1	Shrubs		(2)
+	Herbs		(3)
2 (1)	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.		2 Euphorbia antisyphilitica
+	Stems gnarled, not waxy; cyathia solitary on short shoots; s California.		16 Euphorbia misera
3 (1)	Annual herbs with taproots (E. graminea rarely perennial)		(4)
+	Perennial herbs with rootstocks, tubers, or taproots		(9)
4 (3)	Leaves opposite		(5)
+	Leaves mostly alternate (opposite at proximal nodes in E. bilobata; some opposite in E. graminea)		(6)
5 (4)	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.		10 Euphorbia hexagona
+	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.		17 Euphorbia nephradenia
6 (4)	Dichasial bracts with conspicuous white margins		(7)
+	Dichasial bracts wholly green or distal ones white		(8)
7 (6)	Dichasial bracts linear to narrowly oblanceolate; leaf blades pilose.		3 Euphorbia bicolor
+	Dichasial bracts narrowly elliptic to oblanceolate; leaf blades glabrous.		14 Euphorbia marginata
8 (6)	Stems 10–35 cm; leaves opposite proximally, alternate distally; dichasial bracts wholly green; involucral glands 5; involucral gland appendages usually 2-fid.		4 Euphorbia bilobata
+	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.		9 Euphorbia graminea
9 (3)	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.		13 Euphorbia macropus
+	Stem leaves alternate; plants usually with rootstocks or taproots, rarely with elongated tubers; Arizona to e North America		(10)
10 (9)	Involucral glands 4, appendages green; leaf blade adaxial surfaces densely pilose, bases cordate; s coastal Texas.		11 Euphorbia innocua
+	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 (10)	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)
+	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)
12 (11)	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.		1 Euphorbia aaron-rossii
+	Cyathia in terminal dichasia (rarely in pleiochasia in E. wrightii); petiole absent; stem leaves spreading or ascending; New Mexico, Texas		(13)
13 (12)	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.		20 Euphorbia strictior
+	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.		21 Euphorbia wrightii
14 (11)	Involucral gland appendages 0–0.2 mm; peduncles 10–50(–70) mm		(15)
+	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)
15 (14)	Involucres and glands typically dark red; plants with spreading rootstocks; stems erect or ascending.		8 Euphorbia exserta
+	Involucres and glands yellow or yellow-green; plants with deep, stout taproots; stems decumbent or slightly ascending.		12 Euphorbia ipecacuanhae
16 (14)	Involucral glands red; leaf blades linear to filiform, 10–20 × 0.8–1.5(–4) mm; c, s peninsular Florida.		18 Euphorbia polyphylla
+	Involucral glands green; leaf blades not linear to filiform, or if linear then 10–55 × 1.5–6 mm; not peninsular Florida		(17)
17 (16)	Involucral gland appendages 0.3–0.6 mm; proximal leaves greatly reduced and often scalelike and appressed		(18)
+	Involucral gland appendages (0.5–)1–3.5 mm; proximal leaves not reduced, neither scalelike nor appressed		(19)
18 (17)	Leaf blades usually linear, occasionally elliptic, rarely ovate, 1.5–6 mm wide, margins occasionally sparsely ciliate; seeds smooth.		6 Euphorbia curtisii
+	Leaf blades elliptic to ovate-deltate, 20–26 mm wide, margins densely ciliate; seeds with shallow and coarse depressions.		15 Euphorbia mercurialina
19 (17)	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.		7 Euphorbia discoidalis
+	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 (19)	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.		5 Euphorbia corollata
+	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.		19 Euphorbia pubentissima