In botany, succulent plants, also known as succulents, are plants with parts that are thickened, fleshy, and engorged, usually to retain water in arid climates or soil conditions. The word succulent comes from the Latin word sucus, meaning "juice" or "sap". [1]
Succulents may store water in various structures, such as leaves and stems. The water content of some succulent organs can get up to 90–95%, [2] such as Glottiphyllum semicyllindricum and Mesembryanthemum barkleyii. [3] Some definitions also include roots, thus geophytes that survive unfavorable periods by dying back to underground storage organs may be regarded as succulents. The habitats of these water-preserving plants are often in areas with high temperatures and low rainfall, such as deserts, but succulents may be found even in alpine ecosystems growing in rocky soil. Succulents are characterized by their ability to thrive on limited water sources, such as mist and dew, which makes them equipped to survive in an ecosystem that contains scarce water sources.
Succulents are not a taxonomic category, since the term describes only the attributes of a particular species; some species in a genus such as Euphorbia , or family such as Asphodelaceae may be succulent, whereas others are less so or not at all. Many plant families have multiple succulent species found within them, more than 25 plant families. [4] In some families, such as Aizoaceae, Cactaceae, and Crassulaceae, most species are succulents. In horticultural use, the term is sometimes used in a way that excludes plants that botanists would regard as succulents, such as cacti. Succulents are often grown as ornamental plants because of their striking and unusual appearance, as well as their ability to thrive with relatively minimal care.
By definition, succulent plants are drought-resistant plants in which the leaves, stem, or roots have become more than usually fleshy by the development of water-storing tissue. [5] Other sources exclude roots as in the definition "a plant with thick, fleshy and swollen stems and/or leaves, adapted to dry environments". [6] The difference affects the relationship between succulents and "geophytes"–plants that survive unfavorable seasons as a resting bud on an underground organ. [7]
The underground organs, such as bulbs, corms, and tubers, are often fleshy with water-storing tissues. Thus, if roots are included in the definition, many geophytes would be classed as succulents. Plants adapted to living in dry environments such as succulents, are termed xerophytes . Not all xerophytes are succulents, since there are other ways of adapting to a shortage of water, e.g., by developing small leaves which may roll up or having leathery rather than succulent leaves. [8] Nor are all succulents xerophytes, as plants such as Crassula helmsii are both succulent and aquatic. [9] Succulents allow themselves to go a long ways without any or low water necessary.
Some who grow succulents as a hobby may use the term in a different way from botanists. In horticultural use, the term succulent regularly excludes cacti. For example, Jacobsen's three volume Handbook of Succulent Plants does not include cacti. [10] Many books covering the cultivation of these plants include "cacti (cactus) and succulents" as the title or part of the title. [11] [12] [13] In botanical terminology, cacti are succulents, [5] but not the reverse, as many succulent plants are not cacti. Cacti form a monophyletic group and apart from one species are native only to the New World, the Americas, but through parallel evolution similar looking plants in completely different families like the Apocynaceae evolved in the Old World.[ citation needed ]
A further difficulty for general identification is that plant families are neither succulent nor non-succulent and can contain both. In many genera and families, there is a continuous gradation from plants with thin leaves and normal stems to those with very clearly thickened and fleshy leaves or stems. The succulent characteristic becomes meaningless for dividing plants into genera and families. Different sources may classify the same species differently. [14] Species with intermediate characteristics such as somewhat fleshy leaves or stems may be described as semi-succulent. [15]
