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Published In: Species Plantarum 1: 450. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

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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.


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1 1. At least the uppermost leaves with conspicuous, broad, white margins, these occasionally slightly pinkish-tinged toward the outer edge ... 12. E. MARGINATA

Euphorbia marginata
2 1. None of the leaves with broad, white margins (the inflorescence bracts often bright red to reddish purple, at least toward the base, in E. cyathophora)

3 2. Stem leaves alternate above the lowest node and below the inflorescence branches

4 3. Inflorescence not an umbellate panicle with a whorl of leaves at the base, instead consisting of small clusters (these frequently paired) at the stem or branch tips; uppermost leaves sometimes pinnately few-lobed and bright red to reddish purple, at least toward the base; cyathia with the involucre having only 1(2) more or less 2-lipped marginal gland(s) ... 3. E. CYATHOPHORA

Euphorbia cyathophora
5 3. Terminal portion of the inflorescence an umbellate panicle with a whorl of leaves at the base (additional smaller inflorescences often produced on branches below the main umbellate inflorescence); leaves all unlobed, the margins entire or finely toothed; cyathia with the involucre having 4 or 5 oval to crescent (viewed from the top) marginal glands

6 4. Leaves with the margins finely toothed (the teeth sometimes minute and visible only with magnification); cyathia with the involucre having oblong-oval to elliptic or nearly circular (viewed from the top) marginal glands

7 5. Seeds 1.7–2.3 mm long, the surface smooth; cyathia with the involucre 1.2–1.5 mm long ... 15. E. OBTUSATA

Euphorbia obtusata
8 5. Seeds 1.3–1.7 mm long, the surface with a fine network of low ridges; cyathia with the involucre 0.6–0.9 mm long ... 19. E. SPATHULATA

Euphorbia spathulata
9 4. Leaves with the margins entire; cyathia with the involucre having kidney-shaped to crescent or linear to narrowly oblong (viewed from the top) marginal glands, these sometimes with the central portion appearing tapered into a pair of horns

10 6. Leaves closely spaced (crowded, especially toward the stem tip), those below the inflorescence linear to narrowly oblanceolate ... 4. E. CYPARISSIAS

Euphorbia cyparissias
11 6. Leaves more widely spaced (not appearing crowded), lanceolate, oblong, obovate, or nearly circular, rarely a few of the leaves narrowly lanceolate in E. corollata

12 7. Leaves along the inflorescence branches narrowly elliptic to elliptic or narrowly ovate (much longer than wide), not cupped around the cyathia; cyathia with the involucre having narrowly elliptic to nearly linear (viewed from the top) marginal glands, these lacking horns, but with showy, white petaloid appendages ... 2. E. COROLLATA

Euphorbia corollata
13 7. Leaves along the inflorescence branches kidney-shaped to broadly triangular-ovate or broadly ovate (slightly longer than wide to somewhat wider than long), somewhat cupped around the cyathia; cyathia with the involucre having kidney-shaped to crescent (viewed from the top) marginal glands, these usually with the central portion appearing tapered into a pair of horns, but lacking petaloid appendages

14 8. Stems 10–40 cm long; leaves 5–30 mm long; surface of the fruits smooth; seeds with the surface strongly pitted ... 1. E. COMMUTATA

Euphorbia commutata
15 8. Stems 30–90 cm long; leaves (excluding the inflorescence bracts) 30–100 mm long; surface of the fruits finely warty or appearing roughened, especially on the angles; seeds with the surface smooth ... 7. E. ESULA

Euphorbia esula
16 2. Stem leaves all or mostly opposite

17 9. Cyathia with the involucre having a more or less fringed margin and only 1 or 2 more or less 2-lipped marginal gland(s), these yellow to greenish yellow, lacking petaloid appendages; stems pubescent with a mixture of dense, minute, downward-curved hairs and scattered, relatively long, spreading to downward-angled hairs; leaves short- to long-petiolate, the blade symmetrically tapered at the base, the margins coarsely toothed to nearly entire, mostly 15–60 mm long; seeds 2–3 mm long

18 10. Leaf blades with the undersurface moderately pubescent with relatively stout hairs, these often with a minute, persistent pustular base; seeds angled in cross-section (both the oblique apical portion surrounding the caruncle and the longitudinal inner faces appearing angular), the surface appearing relatively coarsely wrinkled or with poorly differentiated, low, broad warts (appearing lumpy or irregularly swollen) ... 5. E. DAVIDII

