Some euphorbias are widely available commercially, such aspoinsettias at Christmas. Some are commonly cultivated as ornamentals, or collected and highly valued for the aesthetic appearance of their unique floral structures, such as the crown of thorns plant (Euphorbia milii).Succulent euphorbias from the deserts of SouthernAfrica andMadagascar have evolved physical characteristics and forms similar tocacti of North and South America, so they are often incorrectly referred to as cacti.[10] Some are used as ornamentals in landscaping, because of beautiful or striking overall forms, and drought and heat tolerance.[6][2]
Euphorbia all share the feature of having a poisonous,latex-like sap and unique floral structures.[6] When viewed as a whole, the head of flowers looks like a single flower (apseudanthium).[6] It has a unique kind of pseudanthium, called acyathium, where each flower in the head is reduced to its barest essential part needed for sexual reproduction.[6] The individual flowers are either male or female, with the male flowers reduced to only thestamen, and the females to thepistil.[6] These flowers have nosepals,petals, or other parts that are typical of flowers in other kinds of plants.[6] Structures supporting the flower head and other structures underneath have evolved to attract pollinators with nectar, and with shapes and colors that function in a waypetals and other flower parts do in other flowers. It is the only genus of plants that has all three kinds ofphotosynthesis,CAM,C3 andC4.[6]
The common name "spurge" derives from theMiddle English/Old Frenchespurge ("to purge"), due to the use of the plant's sap as apurgative. Thebotanical nameEuphorbia derives fromEuphorbos, theGreek physician of KingJuba II ofNumidia andMauretania (52–50 BC – 23 AD), who married the daughter of Anthony and Cleopatra.[11] Juba was a prolific writer on various subjects, includingnatural history. Euphorbos wrote that one of the cactus-like euphorbias (now calledEuphorbia obtusifolia ssp.regis-jubae) was used as a powerfullaxative.[11] In 12 BC, Juba named this plant after his physician Euphorbos, asAugustus Caesar had dedicated a statue to the brother of Euphorbos,Antonius Musa, who was the personal physician of Augustus.[11] In 1753, botanist and taxonomistCarl Linnaeus assigned the nameEuphorbia to the entire genus in the physician's honor.[12]
The plants areannual,biennial orperennial herbs, woody shrubs, or trees with a caustic, poisonous milky latex. Theroots are fine or thick and fleshy or tuberous. Many species are more or lesssucculent, thorny, or unarmed. The main stem and mostly also the side arms of the succulent species are thick and fleshy, and often winged, 15–91 cm (6–36 in) tall. The succulent trees and large shrubs are mostly confined to southern and eastern Africa, butEuphorbia neutra is native to theCaatinga ofBrazil and Euphorbia royleana is found in theHimalayan foothills.[13] The deciduous[citation needed]leaves may beopposite,alternate, or inwhorls. In succulent species, the leaves are mostly small and short-lived. Thestipules are mostly small, partly transformed intospines orglands, or missing.
Like all members of the family Euphorbiaceae, spurges have unisexual flowers.
InEuphorbia, flowers occur in a head, called thecyathium (plural cyathia). Each male or female flower in the cyathium head has only its essential sexual part, in males thestamen, and in females thepistil. The flowers do not havesepals,petals, ornectar to attract pollinators, although other nonflower parts of the plant have an appearance and nectar glands with similar roles. Euphorbias are the only plants known to have this kind of flower head.[14]
Nectar glands and nectar that attract pollinators are held in the involucre, a cup-like part below and supporting the cyathium head. The "involucre" in the genusEuphorbia is not to be confused with the "involucre" in family Asteraceae members, which is a collection of bracts calledphyllaries, which surround and encase the unopened flower head, then support the receptacle under it after the flower head opens.
The involucre is above and supported bybract-like modified leaf structures (usually in pairs)[citation needed] called cyathophylls', or cyathial leaves. The cyathophyll often has a superficial appearance of being petals of a flower.
Euphorbia flowers are tiny, and the variation attracting different pollinators, with different forms and colors occurs, in the cyathium, involucre, cyathophyll, or additional parts such as glands that attached to these.
The collection of many flowers may be shaped and arranged to appear collectively as a single individual flower, sometimes called apseudanthium in the Asteraceae, and also inEuphorbia.
The majority of species aremonoecious (bearing male and female flowers on the same plant), although some aredioecious with male and female flowers occurring on different plants. It is not unusual for the central cyathia of acyme to be purely male, and for lateral cyathia to carry both sexes. Sometimes, young plants or those growing under unfavorable conditions are male only, and only produce female flowers in the cyathia with maturity or as growing conditions improve.[citation needed]
The female flowers reduced to a single pistil usually split into three parts, often with two stigmas at each tip.[citation needed] Male flowers often have anthers in twos.[citation needed] Nectar glands usually occur in fives,[15] may be as few as one,[15] and may be fused into a "U" shape.[14] The cyathophylls often occur in twos, are leaf-like, and may be showy and brightly coloured and attractive to pollinators, or be reduced to barely visible tiny scales.[citation needed]
The fruits are three- or rarely two-compartmentcapsules, sometimes fleshy, but almost always ripening to a woody container that then splits open, sometimesexplosively. Theseeds are four-angled, oval, or spherical, and some species have acaruncle.[citation needed]
In the genusEuphorbia, succulence in the species has often evolved divergently and to differing degrees. Sometimes, it is difficult to decide, and is a question of interpretation, whether or not a species is really succulent or "only"xerophytic. In some cases, especially withgeophytes, plants closely related to the succulents are normal herbs. About 850 species are succulent in the strictest sense. If one includes slightly succulent and xerophytic species, this figure rises to about 1000, representing about 45% of allEuphorbia species.
