Herbs or shrubs [trees, rarely lianas], annual or perennial, with taproot or thickened or tuberous rootstock. Stems erect, ascending, decumbent, or prostrate, branched or unbranched, terete, glabrous or variously hairy, covered by exfoliating waxy coat in E. antisyphilitica. Leaves alternate or opposite; stipules present (sometimes rudimentary in E. graminea and E. hexagona), at base of petiole; petiole usually present, rarely absent or rudimentary, glabrous or hairy; blade monomorphic (dimorphic in E. curtisii, E. exserta, E. ipecacuanhae, and E. mercurialina), base symmetric, margins entire [rarely toothed], surfaces glabrous or hairy; venation pinnate, occasionally inconspicuous. Cyathial arrangement: solitary or in terminal monochasia, dichasia, or pleiochasia; individual dichasial or pleiochasial branches unbranched or few-branched at one or more successive nodes; bracts subtending dichasia and pleiochasia (pleiochasial bracts) opposite or whorled, green or with white margins, similar in shape and size to distal stem-leaves, those on branches (dichasial or subcyathial bracts) opposite (rarely whorled or alternate in E. corollata), distinct; additional cymose branches occasionally present in distal axils, but not subtended by opposite or whorled bracts. Involucre ± actinomorphic, not spurred; glands [0–] (2–) 5, slightly concave, flat, or slightly convex; appendages usually petaloid, occasionally rudimentary. Staminate flowers (5–) 20–25 (–70). Pistillate flowers: ovary glabrous or hairy; styles connate basally, 2-fid. Seeds: caruncle present or absent.

Distribution

North America, Mexico, West Indies, Central America, South America, in Asia, Pacific Islands

Discussion

Species ca. 120 (21 in the flora).

Selected References

Huft, M. J. 1979. A Monograph of Euphorbia Section Tithymalopsis. Ph.D. dissertation. University of Michigan.

Park, K. R. 1998. Monograph of Euphorbia sect. Tithymalopsis (Euphorbiaceae). Edinburgh J. Bot. 55: 161–208.

Lower Taxa

Euphorbia aaron-rossii, Euphorbia antisyphilitica, Euphorbia bicolor, Euphorbia bilobata, Euphorbia corollata, Euphorbia curtisii, Euphorbia discoidalis, Euphorbia exserta, Euphorbia graminea, Euphorbia hexagona, Euphorbia innocua, Euphorbia ipecacuanhae, Euphorbia macropus, Euphorbia marginata, Euphorbia mercurialina, Euphorbia misera, Euphorbia nephradenia, Euphorbia polyphylla, Euphorbia pubentissima, Euphorbia strictior, Euphorbia wrightii

