Terrestrial, hemiepiphyte or epilith. Internodes 0.1-0.2 x 10-22 cm, stem covered with very long and lignified intravaginal squamules. Prophyll deciduous, 55-60 x 12-14 cm. Petioles slightly canaliculate adaxially, 43-63 x 0.8-1.2 cm. Leaf blade sagittate with shallowly pinnatifid margins, ovate in outline, 39-80 x 25-60 cm, chartaceous to slightly coriaceous, adaxial surface dark green, paler adaxially, anterior division 25-50 x 25-60 cm, apex acute to acuminate, secondary nerves 5-8 per side, arising at an angle of 40-85°, posterior divisions 11-24 x 12-27 cm, denuded portion of the basal rib 2-5 cm, acroscopic lobes 1-2, basiscopic 2-3. Inflorescences 1-3 per leaf axil, peduncle 15-30 x 0.8-1 cm; spathe white inside, green outside, 8-17 cm long; spadix 8-16 x 0.7-2 cm, fertile male portion tapering to the apex, 5-9 x 0.7-1.5 cm, sterile male portion 0.5 x 1-1.8 cm, female portion adnate up to 1/2 of its length, 3-5.5 x 1.3-2 cm. Male flowers 4-androus, stamens prismatic ca. 1 x 0.5 mm, staminodes clavate, 1-2 x 0.5 mm, gynoecium 1-2 x 0.5-1 mm, 7-8-locular, locules 2-3-ovuled, inserted near base, funicles ca. 1/2 of the ovule length. Berry not seen.
STEM: erect or decumbent, trunk-like and stout, up to 2.5 m tall, 9-20 cm thick, lower part grey: adventitious roots running ± down stem, not spreading widely, about 1.4 cm diam., with rugose, ± corky epidermis and prominent lenticels; internodes very short; prophyll scars similar in length to internodes; intravaginal squamules 2-8 mm long, 3-4 mm wide at base, ± flattened, triangular, sometimes falcate, acute-acuminate, common to numerous, persistent and quite conspicuous; foliage leaf scars transverse-elliptic to subrhombic, ± oblique, c. 3 cm long, c. 5-7 cm wide. LEAF: 8-10 per crown; prophyll up to 60 cm long, pale yellowish green; petiole 35-63(-100) cm long, subequalling or longer than leaf blade, c. 1 cm thick at apex, sulcate with sharply angled margins; leaf blade triangular-sagittate in outline, upper surface dark dull green, margins sinuately lobed, overall length (30-)40-80 cm, overall width (21 -)29-65(-72) cm, usually widest below petiole attachment; anterior division (19-)26-55 cm long, (20.5-)25-65(-70) cm wide, apex acute, more rarely acute-acuminate, subacute or rounded; primary lateral lobes 4-7( -8) per side, subtriangular to hemispheric, usually ± oblique and pointing slightly towards leaf apex, tips rounded, basal primary lateral lobes 1.5 - 6.5(- 14) cm, shorter towards apex of division; primary lateral veins 4-7(-8) per side, ± regularly spaced, paler than lamina abaxially, arising at an angle of 30-90°; posterior divisions 11-25 cm long, 0.2-0.4 x overall blade length, (10-)13-30(-36) cm wide, basal ribs denuded for 1-5(-6.5) cm, tip subacute to rounded, primary acroscopic lobes (l-)2-3, primary basiscopic lobes (0-)2-5, much shallower than acroscopic lobes, basiscopic lamina area nearly always well developed, sinus between posterior divisions usually ± oblong, sometimes broadly parabolic. INFLORESCENCE: peduncle (4-)7 -15 cm long, 0.8-2 cm diam. at apex; spathe (11.5-)12.5-25 cm long, c. (1.9-)2.5 - 5 cm diam. at middle, subcylindric, decurrent for (1.5-) 2.8-6 cm, green or reddish-green or dull purplish green basally on outer surface, white or crimson on inner surface; spadix (9.5-)13-19 5 cm long, fertile male zone 3.5 - 5.5( - 8) cm long, (0.9-)l.3-3 cm diam., subcylindric, sterile male zone (3.5 -)5.5 - 7 cm long, usually longer than fertile male zone, (0.9-)l.2-2.5 cm diam., thicker basally, female zone (1.5-)4-5 cm long, (1.0-)2-3.5 cm diam. FLOWERS: stamens 3.5-4.8(-6) mm long, (0.5-) 0.7-1 mm diam. at apex; staminodes 3-6(-7.7) mm long, longest at base of sterile zone, at apex (0.6-)0.8-1.2(-1.4) mm diam., clavate, usually flattened, tapering gradually and then abruptly to base; gynoecium 5-6.5 mm long, ovary flask-shaped to subcylindric, 3-4 mm diam., raphides common in subepidermal tissue of upper ovary walls and style body, locules 6-9(-10) per ovary, ovules 2-3(-4) per locule, inserted from base to upper third of ovary axis, distant, evenly spaced, style body distinctly narrower than ovary or style crown, packed with raphide cells around stylar canals, compitum not penetrating ovary, style crown consisting of well-developed spreading, usually tanniniferous style lobes, central dome lacking or very short or very slender, clavate and slightly overtopping crown lobes, stylar canals entering locules subapically or in upper third of locules, stigma (2.5-)3-4.5 mm diam., covering entire style crown including dorsal surfaces of style lobes, tannin hairs numerous or lacking. INFRUCTESCENCE: fertile berry c. 1.2 cm long, 0.7-0.8 cm diam., cylindric (yellow fide Croat & Mount 1988); seed galled in all but one collection seen, 2.6-3 mm long, 1.3 — 1.5 mm diam., subcylindric. Fig. 17A, Map 2H.
Most collections lack ecological details; Chodat & Vischer (1920) give a detailed habitat description for Paraguay, where P. undulatum grows as an emergent aquatic in open marshes and also thrives in damp marshy woodlands where the stem becomes much taller; it seems not to be restricted to marsh habitats, however, since Schinini & Bordas (21251) collected the species growing on rocks in the Cordillera de Altos of Paraguay. In Brazil three collections give the habitat as riverine marshes (Duarte 3526, Eiten & Eiten 2330, appi & Mayo 77).