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Cactus et plantes succulentes (Mai 2024)
Cactus, (famille des Cactaceae), cactus pluriels ou cactus, famille des plantes à fleurs (ordre des Caryophyllales) avec près de 2000 espèces et 139 genres. Les cactus sont originaires de la majeure partie de la longueur de l'Amérique du Nord et du Sud, de la Colombie-Britannique et de l'Alberta vers le sud; la limite la plus au sud de leur aire de répartition s'étend jusque dans le Chili et l'Argentine. Le Mexique possède le plus grand nombre et la plus grande variété d'espèces. Les seuls cactus possiblement originaires de l'Ancien Monde sont des membres du genre Rhipsalis, présents en Afrique de l'Est, à Madagascar et au Sri Lanka. Bien que quelques espèces de cactus habitent des zones tropicales ou subtropicales, la plupart vivent dans des régions sèches et sont bien adaptées à celles-ci. Voir aussi la liste des plantes de la famille des Cactaceae.
Caryophyllales: Cactaceae
Les cactus sont des plantes curieuses, souvent épineuses (épineuses), à tige succulente constituant la famille des Cactaceae, caractéristiques et bien adaptées
Caractéristiques physiques
Les cactus sont des plantes vivaces succulentes. Les cactus ont généralement des tiges épaisses herbacées ou ligneuses contenant de la chlorophylle. Les cactus se distinguent des autres plantes succulentes par la présence d'aréoles, de petites structures en forme de coussin avec des trichomes (poils végétaux) et, dans presque toutes les espèces, des épines ou des poils barbelés (glochides). Les aréoles sont des branches modifiées, à partir desquelles des fleurs, plus de branches et des feuilles (lorsqu'elles sont présentes) peuvent pousser.
In most species, leaves are absent, greatly reduced, or modified as spines, minimizing the amount of surface area from which water can be lost, and the stem has taken over the photosynthetic functions of the plant. Only the tropical genera Pereskia and Pereskopsis, both vines, have conventional-looking functional leaves, while the leaves of the Andean Maihuenia are rounded, not flattened. The root systems are generally thin, fibrous, and shallow, ranging widely to absorb superficial moisture.
Cacti vary greatly in size and general appearance, from buttonlike peyote (Lophophora) and low clumps of prickly pear (Opuntia) and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro (Carnegiea gigantea). Most cacti grow in the ground, but several tropical species—including leaf cactus (Epiphyllum), Rhipsalis, and Schlumbergera—are epiphytes, growing on other plants; others live on hard substrates such as rocks, while yet others climb far up trees. Epiphytic species tend to have thin, almost leaflike flattened stems. The appearance of the plant varies also according to whether the stem surface is smooth or ornamented with protruding tubercles, ridges, or grooves.
The primary method of reproduction is by seeds. Flowers, often large and colourful, are usually solitary. All genera have a floral tube, often with many petal-like structures, and other less colourful and almost leaflike structures; the tube grows above a one-chambered ovary. A style topped by many pollen-receptive stigmas also arises from the top of the ovary. The fruit is usually a berry and contains many seeds. Soon after pollination, which may be effected by wind, birds, insects, or bats, the entire floral tube detaches from the top of the ovary to leave a prominent scar.
Several cacti develop plantlets at ground level that, as offsets, reproduce the species vegetatively. Many others can reproduce by fragmentation, whereby segments broken from the main plant will readily root to form clonal individuals. Tissues of cacti are broadly compatible so that terminal portions of one species may be grafted on top of another.
The internal structure of cacti stems conforms to the pattern of broad-leaved angiosperms; a cambium layer of dividing cells, located between the woody inner tissues and those near the outside of the stem, is present. The bulk of the stem, however, consists of thin-walled storage cells that contain mucilaginous substances that prevent the loss of moisture. The stem of cacti is the main food-manufacturing and food-storage organ for most species.
Uses
Cacti are widely cultivated as ornamentals. In addition, various species, notably prickly pears and chollas (Opuntia and Cylindopuntia, respectively), are cultivated as food. In Central and South America, species of Opuntia, Cereus, and others are used as living fences, and wood from columnar cacti is used as fuel in some desert regions. In times of drought, the spines are removed from cacti such as mandacaru (Cereus jamacaru) to use as fodder for livestock. Peyote, from Lophophora williamsii, has been used ceremonially since pre-Columbian times for its hallucinogenic properties, and many cactus species are of local importance in traditional medicine.
