gila woodpecker at its nesting hole.]
In a century of maturity, a saguaro may produce 50 million seeds;
replacement of the parent plant would require only that one of these
germinate and grow. But in the cactus forest of the Rincon Mountain
Section, the rate of survival has been even lower, so that over the last
few decades the stand has been dwindling. What is wrong?
Many answers to this question have been advanced, but like all
interrelationships in nature, the saguaro’s role in the desert web of
life is very complex, and involves past events as well as present ones;
a partial answer to the problem may be all we can hope for. The
following reasons for the decline of the saguaros have been suggested by
researchers.
[Illustration: Saguaro, 1 foot high, in a rocky habitat.]
[Illustration: A typical 4-foot saguaro.]
There is some evidence to suggest that the Southwest has been getting
drier since at least the late 19th century, and while the saguaro is
adapted to extreme aridity, some of the “nurse” plants that shelter it
during infancy are not. If such shrubs as paloverdes and mesquites
dwindle, it is argued, so must the saguaro, which in its early years
depends on them for shade.
Other culprits in the saguaro problem are man himself and his livestock.
Around 1880, soon after the first railroad reached Tucson, a cattle boom
began in southern Arizona. The valleys were soon overstocked, and cattle
scoured the mountainsides in search of food. By 1893, when drought and
starvation decimated the herds, the land had been severely overgrazed.
Though the monument was established in 1933, grazing in the Rincon
Mountain Section’s main cactus forest continued until 1958. (Elsewhere
in the monument, it still goes on.) Compounding the problem, woodcutters
removed acres of mesquite and other trees. In the center of the present
Cactus Forest Loop Drive, lime kilns devoured quantities of woody fuel.
Further upsetting the desert’s natural balance, ranchers and Government
agents poisoned coyotes and shot hawks and other predators—in the belief
that this would benefit the owners of livestock.
This unrestrained assault on the environment had unfortunate effects on
saguaros as well as on the human economy. Overgrazing may have resulted
in an increase in kangaroo rats (which benefit from bare ground on which
to hunt seeds) and certain other rodents adapted to an open sort of
ground cover. Man’s killing of predators, their natural enemies, further
encouraged proliferation of these rodents, which some people say are
especially destructive of saguaro seeds and young plants. Whatever the
effect these rodents have on the saguaros, the removal of ground cover
intensified erosion and reduced the chances for the seeds to germinate
and grow. And certainly the cutting of desert trees removed shade that
would have benefited young saguaros. In the Tucson Mountain Section,
which is near the northeastern edge of the Sonoran Desert, freezing
temperatures are perhaps the most important environmental factor in
saguaro mortality.
[Illustration: Looking toward the Santa Catalina Mountains from
Cactus Forest Drive in September 1942.]
Although the causes of decline of the cactus forest lying northwest of
Tanque Verde Ridge are still something of a puzzle, several facts are
clear: the saguaro is not becoming extinct; in rocky habitats many young
saguaros are surviving, promising continued stands for the future; in
non-rocky habitats, some young saguaros are surviving, ensuring that at
least thin stands will endure in these areas. Furthermore, since grazing
was stopped here, ground cover has improved—a plus factor for the
saguaro’s welfare. On the negative side, it is possible that, in
addition to suffering from climatic, biotic, and human pressures, the
once-dense mature stands of the monument are in the down-phase of a
natural fluctuation. It is possible, too, that these stands owed their
exceptional richness to an unusually favorable past environment which
may not occur again. We can hope, however, that sometime in the
not-distant future the total environmental balance will shift once again
in favor of the giant cactus.
[Illustration: A photograph taken from the same spot in January
1970.]
_Other Common Cactuses_
Many other cactuses share the saguaro’s environment. The BARREL CACTUS
is sometimes mistaken for a young saguaro, but can easily be
distinguished by its curved red spines. Stocky and unbranching, this
cactus rarely attains a height of more than 5 or 6 feet. It bears
clusters of sharp spines, called “areoles,” with the stout central spine
flattened and curved like a fishhook. In bloom, in late summer or early
autumn, this succulent plant produces clusters of yellow or orange
flowers on its crown. The widely circulated story that water can be
obtained by tapping the barrel cactus has little basis in fact, although
it is possible that the thick, bitter juice squeezed from the plant’s
moist tissues might, under extreme conditions, prevent death from
thirst. Desert rats, mice, and rabbits, carefully avoiding the spines,
sometimes gnaw into the plant’s tissues to obtain moisture.
The group of cactuses called opuntias (oh-POON-cha) have jointed stems
and branches. They are common and widespread throughout the desert and
are well represented in the monument.
Those having cylindrical joints are known as chollas (CHO-yah), while
those with flat or padlike joints are called pricklypears.
Chollas range in size and form from low mats to small trees, but most of
those in the monument are shrublike. TEDDY BEAR CHOLLA, infamous for its
barbed, hard-to-remove-from-your-skin spines, forms thick stands on warm
south- or west-facing slopes. Its dense armor of straw-colored spines
and its black trunk identify it. Because its joints break off easily
when in contact with man or animal, this uncuddly customer is popularly
called “jumping cactus.” A similar species is CHAIN FRUIT CHOLLA,
notable for its long, branched chains of fruit, which sometimes extend
to the ground. Each year, the new flowers blossom from the persistent
fruit of the previous year. There is a common variety of this species
that is almost spineless. STAGHORN CHOLLA, an inhabitant primarily of
washes and other damp places, is named from its antler-shaped stems.
This cactus’ scientific name—_Opuntia versicolor_—refers to the fact
that its flowers, which appear in April and May, may be yellow, red,
green, or brown. (Each plant sticks with one color through its
lifetime.) Among the smaller chollas, thin-stemmed PENCIL CHOLLA grows
from 2 to 4 feet high on plains and sandy washes. DESERT CHRISTMAS
CACTUS, almost mat-like in form, blooms in late spring but develops
brilliant red fruits which last through the winter.
[Illustration: Barrel cactus blossoms.]
[Illustration: Barrel cactus spines.]
[Illustration: Chain fruit cholla at Tucson Mountain Section
headquarters.]
PRICKLYPEARS, like many of the chollas, produce large blossoms in late
spring. Those on the monument are principally the yellow-flowered
species. The reddish brown-to-mahogany colored edible fruits, called
tunas, attain the size of large strawberries. When mature in autumn,
they are consumed by many desert animals.
Some of the smaller cactuses are so tiny as to be unnoticeable except
when in bloom; examples are the HEDGEHOGS, the FISHHOOKS, and the
PINCUSHIONS. Blossoms of some of these ground-hugging species are large,
in some cases larger than the rest of the plant, and spectacular in form
and color. All add to the monument’s spring and early summer display of
floral beauty.