Midribs of P. subg. Philodendron are more variable than other aroid genera, such as Stenospermation, Rhodospatha, Spathiphyllum and Monstera, yet not as taxonomically significant for the genus as in the related genus Dieffenbachia.
Upper midribs of P. subg. Philodendron are highly variable, being flattened or variously sunken or raised. A total of 56 species have upper midribs which are at least sometimes raised whereas 26 species have upper midribs which are at least sometimes sunken (only P. cretosum is deeply sunken) and 54 species have upper midribs which are sometimes flat. Only 12 species, including P. antonioanum, P. aromaticum, P. albisuccus, P. coloradoense, P. dressleri, P. granulare, P. pirrense, P. purulhaense, P. strictum, P. warszewiczii, P. wilburii var. longipedunculatum and P. zhuanum are described as having strictly flattened upper midribs. Fifteen species are described as having only convex midribs and 15 species are described as having only sunken midribs. The flat condition is shared with the convex condition in 37 species and with the concave condition in 14 species. A few variable species are described as having the upper midrib at least sometimes raised, flat or sunken. These are P. angustilobum, P. chiriquense, P. jodavisianum, P. platypetiolatum, and P. wilburii var. wilburii.
An additional 12 species are at least sometimes flat on the upper midrib but are also sometimes sunken (but not at also sometimes raised). These are P. anisotomum, P. bakeri, P. basii, P. brunneicaule, P. dodsonii, P. pterotum, P. tenue, P. thalassicum, P. jodavisianum, P. hederaceum, P. squamicaule, and P. verrucosum. Only two species, P. morii and P. niqueanum, are described as having upper midribs prominently raised.
The upper midrib is nearly always to some extent paler than the surface. In only 12 species are the midrib and the blade concolorous. An additional eight species have midribs which are concolorous to slightly paler.
The lower midrib of Philodendron blades exhibits less variability. All species have midribs which are to some extent raised. A few species, e.g., P. crassispathum, have the lower midrib so broadly convex as to appear nearly flat but most species have the midrib noticeably raised. The lower midrib is generally convex but often narrowly rounded, thicker than broad, and sometimes bluntly acute. Species with the lower midrib weakly raised are rare in P. subg. Philodendron. Only seven species have lower midribs which are at least sometimes described as broadly convex or weakly raised. These are: P. anisotomum, P. crassispathum, P. fortunense, P. microstictum, P. platypetiolatum, P. purpureoviride, and P. straminicaule.
Many more species have midribs which are more prominent. In the largest category, 47 species have lower midribs that are more or less convex, i.e., more or less hemispherical in cross-section. Somewhat fewer, 36 species, have lower midribs which are described as thicker than broad, narrowly convex, narrowly rounded or bluntly acute at least part of the time. A number of these species have lower midribs that are so prominently raised as to be noticeably thicker than broad. These include: P. copense, P. cretosum, P. dolichophyllum, P. hebetatum, P. heleniae, P. jodavisianum, P. panamense, and P. roseospathum. Species which have lower midribs sometimes thicker than broad are P. scalarinerve and P. wilburii var. longipedunculatum.
The most extreme cases of raised midribs are those which are so raised as to appear to be a cylinder attached to the surface of the leaf. These types of veins have been referred to as "round-raised". Examples of species with such midribs are P. brunneicaule, P. dressleri, P. madronoense, P. squamipetiolatum, and P. radiatum.
Species which have the lower midrib at least sometimes round-raised are P. hederaceum, P. ligulatum, and P. tripartitum. In cases where the midrib is round-raised, the primary lateral veins are also sometimes round-raised.
While the lower midrib is more likely than the upper surface to be concolorous with the rest of the blade or to be darker than the remaining blade surface, 45 species still are reported as having the midrib paler than the surface. Twenty-four species have the lower midrib described as darker than the surface.
A number of species, including P. edenudatum, P. grayumii, P. ligulatum var. ligulatum, and P. sagittifolium, have one or both midribs maroon or purplish spotted.