A cactus (plural: cactuses, cacti) is any member of the plant family Cactaceae, native to the Americas (with one exception, Rhipsalis bacifera, which is native to parts of the Old World). They are often used as ornamental plants, and some are also crop plants for fodder, forage, fruits, cochineal, and other uses. Cactuses are part of the plant order Caryophyllales, which also includes members like beets, gypsophila, spinach, amaranth, tumbleweeds, carnations, rhubarb, buckwheat, plumbago, bougainvillea, chickweed and knotgrass.
Cactuses are unusual and distinctive plants, which are adapted to extremely arid and/or semi-arid hot environments, as well as tropical environments as epiphytes or hemi-epiphytes . They show a wide range of anatomical and physiological features which conserve water. Their stems have adapted to become photosynthetic and succulent, while the leaves have become the spines for which cacti are well known.
Cactuses come in a wide range of shapes and sizes. The tallest is Pachycereus pringlei, with a maximum recorded height of 19.2 m, and the smallest is Blossfeldia liliputiana, only about 1 cm diameter at maturity. Cactus flowers are large, and like the spines and branches arise from areoles. Many cactus species are night blooming, as they are pollinated by nocturnal insects or small animals, principally moths and bats. Cactuses range in size from small and globular to tall and columnar.