Horticulturists often follow commercial conventions and may exclude other groups of plants such as bromeliads, that scientifically are considered succulents. [16] A practical horticultural definition has become "a succulent plant is any desert plant that a succulent plant collector wishes to grow", without any consideration of scientific classifications. [17] Commercial presentations of "succulent" plants will present those that customers commonly identify as such. Plants offered commercially then as "succulents", such as hen and chicks, will less often include geophytes, in which the swollen storage organ is wholly underground, but will include plants with a caudex, [18] that is a swollen above-ground organ at soil level, formed from a stem, a root, or both. [7]
The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants, a characteristic known as succulence. In addition to succulence, succulent plants variously have other water-saving features. These may include:
Other than in Antarctica, succulents can be found within each continent. According to the World Wildlife Fund, South Africa is home to around a third of all succulent species, most residing in the succulent Karoo biome. [21] [22] While it is often thought that most succulents come from dry areas such as steppes, semi-desert, and desert, the world's driest areas do not make for proper succulent habitats, mainly due to the difficulty such low growing plants or seedlings would have to thrive in environments where they could easily be covered by sand. [23]
Australia, the world's driest inhabited continent, hosts very few native succulents due to the frequent and prolonged droughts.[ citation needed ] Even Africa, the continent with the most native succulents, does not host many of the plants in its most dry regions. [24] While succulents are unable to grow in these harshest of conditions, they are able to grow in conditions that are uninhabitable by other plants. In fact, many succulents are able to thrive in dry conditions, and some are able to last up to two years without water depending on their surroundings and adaptations. [25]
Occasionally, succulents may occur as epiphytes, growing on other plants with limited or no contact with the ground, and being dependent on their ability to store water and gaining nutrients by other means; it is seen in Tillandsia . Succulents also occur as inhabitants of sea coasts and dry lakes, which are exposed to high levels of dissolved minerals that are deadly to many other plant species. California is home to close to hundred succulent species that are native to the state, many of them live in coastal environments. [26] Potted succulents are able to grow in most indoor environments with minimal care. [27]
There is a thriving illegal trade in cacti and succulents. [28] [29] In South Africa, several species of succulent have been threatened with extinction due to poaching from the wild for the black market and mining related activities. The plants are mainly sold to collectors in Asian countries, where there has been a high demand for them. [30] [22] Since 1974, it is illegal to be in possession of protected succulents such as the Conophytum without authorisation in the Western Cape and Northern Cape, the two South African provinces where they grow. [30]
There are approximately sixty different plant families that contain succulents. [31]
Plant orders, families, and genera in which succulent species occur are listed below.
Order Alismatales
Order Apiales
Order Arecales (also called Principes)
Order Asparagales
Order Asterales
Order Brassicales
Order Caryophyllales
Order Commelinales
Order Cornales
Order Cucurbitales
Order Dioscoreales
Order Ericales
Order Fabales
Order Filicales
Order Gentianales
Order Geraniales
Order Lamiales
Order Malpighiales
Order Malvales
Order Myrtales
Order Oxalidales
Order Piperales
Order Poales
Order Ranunculales
Order Rosales
Order Santalales
Order Sapindales
Order Saxifragales
Order Solanales
Order Vitales
Order Zygophyllales
(unplaced order)* Boraginaceae : Heliotropium (unplaced order)* Icacinaceae : Pyrenacantha (geophyte)
There also were some succulent gymnosperms (but extinct since the end of the Cretaceous):
Order Pinales
Frenelopsis , Pseudofrenelopsis , Suturovagina , Glenrosa
For some families and subfamilies, most members are succulent; for example the Cactaceae, Agavoideae, Aizoaceae, and Crassulaceae.
