什麼是仙人掌

石竹目仙人掌科顯花植物。約1,650種。原產於南、北美洲大部分地區，以墨西哥的仙人掌種類和數目最多。仙人掌爲肉質多年生植物，多生活在已適應的乾燥地區。莖通常肥厚，含葉綠素，草質或木質。多數種類的葉或消失或極度退化，從而減少水分所由丟失的表面積，而光合作用由莖代行。根系通常纖細，纖維狀，淺而分佈範圍廣，用以吸收表層的水分。仙人掌類植株的大小及外形千差萬別，小者如鈕釦狀的佩奧特掌，矮小團塊狀的仙人果，大者如高柱狀的圓桶掌（猛仙人掌屬和仙人球屬）和高大喬木狀的巨山影掌（Cereus giganteus）。仙人掌與其他肉質植物不同之處爲莖上具墊狀的構造--小區。幾乎所有種類的小區內生長棘刺或鉤毛，花、枝和葉（如果有葉的話）亦由此生出。花通常形大而豔麗，多爲單生。仙人掌廣泛栽作觀賞植物，有些種類（特別是仙人果和喬利亞掌）被栽培爲食用作物。圓桶掌在緊急情況下可當作人類飲水的來源。

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.