P. sect. Baursia (Reichb. ex Schott) Engl.
in Martius, Fl.Bras. 3(2): 134. 1878.
Basionym: P. crassinervium Lindl.
As defined by Engler and Krause, P. sect. Baursia consists of species with generally inconspicuous primary lateral veins, but the group as constituted by Krause remains highly variable in terms of its ovules, habit and leaf shape. The group comprises species with moderately many ovules and axile placentation, moderately many ovules with basal placentation, a few ovules with basal placentation, and solitary ovules with basal placentation. All of these species are purported devoid of primary lateral veins. In reality this is not true of all species included in the group. Most species have elongate, simple blades but there are also three species included which have 3-lobed or tripartite blades.
With 33 species included by Krause the section was third in size of the three major sections (P. sect. Philodendron with 64 and P. sect. Calostigma with 53 species). One species, P. acreanum K. Krause, was actually a member of P. sect. Pteromischum. Of the remaining species in the section, those which best fit the description of the group occur principally in eastern South America and in the upper Amazon basin and have more or less oblong leaf blades. Philodendron crassinervium is the type species of the section. Except for P. crassinervium, P. longilaminatum Schott (with axile placentation), P. bahiense Engl. and P. paxianum K. Krause (each with solitary ovules per locule), species of P. sect. Baursia have a few basal ovules per locule and oblong to oblong-elliptic blades. Many but not all have indistinct primary lateral veins.
Some of the members of Krause's P. sect. Baursia, especially those species which are vines with a solitary ovule per locule and occurring in the Andes west of the Continental Divide, such as P. lehmannii Engl., P. ellipticum Engl., P. pachycaule K. Krause, P. chimboanum Engl., P. longipes Engl. and P. graveolens Engl., do not seem to belong with the remainder and should perhaps be put into another section. The same is true of the three-lobed and tripartite species, P. deltoideum Poepp., P. panduriforme (Kunth) Kunth (Krause also included here P. reichenbachianum Schott, now a variety of P. panduriforme) and P. micranthum Poepp. ex Schott. With the exception of P. micranthum which has primary lateral veins lacking or weak these species have primary lateral veins at least some of the time (though they are indistinct in P. deltoideum. None of the three species appear to share any other features in common with the more typical members of the P. sect. Baursia, e.g., P. crassinervium Lindl., P. linnaei Kunth, and P. callosum K. Krause, among others.
Mayo (1986) believed that P. sect. Baursia contained two groups of species and he would also separate P. deltoideum and related species from the remainder, suggesting that Schott's grex Oligophlebium be recognized to accommodate these species.
Some species which were placed in P. sect. Baursia will have to be reinvestigated to determine if they belong instead in P. sect. Philopsammos, a section newly created by George Bunting (Bunting, 1986). That group is often similar in having elongated leaf blades, but it differs in having bilocular ovaries whereas those of P. sect. Baursia are plurilocular.
By no means all of the species with the more or less oblong blades in P. sect. Baursia lacking or have weak primary lateral veins. At least one species, P. wendlandii, the only Central American species placed in P. sect. Baursia by Engler, should be placed with P. sect. Calostigma. It has distinct primary lateral veins and a spongiose petiole with a distinct dark green annular ring like the other members of P. subsect. Belocardium in Central America.