LEAVES with petioles erect-spreading, 23-61 cm long, 5-8 mm diam., subterete, sometimes faintly 2-3- or more ribbed; geniculum 2-3.5 cm long; blades narrowly ovate to broadly ovate, thin, ± membranous, long acuminate to cuspidate at apex, lobed at base, 26-48 cm long, 19.5-36 cm wide, broadest at middle; anterior lobe 21-35 cm long, the margins broadly rounded; posterior lobes 9-16 cm long from apex of sinus to outermost point; sinus triangular, sometimes closed, acute at apex; matte to semiglossy on both surfaces; midrib weakly and convexly raised above, prominently raised below; basal veins 6-7 pairs, usually all free to base; posterior ribs absent; primary lateral veins 8-10 per side, departing midrib at 35°-50° angle, sunken in valleys above, prominently raised below, basal and primary lateral veins drying conspicuously yellow; lesser veins scarcely visible; collective vein arising from one of the uppermost basal veins, 2-3 mm from the margin.

INFLORESCENCE erect; peduncle (16)40-53 cm long, 4-5 mm diam., shorter or equalling petiole; spathe membranous, pale green (B & K Green 7/5), sometimes withering early, lanceolate, 7-12.5 cm long, 1.5-2.8 cm wide, caudate-acuminate at apex, subcordate at base, inserted on peduncle at 25° angle; stipe 3-5 mm long in front, 1-3 mm long in back; spadix violet-purple or green tinged violet-purple, (7)11-20 cm long, (6)7-10 mm diam. at base, 4-5 mm diam. at apex; flowers square, 2.5-3 mm long, 2.1-3 mm wide, the sides weakly sigmoid; 4-5 flowers visible in the principal spiral, 7-9 flowers visible in the alternate spiral; tepals glossy, lateral tepals 1.7-2 mm wide, the inner margin convex; pistils dark purple, scarcely emergent before anthers emerge; stigmas minute, scarcely visible; stamens emerging rapidly from the base, the leading stamens emerging at apex as third and fourth are emerged two-thirds the length of spadix, exserted on white, translucent, flattened filaments, ca. 0.4 mm long, retracting to hold anthers over pistil; anthers purple to red-violet 0.4-0.5 mm in both directions; thecae ellipsoid; pollen orange (B & K. Yellow-red 7/5).

INFRUCTESCENCE pendent; berries deep red-violet, obovoid; mesocarp gelatinous with numerous raphide cells; seeds 2, pale yellow, tinged reddish brown at apex, green at base, ovoid, flattened, 4.6 mm long, 3.3-3.7 mm wide, 2 mm thick, encased in a sticky substance. Fig. 221.

Anthurium williamsii ranges from Costa Rica to Ecuador from near sea level to 1,800 m elevation. In Costa Rica it has been collected in premontane rain forest. In Panama it has been collected in premontane wet forest, tropical wet forest, and premontane rain forest. The species is placed in section Polyneurium and is closest to A. cuspidatum (see that species for differences). One collection from Costa Rica(Croat 36721) is somewhat intermediate between A. williamsii and A. cuspidatum and may be a hybrid.

