Hemiepiphytic; leaf scars conspicuous, 1-1.5 cm long, 1÷2 cm wide; internodes 1.2÷6 cm long, 2.7-3 cm diam., broader than long, tan to reddish brown, epidermis with loose flakes; roots few to numerous per node; cataphylls moderately spongy, 23÷34 cm long, unribbed, drying sharply 1-ribbed in lower one-third, pale green, drying tan to reddish brown, persisting semi-intact, eventually fibrous; petioles 55.5-68 cm long, 1.4-1.5 cm diam., terete, medium green, drying yellowish tan to reddish tan, surface dark green diffuse-lineate; blades narrowly ovate, coriaceous, semiglossy, moderately bi-colorous, long acuminate at apex, sagittate at base, (40)57.5-72 cm long, (24)32.5-37 cm wide (1.7-1.8 times longer than wide), (ca. 1 time longer than petiole), margins hyaline, markedly sinuate, upper surface drying brown, lower surface drying yellow-red to brown; anterior lobe (32.5)40-49 cm long, (10)17.6-18 cm wide (2.3-3.3 times longer than posterior lobes); posterior lobes (10)17÷18 cm long, 16-16,5 cm wide, obtuse; sinus hippocrepiform or closed and obovate with lobes overlapping, acute when young; midrib broad, flat, paler than surface above, convex, concolorous below; basal veins 4÷9 per side, first (second) pair free to base, most coalesced 1÷4- cm, 2÷3 coalesced to 5.5 cm; posterior rib not naked or obscurely naked for ca. 1 cm; primary lateral veins 5÷7 per side, departing midrib at a 50÷60¡ angle, sunken, paler than surface above, convex, concolorous below; minor veins flat, darker than surface below, arising from both the midrib and primary lateral veins.

INFLORESCENCES 2 per axil, ± erect; peduncle 7.5-10 cm long, 1.3 cm diam., medium green; spathe 16.5-20 cm long, 3.5 cm diam. (closed), 5 cm diam. (opened), constricted above the tube, acuminate at apex, margins paler; spathe blade semiglossy, greenish outside, white inside; spathe tube more ellipsoid, maroon-purple outside, bright cherry-red inside; spadix not seen.

The flowering phenology of Philodendron subincisum is unknown. The only modem collection {Moore & Bunting 8952) is sterile.

Philodendron subincisum is endemic to Mexico, known only from northern Veracruz in the Poza Rica region, at less than 500 m elevation in "Selva Mediana Subperennifolia." Philodendron subincisum is a member of P. sect. Calostigma subsect. Macrobelium ser. Macrobelium. This species is characterized by its large, thick stems, short internodes, deciduous cataphylls, subterete petioles, ovate-cordate blades with markedly sinuate margins, two short-pedunculate inflorescences per axil, spathe tube maroon-purple outside and bright cherry-red inside, and spathe blade green outside and white within.

Philodendron subincisum is apparently rare, having been collected only once (Moore & Bunting 8952) since the type gathering. Philodendron subincisum appears to be most closely related to P. .sagittifolium based on most features, but it may represent a hybrid. Bunting (1965) even suggested that P. radiatum was closely allied to P. subincisum. Judging from its rarity, the latter taxon might be a hybrid between P. radiatum and P. sagittifolium. Because of its ovate blade with sinuate margins, it is not confused with either of the above species. Perhaps most easily confused with P. subincisum is P. radiatum var. pseudoradiatum. That taxon differs in occurring only on the Pacific slope and in having ovate-triangular blades with proportionately narrower and longer lobes.

The inflorescence of this species remains poorly known. Collected in sterile condition by Moore and Bunting (8956), the species later flowered in cultivation at Comell University. The flowering specimen was apparently vouchered by Bunting, but the whereabouts of the collection are unknown. Much of the information concerning the species is based on photographs published by Bunting (1965).