Just LOOK at that lower corolla lobe – absolutely amazing, and one of a kind. Kyle and I discovered and named this species after the acetabulum…a fine wine drinking vessel from ancient Rome. Nothing else in Ruellia comes close to this sort of odd morphology.
Ruellia acetabula is at present known only from Namibia, where it loves the very dry rocky hillsides of the northwestern province Kunene. This species is clearly related to a group of SW African taxa that have a unique “2+2+1” calyx morphology, and smell of citronella. This group includes Ruellia marlothii, R. kaokoana, R. aspera, and R. diversifolia. See Tripp & Dexter 2012, Systematic Botany.
Wild collected, Namibia, Tripp & Dexter #871 & #1991 (RSA-POM); Photo by Erin Tripp