Key to sections, subsections and series of P. subg. Philodendron

1a. Pistils usually 2-locular, rarely 3-locular; eastern South America and Amazon basin.
P. sect. Philopsammos

1b. Pistils 4-10-locular; throughout the range of the genus.

2a. Plants with primary lateral veins moderately obscure; all South American (excludes P. wendlandii which is transferred to P. sect. Calostigma).
P. sect. Baursia
2b. Plants with primary lateral veins usually much more prominent than the minor veins.
  3a. Pistil with style much prolonged beyond the body of the ovary and much narrower than ovary.
  4a. Style turned upward, perpendicular to the body of the ovary; South American species only. P. sect. Camptogynium

4b. Style straight, directed in the same axis as the ovary; Central and South America. P. sect. Macrogynium
                                                              P. jacquinii

3b. Pistil with style about as broad as the ovary.   5a. Blades 3-lobed or deeply incised-lobate
6a. Blades 3-lobed; ovules 1-3 per locule, basal or sub-basal. [Note: P. subsect. Doratophyllum with 3-lobed blades would also key here. See the discussion after that subsection.]P. sect. Tritomophyllum P. angustilobum (perhaps this belongs in P. subsect. Macrobelium (Schott) Engl.)
P. anisotomum
P. cotobrusense
P. madronoense
P. rothschuhianum
P. tripartitum
6b. Blades incised-lobate; ovules 2-8 per locule, sub-basal or axile.
  7a. Blades divided along the anterior lobe into two or more divisions, each division in turn itself terminating with much smaller lobes; petioles sometimes with trichome-like scales; South America only (unless P. pedatum really occurs in Central America).
P. sect. Schizophyllum.

7b. Blades pinnately or bipinnately lobed; the divisions more or less uniform along the anterior lobe; Central and South American species.
P. sect. Polytomium
P. dressleri
P. radiatum
P. warszewiczii

5b. Blades entire, sometimes with the anterior lobes markedly concave, but the blade not markedly 3-lobed. 8a. Pistils with axile placentation; ovules usually 15 or more per locule, seldom with as few as 10 per locule, rarely fewer.
P. sect. Philodendron
9a. Blades cordate to subcordate or sagittate at base; petioles various. 10a. Petioles sharply flattened, sometimes also with the lateral margins raised.   11a. Stems with internodes much broader than long.
P. subsect. Macrolonchium (Schott) Engl.
P. fragrantissimum
11b. Stems with internodes about as broad as long or longer than broad.
P. subsect. Platypodium (Schott) Engl.
P. brunneicaule
P. copense
P. findens
P. fortunense
P. pterotum

10b. Petioles terete or subterete.

12a. Petioles warty at apex; South American species; (excluding P. grandipes erroneously placed here by Krause); P. ornatum Schott and relatives.
P. subsect. Psoropodium (Schott) Engl.
12b. Petioles smooth or conspicuously scaly but not merely warty at apex.
  13a. Plants scandent; internodes much longer than broad; cataphylls usually deciduous; blades cordate.
P. subsect. Solenosterigma (Klotzsch ex Schott) Engl.
P. purpureoviride
P. hederaceum
13b. Plants usually not scandent, either terrestrial or appressed-climbing with internodes broader than long or not much longer than broad; blades ovate to sagittate.   14a. Plants with petioles scaly.
P. subsect. Achyropodium Schott P. glanduliferum
P. hammelii
P. malesevichiae
P. squamipetiolatum
P. squamicaule
P. verrucosum
14b. Plants with petioles smooth, not scaly.
P. subsect. Philodendron
15a. Cataphylls decidduous intact or sometimes becoming fibrous but soon deciduous

P. ser. Philodendron Schott
P. giganteum (not in Central America)

15b. Cataphylls persisting as a mass of fibers

16a. Blades matte on lower surface.

17a. Blades matte, not velvety on upper surface, whitish and glaucous on lower surface P. ser. Impolita Croat [P. hebetatum, P. strictum., P. thalassicum]

17b. Blades velvety on upper surface, not whitish and glaucous on lower surface P. ser. Velutina Croat [P. gigas]

16b. Blades usually semiglossy, never whitish or glaucous, on lower surface.

