Carnegiea gigantea - (Cereus giganteus)
The saguaro cactus is composed of a tall, thick, fluted, columnar stem, 18 to 24 inches in diameter, often with several large branches (arms) curving upward in the most distinctive conformation of all Southwestern cacti.
The skin is smooth and waxy, and the trunk and stems have stout, two inch spines clustered on their ribs. When water is absorbed, the outer pulp of the saguaro can expand like an accordion, increasing the diameter of the stem and, in this way, can increase its weight by up to a ton.
The saguaro often begins life in the shelter of a "nurse" tree or shrub which can provide a shaded, moist habitat for the germination of life. The saguaro grows very slowly -- perhaps an inch a year -- but to a great height, 15 to 50 feet. The largest plants, with more than five arms, are estimated to be 200 years old. The average old saguaro has five arms and is about 30 feet tall.
The saguaro has a surprisingly shallow root system, considering its great height and weight. It is supported by a tap root that is only a pad about three feet long, as well as numerous stout roots no deeper than a foot, emanating radially from its base. More smaller roots run radially to a distance equal to the height of the saguaro. These roots wrap about rocks providing anchorage from winds across the rocky bajadas.
Sonoran Desert of extreme southeastern California, southern Arizona and adjoining northwestern Mexico.
Desert slopes and flats, especially rocky bajadas.
Creamy-white, three inch wide flowers with yellow centers bloom May and June. Clustered near the ends of branches, the blossoms open during cooler desert nights and close again by the next midday. The saguaro flower is the state flower of Arizona.
The slow growth and great capacity of the saguaro to store water allows it to flower every year, regardless of rainfall. The night-blooming flowers, about three inches wide, have many creamy-white petals around a tube about four inches long. Like most cactus, the buds appear on the southeastern exposure of stem tips, and flowers may completely encircle stems in a good year.
A dense group of yellow stamens forms a circle at the top of the tube; the saguaro has more stamens per flower than any other desert cactus. A sweet nectar accumulates in the bottom of this tube. The saguaro can only be fertilized by cross-pollination -- pollen from a different cactus. The sweet nectar, together with the color of the flower, attracts birds, bats and insects, which in acquiring the nectar, pollinate the saguaro flower.
Unlike the Queen of the Night cactus, not all of the flowers on a single saguaro bloom at the same time. Instead, over a period of a month or more, only a few of the up to 200 flowers open each night, secreting nectar into their tubes, and awaiting pollination. These flowers close about noon the following day, never to open again. If fertilization has occurred, fruit will begin to form immediately.
The three inch, oval, green fruit ripens just before the fall rainy season, splitting open to reveal the bright-red, pulpy flesh which all desert creatures seem to relish. This fruit was an especially important food source to Native Americans of the region who used the flesh, seeds and juice. Seeds from the saguaro fruit are prolific -- as many as 4,000 to a single fruit -- probably the largest number per flower of any desert cactus.
While the whitewing dove (whose northern range coincides with range of the saguaro) is one of its primary pollinators, it is the Gila woodpecker and the gilded flicker who can be observed making their home in the saguaro by chiseling out small holes in the trunk.
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