Mem. Tour N. Mexico, 113. 1848.
Illustrator: Barbara Alongi
Copyright: Flora of North America Association
Shrubs or trees, to 3 (–7) m; trunk well defined, to 0.3 m diam.; bark fissured vertically; branches many, spreading or straggling, knotty; crown diffuse. Leaves opposite or fascicled, 1–3 cm, folded at night and often also in heat of day; stipules persistent, subulate, 1 mm, apex acute, somewhat spinescent, glabrous; petiole absent or nearly so; leaflets 8–16, dark green, linear-oblong to linear-spatulate, 5–15 × 2–3 mm, apex obtuse, coriaceous, surfaces reticulate. Pedicels hairy. Flowers axillary, mostly solitary, sometimes clustered, 1.2–2 cm diam.; sepals (4–) 5, obovate, to 5 mm, outer smaller; petals (4–) 5, usually blue to purple, rarely white, obovate to elliptic, 6–10 × 2–3 mm, base short-clawed, apex often notched; stamens (8–) 10, shorter than petals; filaments each with small crenate scale at base; ovary obcordiform, flattened, 2 (–4) -lobed, 2 (–4) -locular, hairy; style subulate. Capsules becoming orange, obcordiform, flattened, 10–20 mm diam., 2 (–4) -lobed, 2 (–4) -locular, ± 2 (–4) -winged, apex abruptly attenuate-apiculate, reticulate, hairy. Seeds yellowish-brown, ovoid to reniform, 10–11 mm.
Phenology: Flowering Mar–Sep.
Habitat: Shrubby vegetation, limestone soils.
Elevation: 0–1200 m.
Tex., Mexico (Chihuahua), Mexico (Coahuila), Mexico (Nuevo León), Mexico (San Luis Potosí), Mexico (Tamaulipas)
Guaiacum angustifolium occurs in southern, central, and western Texas. The root bark is used as a source of soap, and root extracts are used in folk medicine to treat various diseases. The stems are used for fence posts, tool handles, and firewood.