Saguaro cactus, giant sponges on wooden frames, stand desert watch

In silent majesty, the giant saguaro cactus stands as guardian of the Sonoran Desert and provides homes for animals and food for wildlife and humans. The saguaro, found only in Baja and southeastern California, southwestern Arizona, and most of the Mexican state of Sonora, are noted for their lovely waxy white blossoms, the state flower of Arizona.

The cactus may live up to 200 years, but the ones that survive to this "old age" are a very select group. The road from seed to giant is filled with obstacles.

The first hurdle for the seed to overcome is to avoid getting eaten. Fruits produced by the plant mature in late June and July. An average two-ounce fruit may contain about 2,000 seeds. The pulp and seeds are eaten by several kinds of birds, particularly the white-winged doves. Fruits that fall to the ground are eaten by rodents and other animals.

Indians eat the fruits, too. In fact, the harvest was so important to the Papago Indians that they adopted this season as the start of their new year. Even today, the Pima and Papago tribes collect the saguaro's produce.

Ultimately, only one seed in a thousand germinates. And these still don't have it made. Drought, freezing, and predators, such as wood rats and rodents, eat the young plants for the water stored in them. And because they are so small -- a five-year-old cactus may be only one-half to an inch tall -- they are often uprooted by busy diggers, such as the ground squirrel.

By the time the survivors reach maturity at 64 to 94 years, they will be 16 to 22 feet tall and will have begun to put out the "arms" that give them their distinctive shape. At 200 years, the saguaro may be 50 feet tall.

In some respects, the cactus are like giant sponges with wooden frames.

The plants are sustained by a broad network of shallow roots that not only hold them upright, but also capture as much liquid as possible during the irregular desert rainfalls. These root systems, some as large as 70 feet in diameter, account for the almost parklike spacing among desert flora. The number of plants is strictly regulated by their ability to establish root systems wide enough to catch sufficient rainfall for survival.

Moisture is carried from the ground to the spongelike tissue inside the saguaro. This tissue is so absorbent that it can retain enough moisture to keep the plant flowering and fruiting even during years of drought.

As the "sponge" gets fuller, the plant expands like an accordion bellows. As the water is used up, it contracts. A 150- to 200-year-old plant may weight up to 10 tons when its reservoir is full.

Supporting this mass is a skeleton of 12 to 30 wooden ribs. Even the arms of the plant have a wooden framework, giving the naked skeleton of a dead saguaro a strangely humanoid appearance.

The outside of the cactus is covered by a tough, thorny, waxy "skin." This substance provides excellent protection for the moisture stored inside. But it is vulnerable to birds, such as the Gila woodpecker and other flickers that regularly make their nests in the cactus.

Fortunately, the plant is able to wall off these wounds in its exterior, giving the bird a home and yet allowing the plant to retain its precious moisture.

Since the woodpecker generally stays only for a season, other creatures, such as screech and elf owls as well as purple martins, move in.

The saguaro offers wildlife a kind of natural high-rise dwelling place because other birds -- white-winged doves, red-tailed hawks, and horned owls, for instance -- build their nests in the forks where the arms and trunk of the cactus are joined.

Recent inroads into the fragile desert environment may be endangering the saguaro's future. No one knows quite why the number of plants is decreasing, but many of the disruptive elements can be traced to humans.

A cattle boom began in southern Arizona around 1880 and much of the land was overgrazed. Woodcutters have removed large numbers of mesquite and other trees that would have provided shade for the young cactus seedlings during their tendest years. The removal of ground cover has increased the damage caused by erosion and had made it more difficult for seeds to take root.

Even the poisoning of coyotes and the destruction of hawks and similar predators have had an impact. With these natural controls gone, or greatly reduced, kangaroo rats and othere rodents have been on the rise. These animals eat more seeds, reducing the possibilities that a seed will germinate.

Although the number of saguaros has been declining, the cactus becoming extinct. Young plants are surviving, particularly in rocky areas that are not so vulnerable to erosion. Because the cactus has such a long life span, it's hard to determine if this period is just a "down" cycle. Further generations, faced with more hospitable conditions, could be more abundant.

Two sections of the desert have been set aside near Tucson, Ariz., as the Saguaro National Monument to preserve particularly fine stands of cactus. The western portion is less developed, and not all roads are paved. But the eastern half, called the Rincon Mountain Unit, has a nine-mile paved cactus drive that gives a visitor ample opportunities to stop, walk the nature trails, and enjoy the elegant silence of life among the giants.