4. Euphorbia L. (spurge, euphorbia)

Plants annual or perennial herbs (shrubs or trees elsewhere), monoecious but the flowers densely clustered into headlike units and appearing perfect, with milky sap, glabrous or pubescent with unbranched, nonglandular hairs; stinging hairs absent. Stems solitary to several, prostrate to erect, unbranched or more commonly branched, the branches alternate, opposite, or whorled. Leaves alternate, opposite, or whorled, sessile or short-petiolate, the petiole lacking glands, attached at the base of the nonpeltate (but sometimes cordate-clasping) blade. Leaf blades variously shaped, the margins entire, finely toothed, or pinnately few-lobed, appearing pinnately veined or with only the midvein apparent. Stipules absent or, if present, then either minute (less than 1 mm long), triangular scales (these usually persistent at flowering) or small glandular dots. Inflorescences with the basic unit a cyathium consisting of 1 central pistillate flower and several to numerous staminate flowers in a small, cup-shaped involucre with 1–5 conspicuous glands along the rim, the cyathia terminal, axillary, and/or from the stem branch points, appearing solitary or more commonly varying from small clusters to relatively large panicles. Flowers lacking a nectar disc (but the cyathium glands producing nectar), without a calyx and corolla. Staminate flowers consisting of 1 small stamen, this with a minute, jointed stalk, usually subtended by a minute bract at the base. Pistillate flowers consisting of 1 pistil, this with a short, jointed stalk (longer than those of the staminate flowers); the ovary with 3 locules and 1 ovule per locule, the styles free or fused at the very base, each shallowly to deeply 2(3)-lobed. Fruits somewhat 3-lobed (bluntly triangular in cross-section), often depressed-globose (slightly wider than long), dehiscent. Seeds nearly circular to oblong or oblong-ovate in outline, usually somewhat flattened in profile, sometimes slightly wedge-shaped, the caruncle absent or a small, light-colored knob or somewhat flattened bump at the end adjacent to the attachment point, the surface variously smooth, wrinkled, pebbled, or with minute warts or tubercles, reddish brown to dark brown, often all or partially covered with a thin, white to light gray, glaucous coating. In the broad sense, 1,500–2,000 species, worldwide.

The inflorescence unit known as the cyathium is unique in the Missouri flora. In a family noted for its small, inconspicuous flowers, the members of the tribe Euphorbieae have achieved the most extreme floral reduction imaginable. Staminate and pistillate flowers consist of a tiny solitary stamen and pistil respectively, without any calyx or corolla. However, as in other families in which individual flowers have become reduced to the point where they are ineffective at attracting insect pollinators, the flowers of Euphorbieae have become aggregated into dense clusters, each of which is associated with an involucre and mimics the function of a larger, showier flower. In the cyathium, the associated bracts are fused into a shallowly to deeply cup-shaped involucre containing a single central pistillate flower that appears stalked and several series of minutely stalked staminate flowers (each subtended by a very minute bract) radiating outward from the center. The rim of the involucre has 1–5 conspicuous glands of varying color and shape, and in some groups each of the glands has a white or pinkish-tinged petaloid appendage. The entire unit thus mimics a single flower. Because most of the Missouri species have small cyathia, care must be taken to recognize the nature of the floral assemblage to avoid miskeying the group.

Generic limits in the tribe Euphorbieae have remained controversial. Traditionally, the large and morphologically variable genus Euphorbia was regarded as comprising several well-marked subgenera but with a large number of residual unclassified species groups (Steyermark, 1963; Gleason and Cronquist, 1991; see also discussion in Webster [1967]). Over time, the trend became to recognize some of the most easily recognized infrageneric groups as segregate genera, especially Chamaesyce Gray (species 8–11, 13, 14, 16–18, 20 below; Webster, 1967, 1994; Yatskievych and Turner, 1990). Recent phylogenetic studies involving morphological characters (Park and Elisens, 2000) and molecular sequence data (Steinmann and Porter, 2002; Wurdack et al. (2005) have resulted in the recognition that, on the one hand, some of the segregate genera (like Poinsettia Graham) are unnatural assemblages of species related to different groups within Euphorbia in the broad sense, and, on the other hand, the removal of various segregate genera results in an unnatural classification of those species remaining in Euphorbia, with various species of Euphorbia in the strictest sense more closely related to some of the segregates than to other species of Euphorbia. The practical consequence of using a broad circumscription of Euphorbia to include nearly all of the members of the family that produce cyathia is that the genus becomes one of the largest genera of flowering plants in the world and includes incredible morphological diversity. In the future, we may expect further refinement of the classification within the tribe that will result in the breakup of Euphorbia into smaller generic units, but these genera will in most cases be circumscribed differently than the traditional segregates.

In the broad sense, Euphorbia includes plants ranging from tiny annuals to large trees. In dry portions of the Old World, succulent members of the genus have independently evolved growth forms convergent with those found in cacti (a nearly entirely New World group), differing conspicuously in their milky latex, paired (stipular) spines, and minute, imperfect flowers in cyathia. A large number of succulent euphorbias are grown as specimen plants by succulent plant enthusiasts, and some of the hardier woody and succulent species are cultivated outdoors in the southern United States. The genus also includes the cultivated poinsettia (E. pulcherrima Willd. ex Klotzsch), which is an economically important ornamental in the United States during the Christmas season. Other commercial uses include waxes removed by boiling from the stem surfaces of some species, including candelilla (Euphorbia antisyphilitica Zucc.), a native of the Chihuahuan Desert region. These waxes are used primarily in cosmetics and skin care products, varnishes, and polishes for shining certain kinds of leather. A number of species have been used variously for medicinal purposes around the world. Among the negative economic impacts, some species of Euphorbia are considered noxious weeds. In the northern and western United States, the invasive exotic leafy spurge (the Eurasian E. esula L.) aggressively outcompetes native grassland species, impacting the native biodiversity of more than 2.5 million acres and also rendering these areas less productive for grazing by cattle (Dunn, 1979, 1985). The spurges are notable for containing abundant and diverse diterpenoid esters that result in the plants potentially causing irritation to the skin, mucus membranes, and digestive tract. However, as noted by Burrows and Tyrl (2001), reports of severe poisoning by members of the genus are to some extent exaggerated.

Gleason and Cronquist (1991) and Kartesz and Meacham (1999) included Missouri in the range of E. hexagona Nutt. ex Spreng. (six-angled spurge). However, to date, no voucher specimens to substantiate this claim have been discovered. The source of confusion may be a report by Henry and Scott (1983) of an introduced population of the species from Mercer County (but Illinois, not Missouri). Although these authors included a statement in their discussion that E. hexagona had not been reported from Missouri, later authors unfamiliar with the local geography may have misinterpreted the report. Euphorbia hexagona occurs from Montana to New Mexico east to South Dakota, Iowa, Kansas, and Arkansas, and thus may eventually be discovered in western Missouri. The species is not particularly closely related to any of the Missouri spurges but resembles E. corollata superficially. It differs from that species in its relatively delicate annual habit, opposite leaves that are sharply pointed at the tip, and seeds with the surface pebbled, roughened, or finely tuberculate. Plants of E. hexagona would not key well to any of the Missouri species in the following key.