The table below shows the number of succulent species found in some families and their native habitat:[ citation needed ]
Family or subfamily | Succulent # | Modified parts | Distribution |
---|---|---|---|
Agavoideae | 300 | Leaf | North and Central America |
Cactaceae | 1600 | Stem (root, leaf) | The Americas |
Crassulaceae | 1300 | Leaf (root) | Worldwide |
Aizoaceae | 2000 | Leaf | Southern Africa, Oceania, Chile |
Apocynaceae | 500 | Stem | Africa, Arabia, India, Australia |
Asphodelaceae | 500+ | Leaf | Africa, Madagascar, Australia |
Didiereaceae | 11 | Stem | Madagascar (endemic) |
Euphorbiaceae | > 1000 | Stem or leaf or root | Australia, Africa, Madagascar, Asia, the Americas, Europe |
Portulacaceae | ~500 | Leaf and stem | The Americas, Australia, Africa |
Cheirolepidiaceae | 4, maybe more | Leaf | Worldwide, except Antarctica |
Succulents are favored as houseplants for their attractiveness and ease of care. They have been cultivated as houseplants since at least the 17th century. [39] If properly potted, succulents require little maintenance to survive indoors. [40] Succulents are very adaptable houseplants and will thrive in a range of indoor conditions. [41] For most plant owners, over-watering and associated infections are the main cause of death in succulents. [42]
Succulents can be propagated by different means. The most common is vegetative propagation. This includes cuttings where several inches of stem with leaves are cut and after healing, produce a callus. After a week or so, roots may grow. A second method is division consisting of uprooting an overgrown clump and pulling the stems and roots apart. [43]
A third method is propagation by leaf by allowing the formation of a callus. During this method, a bottom leaf is fully removed from the plant often by twisting or cutting. The leaf then dries out and a callus forms preventing the leaf from absorbing too much moisture and rotting. This method typically takes up to a few weeks to produce healthy roots that would eventually create new plants. [44] The vegetative propagation can be different according to the species. [45]
A cactus is a member of the plant family Cactaceae, a family of the order Caryophyllales comprising about 127 genera with some 1,750 known species. The word cactus derives, through Latin, from the Ancient Greek word κάκτος (káktos), a name originally used by Theophrastus for a spiny plant whose identity is now not certain. Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north, with the exception of Rhipsalis baccifera, which is also found in Africa and Sri Lanka. Cacti are adapted to live in very dry environments, including the Atacama Desert, one of the driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of true leaves, cacti's enlarged stems carry out photosynthesis.
Peperomia is one of the two large genera of the family Piperaceae. It is estimated that there are at least over 1,000 species, occurring in all tropical and subtropical regions of the world. They are concentrated in South and Central America, but may also be found in southern North America, the Caribbean islands, Africa, Oceania, and southern and eastern parts of Asia. The exact number of species is difficult to determine, as some plants have been recorded several times with different names, and new species continue to be discovered. Peperomias have adapted to many different environments and their appearances vary greatly. Some are epiphytes or lithophytes, and many are xerophytes or possess underground tubers (geophytes). Most species are compact perennial shrubs or vines.
The Crassulaceae, also known as the stonecrop family or the orpine family, are a diverse family of dicotyledon flowering plants characterized by succulent leaves and a unique form of photosynthesis, known as Crassulacean acid metabolism (CAM). Flowers generally have five floral parts. Crassulaceae are usually herbaceous but there are some subshrubs, and relatively few treelike or aquatic plants. Crassulaceae are a medium-sized monophyletic family in the core eudicots, among the order Saxifragales, whose diversity has made infrafamilial classification very difficult. The family includes approximately 1,400 species and 34–35 genera, depending on the circumscription of the genus Sedum, and distributed over three subfamilies. Members of the Crassulaceae are found worldwide, but mostly in the Northern Hemisphere and southern Africa, typically in dry and/or cold areas where water may be scarce, although a few are aquatic.
Pachypodium is a genus of succulent spine-bearing trees and shrubs, native to Madagascar and Africa. It belongs to the family Apocynaceae.
The Aizoaceae, or fig-marigold family, is a large family of dicotyledonous flowering plants containing 135 genera and about 1,800 species. Several genera are commonly known as 'ice plants' or 'carpet weeds'. The Aizoaceae are also referred to as vygies in South Africa. Some of the unusual Southern African genera—such as Conophytum, Lithops, Titanopsis and Pleiospilos —resemble gemstones, rocks or pebbles, and are sometimes referred to as 'living stones' or 'mesembs'.
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but they open at night to collect carbon dioxide and allow it to diffuse into the mesophyll cells. The CO2 is stored as four-carbon malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO2, which is then used during photosynthesis. The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This mechanism of acid metabolism was first discovered in plants of the family Crassulaceae.
A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have an underground storage organ are called geophytes in the Raunkiær plant life-form classification system. Storage organs often, but not always, act as perennating organs which enable plants to survive adverse conditions.
Ariocarpus is a small genus of succulent, subtropical plants of the family Cactaceae.