18a. Sap brown to clear, never P. ser. Fibrosa, Croat [P. alticola, P. antonioanum, P. breedlovei, P. chiriquense, P. dodsonii, P. grandipes, P. jodavisianum, P. lazorii, P. llanoense, P. panamense, P. pirrense, P. purulhense, P. scalarinerve, P. schottianum. P. tenue.]

18b. Sap white and chalky

P. ser. Albisuccosa Croat [P. albisuccus]

9b. Blades acute to obtuse at base; petioles subterete and smooth
P. subsect. Canniphyllum (Schott) Mayo [P. chirripoense, P. cretosum, P. roseospathum]
8b. Pistils with basal or sub-basal placentation; ovules few per locule, usually fewer than 5, seldom up to 8, rarely as many as 12 (but with some locules in the same inflorescence with us few as 6 ovules per locule)
P. sect. Calostigma
19a. Plants usually appressed hemiepiphytic climbers; internodes frequently as long as wide or longer than wide; widespread in both Central and South America.
20a. Blades deeply 3-lobed; cataphylls persisting in a dense, reddish brown layer of fibers; South American species only; P. acuminatissimum Engl. P bulaoanum Engl  P. subsect. Bulaoana Mayo

20b. Blades not at all 3-lobed, cordate, sagittate, or ± oblong; cataphylls deciduous or persistent, fibrous or intact; widespread in both Central and South America.

21a. Inflorescences both small and numerous, usually more than 6 per axil, less than 5 cm long
P. subsect. Oligocarpidium (Engl.) Mayo [P. clewellii, P. heleniae]

21b. Inflorescences of normal size, typically I few and more than 5 cm long.

22a. Pistils with several ovules per locule; petioles rarely with purple ring at apex; blades sagittate or cordate at base P. subsect. Macrobelium

23a. Cataphylls deciduous.

24a. Leaf blades with minor veins not at all etched into upper surface P. ser. Macrobelium (Schott) Croat [P. advena, P. annulatum, P. aromaticum, P. coloradense, P. dwyeri, P. edenudatum, P. ferrugineum, P. grayumii, P. knappiae, P. mexicanum, P. platypetiolatum, P. sagittifolium, P. sousae, P. subincisum, P. verapazense, P. zhuanum]

24b. Leaf blades with minor veins etched into upper surface P. ser. Ecordata Croat [P. brenesii, P. crassispathum, P. davidsonii, P. lentii, P. niqueanum]

23b. Cataphylls persistent.

25a. Cataphylls weathering and persisting as fibers P. ser. Reticulata Croat [P. jefense, P. tysonii]

25b. Cataphylls persisting intact; stems markedly succulent P. ser. Pachycaulia Croat [P. basii]
22b. Pistils with ovules solitary (or sometimes 2, as in P. wilburii) in each locule; petioles frequently with purple ring at apex (P. subsect. Glossophyllum); blades elongate (P. ser. Glossophyllum) or ± ovate [P. subsect. Glossophyllum ser. Ovata].

26a. Blades mostly ± oblong and acute to narrowly subcordate at base; petioles frequently with a purple ring at apex
P. subsect. Glossophyllum(Schott) Engl.;  P. ser. Glossophyllum (Schott) Croat [P. auriculatum, P. bakeri, P. brewsterense, P. correae, P. dolichophyilum, P. folsomii, P. granulare, P. immixtum, P. illgulatum, P. morii, P. pseudauriculatum; P. ubigantupense, P. utleyanum, P.wendlandii]

26b. Blades ovate to ovate-triangular, cordate to subcordate at base; petioles usually lacking a purple ring at apex
P. subsect. Glossophyllum; P. ser. Ovata Croat  [P. cotonense, P. dominicalense, P. microstictum, P. smithii, P. straminicaule, P. sulcicaule, P. wilburii]

19b. Plant habit unknown; internodes much broader than long; Colombia (exact locality unknown) P. subsect. Eucardium (Engl.) Mayo

Note: for an outline of the sectional classification of Philodendron subg. Philodendron in Central America see apendix 3.