Hemiepiphytic or terrestrial; stem appressed-climbing or scandent, sometimes upright to 80 cm; sap spicy; internodes 0.5–4 3 3–6 cm, usually longer than broad, medium green, semiglossy, drying golden tan to tan with flaking epidermis, lower portions obscured by cataphyll fibers; roots few to several per node, often highly branching, drying medium to reddish brown; cataphylls 16–35 cm, sharply 2- ribbed, rarely bluntly 2-ribbed, maroon to reddish violet or at least red at base, drying with very pale fibers and reddish brown epidermis attached, persisting semi-intact as robust sometimes wavy fibers with pieces of attached epidermis. LEAVES erect; petioles 26.5–108 cm, tapering from base to apex, from 0.4– 1.3 cm diam. at the base to 2–5 mm diam. at the apex (dry), firm, terete to obtusely flattened adaxially, sometimes with 1 or 2 blunt ribs, slightly sulcate near base, pale to medium green, sometimes striate, drying dark olive-green, geniculum not obvious living or dried; blades ovate, subcoriaceous, obtuse, short-apiculate at apex, deeply cordate at base, 23–58.5 3 15–50 cm, 1.2–1.73 longer than wide, 0.5– 0.8(–1)3 as long as petiole, adaxial surface glossy, dark green, slightly bicolorous, drying glossy, dark reddish green, abaxial surface semiglossy, moderately paler than adaxially, drying shiny and barely paler than adaxially; anterior lobe 22.2–43 3 15–50 cm, 1.9–2.4(–2.8)3 longer than posterior lobes; posterior lobes 6.5–22.5 3 7.5–27 cm, rounded on inside margin to rectangular on outside margin; sinus usually spatulate, 2.5–10.5 cm deep; midrib flat to sunken, paler or concolorous adaxially, narrowly rounded or convex, sometimes matte and darker than blade abaxially, usually drying darker than blade; basal veins 7 to 8 per side, 1 to 2 free to base, the remainder coalesced for 6 cm; posterior rib naked 1–2 cm; primary lateral veins 4 to 8 per side, 2–4 cm apart, spreading from midrib at 35u–55u angles, proceeding straight toward margin to within 1–2 cm then curving toward apex, downturned at midrib, obtusely sunken adaxially, convex abaxially; interprimary veins prominent but still less obvious than primary veins, drying darker than blade to concolorous, raised abaxially; cross veins numerous, drying with a stitched appearance toward margins. INFLORESCENCES erect, 1 to 2 per axil; peduncle 17.5–23.5 cm 3 5–10 mm (dry), much shorter than petiole, medium green with a red base, somewhat white-streaked, drying dark to medium brown, somewhat red, longitudinally wrinkled; spathe 10–13.5 cm, moderately glossy, bluntly rounded at apex, somewhat constricted above tube; spathe blade 6–7.5 cm, medium green outside, paler inside; spathe tube 3.5–5.5 cm, dark green becoming greenish pink or tinged with red outside; spadix 9.5– 11.5 cm; pistillate portion 2.3–4.5 cm, ca. 1.2 cm diam., white; staminate portion 7.5–10.5 3 ca. 1.4 cm at base, ca. 1.7 cm diam. 1 cm from apex; sterile staminate portion 2.5–3.5 cm; pistils 2.2–2.4 mm; ovary 5- to 6-locular, ovules with axile placentation, about 20 per locule. INFRUCTESCENCE to 13 cm, pistillate portion ca. 5 cm. JUVENILE PLANTS differ in having leaf blades that are oblong, rounded and not cordate at base, acuminate at apex, but soon developing a lobe at the base.
The species is a member of subgenus Philodendron, section Philodendron, subsection Philodendron, series Fibrosa and is characterized by its large 2-ribbed cataphylls weathering to coarse fibers with pieces of attached epidermis, its long-tapered petioles, and glossy ovate-cordate blades with few primary lateral veins, but especially by the numerous fine cross veins for which it is named (‘‘venulosum’’ from the Latin ‘‘venosus’’ meaning having many branched veins). Also characteristic are the bluntly rounded inflorescences with pale green spathe blades and rose-colored spathe tubes. The species most likely to be mistaken for Philodendron venulosum is P. striatum because these two species can both grow terrestrially and have similarly shaped blades, few primary lateral veins, and short peduncles. The latter species, however, differs in having coarse white striations on the internodes, petioles, peduncles, and inside the spathe, as well as unribbed cataphylls that weather to thin fibers. Philodendron striatum also differs in having blades that are matte on the abaxial surface (rather than glossy) and long-tapered inflorescences with spathe tubes that are yellow-green outside.