The last thorough taxonomic revision of the Araceae was that by Engler (1905-1920), conducted in part by K. Krause (Engler & Krause, 1908, 1920; Krause, 1913). That revision included eight subfamilies of Araceae with Philodendron in its own subfamily and tribe, Philodendroideae and Philodendreae respectively. Philodendreae (Engler, 1911) shared the subfamily with six other tribes, including Schismatoglottidinae, Anubideae, Aglaonemateae, Dieffenbachieae, Zantedeschieae, Typhonodoreae and Peltandreae. Engler (1912) grouped Philodendron with Homalomena and the genera of the largely Asian subtribe Schismatoglottidinae (Bucephalandra Schott, Gamogyne N.E. Br., Microcasia Becc. and Piptospatha N.E. Br.) into subtribe Philodendrineae. Bogner and Nicolson (1991) left Engler's subfamily Philodendroideae intact but Grayum (1984) made substantive changes including an incorporation of the subfamily Calloideae and the Colocasioideae (thus forcing a change in the subfamilial name to Colocasioideae, because of nomenclatural priority). The subfamily is divided up into Nephthytis Schott, Aglaonema, Peltandra, and Philodendron Alliances. In Grayum's system Philodendron, in its own tribe Philodendreae, originally shared the alliance with the tribes Spathicarpeae, Dieffenbachieae and Bognereae. Both Dieffenbachia and Bognera Nicolson are in their own tribe but the Spathicarpeae has eight small genera mostly occurring in southern South America. These are: Asterostigma F.E.L. Fischer & C.A. Mey., Gearum N.E. Br., Gorgonidium Schott, Mangonia Schott, Spathantheum Schott, Spathicarpa Hook., Synandrospadix Engl., and Taccarum Brongn. ex Schott. Grayum (1990) later placed Philodendron close to Homalomeninae and the African genera Culcasia P. Beauv. and Cercestis Schott (which had been placed in the Pothoideae and Lasioideae respectively by Engler). They all share similar stem and stamen vasculature as well as extrafloral nectaries and uniquely share resin canals in their roots and a sclerotic root hypodermis. Most (excluding Homalomeninae) also lack an anther endothecium that is present in all other Araceae (Grayum, 1990). Grayum believed that the Philodendroideae is a sister group to the Pothoideae (including Engler's Monsteroideae) which share in common the exclusive characteristics of geniculate petioles, cork formation in aerial roots, compound vascular bundles, collateral bundles and other features (Grayum, 1984).
While there is reasonable agreement on the classification of tribes and subtribes, subfamilial concepts are still evolving. A comparison of the major systems of classification at the suprageneric level was made by Croat (1990). It included the system of Hotta (1970), Grayum (1990) and Bogner & Nicolson (1991).
Phenetic analysis on the Philodendroideae by Mayo (1986) show the genus to be distinct but without any definitive diagnostic features with which to distinguish it completely from other genera. He reported the genus to be only distantly related to other genera in the subfamily but that its closest relatives were the African genera Culcasia and Cercestis. In his survey of sclerotic hypodermis in the roots of Araceae (French, 1987a) provided evidence to link Philodendron to the West African genera Anubias Schott, Culcasia, and Cercestis and to the neotropical Montrichardia Crueg..
More recently in an attempt to bridge differences in the system of Bogner & Nicolson and the system of Grayum as well as that of Hay & Mabberly (1991), Mayo, Bogner & Boyce (1995) conducted another sweeping survey and produced a cladistic analysis. While maintaining essentially the same alliances suggested by Grayum (1990), Mayo et al. (1995) have placed all araceous genera with unisexual flowers in subfamily Aroideae. Philodendron will be placed in the tribe Philodendreae in the Philodendron Alliance along with tribe Homalomeneae with Furtadoa M. Hotta and Homalomena and tribe Anubiadeae (Anubias only). Their cladistic analysis also will show Bognera, a close ally of Dieffenbachia.
Another cladistic analysis resulting from a study of chloroplast DNA restriction site variation in the Ariflorae by French et al. (1995) places Philodendron as a sister-group with Homalomena, suggesting, according to Grayum (1996) that Homalomeninae is paraphyletic. According to French's findings Anubias is a sister-taxon of Homalomena, Furtadoa, and Philodendron taken together while Montrichardia is a sister-taxon with all four of these genera.
While questions still remain about the closest generic relatives to Philodendron, the classification by Mayo et al. (1995) has taken into account all the evidence to date including the extensive molecular studies by French.
The genera of Araceae most easily confused with P. subg. Philodendron are Homalomena and Dieffenbachia with Schismatoglottis Zoll. & Moritzi coming a distant third. Despite its somewhat more distant placement from Homalomena, herbarium material of Dieffenbachia is most easily confused with Philodendron, sometimes requiring the opening of the spathe to determine the genus. Since the pistillate flowers of Dieffenbachia are distant from one another, surrounded by clavate staminodia and borne on a spadix that is fused throughout its length to the spathe, while those of Philodendron are closely compacted, devoid of staminodia and borne on a spadix that is largely free from the spathe, separation from Dieffenbachia is immediate as long as the flowers are visible. Dried sterile material without field notes denoting terrestrial habit (consistently true of Dieffenbachia but rarely so of Philodendron) or scent (usually foul and of oxalic acid in Dieffenbachia) are much more problematic. Dieffenbachia leaf blades are rarely ovate and never truly cordate whereas this blade shape is common in Philodendron. Philodendron may, however, have blade shapes that closely match those of some Dieffenbachia. If the petiole is well preserved the presence of the petiole sheath is the best means to separate Dieffenbachia and P. subg. Philodendron, since the latter generally has a very short sheath while it is rare that the sheath of Dieffenbachia does not extend to the middle or above the middle of the petiole.
Live material of neotropical Homalomena is not easily confused with Philodendron because the former usually has anise-scented sap, while Philodendron usually has a distinct turpentine-like aroma, sometimes also like fresh carrots, but never anise-scented. Both Philodendron and Homalomena may have similar leaves but the latter often has pubescence on the blades and pubescence and/or scales and spines on the petioles. Both genera have similar inflorescences with unisexual flowers, sterile and fertile sections of the staminate spadix and a close arrangement of pistillate flowers, not to mention the similar constricted spathe which persists after anthesis. However Homalomena can usually be determined by the presence of minute, club-shaped staminodia sparsely scattered among the pistils.
Sterile specimens of neotropical Schismatoglottis may be confused by the novice because the two genera do share similar venation. However, Schismatoglottis occurs always terrestrially, often in somewhat marshy situations where Philodendron rarely occur. In fertile condition they are easily separated by the spathe promptly dehiscing above the tube in Schismatoglottis with the staminate spadix falling free. By contrast, in Philodendron the spathe is thick and persistent, usually reclosing over the staminate spadix, which in turn rots away inside, never really falling free until the spathe opens in fruit.
Sterile material of Spathiphyllum has been confused by some with Philodendron but that genus differs by its consistently terrestrial habit, long-sheathed petioles (exhibited in Philodendron only in P. subg. Pteromischum) and by its distinctive closely spaced primary lateral veins.