P. subsect. Macrobelium (Schott) Engl.,
Fl. Bras. 3(2): 143. 1878. Basionym: P. grex Macrobelium Schott, Syn. Aroid. 96. 1856. Type species: P. sagittifolium Liebm.
Philodendron subsect. Macrobelium was the largest subsection in P. sect. Calostigma reported by Krause with 22 species. It is a poorly defined group, roughly defined by Krause as consisting of species with somewhat spongy petioles, 6-12 locular ovaries with relatively few ovules per locule and basal to sub-basal placentation. In addition, though not stated by Krause the blades are all cordate, sagittate or hastate. Though Krause described the subsection as having 2-5 ovules per locule, many more species have been added to the group since the last revision and several species which logically belong in P. subsect. Macrobelium have more than ten ovules per locule, despite having sub-basal placentation.
Because of its size (28 species) and the diverse composition of the species inhabiting P. subsect. Macrobelium it was deemed necessary to divide the subsection into a group of series. A discussion of the new series of P. subsect. Macrobelium follows: