Petalidium namibense (Acanthaceae), a new species from Namibia

Abstract

Petalidium namibense, hitherto confused with P. englerianum, P. rossmannianum, and the widespread P. variabile, is here described as a new species. It is a range-restricted species, only known from the area to the southwest, west and northwest of Puros in the Kaokoveld Centre of Endemism, northwestern Namibia, where it grows at the base of rocky outcrops, on arid hillsides, and along ephemeral riverbeds and drainage lines. Diagnostic characters for P. namibense include the pale grey appearance of the plants, corky bark on older stems, vegetative parts with a dense white indumentum of relative long dendritic trichomes appearing lanate, flowers borne in short few-flowered dichasia, and bracteoles narrowly ovate. The flowers of P. namibense are distinctive in having the two upper corolla lobes discolorous, abaxially light brown, adaxially vermillion, without nectar guides, and connate towards the base for 25–40% of their length. The anterior lobe is adaxially magenta with two relative long, narrowly triangular yellow nectar guides. The two lateral lobes are adaxially vermillion, or distally magenta, grading to vermillion towards the throat, and lack conspicuous nectar guides. A comparison of key morphological features distinguishing P. namibense from P. sesfonteinense, its closest relative in appearance, as well as from P. englerianum, P. rossmannianum, and P. variabile, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Vulnerable (VU) is recommended for the new species.