Pereskiopsis is a genus of cactus in the subfamily Opuntioideae. Unlike typical cacti, it has persistent fleshy leaves. The genus name refers to its resemblance to the genus Pereskia. Most species are found in Mexico south through Guatemala to Honduras, with one species in Bolivia. The incorrect spelling Peireskiopsis has also been used.
Delosperma cooperi, the trailing Iceplant, hardy iceplant or pink carpet, is a dwarf perennial plant native to South Africa. It forms a dense lawn with abundant, long-lasting flowers. It reaches sizes of approximately 10–15 cm (4–6 in) tall, with fleshy leaves that are linear and simple and can grow up to 1.5 inches long and a trailing stem that hangs down. These fleshy roots help provide the ability for the plants to recover and grow rapidly if a disturbance has occurred.
Conophytum is a genus of South African and Namibian succulent plants that belong to the family Aizoaceae. The name is derived from the Latin conus (cone) and Greek phytum (plant). The plants are also known as knopies, waterblasies, sphaeroids, conos, cone plants, dumplings, or button plants.
A xerophyte is a species of plant that has adaptations to survive in an environment with little liquid water. Examples of xerophytes include cacti, pineapple and some gymnosperm plants. The morphology and physiology of xerophytes are adapted to conserve water during dry periods. Some species called resurrection plants can survive long periods of extreme dryness or desiccation of their tissues, during which their metabolic activity may effectively shut down. Plants with such morphological and physiological adaptations are said to be xeromorphic. Xerophytes such as cacti are capable of withstanding extended periods of dry conditions as they have deep-spreading roots and capacity to store water. Their waxy, thorny leaves prevent loss of moisture.
Mesembryanthemum cordifolium, formerly known as Aptenia cordifolia, is a species of succulent plant in the iceplant family. It is a creeping plant that forms a carpet of flat-growing perennial herbs in groups on the ground from a base. The genus name means middle-embryo flower in reference to the position of the ovary in the flower. The specific epithet is derived from Latin for heart-shaped leaves.
The Huntington Desert Garden is part of The Huntington Library, Art Collections and Botanical Gardens in San Marino, California. The Desert Garden is one of the world's largest and oldest collections of cacti, succulents and other desert plants, collected from throughout the world. It contains plants from extreme environments, many of which were acquired by Henry E. Huntington and William Hertrich in trips taken to several countries in North, Central and South America.
Crassula muscosa, synonyms Crassula lycopodioides and Crassula pseudolycopodioides, is a succulent plant native to South Africa and Namibia, belonging to the family of Crassulaceae and to the genus Crassula. It is a houseplant grown worldwide and commonly known as rattail crassula, watch chain, lizard's tail, zipper plant and princess pines.
Crassula marnierana, common name Jade Necklace or Chinese Pagoda, is a species of succulent flowering plant in the family Crassulaceae.
Fockea is a genus of succulent scrubs native to Africa south of the equator. They are members of the Asclepiadoideae (milkweeds), a subfamily of the dogbane family Apocynaceae. Of the six recognized species, only the two most widely distributed extend north of southern Africa, with F. multiflora reaching as far north as Tanzania and F. angustifolia reaching to southern Kenya. Fockea are known as water roots, a reference to the bulbous caudex characteristic of most species, which is also edible in at least some species.
Crassula ovata, commonly known as jade plant, lucky plant, money plant or money tree, is a succulent plant with small pink or white flowers that is native to the KwaZulu-Natal and Eastern Cape provinces of South Africa, and Mozambique; it is common as a houseplant worldwide. Much of its popularity stems from the low levels of care needed; the jade plant requires little water and can survive in most indoor conditions. It is sometimes referred to as the money tree; however, Pachira aquatica also has this nickname.
Crassula atropurpurea is a succulent plant, very common and widespread in the southern Karoo regions of South Africa and Namibia.
Crassula perfoliata is the type species of the genus Crassula, in the succulent/flowering plant family Crassulaceae, where it is placed in the subfamily Crassuloideae. Formally described by Linnaeus in 1753 as one of 10 species of Crassula, the plant is endemic to Southern Africa, where it may be found in Eswatini, Mozambique, South Africa and Zimbabwe.
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