Succulent plants have thickened stems, or leaves, such as this Aloe.
Succulent plants have thickened stems, or leaves, such as this Aloe.

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]

Succulent plants 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] 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 scientific category, since the term describes only the attributes of a particular species; some species in a genus or family 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).[3] 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.

Definition

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.[4] Other sources exclude roots as in the definition "a plant with thick, fleshy and swollen stems and/or leaves, adapted to dry environments".[5] The difference affects the relationship between succulents and "geophytes"–plants that survive unfavorable seasons as a resting bud on an underground organ.[6] 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. However, 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.[7] Nor are all succulents xerophytes, as plants such as Crassula helmsii are both succulent and aquatic.[8]

Center rosette of (Aloe polyphylla)
Center rosette of (Aloe polyphylla)

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.[9] Many books covering the cultivation of these plants include "cacti and succulents" as the title or part of the title.[10][11][12] However, in botanical terminology, cacti are succulents,[4] 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.[13]

Horticulturists often follow commercial conventions and may exclude other groups of plants such as bromeliads, that scientifically are considered succulents.[14] 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.[15] 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,[16] that is a swollen above-ground organ at soil level, formed from a stem, a root, or both.[6]

Appearance

A collection of succulent plants, including cacti, from the Jardin botanique d'Èze, France
A collection of succulent plants, including cacti, from the Jardin botanique d'Èze, France

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:

Habitat

Succulents, such as these Adromischus marianae, Crassula deceptor and Conophytum, share an affinity for arid, fast-draining soils, often growing directly on rocks
Succulents, such as these Adromischus marianae, Crassula deceptor and Conophytum, share an affinity for arid, fast-draining soils, often growing directly on rocks

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.[19] 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.[20] 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.[21] However, 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.[22] 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.[23] Potted succulents are able to grow in most indoor environments with minimal care.[24]

Conservation

In South Africa, several species 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.[25][19] It is illegal since 1974 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.[25]

Families and genera

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Apocynaceae: Pachypodium lealii, stem succulent
Asphodelaceae: Haworthia arachnoidea, leaf succulent
Asphodelaceae: Astroloba tenax, leaf succulent
Asphodelaceae: Astroloba tenax, leaf succulent
Cactaceae: Rebutia muscula, stem succulent
Cactaceae: Rebutia muscula, stem succulent
Crassulaceae: Crassula ovata, stem and leaf succulent
Crassulaceae: Crassula ovata, stem and leaf succulent
Euphorbiaceae: Euphorbia obesa ssp. symmetrica, stem succulent
Cylindropuntia imbricata: stem, woody succulent
Cylindropuntia imbricata: stem, woody succulent
Malvaceae: Adansonia digitata, stem succulent
Malvaceae: Adansonia digitata, stem succulent
Moringaceae: Moringa ovalifolia, stem succulent
Moringaceae: Moringa ovalifolia, stem succulent
Asparagaceae: Beaucarnea recurvata, stem succulent
Asparagaceae: Dracaena draco, stem succulent
Asparagaceae: Dracaena draco, stem succulent
Euphorbia resinifera
Succulents kept at 25 °C (77 °F) in a Connecticut greenhouse
Succulents kept at 25 °C (77 °F) in a Connecticut greenhouse
Kalanchoe longiflora
Echeveria derenbergii
Senecio angulatus

There are approximately sixty different plant families that contain succulents.[26] 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 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

Cultivation

Succulent wall in a nursery in San Francisco, United States consisting of Sempervivum, Echeveria, and Crassula
Succulent wall in a nursery in San Francisco, United States consisting of Sempervivum, Echeveria, and Crassula

Succulents are favored as houseplants for their attractiveness and ease of care. They have been cultivated as houseplants since at least the 17th century.[34] If properly potted, succulents require little maintenance to survive indoors.[35] Succulents are very adaptable houseplants and will thrive in a range of indoor conditions.[36] For most plant owners, over-watering and associated infections are the main cause of death in succulents.[37]

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. 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 thus rotting. This method typically takes up to a few weeks to produce healthy roots that would eventually create new plants.[38] The vegetative propagation can be different according to the species.[39]

