Many plants of the spurge family have reduced fleshy leaves, in particular the vast Euphorbia genus of approximately 1,600 subtropical and warm-temperate species. These cactuslike plants, comprising most of the species commonly called spurge, have spiny, jointed stems and are among the most common Old World desert succulents. The euphorbias and the cacti illustrate the biological phenomenon of convergent evolution, in which unrelated groups of organisms, subject to the same environmental pressures, gradually develop similar structures. The euphorbias exhibit another family trait: "naked flowers" (i.e., flowers lacking petals and sometimes sepals) that are enclosed in a bract envelope, from which they emerge during the flowering period to permit pollination.
Many species are cultivated for their brilliant, showy bracts as well as for their frequently colorful foliage. These include snow-on-the-mountain (E. marginata), native to the United States; the cypress spurge (E. cyparissias), a favored cemetery plant that was introduced from Europe and naturalized; the scarlet-bracted greenhouse plant crown-of-thorns (E. splendens), native to Madagascar; and the poinsettia (for J. R. Poinsett), an ornamental shrub native to Central America. The poinsettia (E. pulcherrima), whose several species are sometimes considered a separate genus (Poinsettia), is a popular Christmas decoration with its large rosettes of usually bright-red bracts.
Many spurges are of great economic importance as a source of food, drugs, rubber, and other products. The sap of most species is a milky latex, and the source of a very large part of the world's natural rubber is the latex of the Pará rubber tree. Pará rubber and several other latexes also come from plants of the spurge family. The tropical American Manihot genus includes the cassava, the source of tapioca and the most important tropical root crop next to the sweet potato.
Other valuable commercial products of this family are castor oil and tung oil, expressed from the seeds of Ricinus communis and Aleurites fordii respectively. The castor bean, the source of castor oil, is native to tropical Africa, where it grows as a tree, but is now widespread and is sometimes cultivated in temperate regions as an annual ornamental. The tung tree, indigenous to E Asia and Malaysia, is the only important plant of the spurge family cultivated commercially in the United States. The candlenut tree (A. moluccana) and the Japanese wood oil tree (A. cordata), of the same genus as the tung tree, also yield oils, as does the Chinese tallow tree (Sapium sebiferum), a source of grease for candles and soap.
Various spurges provide medicines, dyes, oils, and other products; primitive peoples utilized the poisonous saps of other spurges on arrow tips and to poison fish. The presence of poisonous substances in many euphorbias and in a number of other spurges has led these to be classed as noxious pests, especially when they grow as weeds on livestock ranges.
Spurge is classified in the division Magnoliophyta, class Magnoliopsida, order Euphorbiales, family Euphorbiaceae.