Map of Mesoamerican specimens with coordinates

Costa Rica Heredia: 500 m, 10.19.45N 84.04.50W, 12 December 1989, Abelardo Chacón 622 (MO).
Costa Rica Heredia: 1060 m, 10.16N 84.05W, 18 April 1988, B.A. Loiselle 378 (MO).
Costa Rica Heredia: 1060 m, 10.16N 84.05W, February, 1987, B.A. Loiselle 287 (MO).
Costa Rica Heredia: 400-600 m, 10.17N 84.02W, 1 November 1989, Orlando Vargas 270 (MO). Costa Rica Lim—n: 800 m, 9.27.30N 83.00.00W, 26 July 1989, Abelardo Chacón 295 (MO).
Panama Bocas del Toro: 590 m, 8.46N 82.14W, 10 March 1985, T.B. Croat & M.H. Grayum 60207 (MO).
Panama Bocas del Toro: 700 m, 8.45N 82.15W, 10 March 1985, T.B. Croat & M.H. Grayum 60238 (MO).
Panama Bocas del Toro: 1200 m, 8.44N 82.17W, 11 March 198, T.B. Croat & M.H. Grayum 60340 (MO).
Panama Bocas del Toro: 1170 m, 8.44N 82.17W, 22 June 1987, Thomas B. Croat 66652 (MO).
Panama Bocas del Toro: 950-1100 m, 8.48N 82.10W, 7 March 1986, Gordon McPherson 8650 (MO).
Panama Bocas del Toro: 650 m, 8.45N 82.15W, 25 Oct. 1985, Gordon McPherson 7336 (MO).
Panama Bocas del Toro: 825 m, 8.45N 82.15W, 25 Oct. 1985, Gordon McPherson 7363 (MO).
Panama Bocas del Toro: 1000 m, 8.48N 82.12W, 5 Feb. 1984, H.W. Churchill, G. de Nevers & H. Stockwell 4640 (MO).
Panama Bocas del Toro: 770 m, 08.45N 82.17W, 27 June 1994, Thomas B. Croat & Guanghua Zhu 76531 (MO).
Panama Chiriquí: 1100-1135 m, 8.43N 82.17W, 8 March 1985, T.B. Croat & M.H. Grayum 60074 (MO).
Panama Chiriquí: 1200 m, 8.43N 82.14W, 14 June 1984, H.W. Churchill 5382 (MO).
Panama Chiriquí: 1100 m, 8.45N 82.14W, 18 Sept. 1984, H.W. Churchill & A. Churchill 6077 (MO).
Panama Chiriquí: 08.45N 82.18W, 23 June 1994, Thomas B. Croat & Guanghua Zhu 76317 (AAU, CM, F, MEXU, MO, US).
Panama Chiriquí: 08.45N 82.18W, 24 July 1994, Thomas B. Croat & Guanghua Zhu 76402 (MO).
Panama Chiriquí: 1080 m, 08.45N 82.18W, 27 June 1994, Thomas B. Croat & Guanghua Zhu 76501 (MO, SEL, WIS).
Panama CoclŽ: 600-800 m, 08.38N 08.35W, 8 July 1994, Thomas B. Croat & Guanghua Zhu 77213 (CAS, MO).
Panama CoclŽ: 580-740 m, 8.38N 80.36W, 12 Sept. 1987, Thomas B. Croat 67529 (MO).
Panama CoclŽ: 89.39N 80.36W, 23 June 1988, Thomas B. Croat 68770 (MO). P
anama CoclŽ: 200-400 m, 8.45N 80.35W, 4 Feb. 1983, C. Hamilton & G. Davidse 2760 (MO).
Panama Darien: 1030 m, 7.58N 77.42W, 12 Aug 1986, McDonagh et al. 650 (BM).
Panama Darien: 800-1050 m, 7.56N 77.45W, 29 June 1988, Thomas B. Croat 68938 (CM,MO).
Panama Darien: 700-1000 m, 08.00N 77.45W, 7 Febrero 1991, H. Herrera, L. Rojas y N. Bristán Jr. 870 (MO).
Panama San Blas: 800-850 m, 9.18N 79.16W, 20 Nov. 1985, G. de Nevers, A. Henderson, H.Herrera, G.McPherson & L.Brako 6288 (MO). P
Panama Veraguas: 700 m, 8.33N 81.08W, 28 June 1987, Thomas B. Croat 66928 (MO, NY).
Panama Veraguas: 1100-1400 m, 8.35N 81.05W, 15 July 1983, C. Hamilton & K. Krager 3985 (MO).
Panama Veraguas: Santa Fe Region, 600 m, 8.33N 81.08W, 28 Jun 1987, Croat 66932 (MO).
Panama Veraguas: 600-800 m, 8.30N 81.7W, 17 Oct. 1985, Gordon McPherson 7165 (MO).
Panama Veraguas: 800-1030 m,, 15 July 1994, Thomas B. Croat & Guanghua Zhu 76922 .

Map of South American Specimens with coordinates
 

Colombia Antioquia: Las Orquideas, 1095 m, 6.34N 76.19W, 23 July 1988, A. Cogollo et al. 3427 (MO).
Colombia Cauca: 900 m, 02.30.48N 077.02.33W, 19 July 1997, Thomas B. Croat & John F. Gaskin 80084 . Colombia Choco: Mpo. Tado, 600 m, 5.27N 76.13W, 24 Oct. 1991, Betancur et al. 2973 (MO).
Colombia Choco: Río Torito, 730-830 m,, 29 Feb 1980, Forero et al. 6253 (MO).
Colombia Narino: Reserva Natural Río Nambi, 1325 m, 1.18N 78.08W, 11 Dec. 1993, Betancur, J. et al. 4822 (MO).
Colombia Valle del Cauca: 500 m,, , Koie 4796 (C).
Ecuador Esmeraldas: Bilsa, 500 m, 00.21N 79.44W, 18 Nov 1996, John L. Clark, F. Gurumendí & H. Weinert 3436 (MO, QCNE).