Urophyllum, Wallich, 1824

Wong, Khoon Meng, 2018, Flora of Singapore Precursors, 11. A new species of Urophyllum (Rubiaceae) from the Malay Peninsula, Phytotaxa 373 (3), pp. 236-240 : 236

Characteristics of Urophyllum s.l.

Bremekamp (1940a) distinguished Maschalocorymbus as having inflorescences trichotomously corymbose, a corolla throat with stiff-pointed hairs forming a ring and “5 ovate-subobtuse stigmas”. He characterized Pleiocarpidia as having inflorescences trichotomously corymbose or paniculate, corolla throat with weak moniliform hairs and a “peltate stigma”. On the other hand, he separated Urophyllum as having inflorescences either consisting of a terminal umbel preceded by a whorl of flowers or simply umbels, a corolla throat with hairs torulose or moniliform in the upper part only and “stigmata not cohering in a flat disk”. However, Bremekamp (1940a) did not comment on Urophyllum trifurcum H.Pearson ex King & Gamble (1904: 194) , which has the features of Urophyllum as he defined but also has trichotomously branched inflorescences.

Our own observations are that some (not all) Urophyllum s.s. species also have hairs in a ring in the corolla throat that may be either stiff, flexuous and moniliform or not, so this character does not appear to be sufficiently distinctive at the generic level among these taxa. Also, Maschalocorymbus corymbosus (Blume) Bremek. (1940a: 182) , or its synonym Urophyllum corymbosum (Blume) Korth. (1851: 194) , as well as Urophyllum s.s. have 3–8 stigmatic lobes, whereas Pleiocarpidia indeed has a peltate stigma with a central depression and not divided into lobes. Further, we have observed that in Maschalocorymbus and Urophyllum s.s., the stipules are of various shapes but always entire, whereas in Pleiocarpidia they are apically cleft or bifid (as also noted by Bremekamp 1940a).

Notwithstanding this, we note that the type species Pleiocarpidia enneandra (Wight) K.Schum. (in Engler & Prantl 1879: 314) and another, P. kinabaluensis Bremek. (1940a: 217) , were recovered nested inside Urophyllum in the molecular phylogenetic analyses by Smedmark & Bremer (2011), so that the implication is that the stigma and stipule conditions in Pleiocarpidia could be unimportant at the generic level in this case. Smedmark & Bremer (2011) also reported conflicting nuclear and chloroplast gene topologies from their analyses, which suggest complex genetic history that include possibilities of hybridization as well as incomplete lineage sorting; they also did not rule out the possibility of polyploidy although this was not demonstrated. Ridley (1932) would seem to have been particularly insightful when he remarked, “The species are often very difficult to separate...it is probable that a certain amount of hybridization takes place...” What effects these phenomena could have on the resultant morphology of taxa so derived is not presently understood and might be a useful caveat in considering what may seem to be odd characters for individual taxa or infra-generic groups within this taxonomic complex. Based on these considerations, we adopt a wider circumscription of Urophyllum to include Maschalocorymbus and Pleiocarpidia .