Flora of North America species comparison

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Euphorbia sect. Alectoroctonum
Habit 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.
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.
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.

Euphorbia sect. Alectoroctonum
Distribution
North America; Mexico; Central America; South America; West Indies [Introduced in Asia, Pacific Islands]
Discussion

Species ca. 120 (21 in the flora).

(Discussion copyrighted by Flora of North America; reprinted with permission.)
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.
E. antisyphilitica
2. Stems gnarled, not waxy; cyathia solitary on short shoots; s California.
E. 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.
E. 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.
E. 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.
E. bicolor
7. Dichasial bracts narrowly elliptic to oblanceolate; leaf blades glabrous.
E. 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.
E. 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.
E. 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.
E. 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.
E. 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.
E. 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.
E. 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.
E. 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.
E. exserta
15. Involucres and glands yellow or yellow-green; plants with deep, stout taproots; stems decumbent or slightly ascending.
E. 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.
E. 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.
E. curtisii
18. Leaf blades elliptic to ovate-deltate, 20–26 mm wide, margins densely ciliate; seeds with shallow and coarse depressions.
E. 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.
E. 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.
E. 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.
E. pubentissima
Source FNA vol. 12, p. 240. Authors: Jess A. Peirson, Victor W. Steinmann, Jeffery J. Morawetz.
Parent taxa Euphorbiaceae > Euphorbia
Subordinate taxa
E. aaron-rossii, E. antisyphilitica, E. bicolor, E. bilobata, E. corollata, E. curtisii, E. discoidalis, E. exserta, E. graminea, E. hexagona, E. innocua, E. ipecacuanhae, E. macropus, E. marginata, E. mercurialina, E. misera, E. nephradenia, E. polyphylla, E. pubentissima, E. strictior, E. wrightii
Synonyms Alectoroctonum, Agaloma, Tithymalopsis
Name authority (Schlechtendal) Baillon: Étude Euphorb., 284. (1858)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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