The milky sap of spurges (called "latex") evolved as a deterrent toherbivores. It is white, and transparent when dry, except inE. abdelkuri, where it is yellow. The pressurized sap seeps from the slightest wound and congeals after a few minutes in air. The skin-irritating and caustic effects are largely caused by varying amounts ofditerpenes.Triterpenes such asbetulin and correspondingesters are other major components of the latex.[16] In contact with mucous membranes (eyes, nose, mouth), the latex can produce extremely painfulinflammation. The sap has also been known to cause mild to extreme Keratouveitis, which affects vision.[17] Therefore, spurges should be handled with caution and kept away from children and pets. Wearing eye protection while working in close contact withEuphorbia is advised.[17] Latex on skin should be washed off immediately and thoroughly. Congealed latex is insoluble in water, but can be removed with an emulsifier such as milk or soap. A physician should be consulted if inflammation occurs, as severe eye damage including permanent blindness may result from exposure to the sap.[18]
The poisonous qualities were well known: in the EthiopianKebra Nagast, the serpent kingArwe is killed with juice from theEuphorbia.[19]
Among laypeople,Euphorbia species are among the plant taxa most commonly confused withcacti, especially thestem succulents.[21] Euphorbias secrete a sticky, milky-white fluid with latex, but cacti do not.[21] Individual flowers of euphorbias are usually tiny and nondescript (although structures around the individual flowers may not be), without petals and sepals, unlike cacti, which often have fantastically showy flowers.[21]
The present taxon "Euphorbia" corresponds to its own former subtribe, theEuphorbiinae.[citation needed] It has over 2000 species.[6]Morphological description using the presence of a cyathium (see section above) is consistent with nuclear and chloroplast DNA sequence data in testing of about 10% of its members. This testing supports inclusion of formerly other genera as being best placed in this single genus, includingChamaesyce, Monadenium, Pedilanthus, and poinsettia (E. pulcherrima).
Genetic tests have shown that similar flower head structures or forms within the genus, might not mean close ancestry within the genus. The genetic data show that within the genus, convergent evolution ofinflorescence structures may be from ancestral subunits that are not related. So using morphology within the genus becomes problematic for further subgeneric grouping. As stated on the Euphorbia Planetary Biodiversity Inventory project webpage:[6]
Previous morphologically based delimitations of subgenera or sections within the genus should not be taken at face value. The genus is in fact rife with striking examples of morphologicalconvergence in cyathial and vegetative features, which justifies a global approach to studying the genus to obtain a proper phylogenetic understanding of the whole group.... The bottom line is that a number of clades have been placed inside or outside of Euphorbia at different times... few of the subgeneric circumscriptions hold up under DNA sequence analysis.
According to a 2002 publication on studies ofDNA sequence data,[22][23][24] most of the smaller "satellite genera" around the huge genusEuphorbia nest deep within the latter. Consequently, thesetaxa, namely the never generally accepted genusChamaesyce, as well as the smaller generaCubanthus,[25]Elaeophorbia,Endadenium,Monadenium,Synadenium,andPedilanthus were transferred toEuphorbia. The entiresubtribe Euphorbiinae now consists solely of the genusEuphorbia.
Euphorbia has been extensivelyhybridised for garden use, with manycultivars available commercially. Moreover, some hybrid plants have been found growing in the wild, for instanceE. ×martini Rouy,[31] a cross ofE. amygdaloides ×E. characias subsp.characias, found in southern France.
Simplified diagram of relations in subtribeEuphorbiinae, 2006[24]
The genusEuphorbia is one of the largest and most complex genera of flowering plants, and several botanists have made unsuccessful attempts to subdivide the genus into numerous smaller genera. According to the recentphylogenetic studies,[22][23][24]Euphorbia can be divided into four subgenera, each containing severalsections and groups. Of these, subgenusEsula is the mostbasal. The subgeneraChamaesyce andEuphorbia are probablysister taxa, but very closely related to subgenusRhizanthium. Extensivexeromorph adaptations in all probabilityevolved several times;[32] it is not known if the common ancestor of the cactus-likeRhizanthium andEuphorbia lineages had been xeromorphic—in which case a more normal morphology would have re-evolved namely inChamaesyce—or whether extensive xeromorphism is entirelypolyphyletic even to the level of the subgenera.
^"Euphorbia L."Plants of the World Online. Board of Trustees of the Royal Botanic Gardens, Kew. 2017. Retrieved7 July 2020.
^ab"Euphorbia".Fine Gardening. The Taunton Press, Inc.
^Brenan, J.P.M.; Greenway, P.J. (1949).Check-lists of the Forest Trees and Shrubs of the British Empire #5 - Tanganyika Territory. Oxford, England: Imperial Forestry Institute. p. 214 (part 2).
^Hargreaves, Bruce (n.d.). "Euphorbia ingens in Malawi...etc".Euphorbia Journal.7: 78 plus diagram p. 63.
^abcBruyns PV, Mapaya RJ, Hedderson TJ (2006). "A new subgeneric classification forEuphorbia (Euphorbiaceae) in southern Africa based on ITS andpsbA-trnH sequence data".Taxon.55 (2):397–420.doi:10.2307/25065587.JSTOR25065587.
^Steinmann VW, van Ee B, Berry PE, Gutiérrez J (2007). "The systematic position ofCubanthus and other shrubby endemic species ofEuphorbia (Euphorbiaceae) in Cuba".Anales Jard. Bot. Madrid.64 (2):123–133.doi:10.3989/ajbm.2007.v64.i2.167.
Carter S, Eggli U (1997).The CITES Checklist of Succulent Euphorbia Taxa (Euphorbiaceae). Germany: German Federal Agency for Nature Conservation.ISBN9783896246097.
