Herbs or shrubs, annual, biennial, or perennial, with taproot or thickened rootstock. Stems woody or herbaceous (succulent in Euphorbia myrsinites), erect or ascending, branched or unbranched, terete, glabrous or hairy. Leaves alternate (opposite in E. lathyris); stipules absent; petiole present or absent, glabrous or hairy; blade monomorphic, herbaceous (fleshy in E. myrsinites), base symmetric, margins entire or toothed, surfaces glabrous or hairy; venation pinnate, sometimes obscure, midvein often prominent. Cyathial arrangement: terminal pleiochasia with (1–)2–17 primary branches; individual pleiochasial branches unbranched or 2–4 branched at 1 or more successive nodes; bracts subtending pleiochasia (pleiochasial bracts) whorled, green, similar in shape and size to distal stem leaves or distinctly different, those on branches and subtending cyathia (dichasial and subcyathial bracts) opposite (alternate in E. trichotoma), distinct or connate; additional cymose branches often present in axils of distal leaves, but alternately arranged and without whorled bracts. Involucre ± actinomorphic, not spurred; glands 4–5 (2–3 in E. oblongata), flat or slightly convex; appendages absent or hornlike, 2, slender with attenuate or rounded tips (thick with rounded and dilated tips in E. myrsinites). Staminate flowers 5–40. Pistillate flowers: ovary glabrous or hairy; styles distinct or partly connate, usually 2-fid, sometimes unbranched. Seeds: caruncle present.

Distribution

Nearly worldwide.

Discussion

Species ca. 480 (34 in the flora).

The European natives Euphorbia paralias and E. segetalis were recorded from Pennsylvania in the early twentieth century. Neither species has been collected more recently, and those early occurrences probably represent waifs that never became established. Other European species here considered waifs in the flora area include E. amygdaloides, E. epithymoides (sometimes treated as E. polychroma), and E. lucida. The widespread, introduced leafy spurge is treated here as E. virgata, which in North America previously has been mostly misidentified as E. esula Linnaeus. Euphorbia esula is a related, but more restricted and less weedy, European species that has been recorded historically in the flora area as a waif but is excluded because it is not persistent. Typifications and synonymy of native North American species of subg. Esula were published by D. V. Geltman et al. (2011).

Selected References

Geltman, D. V. et al. 2011. Typification and synonymy of the species of Euphorbia subgenus Esula (Euphorbiaceae) native to the United States and Canada. J. Bot. Res. Inst. Texas 5: 143–151.

Norton, J. B. S. 1900. A revision of the American species of Euphorbia of the section Tithymalus occurring north of Mexico. Rep. (Annual) Missouri Bot. Gard. 11: 85–144.

Peirson, J. A. et al. 2014. Phylogeny and taxonomy of the New World leafy spurges, Euphorbia section Tithymalus (Euphorbiaceae). Bot. J. Linn. Soc. 175: 191–228.

Riina, R. et al. 2013. A worldwide molecular phylogeny and classification of the leafy spurges, Euphorbia subgenus Esula (Euphorbiaceae). Taxon 62: 316–342.

Lower Taxa

Euphorbia agraria, Euphorbia alta, Euphorbia austrotexana, Euphorbia brachycera, Euphorbia chamaesula, Euphorbia commutata, Euphorbia crenulata, Euphorbia cyparissias, Euphorbia dendroides, Euphorbia exigua, Euphorbia falcata, Euphorbia georgiana, Euphorbia helioscopia, Euphorbia helleri, Euphorbia lathyris, Euphorbia longicruris, Euphorbia lurida, Euphorbia myrsinites, Euphorbia oblongata, Euphorbia ouachitana, Euphorbia peplidion, Euphorbia peplus, Euphorbia platyphyllos, Euphorbia purpurea, Euphorbia roemeriana, Euphorbia schizoloba, Euphorbia serrata, Euphorbia spathulata, Euphorbia terracina, Euphorbia tetrapora, Euphorbia texana, Euphorbia trichotoma, Euphorbia virgata, Euphorbia yaquiana

