Petalidium sesfonteinense, P. kaokoense, P. variabile, and other species of Petalidium, Nees von Esenbeck, 1832

Swanepoel, Wessel & Manzitto-Tripp, Erin A., 2022, Petalidium sesfonteinense (Acanthaceae), a new species from the Kaokoveld, Namibia, Phytotaxa 549 (2), pp. 127-135 : 134

Identification key to Petalidium sesfonteinense, P. kaokoense, P. variabile, and other species of Petalidium within the distribution range of P. sesfonteinense

1. Plants usually prostrate or at least some branches prostrate...............................................................................................................2

1’. Plants erect..........................................................................................................................................................................................4

2. Inflorescence axis elongated. Flowers alternate on axis (up to 40)............................................................................ P. angustitubum

2’. Inflorescence compact balls ...............................................................................................................................................................3

3. Leaf lamina ca. 1.5× as long as wide or more compact, broadly ovate.......................................................................... P. halimoides

3’. Leaf lamina ca. 2.5× as long as wide or thinner, narrowly ovate........................................................................................ P. lanatum

4. Indumentum on leaves consisting of simple trichomes, erect or appressed (strigose).......................................................................5

4’. Indumentum on leaves consisting of dendritic or stellate trichomes interspersed with very short-stalked glandular trichomes ........ ............................................................................................................................................................................................................6

5. Trichomes erect, corolla tube distinctively curved, corolla lobes one-coloured red....................................................... P. coccineum

5’. Trichomes appressed, corolla tube straight, front corolla lobe differently coloured or of darker shade than other lobes: white, pink, or crimson; other lobes: brown, orange, pink, or crimson................................................................................................. P. variabile

6. Indumentum consisting of short-stalked stellate trichomes interspersed with very short-stalked glandular trichomes; expanded portion of corolla strigose outside ................................................................................................................................... P. kaokoense

6’. Indumentum consisting of dendritic trichomes only; expanded portion of corolla glabrous outside ....................... P. sesfonteinense