Cactus

Cacti are perennial plants. Their stems are fleshy or succulent, and are cylindrical or flattened in shape. The stems are green-colored, and are photosynthetic, usually performing this function instead of leaves, which are greatly reduced in abundance or even absent in most mature cacti. Most species of cactus are well-protected by sharp bristles and spines, which serve to deter most herbivores.

The stems of cactus plants have numerous cushion- or pit-like structures known as areoles on their surface, from which usually emerge clusters of spines. In terms of developmental biology, areoles are usually interpreted as being incompletely developed, axillary stem branches. The spines are actually modified leaves. The areoles may also be protected by hook-like barbs known as glochidia. The roots of cacti are shallow and may be widely spread in the soil.

The flowers of cacti are usually perfect (bisexual), containing both male reproductive organs (stamens) and female parts (a pistil). The flowers occur singly, rather than in groups, although many discrete flowers may be present on a cactus at the same time. The flowers of most species of cacti are large and showy, and they can be colored white, red, pink, orange, or yellow, but not blue. The sepals of the calyx are petal-like in shape and color, and they combine with the numerous petals to form an attractive, often richly scented, nectar-producing flower, designed to lure such pollinators as hawk-moths, bees, bats, and birds, especially hummingbirds and small doves. The fruit is a many-seeded berry.

Cacti are xerophytic plants, meaning they are physiologically and morphologically adapted to coping with the extreme water deficiencies of dry habitats, such as deserts. The xerophytic adaptations of cacti include: (1) their succulent, water-retaining stems, (2) a thick, waxy cuticle and few or no leaves to greatly reduce the losses of water through transpiration, (3) stems that are photosynthetic, so leaves are not required to execute this function, (4) stems that are cylindrical or spherical in shape, which reduces the surface to volume ratio, and helps to preserve moisture, (5) tolerance of high tissue temperatures, (6) protection of the biomass and moisture reserves from herbivores by an armament of stout spines, (7) a physiological tolerance of long periods of drought, and (8) a periodic pattern of growth, productivity, and flowering, which takes advantage of the availability of moisture during the brief, rainy season, while the plant remains dormant at drier times of the year.

Cacti have a so-called crassulacean-acid metabolism, in which atmospheric carbon dioxide is only taken up during the night, when the stomates are open. The carbon dioxide is fixed into four-carbon, organic acids, and can later be released within the plant, to be fixed into sugars by photosynthesis when the sun is shining during the daylight hours. Because this system allows stomates to be kept tightly closed during the day, crassulacean-acid metabolism is an efficient way of conserving water in dry environments.

A prickly pear cactus in Big Bend National Park, Texas. The prickly pear is the only widespread eastern cactus in the United States. It can be found as far north as southern Ontario. Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission.

Some plant species of dry habitats that are not related to cacti are nevertheless remarkably similar in appearance (at least, apart from their flowers and fruits, which are always distinctive among plant families). This is the result of convergent evolution, the similar evolutionary development of unrelated species or families that are subjected to comparable types of environmental selective pressures. Some species of spurges (family Euphorbiaceae) that grow in dry habitats are commonly thought by non-botanists to be cacti, even though they are quite unrelated.