93. Euphorbia alta Norton, N. Amer. Euphorbia. 24, plate 24. 1899.
Giant spurge Giant spurge
Tithymalus altus (Norton) Wooton & Standley
Herbs, annual or biennial, with taproot. Stems erect, branched, 20–60 cm, glabrous. Leaves: petiole 0–1 mm; blade oblong-spatulate, 20–50 × 7–18 mm, base broadly attenuate, margins serrulate, apex rounded to obtuse, surfaces glabrous, ± glaucous; venation pinnate, midvein prominent. Cyathial arrangement: terminal pleiochasial branches 3, 2–3 times 2-branched; pleiochasial bracts elliptic-oblanceolate to oblong, similar in size to distal leaves; dichasial bracts distinct, broadly ovate to orbiculate/reniform, base obtuse, margins serrulate, apex obtuse to rounded and often mucronulate; axillary cymose branches 6–20(–25). Cyathia: peduncle 0.5–1 mm. Involucre narrowly campanulate, 0.8–1.1 × 1.1–1.3 mm, glabrous; glands 4, elliptic, 0.3–0.5 × 0.5–0.7 mm; horns absent. Staminate flowers 5–10. Pistillate flowers: ovary glabrous; styles 0.5–0.9 mm, 2-fid. Capsules depressed-globose, 2–3 × 2.5–3.5 mm, 3-lobed; cocci rounded, papillate, papillae 0.2–0.5 mm, glabrous; columella 1.5–1.9 mm. Seeds purple-black, ovoid, 1.6–2 × 1.3–1.7 mm, reticulate and areolate; caruncle reniform, flat, 0.5 × 0.7 mm.
Flowering and fruiting summer. Montane pine-oak and mixed conifer forests, disturbed roadsides, logged areas; 1500–3000 m; Ariz., N.Mex.; Mexico.
Euphorbia alta is a montane species from southern Arizona, New Mexico, and northern and central Mexico that is very similar to and sometimes difficult to distinguish from E. spathulata. Euphorbia alta tends to be a robust biennial, whereas E. spathulata is strictly annual. The most consistent characteristic to separate these two species is that the ovaries and capsules of E. alta are distinctly papillate, with the papillae rising sharply above the surface, whereas the ovaries and capsules of E. spathulata are merely verrucose, with the protuberances lower and rounded.