Thyrsacanthus Moricand (1847: 165)

Ezcurra, Cecilia, Côrtes, Ana Luiza A. & Daniel, Thomas F., 2019, A synopsis of Thyrsacanthus (Acanthaceae) in Argentina and Paraguay, with notes on generic delimitation, a new combination, and lectotypifications, Phytotaxa 395 (2), pp. 81-88 : 83-84

Thyrsacanthus Moricand (1847: 165) .

Type:— Thyrsacanthus ramosissimus Moricand (1847: 165) .

Drejera Nees (1847a: 112) . Type:— Drejera ramosa Nees(1847a: 112) .

Shrubs, subshrubs or perennial herbs, erect or ascending, in some cases deciduous during flowering, with quadrangular or cylindrical branches, longitudinally striate or sulcate, glabrous or pubescent. Leaves sessile or shortly petiolate, entire. Inflorescences of sessile flowers arranged in thyrses or spikes that are sometimes secund and grouped in clusters. Bracts and bracteoles opposite, frequently smaller than calyx, inconspicuous. Flowers nototribic. Calyx much shorter than the corolla tube, 5-parted, with apically acute, subequal segments, the posterior one sometimes smaller. Corolla red with a straight or curved tube that widens to the throat, or white, lilac or pale blue, with a straight narrow tube apically not ampliate; limb bilabiate, posterior lip narrow and entire or very slightly 2-lobed, internal in bud, and anterior lip 3-lobed with the lobes narrow and similar to the posterior lip, subequal, the middle one external in bud. Stamens 2, inserted on the supra-median portion of the corolla tube, slightly or completely exserted; anthers dorsifixed, with 2 thecae, the thecae parallel, narrowly oblong, equal, muticous, inserted at or nearly at the same height, with reduced connective; staminodes absent. Pollen 3-colporate, spherical to prolate, with colpi flanked by pseudocolpi. Disk annular or shortly cupular. Ovary with 2 ovules in each locule, style filiform, and stigma shortly 2-lobed. Fruit an obovoid capsule with the base markedly contracted into a solid stipe, apically apiculate. Seeds 4, suborbicular, laterally compressed, sparsely tuberculate to rugose or smooth or glochidiate; retinacula curved, pointed.

A genus with at least six species distributed in Venezuela, Guyana, Brazil, Bolivia, Paraguay, and northern Argentina ( Côrtes et al. 2010). Thyrsacanthus is morphologically very similar to Anisacanthus Nees (1842: 307) of North America, with which it has been confused until recently (e.g. Wasshausen & Wood 2004), especially due to its red ornitophilous flowers, probably associated with pollination by hummingbirds ( Côrtes et al. 2010). However, these genera can be distinguished by the position of the stamens and the direction of anther dehiscence. In Thyrsacanthus , the flowers are nototribic, with the stamens close to the upper lip of the corolla and the anthers dehiscing toward the lower lip. In contrast, Anisacanthus has sternotribic flowers with the stamens situated near the lower lip and the anthers dehiscing toward the upper lip. Based on molecular phylogenetic results, Daniel et al. (2008) showed that these genera pertain to different clades of the Tetramerium . lineage.