See also

References

  1. ^ Merriam-Webster: succulent, retrieved 2015-04-13
  2. ^ Griffiths, Howard; Males, Jamie (2017-09-11). "Succulent plants". Current Biology. 27 (17): R890–R896. doi:10.1016/j.cub.2017.03.021. ISSN 0960-9822. PMID 28898660.
  3. ^ Dimmitt, Mark. "The Tucson Cactus and Succulent Society". www.tucsoncactus.org. Archived from the original on 22 August 2017. Retrieved 5 February 2017.
  4. ^ a b Rowley 1980, p. 1
  5. ^ Beentje 2010, p. 116
  6. ^ a b Beentje 2010, p. 32
  7. ^ "xerophyte", Dictionary of Botany, 2001, retrieved 2012-09-23
  8. ^ "Crassula helmsii (aquatic plant, succulent)", Global Invasive Species Database, ISSG, April 15, 2010, retrieved 2012-09-23
  9. ^ Jacobsen 1960
  10. ^ Anderson 1999
  11. ^ Hecht 1994
  12. ^ Hewitt 1993
  13. ^ Rowley 1980, p. 2
  14. ^ Innes & Wall 1995
  15. ^ Martin & Chapman 1977
  16. ^ Martin & Chapman 1977, pp. 19–20
  17. ^ a b c Compton n.d.
  18. ^ Speck, Olga; Schlechtendahl, Mark; Borm, Florian; Kampowski, Tim; Speck, Thomas (2018-01-16). "Humidity-dependent wound sealing in succulent leaves of Delosperma cooperi – An adaptation to seasonal drought stress". Beilstein Journal of Nanotechnology. 9 (1): 175–186. doi:10.3762/bjnano.9.20. ISSN 2190-4286. PMC 5789399. PMID 29441263.
  19. ^ a b Trenchard, Tommy (2021-07-31). "In South Africa, Poachers Now Traffic in Tiny Succulent Plants". The New York Times. ISSN 0362-4331. Retrieved 2022-03-16.
  20. ^ GINNS, R. (1961). "The Habitat of Succulent Plants". The National Cactus and Succulent Journal. 16 (2): 29–30. ISSN 0027-8858. JSTOR 42788160.
  21. ^ "Succulents in their natural environment". November 2021.
  22. ^ "Cactuses and Succulents".
  23. ^ "California's Native Succulents".
  24. ^ "Succulent Care Tips". 17 April 2019.
  25. ^ a b "These tiny succulents are under siege from international crime rings". Animals. 2022-03-08. Retrieved 2022-03-16.
  26. ^ "10 Things You Never Knew About Succulents". 16 September 2018.
  27. ^ "Apiaceae". succulent-plant.com. Retrieved 2018-02-07.
  28. ^ Plants of Southern Africa Retrieved on 2010-1-1
  29. ^ FloraBase – The Western Australian Flora Retrieved on 2010-1-1
  30. ^ Parakeelya. The Plant List.
  31. ^ Dregeochloa pumila. South African National Biodiversity Institute
  32. ^ Haevermans, Thomas; Mantuano, Dulce; Zhou, Meng-Yuan; Lamxay, Vichith; Haevermans, Agathe; Blanc, Patrick; Li, De-Zhu (2020). "Discovery of the first succulent bamboo (Poaceae, Bambusoideae) in a new genus from Laos' karst areas, with a unique adaptation to seasonal drought". PhytoKeys (156): 125–137. doi:10.3897/phytokeys.156.51636. PMC 7455575. PMID 32913413.
  33. ^ "Crassulaceae Genera". Missouri Botanical Garden. Retrieved 2017-10-26.
  34. ^ Through England On a Side Saddle in the Time of William and Mary. London: Field & Tuer, The Leadenhall Press, E.C. 1888. p. 91.
  35. ^ Kramer, Jack (1977). Cacti and Other Succulents. New York: Abrams. p. 9.
  36. ^ Kramer, Jack (1977). Cacti and Other Succulents. New York: Abrams. p. 49.
  37. ^ SproutingIndoors (2020-06-13). "Succulent Root Rot: What it is and How to Treat it". Sprouting Indoors. Retrieved 2020-06-15.
  38. ^ "Propagating Succulents".
  39. ^ Lee, Debra (2007). Designing with Succulents. Portland, Obregon: Timber Press. p. 133.

Bibliography