The identity of Pachystigma bowkeri (Rubiaceae, tribe Vanguerieae)

Since its publication the name Pachystigma bowkeri Robyns has not been taken up and various authors suggested that the taxon to which it was applied might be conspecific with P. macrocalyx (Sond.) Robyns. In a recent study of the genus in southern Africa it was established that P. macrocalyx constitutes a heterogeneous taxon. Two species should be recognized with P. bowkeri Robyns being the correct name for one of these. The name P. bowkeri Robyns is therefore reinstated. Sedert sy publikasie is die naam Pachystigma bowkeri Robyns nooit gebruik nie en verskeie outeurs het voorgestel dat die spesie waarop die naam betrekking het gelyksoortig met P. macrocalyx (Sond.) Robyns is. Tydens 'n onlangse studie van die genus in suidelike Afrika is vasgestel dat P. macrocalyx 'n heterogene takson verteenwoordig. Twee spesies moet erken word met P. bowkeri Robyns die korrekte naam vir een van hierdie spesies. Die naam P. bowkeri Robyns word dus heringestel.


Introduction
The demarcation of Pachystigma macrocalyx (Sond.) Robyns and P. bowkeri Robyns is considerably confused. Since its publication by Robyns in 1928 the name P. bowkeri has not been taken up. Various authors including Coates Pal grave (1977) , Palmer & Pitman (1973), Gibbs Russell et al . (1984) and von Breitenbach (1986), relegated P. bowkeri to the synonymy of P. macrocalyx and thus contributed to the disuse of the name. Notes on some species covers in herbaria also state that the two species cannot be separated by the hairiness of the leaves owing to the existence of intermediate specimens. To add to the confusion, the same specimen (Pegler 702) was apparently cited under both P. macrocalyx and P. bowkeri by Robyns (1928). However, a study of Pegler collections showed that the number 702 was assigned to at least three different gatherings -two belonging to P. macrocalyx and the third to P. bowkeri. Pachystigma macrocalyx was first described in the genus Vangueria Juss. by Sonder in 1850. In 1865 Sonder used the hairiness of the leaf to distinguish this species from the other species of Vangueria that also had long calyx lobes and in the description, 'young branches, leaves and flowers tomentose' were printed in italics. Reference was made to the following specimens: Ecklon & Zeyher Ebenac 17; H. Bowker s.n. and Gerrard & McKen 1344. Robyns (1928) reinstated the genus Pachystigma Hochst. to accommodate Vangueria macrocalyx and a few other,species. In the same year Robyns described P. bowkeri and designated Bowker s.n. [the same specimen cited by Sonder (1865) under Vangueria macrocalyx] as the type of the new name. Although some of the differentiating characters mentioned in the diagnosis may also be found in other species of the genus, it is evident that the newly described species was separated from P. macrocalyx mainly by its hairless leaves and calyx.
When referring to some of the diagnostic features of P. macrocalyx (shrub with small densely velvety leaves), Palmer & Pitman (1973) drew attention to the presence of tall trees with large almost smooth leaves in the forests of Zululand. They speculated that although these trees were also at that stage referred to Pachystigma macrocalyx, they may in future be treated as a different taxon .
A recent investigation, Boshoff (1987), revealed that P. macrocalyx represents a heterogeneous taxon comprising at least two distinct species, namely P. macrocalyx and P. bowkeri. The differences between the two species are summarized in Table 1.
In evaluating the taxonomic significance of the characters differentiating between the two species, it is evident that in the Rubiaceae they are being used by various authors at different hierarchical levels. For example, the size of the leaf blade was used to distinguish between species of Alberta E . Mey. (Puff et al. 1984) Calycosiphonia Pierre ex Robbrecht (Robbrecht 1981b) and Vangueria (Verdcourt 1982). Habit and leaf texture ,  (Puff et al . 1984) . Bridson (1987) also employed the texture of the leaf to differentiate between species of Multidentia Gilli . Characters of the leaf indumentum, in particular hairiness , are extensively used in the demarcation of taxa in the Rubiaceae. The two varieties of Otiophora lebruniana (Bamps) Robbrecht & Puff are distinguished on this basis (Robbrecht & Puff 1981), whereas in Argocoffeopsis Leb . (Robbrecht 1981b) , Tricalysia A. Rich . ex DC. (Robbrecht 1981a) Vangueria (Verdcourt 1987) , Psydrax Gaertn. (Bridson 1985) and many other genera, hairiness of the leaf serves as a differentiating character between species. Moreover, the genus Tapiphyllum Robyns is separated from related genera mainly on the 561 basis of the velvety-tomentose character of most parts of the plant (Verdcourt 1987) . In the genus Pachystigma the presence or absence of hairs on the leaves was also used by Verdcourt (1982) to differentiate between P. kenyense Verdc. and P. burttii Verdc. Peduncle length is also considered a distinguishing character between taxa , and it has for example been used in the genera Psydrax (Bridson 1985) and Vangueria (Verdcourt 1982).
The geographical distribution of P. bowkeri ( Figure 2) corresponds with that of several other woody rubiaceous species, for example Psycho tria capensis (Eckl.) Vatke, Rothmannia capensis Thunb., Tricalysia lanceolata (Sond.) Burtt Davy and Canthium suberosum Codd. These characteristically sparse and widely disjunct distribution patterns may be indicative of a previous wider distribution.
Except for P. macrocalyx (Figure 3), P. bowkeri is not closely related to any other species of Pachystigma in southern Africa . In the protologue of P. kenyense, a Figure 2 The know geographical distribution of Pachystigma bowkeri.
If indeed this is the case , the resultant discontinuity in distribution recalls the distribution pattern of Tricalysia africana (Sim) Robbrecht, a Pondoland endemic separated by a wide interval from its putative Guineo-Congolean relatives (Robbrecht 1985) .