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may contain few to many (3,000), seeds, which can be between 0.4 and 12 mm long. The life of a cactus is seldom longer than 300 years, but may be as short as 25 years, (although these flower as early as their second year). The Saguaro cactus (Carnegiea gigantea) grows to a height of up to 15 metres (the record is 17 metres 67 cm), but in its first ten years, it grows only 10 centimeters. The "mother-in-law's cushion" (Echinocactus grusonii) reaches a height of 2.5 meters and a diameter of 1 metre and – at least on the Canaries – is already capable of flowering after 6 years. The diameter of cactus flowers ranges from 5 to 30 cm; the colors are often conspicuous and spectacular. The cactus family is endemic to the Americas with one exception, Rhipsalis baccifera; this species has a pantropical distribution, occurring in the Old World tropical Africa, Madagascar and Sri Lanka as well as in tropical America. This plant is thought to be a relatively recent colonist in the Old World (within the last few thousand years), probably carried as seeds in the digestive tracts of migratory birds. Many other cacti have become naturalized to similar environments in other parts of the world after being introduced by people, e.g. Australia, Hawaii, and the Mediterranean region. The Tehuacán Valley of Mexico has one of the richest occurrences of cacti in the world. Species diversity decreases as one travels north; hundreds of species can be found in areas of Mexico, dozens of species are found in the Sonora and Mojave deserts of
the southwestern U.S., and only several species are found in the eastern plains and dry valleys of Montana and Alberta. Cacti are believed to have evolved in the last 30 to 40 million years. Long ago, the Americas were joined to the other continents, but separated due to continental drift. Unique species in the New World must have developed after the continents had moved apart or began developing just prior to the separation. Significant distance between the continents was only achieved in around the last 50 million years. This may explain why cacti are so rare in Africa as the continents had already separated when cacti evolved. Many succulent plants in both the Old and New World bear a striking resemblance to cacti, and are often called "cactus" in common usage.