Euphorbia is a genus of flowering plants belonging to the family Euphorbiaceae. Consisting of 2008 species, Euphorbia is the fourth largest genus of flowering plants; it also has one of the largest ranges of chromosome counts, along with Rumex and Senecio. Members of the family and genus are commonly referred to as spurges. Euphorbia antiquorum is the type species for the genus Euphorbia; it was described by Carl Linnaeus in 1753 in Species Plantarum. The family is primarily found in the tropical and subtropical regions of Africa and the Americas, but also in temperate zones worldwide. Succulent species originate mostly from Africa, the Americas and Madagascar. There exists a wide range of insular species: on the Hawaiian Islands, where spurges are collectively known as “akoko”, and on the Canary Islands as “tabaibas”.
The botanical name Euphorbia derives from Euphorbus, the Greek physician of king Juba II of Numidia (52–50 BC – 23 AD), who married the daughter of Anthony and Cleopatra. He wrote that one of the cactus-like Euphorbias was a powerful laxative. In 12 B.C., Juba named this plant after his physician Euphorbus in response to Augustus Caesar dedicating a statue to Antonius Musa, his own personal physician. Botanist and taxonomist Carl Linnaeus assigned the name Euphorbia to the entire genus in the physician’s honor.
The plants are annual or perennial herbs, woody shrubs or trees with a caustic, poisonous milky sap (latex). The roots are fine or thick and fleshy or tuberous. Many species are more or less succulent, thorny or unarmed. The main stem and mostly also the side arms of the succulent species are thick and fleshy, 15–91 cm (6–36 inches) tall. The deciduous leaves are opposite, alternate or in whorls. In succulent species the leaves are mostly small and short-lived. The stipules are mostly small, partly transformed into spines or glands, or missing.
Like all members of the family Euphorbiaceae, all spurges have unisexual flowers. In Euphorbia these are greatly reduced and grouped into pseudanthia called cyathia. The majority of species are monoecious (bearing male and female flowers on the same plant), although some are dioecious with male and female flowers occurring on different plants. It is not unusual for the central cyathia of a cyme 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. The bracts are often leaf-like, sometimes brightly coloured and attractive, sometimes reduced to tiny scales. The fruits are three (rarely two) compartment capsules, sometimes fleshy but almost always ripening to a woody container that then splits open (explosively, see explosive dehiscence). The seeds are 4-angled, oval or spherical, and in some species have a caruncle.
Xerophytes and succulents
In the genus Euphorbia, succulence in the species has often evolved divergently and to differing degrees. Sometimes it is difficult to decide, and it is a question of interpretation, whether or not a species is really succulent or “only” xerophytic. In some cases, especially with geophytes, 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 all Euphorbia species.
The milky sap of spurges (called “latex”) evolved as a deterrent to herbivores. It is white and colorless when dry, except in E. abdelkuri, where it is yellow. The pressurized sap seeps from the slightest wound and congeals after a few minutes in air. The specific composition of diterpene or triterpene esters varies, and determines how caustic and irritating a particular species is. In contact with mucous membranes (eyes, nose, mouth), the latex can produce extremely painful inflammation. Therefore, spurges should be handled with caution and kept away from children and pets. Latex on skin should be washed off immediately and thoroughly. Congealed latex is insoluble in water, but can be removed with an emulsifier like 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. When large succulent spurges in a greenhouse are cut, vapours can cause irritation to the eyes and throat several metres away. Precautions, including sufficient ventilation, are required.
Several spurges are grown as garden plants, among them Poinsettia (E. pulcherrima) and the succulent E. trigona. E. pekinensis (Chinese: 大戟; pinyin: dàjǐ) is used in traditional Chinese medicine, where it is regarded as one of the 50 fundamental herbs. Several Euphorbia species are used as food plants by the larvae of some Lepidoptera (butterflies and moths), like the Spurge Hawk-moths (Hyles euphorbiae and Hyles tithymali), as well as the Giant Leopard Moth.
The diterpenoid ingenol is the core structure of a topical drug recently commercialized to treat actinic keratosis, a precancerous skin condition. It is produced by the Euphorbia plants.