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a genus of plants in the Cactaceae family. There is one species—the old-man cactus (C. senilis). The columnar and only slightly branched stem reaches 15 m in height and 30 cm in diameter. Lignification is insignificant. There are 20 to 30 ribs, bearing bundles of small spines and long white straight or curly hairs. Plants entering into the blooming phase form a special organ—the cephalium—a zone on the stem where, instead of ribs, low areoles with bundles of long bristles and hairs are formed. The bell mouth-shaped flowers developing on the cephalium are pinkish and unpleasant in odor; they open at night. The fruits are violet-red berries. The old-man cactus is native to Mexico. The plant grows well on steep limestone slopes in dry, hot valleys. The cactus is cultivated in botanical gardens and as an ornamental house-plant.