Euphorbia sect. Poinsettia

Herbs, annual or perennial [rarely shrubs or small trees], with taproot or tuberous rootstock. Stems erect or ascending, branched, terete, glabrous or hairy. Leaves opposite or alternate; stipules usually present, occasionally absent, at base of petiole; petiole present, glabrous or hairy; blade monomorphic (occasionally polymorphic in E. cyathophora and E. heterophylla), base symmetric, margins entire or toothed, flat to revolute, surfaces glabrous or variously hairy; venation pinnate, midvein often prominent. Cyathial arrangement: terminal monochasia, dichasia, or condensed pleiochasia with 1–3 primary branches; individual pleiochasial branches unbranched or few-branched at 1 or more successive nodes; bracts subtending pleiochasia (pleiochasial bracts) opposite or whorled, usually wholly green or with paler green, white, pink, or red at base, sometimes wholly white, pink, or red, similar in shape and size to distal leaves or distinctly different, those on branches and subtending cyathia (dichasial and subcyathial bracts) opposite, distinct; additional cymose branches occasionally present in axils of distal leaves, but alternately arranged and without whorled bracts. Involucre ± actinomorphic, not spurred; glands 1–3 (sometimes 4–5 in E. eriantha, E. exstipulata, and E. radians), sessile or stipitate, shallowly cupped to deeply concave; appendages absent or petaloid (E. bifurcata, E. eriantha, and E. exstipulata). Staminate flowers 3–25. Pistillate flowers: ovary glabrous or hairy; styles distinct, occasionally appearing connate at base, unbranched or 2-fid. Seeds: caruncle present or absent. x = 7.

Distribution

North America, Mexico, West Indies, Central America, South America, Eurasia, Africa, Indian Ocean Islands, Pacific Islands, Australia.

Discussion

Species ca. 30 (10 in the flora).

Section Poinsettia belongs to Euphorbia subg. Chamaesyce (Gray) Reichenbach. An expanded sect. Poinsettia is recognized here to include three species that have previously often been included in sect. Alectoroctonum (Schlechtendal) Baillon (E. bifurcata, E. eriantha, and E. exstipulata). These three species differ from the so-called core Poinsettia by the presence of involucral gland appendages, but they possess the shallowly to deeply concave involucral glands and toothed leaves that are generally diagnostic for the broader section. Molecular phylogenetic analyses clearly unite these three species with the other members of sect. Poinsettia and not with sect. Alectoroctonum (Y. Yang et al. 2012).

Selected References

Dressler, R. L. 1961. A synopsis of Poinsettia (Euphorbiaceae). Ann. Missouri. Bot. Gard. 48: 329–341.

Mayfield, M. H. 1997. A Systematic Treatment of Euphorbia Subg. Poinsettia (Euphorbiaceae). Ph.D. dissertation. University of Texas.

Lower Taxa

Euphorbia bifurcata, Euphorbia cuphosperma, Euphorbia cyathophora, Euphorbia davidii, Euphorbia dentata, Euphorbia eriantha, Euphorbia exstipulata, Euphorbia heterophylla, Euphorbia pinetorum, Euphorbia radians

Key

1	Leaf blades linear to linear-elliptic, margins entire or with 2–4 inconspicuous teeth near apex; involucral glands densely canescent, appendages divided into subulate segments, incurved and covering glands, densely canescent; styles unbranched.	Euphorbia eriantha
1	Leaf blades linear, lanceolate, ovate, oblong, elliptic, or pandurate, margins conspicuously toothed (sometimes subentire in E. cyathophora, entire or few toothed in E. pinetorum); involucral glands glabrous, appendages absent or entire, undulate, slightly lobed, or divided into triangular segments, not incurved and covering glands, glabrous; styles 2-fid.	> 2
2	Branches often arcuate; involucral gland appendages usually present, rarely absent.	> 3
3	Leaf blades usually ovate, rarely oblong or elliptic, margins finely serrulate; petioles 15–49 mm; involucral glands 1(–3); ovaries glabrous; caruncles absent or rudimentary.	Euphorbia bifurcata
3	Leaf blades linear to narrowly elliptic or ovate, margins coarsely serrate; petioles 1–3 mm; involucral glands 4 (–5); ovaries puberulent on keels; caruncles 0.1 × 0.2 mm.	Euphorbia exstipulata
2	Branches ± straight (except occasionally proximal branches arcuate in E. davidii and E. dentata); involucral gland appendages absent.	> 4
4	Leaves usually opposite, occasionally alternate distally, blade margins coarsely crenate-dentate or doubly crenate; seeds with caruncles; pleiochasial bracts wholly green or with paler green, white, or mauve near base; annual herbs.	> 5
5	Ovaries densely pilose; capsules pilose (often sparsely); involucral gland taller than wide, stipitate.	Euphorbia cuphosperma
5	Ovaries and capsules glabrous or sparsely strigose; involucral glands shorter than wide, sessile.	> 6
6	Hairs of abaxial leaf blade surface stiff, strongly tapered; capsules 4–4.8 mm wide; seeds angular in cross section, unevenly tuberculate.	Euphorbia davidii
6	Hairs of abaxial leaf blade surface weak, filiform; capsules 3.5–4 mm wide; seeds rounded in cross section, evenly tuberculate.	Euphorbia dentata
4	Leaves usually alternate, occasionally opposite proximally, blade margins entire, subentire, or glandular-serrulate; seeds usually without caruncles, occasionally caruncles rudimentary; pleiochasial bracts green (purpurescent in E. pinetorum), often paler green, white, pink, or red at base, occasionally wholly white, pink, or red; annual or perennial herbs.	> 7
7	Stems 5–20(–30) cm, from moniliform tuberous rootstocks; seeds 4–4.6 mm; perennial herbs.	Euphorbia radians
7	Stems 20–100 cm, with taproots or woody rootstocks; seeds 2.1–3.1 mm; annual or perennial herbs.	> 8
8	Pleiochasial bracts wholly green or paler green at base; involucral glands stipitate, opening round (occasionally flattened from pressing), with annular rim.	Euphorbia heterophylla
8	Pleiochasial bracts green or purpurescent, often white, pink, or red at base, occasionally wholly white, pink, or red; involucral glands sessile or substipitate, opening oblong (flattened without pressing), without annular rim.	> 9
9	Annual herbs with spreading taproots; leaf blades 4–40 mm wide, linear, lanceolate, elliptic, or broadly pandurate; bracts usually green with white, pink, or red at base, occasionally distal bracts wholly white, pink, or red, rarely all bracts wholly green; capsules green; involucral gland 1; widespread, including s Florida.	Euphorbia cyathophora
9	Perennial herbs with thickened, woody taproots; leaf blades 2.5–5 mm wide, lanceolate to linear; bracts wholly purpurescent green or pink at base; capsules purpurescent; involucral glands 3(–5); Miami-Dade and Monroe counties, s Florida.	Euphorbia pinetorum