KEY TO PHILODENDRON SUBGENERA
1. Stem of mature flowering plants with a succession of many leaves terminated by a solitary, or rarely several inflorescences; petioles with long sheaths encircling the stem at base, winged to at least midway on adult plants; up to 5 leaves produced on the stem between each successive inflorescence.
1. Stem of mature flowering plants with a succession of short sympodial segments each bearing a cataphyll and a single leaf with the inflorescence(s) 1 to 10 and appearing to be borne in the leaf axils; petioles of adult plants with short, usually inconspicuous petiole sheath and borne on the side of the stem, not encircling it at the base, usually only winged near the base on adult plants (sometimes fully winged on juvenile plants); inflorescences produced with each new leaf (though frequently aborted).
2. Stems often arborescent, with conspicuous leaf scars and frequently interpetiolar scales persisting around at least the upper margins of the petiolar scars; male flowers conspicuously elongate, up to 10 times longer than wide; staminodial zone between staminate and pistillate zones of the spadix subequal or longer than fertile zone.
2. Stems rarely arborescent, often scandent, stout or slender and with interpetiolar scales lacking; male flowers only 2-3 times longer than wide; staminodial zone between staminate and pistillate zones of spadix much shorter than the fertile staminate zone.
There are also a number of anatomical characteristics separating the subgenera. Vegetative buds of Philodendron subg. Philodendron are always located below the point of overlap in the sheath margins of the cataphyll whereas they are lacking in P. subg. Pteromischum (Ray, 1987b). Philodendron subg. Pteromischum is also distinct in having a style with a shallow compitum with a subepidermal concentration of raphide crystals (Mayo, 1986,1989) and a total lack of tannin cells in the stamens (Mayo, 1986). In addition, while hypophyllous stem segments are typical for P. subg. Philodendron they are ambiphyllous, hyperphyllous or peraphyllous in P. subg. Pteromischum. In addition, P. subg. Philodendron is characterized by having continuous parenchyma from the cortex to the center of the stem. In contrast P. subg. Pteromischum has a central cylinder with a solid ring of fibers around the central cylinder.
While not definitive, there are a number of other features that normally are useful to separate P. subg. Pteromischum. Its blades are typically more or less oblong, moderately thin and typically more inequilateral than P. subg. Philodendron. The stems of P. subg. Pteromischum are commonly less than 1 cm diam and frequently with rather long internodes. While there are many members of P. subg. Philodendron with more or less oblong leaf blades, these are often cordate or subcordate at the base in P. subg. Philodendron. In addition many members of P. subg. Pteromischum have stems that branch and spread away from their support before flowering. This behavior is rare in P. subg. Philodendron.
Because of the usually conspicuously sheathed petioles for P. subg. Pteromischum, the subgenus is more likely to be confused with sterile specimens of Rhodospatha Poepp. than with the oblong-bladed species of P. subg. Philodendron (and in such a case the presence of trichoschlereids beneath the epidermis of Rhodospatha easily distinguish it from Philodendron).