Euphorbiaceae

The Spurge family (Euphorbiaceae) is a large family of flowering plants with 240 genera and around 6,000 species. Most are herbs, but some, especially in the tropics, are also shrubs or trees. Some are succulent and resemble cacti.

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This family occurs mainly in the tropics, with the majority of the species in the Indo-Malayan region and tropical America a good second. There is a large variety in tropical Africa, but it is not as abundant or varied as in these two other tropical regions. However, Euphorbia also has many species in non-tropical areas such as the Mediterranean, the Middle East, South Africa and southern USA.

The leaves are alternate, seldom opposite, with stipules. They are mainly simple, but where compound, are always palmate, never pinnate. Stipules may be reduced to hairs, glands, or spines, or in succulent species are sometimes absent.

The radially symmetrical flowers are unisexual, with the male and the female flowers usually occurring on the same plant. As can be expected from such a large family, there is a wide variety in the structure of the flowers. They can be monoecious or dioecious. The stamens (the male organs) can number from 1 to 10 (or even more). The female flowers are hypogynous, that is, with a superior ovary.

  The genera in tribe Euphorbieae, subtribe Euphorbiinae (Euphorbia and close relatives) show a highly specialized form of pseudanthium ("false flower" made up of several true flowers) called a cyathium. This is usually a small cup-like involucre consisting of fused together bracts and peripheral nectary glands, surrounding a ring of male flowers, each a single stamen. In the middle of the cyathium stands a female flower: a single pistil with branched stigmas. This whole arrangement resembles a single flower.

The fruit is usually a schizocarp, sometimes a drupe. A typical schizocarp is the regma, a capsular fruit with three or more cells, each of which splits open at maturity into separate parts and then breaks away explosively, scattering the small seeds.

The family contains a large variety of phytotoxins (toxic substances produced by plants), mainly diterpene esters, alkaloids, glycosides, and ricin-type toxins.

A milky sap or latex is a characteristic of the subfamilies Euphorbioideae and Crotonoideae. This milky sap is poisonous in the Euphorbioideae, innocuous in the Crotonoideae. White mangrove (Excoecaria agallocha), or Blind-Your-Eye Mangrove sap causes blistering on contact and temporary blindness if it contacts the eyes. Other common names are Milky Mangrove, Buta Buta (Malay), Gewa (Bangladesh). The sap of spurge was used as a purgative.

Recent molecular studies have shown that the enigmatic family Rafflesiaceae, which was only recently recognized to belong to order Malpighiales, is derived from within Euphorbiaceae (Davis et al. 2007).

Uses

A number of plants of the Spurge family are of considerable economic importance. Prominent plants include Manioc, the Castor bean, and the Para rubber tree. Many are grown as ornamental plants, such as Poinsettia (Euphorbia pulcherrima). Leafy spurge (Euphorbia esula) is an invasive weed in North America.

References

• Charles C. Davis, Maribeth Latvis, Daniel L. Nickrent, Kenneth J. Wurdack, David A. Baum. 2007. Floral gigantism in Rafflesiaceae. Science Express, published online January 11, 2007 (online abstract here).
• International Euphorbia Society
• Data from GRIN Taxonomy
• Euphorbiaceae in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. http://delta-intkey.com