Petalidium hoarusibense (Acanthaceae), a new species from Namibia

Petalidium hoarusibense, hitherto misidentified as P. rossmannianum and P. ohopohense, is here described as a new species. It is a range-restricted species, only known from the area to the south and southeast of Okandjombo in the Kaokoveld Centre of Endemism, northwestern Namibia, where it grows on arid hillsides and along ephemeral riverbeds and drainage lines. Diagnostic characters for P. hoarusibense include the pale grey appearance of the plants, single or multi-stemmed from a thick rootstock, vegetative parts with a dense white indumentum of short dendritic, simple and bifurcate trichomes appearing matted, flowers borne in short, few-flowered dichasia, and bracteoles narrowly ovate or elliptic, deeply concave, appearing cobwebbed due to a mixture of loosely entangled long simple and dendritic trichomes. The flowers of P. hoarusibense are distinctive in having the lobes magenta with the anterior lobe sometimes slightly lighter shaded than the others and with two separate narrowly triangular yellow nectar guides. A comparison of key morphological features distinguishing P. hoarusibense from P. kaokoense, its closest relative in appearance, as well as from P. ohopohense, P. rossmannianum, P. sesfonteinense, and P. welwitschii, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Vulnerable (VU) is recommended for the new species.