The saguaro ( sə-(G)WAR-oh, Spanish: [saˈɣwaɾo]; Carnegiea gigantea) is a tree-like cactus species in the monotypic genus Carnegiea that can grow to be over 12 meters (40 feet) tall. It is native to the Sonoran Desert in Arizona, the Mexican state of Sonora, and the Whipple Mountains and Imperial County areas of California. Saguaro typically grow at elevations ranging from sea level to 4,500 feet (1,400 m), although they may be found at up to 5,000 feet (1,500 m). The saguaro blossom is the state wildflower of Arizona. Its scientific name is given in honor of Andrew Carnegie. In 1933, Saguaro National Park, near Tucson, Arizona, was designated to help protect this species and its habitat.
Saguaros have a relatively long lifespan, often exceeding 150 years. They may grow their first side arm around 75–100 years of age, but some never grow any arms. Arms are developed to increase the plant's reproductive capacity, as more apices lead to more flowers and fruit. A saguaro can absorb and store considerable amounts of rainwater, visibly expanding in the process, while slowly using the stored water as needed. This characteristic enables the saguaro to survive during periods of drought. It is a keystone species, and provides food and habitat to a large number of species. Saguaros have been a source of food and shelter for humans for thousands of years. Their sweet red fleshed fruits are turned into syrup by native peoples, such as the Tohono Oʼodham and Pima. Their ribs are used as building materials in the wood-poor deserts. The saguaro cactus is a common image in Mexican and Arizonan culture, and American Southwest films.
Description The saguaro is a columnar cactus that grows branches, usually referred to as arms. Over 50 arms may grow on one plant, with one specimen having 78 arms. Saguaros grow from 3–16 m (10–52 ft) tall, and up to 75 cm (30 in) in diameter. They are slow growing, and routinely live 150 to 200 years. They are the largest cactus in the United States.
The growth rate of this cactus is strongly dependent on precipitation; saguaros in drier western Arizona grow only half as fast as those in and around Tucson. Saguaros grow slowly from seed, and may be only 6.4 mm (1⁄4 in) tall after two years. Cuttings rarely root, and when they do, they do not go through the juvenile growth phase, which gives a different appearance. Since 2014, the National Register of Champion Trees listed the largest known living saguaro in the United States in Maricopa County, Arizona, measuring 13.8 m (45 ft 3 in) high with a girth of 3.1 m (10 ft 2 in); it has an estimated age of 200 years and survived damage in the 2005 Cave Creek Complex Fire. The tallest saguaro ever measured was an armless specimen found near Cave Creek, Arizona. It was 78 ft (23.8 m) in height before it was toppled in 1986 by a windstorm. Saguaros are stem succulents and can hold large amounts of water; when rain is plentiful and the saguaro is fully hydrated, it can weigh between 1,500 and 2,200 kg (3,200 and 4,800 lb).
Saguaros have a very large root network that can extend up to 30 m (100 ft), and long taproots of up to 1 m (3 ft 3 in) deep. Saguaros may take between 20 and 50 years to reach a height of 1 m (3 ft 3 in). Individual stomatal guard cells and medulla cells can live and function for as long as 150 years, possibly the longest living of all cells, except nerve cells in some tortoises. As a cactus, it uses crassulacean acid metabolism photosynthesis, which confers high levels of water-use efficiency. This allows the saguaro to transpire only at night, minimizing daytime water loss. A saguaro without arms is called a "spear". Some saguaros grow in rare formations called a cristate, or "crested" saguaro. This growth formation is believed to be found in one in roughly 10,000 saguaros, with 2,743 crested saguaros documented as of 2020. The crest formation, caused by fasciation, creates a seam of abnormal growth along the top or top of the arm of the saguaro.
Ribs
Inside the saguaro, many "ribs" of wood form something like a skeleton, with the individual ribs being as long as the cactus itself and up to a few centimeters in diameter. The rib wood itself is also relatively dense, with dry ribs having a solid density around 430 kg/m3 (27 lb/cu ft), which made the ribs useful to indigenous peoples as a building material. While the ribs of dead plants are not protected by the Arizona native plant law, the Arizona Department of Agriculture has released a memo discussing when written permission is needed before harvesting them because of the importance of the decomposition of cactus remains in maintaining desert soil fertility. The composition of the ribs is similar to that of hardwoods.
Spines
The spines on a saguaro are extremely sharp and can grow to 7 cm (3 in) long, and up to 1 mm (1⁄32 in) per day. When held up to the light or bisected, alternating light and dark bands transverse to the long axis of spines are visible. These bands have been correlated to daily growth. In columnar cacti, spines almost always grow in areoles that originate at the apex of the plant. A spine stops growing in its first season. Areoles are moved to the side and the apex continues to grow upward. Thus, older spines are toward the base of a columnar cactus and newer spines are near the apex. A 2007 study examined the relationship of carbon and oxygen isotope ratios in the tissues of spines of an individual to its climate and photosynthetic history (acanthochronology). The spines may cause significant injury to animals; one paper reported that a bighorn sheep skull had been penetrated by a saguaro spine after the sheep collided with a saguaro. They can also cause severe injury to humans, being as sharp and nearly as strong as steel needles. Their long, unbarbed nature means that partially embedded spines can be easily removed, but their relative length can complicate injuries. The spines can puncture deeply, and if broken off, can leave splinters of spine deep in the tissue that can be difficult to remove. Fully embedded spikes are also difficult to remove. Such injuries do not usually result in infection, though, as the cactus spines are generally aseptic. However, spines that remain embedded may cause inflammatory granuloma.
Flowers
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