Key

1	Shrubs.	> 2
2	Stems pencil-like, covered with flaky, exfoliating layer of wax; cyathia in axillary congested cymes near branch tips or solitary at distal nodes; Arizona, New Mexico, Texas.	Euphorbia antisyphilitica
2	Stems gnarled, not waxy; cyathia solitary on short shoots; s California.	Euphorbia misera
1	Herbs.	> 3
3	Annual herbs with taproots (E. graminea rarely perennial).	> 4
4	Leaves opposite.	> 5
5	Stems 30–70(–100) cm; leaf blades linear-filiform, linear, or elliptic; cyathia solitary in leaf axils or in terminal cymes or dichasia; involucral gland appendages 0.7–1.7 mm; seeds 3.4 × 2.7 mm; c United States, mostly e of Rocky Mountains.	Euphorbia hexagona
5	Stems 4–25 cm; leaf blades usually linear- to narrowly-elliptic, occasionally ovate to obovate; cyathia solitary at distal bifurcations of stems; involucral gland appendages 0.2–0.5 mm; seeds 2.3–2.6 × 1.3–1.5 mm; Colorado Plateau of Utah, sw Colorado.	Euphorbia nephradenia
4	Leaves mostly alternate (opposite at proximal nodes in E. bilobata; some opposite in E. graminea).	> 6
6	Dichasial bracts with conspicuous white margins.	> 7
7	Dichasial bracts linear to narrowly oblanceolate; leaf blades pilose.	Euphorbia bicolor
7	Dichasial bracts narrowly elliptic to oblanceolate; leaf blades glabrous.	Euphorbia marginata
6	Dichasial bracts wholly green or distal ones white.	> 8
8	Stems 10–35 cm; leaves opposite proximally, alternate distally; dichasial bracts wholly green; involucral glands 5; involucral gland appendages usually 2-fid.	Euphorbia bilobata
8	Stems 30–80(–110) cm; leaves usually alternate, sometimes some opposite; distal dichasial bracts often white; involucral glands (1–)2–4; involucral gland appendages undivided.	Euphorbia graminea
3	Perennial herbs with rootstocks, tubers, or taproots.	> 9
9	Stem leaves usually opposite, occasionally whorled distally, rarely with 1 or 2 alternate leaves; plants with thick, globose to elongated tubers; se Arizona, primarily from Huachuca Mountains.	Euphorbia macropus
9	Stem leaves alternate; plants usually with rootstocks or taproots, rarely with elongated tubers; Arizona to e North America.	> 10
10	Involucral glands 4, appendages green; leaf blade adaxial surfaces densely pilose, bases cordate; s coastal Texas.	Euphorbia innocua
10	Involucral glands 5, appendages usually white to pink, if greenish then minute and forming rim around gland; leaf blade adaxial surfaces glabrous, rarely villous or strigose (or pilose when young in E. aaron-rossii), bases cuneate to rounded; not s coastal Texas.	> 11
11	Stems usually densely clumped, previous year's dead stems often persistent; leaf blades filiform to linear or narrowly ovate to lanceolate or oblanceolate, 0.5–6.5 mm wide; Arizona, New Mexico to wc Texas.	> 12
12	Cyathia in terminal monochasia; petiole 0.2–2.2 mm; stem leaves usually reflexed, occasionally spreading; endemic to banks of Colorado River in n Arizona.	Euphorbia aaron-rossii
12	Cyathia in terminal dichasia (rarely in pleiochasia in E. wrightii); petiole absent; stem leaves spreading or ascending; New Mexico, Texas.	> 13
13	Leaf blades (2–)4–5 mm wide; involucral gland appendages 0.2 mm, forming narrow rim around distal margin of gland; capsules 3.2–4.5 × 4–6.5 mm, all 3 locules fertile; seeds 3.8 mm; mid and proximal cyathia early deciduous; Texas Panhandle, adjacent New Mexico.	Euphorbia strictior
13	Leaf blades 1–2.5 mm wide; involucral gland appendages 0.5–1 mm, orbiculate; capsules 2.5(–3) × 2.7–3.3(–5) mm, 1 locule usually aborting; seeds 2.2–2.9 mm; cyathia persistent; wc Texas.	Euphorbia wrightii
11	Stems usually solitary or few, if densely clumped then previous year's dead stems not persistent; leaf blades filiform, linear or elliptic to lanceolate, ovate, oblanceolate, obovate or orbiculate, 0.8–26 mm wide (often greater than 5 mm wide); e Texas and Oklahoma to e North America.	> 14
14	Involucral gland appendages 0–0.2 mm; peduncles 10–50(–70) mm.	> 15
15	Involucres and glands typically dark red; plants with spreading rootstocks; stems erect or ascending.	Euphorbia exserta
15	Involucres and glands yellow or yellow-green; plants with deep, stout taproots; stems decumbent or slightly ascending.	Euphorbia ipecacuanhae
14	Involucral gland appendages 0.3–3.5(–4.5) mm; peduncles 1–17 mm (occasionally peduncle of central cyathium greater than 30 mm; occasionally to 40 mm in early May–Jun flowering E. pubentissima).	> 16
16	Involucral glands red; leaf blades linear to filiform, 10–20 × 0.8–1.5(–4) mm; c, s peninsular Florida.	Euphorbia polyphylla
16	Involucral glands green; leaf blades not linear to filiform, or if linear then 10–55 × 1.5–6 mm; not peninsular Florida.	> 17
17	Involucral gland appendages 0.3–0.6 mm; proximal leaves greatly reduced and often scalelike and appressed.	> 18
18	Leaf blades usually linear, occasionally elliptic, rarely ovate, 1.5–6 mm wide, margins occasionally sparsely ciliate; seeds smooth.	Euphorbia curtisii
18	Leaf blades elliptic to ovate-deltate, 20–26 mm wide, margins densely ciliate; seeds with shallow and coarse depressions.	Euphorbia mercurialina
17	Involucral gland appendages (0.5–)1–3.5 mm; proximal leaves not reduced, neither scalelike nor appressed.	> 19
19	Leaf blades usually linear, rarely ovate, 1.5–4 mm wide, margins revolute; stems usually densely puberulent to sericeous, rarely glabrous; seeds 2 × 1.2–1.3 mm.	Euphorbia discoidalis
19	Leaf blades oblanceolate, obovate, lanceolate, lance-ovate, or elliptic, 5–18 mm wide, margins not revolute or occasionally slightly revolute (E. corollata); stems glabrous, slightly pilose, or rarely villous; seeds 2.2–2.8 × 1.6–2.2 mm.	> 20
20	Involucral gland appendages 2.5–3.5(–4.5) × 2.5–3.2 mm; peduncles (1.5–)5–11(–13) mm (proximal to 70 mm); seeds 2.5–2.8 mm.	Euphorbia corollata
20	Involucral gland appendages 1–2.2 × 1.5 mm; peduncles 1–5 mm (or 15–40 mm in early flowering plants); seeds 2.2–2.4 mm.	Euphorbia pubentissima

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