仙人掌(cactus)
石竹目仙人掌科显花植物。约1,650种。原产於南、北美洲大部分地区，以墨西哥的仙人掌种类和数目最多。仙人掌为肉质多年生植物，多生活在已适应的乾燥地区。茎通常肥厚，含叶绿素，草质或木质。多数种类的叶或消失或极度退化，从而减少水分所由丢失的表面积，而光合作用由茎代行。根系通常纤细，纤维状，浅而分布范围广，用以吸收表层的水分。仙人掌类植株的大小及外形千差万别，小者如钮扣状的佩奥特掌，矮小团块状的仙人果，大者如高柱状的圆桶掌（猛仙人掌属和仙人球属）和高大乔木状的巨山影掌（Cereus giganteus）。仙人掌与其他肉质植物不同之处为茎上具垫状的构造--小区。几乎所有种类的小区内生长棘刺或钩毛，花、枝和叶（如果有叶的话）亦由此生出。花通常形大而艳丽，多为单生。仙人掌广泛栽作观赏植物，有些种类（特别是仙人果和乔利亚掌）被栽培为食用作物。圆桶掌在紧急情况下可当作人类饮水的来源。
English version：
cactus
Any of the flowering plants that make up the family Cactaceae, containing about 1,650 species, native through most of North and South America, with the greatest number and variety in Mexico. Cacti are succulent perennials. Most live in and are well adapted to dry regions. Cacti generally have thick herbaceous or woody stems containing chlorophyll. Leaves usually are absent or greatly reduced, minimizing the surface area from which water can be lost; the stem is the site of photosynthesis. The generally thin, fibrous, shallow root systems range widely in area to absorb superficial moisture. Cacti vary greatly in size and appearance, from buttonlike peyote and low clumps of prickly pear and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro. Cacti can be distinguished from other succulent plants by the presence of small cushionlike structures (areoles) from which, in almost all species, spines arise, as do flowers, branches, and leaves (when present). Flowers, often large and colorful, are usually solitary. Cacti are widely cultivated as ornamentals. Various species, notably prickly pears and chollas, are cultivated as food. Barrel cacti are an emergency source of water for humans.
