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succulent(sŭk`yələnt), any fleshy plant that belongs to one of many diverse families, among them species of cactus, aloe, stonecrop, houseleek, agave, and yucca. Most succulents are indigenous to arid or semiarid regions, and their succulence is simply an evolutionary adaptation to the extreme heat and dryness of the environment. Typically the plants have greatly reduced leaves with a hard and heavily cutinized outer surface which minimizes evaporation from the inner, juicy tissue that can retain and store water over long periods. Many are grown horticulturally for their interesting and often grotesque forms, e.g., the ice plant; a few have very attractive flowers.
See H. Jacobsen, A Handbook of Succulent Plants (3 vol., 1973).
a perennial plant with juicy, fleshy leaves (agave, aloe) or stems (cacti, certain spurges); a special type of xero-phyte. Succulents grow in the deserts of Central, North, and South America; some are also encountered in the deserts of southern Africa. The few succulents that occur in the USSR belong to the family Crassulaceae.
The distinctive physical appearance of succulents, which evolved in the process of historical development, is related to their ability to accumulate water in their leaves or stems and to expend the water sparingly during prolonged drought. The leaves and stems have highly developed water-bearing parenchyma. The stems of some species of cacti store 1,000 to 3,000 kg of water; the plants often serve as a source of water for humans and animals during drought. The ability of succulents to expend small amounts of moisture is made possible by the heavy cutinization of the epidermis, by the presence of hairs, by the low osmotic pressure of the cellular juice, and by the presence of only a small number of low-lying stomata.
Succulents are photophilic. Growing under conditions of intense heat, they have developed an increased resistance to high temperatures as a result of the great viscosity of the plasma and the high content of bound water. At the same time, however, the protoplasm is characterized by low elasticity, and, therefore, the plants cannot tolerate dehydration.
Succulents grow slowly as a result of their economical expenditure of water and the nature of their carbon metabolism. In darkness the leaves store substantial quantities of CO2, which results in formation of organic acids. During dry seasons the stomata are closed, and the CO2 serves as the source of carbon. Part of the CO2 is released upon decomposition of the organic acids under the action of light. The formation of endogenous water during respiration maintains hydration of the cell contents and, thus, is of great importance in the life of succulents.
Succulents are widely used as ornamentals (cacti, aloe) and textile plants (agave). Some, for example, opuntias, are used as animal feed. The leaf and stem structure of succulents is also characteristic of many solonchak plants, although the latter have no adaptations for lowering transpiration.
REFERENCESWarming, E. Raspredelenie rastenii v zavisimosti ot vneshnikh uslovii (ekologicheskaia geografiia rastenii). St. Petersburg, 1902. (Translated from German.)
Genkel’, P. A. “Fiziologiia ustoichivosti rastitel’nykh organizmov.” In Fiziologiia sel’skokhoziaistvennykh rastenii, vol. 3. Moscow, 1967.
Vartapetian, B. B. Molekuliarnyi kislorod i voda v metabolizme kletki. Moscow, 1970.
P. A. GENKEL’