Euphorbia davidii
19 10. Leaf blades with the undersurface sparsely to moderately pubescent with relatively slender hairs, these not expanded at the base; seeds rounded in cross-section (the oblique apical portion surrounding the caruncle angled but the longitudinal inner faces appearing rounded), the surface appearing relatively finely warty or with fine tubercles ... 6. E. DENTATA

Euphorbia dentata
20 9. Cyathia with the involucre having a shallowly lobed or toothed rim and 4 or less commonly 5 unlobed marginal glands, these sometimes reddish purple to dark purple (sometimes green to yellowish green), with white or pinkish- to reddish-tinged petaloid appendages; stems glabrous or, if pubescent, then variously with either dense, minute, appressed or moderate to dense, upward-curved hairs; leaves sessile or very short-petiolate, the blade truncate to more or less rounded or angled at the base, often noticeably asymmetrically so (except in E. geyeri and E. missurica), the margins entire or finely toothed, 2–30 mm long; seeds 0.8–2.0 mm long

21 11. Stems and leaves glabrous

22 12. Leaf blades with the margins minutely toothed (best viewed with magnification), usually only above the midpoint

23 13. Seeds with 3 or 4(–6) coarse transverse ridges; involucres 0.6–0.9 mm long, each with 1–5 staminate flowers surrounding the solitary pistillate flower; stems not flattened or winged toward the tip ... 9. E. GLYPTOSPERMA

Euphorbia glyptosperma
24 13. Seeds smooth or with 1–4 indistinct, low cross-wrinkles, rarely appearing faintly roughened or pitted; involucres 0.8–1.2 mm long, each with 5–18 staminate flowers surrounding the solitary pistillate flower; stems often appearing somewhat flattened or narrowly winged toward the tip ... 18. E. SERPYLLIFOLIA

Euphorbia serpyllifolia
25 12. Leaf blades entire

26 14. Stems erect or ascending; leaf blades linear to narrowly oblong ... 13. E. MISSURICA

Euphorbia missurica
27 14. Stems prostrate; leaf blades oblong or broadly oblong to broadly elliptic or nearly circular

28 15. Stipules from the adjacent leaf in each pair not fused into a single, small, scalelike structure on each side of the stem (or rarely fused on only 1 side at a few nodes), thus a pair of minute, free stipules usually positioned on each side of the stem between the leaf bases, the stipules often appearing irregularly fringed or lobed ... 8. E. GEYERI

Euphorbia geyeri
29 15. Stipules from the adjacent leaf in each pair fused into a single, small, scalelike structure on each side of the stem positioned between the leaf bases, this often appearing irregularly fringed or lobed ... 17. E. SERPENS

Euphorbia serpens
30 11. Stems hairy, at least toward the tip, the pubescence sometimes in longitudinal bands on opposite sides of the stem; leaves usually hairy, at least when young (sometimes becoming glabrous at maturity), sometimes only near the base

31 16. Ovaries and fruits glabrous; stems erect or more commonly ascending, often arched at the branch tips ... 14. E. NUTANS

Euphorbia nutans
32 16. Ovaries and fruits hairy; stems mostly prostrate (sometimes loosely ascending near the tips), usually mat-forming

33 17. Styles entire or inconspicuously notched at the very tip; seeds with the surface usually mottled, finely pitted, some of the pits rarely forming shallow, irregular troughs and the seeds then appearing partially and irregularly few-ridged ... 20. E. STICTOSPORA

Euphorbia stictospora
34 17. Styles 2-lobed or divided at least 1/4 of the way from the tip; seeds with the surface not mottled (sometimes all or partially with a thin, white coating in E. prostrata), smooth, roughened, or with cross-ridges

35 18. Styles about 0.1 mm long, each deeply lobed nearly to the base; fruits moderately to densely pubescent with more or less spreading hairs toward the angles, less densely hairy to nearly glabrous between the angles; seeds with 4–7 relatively sharp, slender cross-ridges ... 16. E. PROSTRATA

Euphorbia prostrata
36 18. Styles 0.3–0.7 mm long, divided 1/4–1/2 of the way from the tip; fruits sparsely to moderately and relatively evenly pubescent with appressed or strongly incurved hairs; seeds with the surface smooth, finely roughened, or with 3 or 4 low, broadly rounded cross-wrinkles (species difficult to distinguish)

37 19. Styles 0.5–0.8 mm long, divided about 1/2 of the way to the base; seeds with the surface smooth or appearing finely roughened, lacking cross-ridges ... 10. E. HUMISTRATA

Euphorbia humistrata
38 19. Styles about 0.3–0.4 mm long, divided 1/4–1/3 of the way to the base; seeds with 3 or 4 low, broadly rounded cross-ridges ... 11. E. MACULATA Euphorbia maculata
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