Planta hemiepiphytica; internodia usque 3.5 cm longa, 8÷15 mm diam.; cataphylla incostata, deridua; petiolus semiteres, 29÷54 cm longus, 8÷12 cm Diam.: lamina ovato-cordata, 28÷55 cm longa, 12.5÷28 cm lata, in sicco brunnea vel olivacea supra, virella vel flavibrunea infra; nervis lateralibus I 3-4; inflorescentia usque 4; spatha 15-21 cm longa, lamina spathae exius cremea, tubo spathae albo, in superficiebus ambabus saturale suffuso rubro aut purpureo; pistilla 6÷8-locularia; loculi 2-ovulati.

Hemiepiphytic; stem probably scandent; inter-nodes to 3.5 cm long, 8-15 mm diam., yellow-brown to dark brown, epidermis drying smooth to wrinkled, sometimes fissured closely; roots drying reddish brown, thin, elongate, drying 3÷4 mm diam., few per node; cataphylls 14 cm long, unrib-bed, deciduous; petioles 29-54 cm long, 8-12 mm diam., ± terete, drying yellow-brown; sheathing 3-6.5 cm; blades ovate-cordate, subcoriaceous, gradually acuminate at apex, conspicuously cordate at base, 28-55 cm long, 12.5-28 cm wide (2.2 times longer than wide), (about as long as petiole),

drying subcoriaceous, upper surface drying brown to olive-green, lower surface drying greenish to yellowish brown, smooth or minutely ridged; anterior lobe 23÷36 cm long, 21÷27 cm wide (2.8÷3 times longer than posterior lobes); posterior lobes 8-12 cm long, 10÷13 cm wide; sinus parabolic; midrib drying broadly convex and usually paler than surface above, drying narrowly convex and usually concolorous below; basal veins 3÷5 per side, free to base, several coalesced to 1÷2 cm, occasionally 2 fused to 3.5 cm; posterior rib usually not at all naked, sometimes obscurely so for about 1 cm; primary lateral veins 3-4 per side, departing midrib at a 60÷70¡ angle, weakly arcuate to the margins, drying weakly raised and usually paler than surface above, drying raised below; minor veins arising from both the midrib and primary lateral veins.

INFLORESCENCES to 4 per axil; peduncle 6.5-13 cm long, ca. 1 cm diam.; spathe 15÷21 cm long (1.6-2.3 times longer than peduncle), with resin canals on inner surface, visibly constricted above the tube; spathe blade cream outside; spathe tube white, heavily tinged red or purple on both surfaces, spathe lube 5÷11 cm long; spadix sessile; 6÷14 cm long; pistillate portion cylindrical-tapered toward the apex, 2.4-4.1 cm long, 4-9 mm diam. at apex, 5÷10 mm diam. at middle, 6-10 mm wide at base; staminate portion 3.6÷10 cm long; fertile staminate portion (4)9 mm diam. al base, 6÷12 mm diam. at middle, 3÷7 mm diam. ca. 1 cm from apex, broadest at or below middle, broader than the pistillate portion; sterile staminate portion not obvious; pistils 0.5-2.7 mm long, (0.7)1.8-2.7 mm diam.; ovary 6÷8-locular, 0.4÷1.4 mm long, 0.7÷2.7 mm diam., with basal (to sub-basal) placentation; locules 0.4÷1.4 mm long, 0.9 mm diam.; ovule sac 0.3 mm long; ovules 2 per locule, contained within a translucent, gelatinous ovule sac, 0.1-0.2 mm long, much shorter than funicle; funicle 0.7÷0.9 mm long, style 0.4 mm long, 0.4÷1.5 mm diam., similar to style type D; stylar canals emerging at base of apical depressions and arranged separately in a ring on orifices around center; style apex flat; style boss moderately shallow to moderately prominent;stigma discoid, 0.4 mm diam., 0.1 mm high, covering entire style apex; thecae oblong to weakly ovate, 0.2÷0.4 mm wide, divaricate.

Flowering in Philodendron sousae occurs in the early rainy season, and specimens have been collected in May and August. Fruits are not known.

Philodendron sousae is endemic to Mexico in Chiapas, at 490 to 1400 m elevation on the Atlantic slope in regions of "Selva Alta Perennifolia."

Philodendron sousae is a member of P. sect. Calostigma subsect. Macrobelium ser. Macrobelium. This species is characterized by its ovate-cordate, light greenish-brown-drying blades with three to four primary lateral veins per side and three to five free to weakly coalesced basal veins, which are usually not at all naked. Also characteristic are the inflorescences, ranging up to four per axil and with the spathe more or less oblong and constricted somewhat above the tube. The spathe tube is white and heavily tinged with red or purple on both surfaces.

Philodendron sousae is most easily confused with some material of P. advena (from the Pacific slope), which dries a similar light brown to greenish brown (not dark brown typical of most material). The latter differs in having a more or less ellipsoid spathe, scarcely or not at all constricted above the tube.

Philodendron sousae may also be confused with P. breedlovei, which has somewhat similar blades that dry a similar color. The latter species has narrower blades, to 1.8 cm long, a single inflorescence per axil, and about 20 ovules per locule (vs. blades usually broadly ovate, averaging 1.4 times longer than wide, four inflorescences per axil, and 1-3 ovules per locule).

Philodendron sousae was first collected by D. E. Breedlove in 1972. It is named in honor of Mario Sousa (MEXU), co-editor of Flora Mesoamericana (Davidse et al., 1995), who (in the company of Gerrit Davidse) collected the type specimen.