Euphorbia bilobata |
Euphorbia revoluta |
|
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black-seed spurge |
revolute spurge, threadstem sandmat, threadstem spurge |
|
Habit | Herbs, annual, with slender taproot. | Herbs, annual, with slender taproot. |
Stems | erect, branched, 10–35 cm, glabrous or strigillose (especially when young and around nodes). |
erect, 5–25 cm, glabrous. |
Leaves | opposite proximally, alternate distally; stipules 0.1–0.2 mm; petiole 1–4(–6) mm, glabrous, sericeous or strigillose; blade linear to narrowly elliptic, 8–52 × 2–7 mm, base attenuate, margins entire, ciliate-strigose, apex acute, abaxial surface sparsely strigillose to sericeous, adaxial surface usually glabrous; venation obscure, only midvein conspicuous. |
opposite; stipules distinct, subulate-filiform, entire, 0.5–0.9 mm, glabrous; petiole 0.6–1.1 mm, glabrous; blade linear, 6–27 × 0.6–1.2 mm, base symmetric, attenuate, margins entire, revolute, apex acute, sometimes mucronate, surfaces glabrous; only midvein conspicuous. |
Involucre | obconic, 0.9–1.5 × 0.9–1.3 mm, strigillose to pilose; glands 5, yellow or pink, U-shaped, 0.2–0.3 × 0.4–0.5 mm; appendages greenish, white, or pink, forming narrow rim around gland, or ovate, oblong, or obovate and usually 2-fid, rarely rudimentary, 0.2–1.4 × 0.2–0.6 mm, entire. |
obconic to campanulate, 0.7–0.9 × 0.5–0.7 mm, glabrous; glands 4, pink to dark purple, nearly circular to oblong or reniform, 0.1 × 0.1–0.2 mm; appendages white, oblong, nearly circular, ovate, deltate, or forming thin margin around gland, rarely absent, (0–)0.1–0.2 × (0–)0.1–0.3 mm, distal margin entire. |
Staminate flowers | 20–25. |
5–10. |
Pistillate flowers | ovary glabrous, puberulent, strigillose, or pilose; styles 0.5–0.8 mm, 2-fid 1/3–1/2 length. |
ovary glabrous; styles 0.2–0.3 mm, unbranched. |
Capsules | oblate, 1.5–2.6 × 2.1–3.3 mm, glabrous or puberulent, strigillose, or pilose; columella 1.2–2.1 mm. |
ovoid to subglobose, 1.5–1.8 × 1.6–1.8 mm, glabrous; columella 1.1–1.5 mm. |
Seeds | brown to grayish black, narrowly ovoid, 3- or 4-angled in cross section, sometimes obscurely so, 1.3–1.9 × 1–1.4 mm, tuberculate, often with shallow depressions; caruncle absent. |
whitish, brick red, light gray, or light brown, narrowly to broadly ovoid, 4-angled in cross section, 0.9–1.4 × 0.7–1 mm, nearly smooth, rugulose, with faint transverse ridges, or with 2–3 well-defined transverse ridges separated by shallow depressions. |
Cyathia | solitary at distal nodes or in weakly defined cymes or dichasia, dichasial bracts and distal stem leaves wholly green; peduncle 0.5–3.6 mm, strigillose. |
solitary at distal nodes; peduncles absent or to 1.5 mm. |
2n | = 32. |
|
Euphorbia bilobata |
Euphorbia revoluta |
|
Phenology | Flowering and fruiting spring–fall. | Flowering and fruiting summer–late fall. |
Habitat | Sandy and rocky soils on slopes and canyon bottoms in pine-juniper woodlands, oak woodlands, grasslands. | Desert scrub, sagebrush scrub, juniper woodlands, Joshua tree-pinyon pine woodlands, oak woodlands, grasslands, chaparral, pine-oak forests. |
Elevation | 1400–2600 m. (4600–8500 ft.) | 600–2500 m. (2000–8200 ft.) |
Distribution |
AZ; NM; TX; Mexico (Chihuahua, Durango, Sonora)
|
AZ; CA; CO; NM; NV; TX; UT; Mexico (Baja California, Chihuahua, Coahuila, Sonora, Zacatecas)
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Discussion | In Texas, Euphorbia bilobata is known only from Jeff Davis County. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Euphorbia revoluta is a distinctive, easily recognizable species by virtue of the combination of unbranched styles and linear leaves with symmetric bases. The species ranges through northern Mexico into the southwestern United States and is composed of three well-marked, geographically distinct races that can be distinguished by their seeds. The first of these races occurs primarily in pine-oak forest of the Sierra Madre Occidental of Mexico (Chihuahua and Sonora) and barely enters the flora area in the Huachuca Mountains of southeastern Arizona; the seeds are brick red and nearly smooth. The second race is widespread in the Chihuahuan Desert from northern Mexico to southeastern New Mexico and southwestern Texas; its seeds are whitish to light gray and possess two or three well-defined transverse ridges separated by shallow depressions. The third race corresponds to the type collection and is characterized by grayish white to light brown seeds that are rugulose or with faint transverse ridges; it occurs in northwestern Mexico (Baja California and Sonora) and throughout the arid southwestern United States. It is probable that further study will justify the taxonomic segregation of these races as either distinct species or subspecies. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Source | FNA vol. 12, p. 243. | FNA vol. 12, p. 286. |
Parent taxa | Euphorbiaceae > Euphorbia > sect. Alectoroctonum | Euphorbiaceae > Euphorbia > sect. Anisophyllum |
Sibling taxa | ||
Synonyms | Chamaesyce revoluta | |
Name authority | Engelmann: in W. H. Emory, Rep. U.S. Mex. Bound. 2(1): 190. (1859) | Engelmann: in W. H. Emory, Rep. U.S. Mex. Bound. 2(1): 186. (1859) |
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