Hemiepiphytic, rarely terrestrial; stem appressed-climbing or scandent; internodes short, semiglossy, usually obscured by cataphyll bases, 1–6 cm x 8– 45 mm, longer than broad, medium brown, drying medium brown to tan, with transversely fissured flaking epidermis; cataphylls 4–12(–17) cm, unribbed, rounded at apex, red to red-violet, especially dark red at base becoming lighter and green at apex, persisting at base, becoming mushy, drying dark reddish brown. LEAVES spreading; petioles 30.5–85.5 cm x 0.5– 1.8 cm (dry), obtusely flattened adaxially, sometimes with slight medial rib, medium green, pale-striate, moderately glossy, drying reddish brown; geniculum slightly swollen, 1–1.5 cm, tan, drying darker than petiole and somewhat shrivelled; blades triangularovate, moderately coriaceous, moderately bicolorous, long-acuminate at apex, cordate at base, margins rounded, 28.5–68.5 x 15–49 cm, 1.4–1.6(–1.9)x longer than wide, 0.8–1.1(–1.5)x as long as petiole, adaxial surface semiglossy, medium to dark green, drying dull olive-green, often tearing between primary lateral veins, abaxial surface paler than adaxially, drying dull and paler than adaxially; anterior lobe 24– 54 x 15–49 cm, 1.8–2.8(–3.3)x longer than posterior lobes, broadest at point of petiole attachment; posterior lobes 7.5–23.5 x 7.5–23 cm, rounded; sinus spatulate to rhombic, 4.5–18 cm deep; midrib flat and paler than blade adaxially, drying olive to medium tan, narrowly rounded and paler than blade abaxially, drying light tan; primary lateral veins 5 to 9 per side, 2–4 cm apart, spreading from midrib at 65º angles, down-turned at midrib, then curving gradually upward to margin, obtusely sunken adaxially, convex and paler than blade abaxially, drying lighter than blade; basal veins 5 to 9 per side, with 2 free to base, 3 coalesced to 8 cm, 5 coalesced to 6 cm, 7 coalesced to 4 cm, well-developed posterior rib not naked or naked up to 3 cm, paler than blade adaxially and abaxially, drying light tan and raised abaxially; minor veins moderately obscure, arising from midrib. INFLORESCENCE erect or spreading, 2 to 6 per axil, enclosed in a persistent prophyll; peduncle 3– 10 cm x 2–3 mm (dry), much shorter than petiole, white in the lower 1/3, red to apex, drying dark reddish tan; spathe thumb-shaped, often caudate-acuminate when immature, 4.5–8.5 cm, slightly constricted; spathe blade 2–3 cm, red to maroon becoming green to white at apex outside, white inside; spathe tube 2.5–5 cm, red to maroon outside, white inside, drying dark rusty tan; spadix cylindrical,3.5–6 cm; pistillate portion pale green, 2–3 cm x ca. 2.6 mm; staminate portion creamy white, 2– 3 cm x ca. 2.5 mm at base, ca. 3.2 mm diam. at apex; sterile staminate portion not apparent; pistils ca. 1.4 x 0.6 mm at base, ca. 0.3 mm diam. at apex;ovary 5-locular, ovules with basal placentation, 1 per locule. INFRUCTESCENCE with orange berries. JUVENILE PLANTS differ in having internodes that are much longer than broad, leaf blades long-triangular, shallowly cordate at base, and lacking posterior rib.
The new species is a member of subgenus Philodendron, section Macrobelium, subsection Glossophyllum, series Ovata characterized by stems with short internodes obscured by persistent cataphyll bases, red cataphylls, large triangular-ovate leaf blades with a highly developed posterior rib, and long petioles. The small thumb-shaped, red to maroon inflorescences with white apices (hence the epithet from the Latin ‘‘pollex’’ meaning thumb and ‘‘forma’’ meaning shape) are most distinctive. This species may sometimes be confused with Philodendron canicaule because they have similar growth habits and leaf blades with similar shapes and proportions. In addition, they both have small inflorescences on short peduncles. However, P. canicaule differs in having longer internodes that are distinctively gray-green, green deciduous cataphylls, leaf blades that are usually matte abaxially and dry slightly red, and inflorescences with green and white spathes (as opposed to red).