Key

1	Leaves opposite; capsules tardily dehiscent and appearing indehiscent, mesocarp spongy.	Euphorbia lathyris
1	Leaves alternate; capsules dehiscent, mesocarp not spongy.	> 2
2	Shrubs.	Euphorbia dendroides
2	Herbs.	> 3
3	Leaves, pleiochasial bracts, and dichasial bracts similar; dichasial bracts alternate; pleiochasial branches 3-branched at nodes; coastal areas of s Florida.	Euphorbia trichotoma
3	Leaves, pleiochasial bracts, and dichasial bracts usually dissimilar; dichasial bracts opposite; pleiochasial branches unbranched or 2-(3- or 4-)branched at each node; not coastal areas of s Florida.	> 4
4	Cocci verrucose-tuberculate, verrucose, or papillate; involucral gland horns absent.	> 5
5	Perennial herbs with thick rootstock or woody taproot.	> 6
6	Leaf blade margins finely serrulate; stems often densely villous; involucral glands 2–3; capsules 3–4.5 mm; w United States.	Euphorbia oblongata
6	Leaf blade margins entire; stems glabrous; involucral glands 5; capsules 4.5–5.2 mm; e United States.	Euphorbia purpurea
5	Annual or biennial herbs with taproot.	> 7
7	Leaves sparsely pilose, apex usually acute; terminal pleiochasial branches usually 5, 3-branched; involucre sparsely pilose.	Euphorbia platyphyllos
7	Leaves glabrous, apex usually obtuse to rounded; terminal pleiochasial branches usually 3, rarely 5, 2-branched; involucre glabrous.	> 8
8	Cocci papillate, papillae raised, 0.2–0.5 mm; montane areas of Arizona, New Mexico.	Euphorbia alta
8	Cocci verrucose, verrucae low and round, 0.1–0.2 mm; widespread, including Arizona, New Mexico.	Euphorbia spathulata
4	Cocci smooth (granulate in E. cyparissias, granulate toward abaxial line in E. agraria and E. virgata, or puncticulate toward abaxial line in E. exigua, but then involucral gland horns always present); involucral gland horns present or absent.	> 9
9	Perennial or biennial herbs with rootstocks (taproot in E. myrsinites).	> 10
10	Stems succulent; leaf blades fleshy, midvein not prominent; involucral gland horns thick, tips rounded, dilated; capsules 5–7 mm.	Euphorbia myrsinites
10	Stems not succulent; leaf blades not fleshy, midvein prominent; involucral gland horns absent or slender, tips attenuate or rounded; capsules 2–5 mm (4.5–6 in E. serrata).	> 11
11	Leaf blade and bract margins serrate, serrulate, or irregularly dentate.	> 12
12	Leaf blade margins irregularly serrate; involucres 2–4 mm, gland horns 0–0.6 mm; capsules 4.5–6 mm.	Euphorbia serrata
12	Leaf blades margins finely serrulate; involucres 1.1–2 mm, gland horns 1–2 mm; capsules 2.5–3 mm.	Euphorbia terracina
11	Leaf blade and bract margins entire (bract margins occasionally slightly crenate in E. brachycera or slightly crenulate in E. chamaesula).	> 13
13	Plants with slender, spreading rootstocks; seeds smooth.	> 14
14	Leaf blades oblong-elliptic, 9–20 mm wide, bases truncate or auriculate, apices obtuse to rounded.	Euphorbia agraria
14	Leaf blades linear, linear-oblong, linear-ovate, or narrowly oblanceolate, 0.5–12 mm wide, bases attenuate, cuneate, rounded, or truncate, apices acute to rounded (sometimes mucronulate in E. virgata).	> 15
15	Leaf blades 0.5–3 mm wide; stems 10–50 cm; cocci granulate.	Euphorbia cyparissias
15	Leaf blades 3–12 mm wide; stems 20–90 cm; cocci smooth except finely granulate toward abaxial line.	Euphorbia virgata
13	Plants from thick rootstocks; seeds shallowly pitted to almost smooth.	> 16
16	Peduncles 1–3 mm; involucral gland horns usually convergent; capsules 4.3–5 mm.	Euphorbia chamaesula
16	Peduncles 0.3–1 mm; involucral gland horns absent or usually divergent; capsules 2–4 mm.	> 17
17	Involucral gland margins usually entire, occasionally slightly crenate to dentate, horns longer than teeth on gland margins.	Euphorbia brachycera
17	Involucral gland margins irregularly to strongly crenate or dentate, horns absent or equaling to slightly longer than teeth on gland margins.	> 18
