EUPHORBIACEAE (Spurge Family)

Contributed by George Yatskievych and Mark H. Mayfield

Plants annual or perennial herbs or less commonly shrubs (trees, or lianas elsewhere), rarely twining, monoecious or dioecious, often with milky sap. Leaves alternate or less commonly opposite, simple (compound elsewhere), occasionally lobed, pinnately or less commonly palmately veined, the margins otherwise entire or toothed. Stipules usually present, but small, scalelike, hairlike, or glandular, and sometimes shed early. Inflorescences axillary or terminal, of solitary flowers, clusters, small panicles, spikes, or racemes, sometimes associated with prominent bracts, the basic unit in Euphorbia a highly modified cluster of small flowers grouped into a small, cuplike involucre called a cyathium (for more details, see the treatment of the genus). Flowers imperfect (appearing perfect in Euphorbia; see discussion under that genus), actinomorphic, hypogynous. Calyces of 3–12 sepals (absent in Euphorbia), these sometimes fused toward the base. Petals absent or 4–7, separate or rarely fused at the base, usually not showy. Staminate flowers with 1 to numerous stamens, the filaments distinct or less commonly fused into a tube toward the base, the anthers attached at the base or appearing attached between the somewhat spreading anther sacs. Nectar disc sometimes present in staminate and/or pistillate flowers, usually conspicuous, sometimes lobed or divided into segments. Pistil of usually 3 fused carpels, the ovary superior, usually 3-locular, the ovules 1 or 2 per locule, the placentation axile or more or less apical. Styles usually 3 (occasionally fused toward the base), each style often forked (unbranched in Tragia, with several irregular branches in Acalypha), the stigmas 1 per style branch (or branch fork), variously shaped but often consisting of an apical band along the inner surface of each style branch. Fruits appearing capsular but actually schizocarps (drupes, samaras, or berries elsewhere), usually lobed, dehiscent (indehiscent in some species of Croton) by the carpels splitting open usually from the tip along the inner margin elastically and longitudinally to expose the seeds, leaving a usually persistent central column. Seeds 1 or 2 per locule, often with a small, hardened, aril-like outgrowth of tissue (known as a caruncle) adjacent to the attachment point. In the broad sense, 300–400 genera, 7,000–8,000 species, worldwide, but most diverse tropical and subtropical regions.

The higher-order taxonomy of the Euphorbiaceae is still somewhat controversial. Some authorities have suggested splitting the family into five or more smaller families (see discussion in Wurdack et al. [2004]). Several authors have suggested, on the basis of molecular phylogenetic studies, that the group of about 55 genera and more than 1,500 species related to Phyllanthus (including Andrachne and Phyllanthus in Missouri) are not very closely related to the remainder of the Euphorbiaceae and should be treated as a separate family, Phyllanthaceae (Savolainen et al., 2000; Chase et al., 2002). However, the relationship of that group to other members of the order Malpighiales has not been settled (Wurdack et al., 2004). The phyllanthoid group is unusual within the Euphorbiaceae in having clear sap (lacking a milky latex) and in having two ovules per locule (Judd et al., 2002) and has been considered a separate subfamily within Euphorbiaceae by other authors (Webster, 1994; Govaerts et al., 2000; Radcliffe-Smith, 2001). The traditional, more inclusive circumscription of the Euphorbiaceae (Cronquist 1981, 1990; Webster, 1994; Govaerts et al., 2000; Radcliffe-Smith, 2001) is used in the present work with some reservation, as future studies likely will result in a better understanding of the phylogenetic relationships within the order Malpighiales. At the generic level, circumscriptions of Andrachne, Euphorbia, and Phyllanthus, and their segregates continue to be less than fully resolved (for further discussion, see the treatments of those genera).

The Euphorbiaceae are a large and morphologically diverse family. Although in the United States most species are herbaceous, the majority of the family consists of woody plants, including a number of succulents. The family is economically important for a variety of uses. Hevea brasiliensis (Willd.) Müll. Arg. (Para rubber; rubber tree) is the principal commercial source of latex for the production of rubber. Manihot esculenta Crantz (cassava) is cultivated widely in tropical regions for its starchy, tuberous roots, which are prepared as a dietary staple. The seeds of some genera contain commercially important oils, such as castor oil (Ricinus communis L.) and tung oil (mainly Aleurites fordii Hemsl.). Waxes have been extracted from some genera for candles and industrial uses. Euphorbia pulcherrima Willd. (poinsettia) is grown for its showy bracteal leaves, especially in conjunction with Christmas, but many other species are cultivated as houseplants, garden ornamentals, and specimen plants (especially the succulent taxa). The family is biochemically quite diverse, and various species have been used medicinally, especially for their alkaloids. By the same token, some species are poisonous, and a few genera have stinging hairs. The Neotropical genus Hura L. (cannonball tree) has woody fruits that are explosively dehiscent, flinging the shrapnel of 5–20 sharp-edged 1-seeded segments up to 15 m (these with sufficient force to shatter a window and cause a large gash in a metal file cabinet when an unwary botanist allows a ripe fruit to dry in his office); the immature fruits are used as go-cart wheels in parts of the Caribbean, and the toxic latex is used as a fish poison.