Acanthus L.

Shrubs or perennial herbs, erect or decumbent, without cystoliths. Leaves opposite [whorled or in a basal cluster], petiolate; leaf blade margin entire, pinnatifid, or spinously toothed. Inflorescences in terminal [or axillary], uninterrupted spikes; bracts imbricate, ovate, growing in size as plant matures, margin spiny; bracteoles 2 or sometimes absent. Calyx 4-lobed, posterior and anterior lobes larger, lateral pair of lobes smaller. Corolla tube very short, horny; limb of a single (lower) lip (upper lip sometimes rudimentary); lip elongate, obtusely 3-lobed. Stamens 4, paired, inserted at top of corolla tube, exserted; filaments stout, thickened, bony; anthers 1-thecous; theca linear-oblong, usually densely bearded, lacking basal appendages; staminodes absent. Ovary with 2 ovules per locule; style slender; stigma 2-cleft. Capsule not stipitate, ellipsoid, shiny, 4-seeded; retinacula present. Seeds ± compressed.