18	Leaf blade apices obtuse to rounded; involucral glands oblong to broadly ovate, 0.5–0.8 × 1–1.6 mm.	Euphorbia lurida
18	Leaf blade apices acute to acuminate; involucral glands semicircular to trapezoidal, 0.8–1.5 × 1–2.2 mm.	> 19
19	Stems and leaf blades usually glabrous and glaucous.	Euphorbia schizoloba
19	Stems and leaf blades puberulent to lanulose.	Euphorbia yaquiana
9	Annual or biennial herbs with taproots.	> 20
20	Leaf blade margins serrulate to crenulate, sometimes only obscurely so; involucral glands without horns; seeds reticulate or areolate.	> 21
21	Terminal pleiochasial branches usually 5; involucres 1.5–2 mm; capsules 2.5–4 mm; seeds 1.6–2.2 mm.	Euphorbia helioscopia
21	Terminal pleiochasial branches 3; involucres 0.5–0.9 mm; capsules 1.6–2 mm; seeds 1.4–1.5 mm.	Euphorbia texana
20	Leaf blade margins entire; involucral glands usually with, occasionally without, horns; seeds smooth, pitted, sulcate, or tuberculate.	> 22
22	Seeds smooth.	Euphorbia helleri
22	Seeds pitted, sulcate, or tuberculate.	> 23
23	Seeds sulcate at least on adaxial faces (large-pitted on abaxial faces in E. peplidion and E. peplus).	> 24
24	Ovaries pilose at base; cocci glabrous or slightly pilose; capsules 2–3 mm; seeds transversely sulcate; involucral glands with or without horns.	Euphorbia falcata
24	Ovaries glabrous; cocci glabrous; capsules 1.3–2.3 mm; seeds longitudinally sulcate on adaxial faces, large-pitted on abaxial faces; involucral glands with horns.	> 25
25	Leaf blades linear-oblanceolate to narrowly cuneate-spatulate, 1–4 mm wide; cocci without wings.	Euphorbia peplidion
25	Leaf blades obovate, oblong or suborbiculate, 4–15 mm wide; cocci with low longitudinal wings.	Euphorbia peplus
23	Seeds pitted or tuberculate.	> 26
26	Seeds tuberculate.	Euphorbia exigua
26	Seeds pitted.	> 27
27	Dichasial bracts connate 1/4–1/2 length (often only on one side or rarely only basally in E. crenulata).	> 28
28	Biennial or occasionally annual herbs; dichasial bract margins erose-denticulate to subentire; involucral glands 1.5–2.3 mm wide, horns 0.4–0.6 mm; capsules 2.5–3 mm; seeds 2–2.5 mm; California, sw Colorado, Oregon.	Euphorbia crenulata
28	Annual herbs; dichasial bract margins entire; involucral glands 0.7–1.2 mm wide, horns 0.1–0.4 mm; capsules 2–2.7 mm; seeds 1.6–2 mm; e of Rocky Mountains.	> 29
29	Petioles 0–3 mm; capsules ovoid-globose, 2.6–2.7 mm; seeds with deep, rounded pits in 3–4 regular, vertical rows; c United States but not Texas.	Euphorbia ouachitana
29	Petioles 2–5 mm; capsules subglobose, 2–2.5 mm; seeds with scattered, deep and broad pits; Texas.	Euphorbia roemeriana
27	Dichasial bracts usually distinct, occasionally subconnate basally.	> 30
30	Dichasial bracts strongly imbricate; seeds gray to purple-gray or sometimes nearly black.	Euphorbia longicruris
30	Dichasial bracts not imbricate; seeds white to gray or red-brown to brown.	> 31
31	Biennial or occasionally annual herbs; petioles of proximal leaves 5–10 mm; leaf blades usually oblanceolate to obovate, rarely ovate, 3–10 mm wide (at least some blades more than 5 mm wide); capsules 2.5–3.2 mm.	Euphorbia commutata
31	Annual herbs; petioles of proximal leaves 0–2 mm; leaf blades linear to oblanceolate, spatulate-cuneate, or slightly lanceolate, 0.5–5 mm wide; capsules 1.8–2.4 mm.	> 32
32	Seeds reddish brown to brown, 1.3–1.4 × 0.8–0.9 mm, with 4–6 shallow pits or irregular oblong grooves on adaxial faces, small-pitted or nearly smooth on abaxial faces.	Euphorbia tetrapora
32	Seeds white to gray, 1.4–1.7 × 1–1.6 mm, with pits scattered over entire surface.	> 33
33	Leaf blades linear to slightly lanceolate or linear-oblanceolate, 0.5–2.5 mm wide; seeds ellipsoid, 1.4–1.7 × 1–1.3 mm; sandy-soiled habitats in Texas.	Euphorbia austrotexana
33	Leaf blades oblanceolate, 3–5 mm wide; seeds ovoid, 1.6–1.7 × 1.4–1.6 mm; granitic outcrops in Georgia.	Euphorbia georgiana