Basal veins are generally to some extent coalesced near their union with the petiole. The union or coalescence of basal veins is referred to as the "posterior rib" (Figs. 22, 25) [See also Croat & Bunting, 1979]. Sixty-three species (65 taxa) have the basal veins at least in part united and thus possess posterior ribs, while 34 species (38 taxa) have the basal veins free to the base (or lack basal veins all together), and thus lack posterior ribs. Both the number and nature of basal veins and the length of the posterior rib are good characters for distinguishing taxa. The basal veins may be completely coalesced for a distance (Fig. 22) or loosely coalesced. The posterior rib may be prominently naked for a distance (Figs. 22, 169) or barely naked (Figs. 26, 211) or not at all naked (Figs. 161, 163, 287). Basal veins are free or nearly so on most species with rounded or cordulate leaf bases (Fig. 20). Such species include: P. auriculatum, P. bakeri, P. correae, P. chirripoense, P. cretosum, P. dolichophyllum, P. granulare, P. immixtum, P. lentii, P. ligulatum, P. pseudoauriculatum, P. roseospathum, P. scalarinerve, P. ubigantupense, P. utleyanum, and P. wendlandii. Even species with short or poorly developed posterior lobes, such as P. annulatum, P. clewelii, P. davidsonii, P. microstictum, P. niqueanum, and P. platypetiolatum usually have basal veins free to the base. Alternatively, those species with well developed posterior lobes usually have the basal veins coalesced to some degree. Posterior ribs range from less than 0.5 cm long as in P. straminicaule and 1 cm long in P. angustilobum, P. basii, P. alticola to more than 14 cm long in P. gigas.
Another important taxonomic character is the extent to which the posterior rib is naked along the sinus. In general, the longer the posterior rib the more likely it is to be naked at least to some extent along the sinus. However, P. tenue is exceptional in both, having a well developed posterior rib and having the primary lateral rib not naked along the sinus. The degree to which the posterior rib is naked along the sinus varies from as little as 0.5 cm in P. straminicaule to 6 cm in P. squamicaule. Examples of species with prominently naked posterior ribs include: P. albisuccus, P. alticola, P. aromaticum, P. basii, P. brunneicaule, P. copense, P. dodsonii, P. dressleri, P. findens, P. hebetatum, P. knappiae, P. panamense, P. purulhaense, P. strictum, P. thalassicum, and P. verapazense.