Hemiepiphyte at (2-)10-25 m; stem with internodes (2-)3.5-7.2 cm long and (3.1-)4-7.2 cm in diam., the uppermost internode bright green, smooth, becoming silver-grey with some longitudinal or transverse markings or lines, then becoming finely fissured to give the surface a greyish, marbled appearance, eventually grey to brownish grey with minute irregular fissures. Cataphylls 35-42 cm long, to 8.5 cm wide, flattened, sharply 2-ribbed (one rib smaller), yellowish with vague purplish spots and short dark-lineate outside with 3 medium green longitudinal veins, milky white inside, deciduous while intact. Petioles (38-)74-92 cm long, averaging 83.7 cm, about as long as the blades (ranging from 0.8-1.1× the length of the blade), 16-20 mm in diam. at apex, 26-33 mm at base, ± oval in cross section, obtusely flattened to obtusely angular adaxially at apex, obtusely sulcate at base, dark green and contrasting prominently with the stem, becoming pale yellow to almost orange in full sun, vaguely purple-spotted, more greenish toward the base, more purplish toward apex, drying yellowish green and finely uniformly ridged; blades narrowly ovate-sagittate (53-)78-93 cm long, (28-)49-54 cm wide, averaging 83 × 51.5 cm, glossy and dark green above, semiglossy and slightly paler below, drying greenish grey on both surfaces, only slightly paler beneath; margins moderately sinuate and undulate; midrib broadly convex and moderately paler above, narrowly raised and paler below; primary lateral veins in 6-10 pairs, departing midrib at 60-70° angle, above concolourous to slightly paler, drying flat to weakly sunken and slightly darker, below narrowly raised and light green, paler than surface, drying greenish yellow, narrowly ridged and paler than surface; anterior lobe 53-60 cm long; posterior lobes ± rounded, directed up at an angle to midrib, prominently incurved and overlapping, 24.5-26 cm long, 23-25.5 cm wide, directed toward the base when flattened; basal veins in 7-8 pairs, 4-5 acroscopic, 2-3 basioscopic, the 1st and sometimes 2nd free to the base, the remainder ± equally spaced along the posterior rib, (4th-)5th and 6th fused for 13-19 cm; posterior rib straight to the apex of the lobe at 155° angle to the midrib, naked 2-3 cm; sinus 19-24 cm deep, closed with overlapping lobes; minor veins moderately visible, drying weakly raised and concolourous. Inflorescences (1-)3-5 per axil, erect, to 41 cm long; peduncle olive-green with minute deep red stripes, 12-15.5 cm long, 1.9-2.5 cm in diam., glossy medium green with fine reddish stripes and dark red spots, reddish towards base, often clearly demarked from the spathe; spathe 22.6-25.5 cm long, 3.3 cm in diam., weakly glossy outside, weakly constricted when furled, medium green, finely striped longitudinally with scattered dark red spots; spathe tube 2.6 × 3 cm wide when furled, medium green and pale whitish lineate-speckled outside, deep red inside throughout; the blade medium green, semiglossy, becoming pale green to whitish toward margins, tinged red in the tube inside, to 5 cm wide and 4 cm deep at anthesis, outer surface medium green, white toward the margins; spadix 20-22 cm long; pistillate portion 5.5 cm long, 1.5 cm in diam. at base, 1.8 cm in diam. midway, 1.6 cm in diam. at apex; fertile staminate portion 14.7 cm long, 1.5-1.8 cm in diam. at constriction, 1.7-1.8 cm wide at widest portion, matte, milky white with small reddish resin droplets; protruding weakly forward out of the spathe at anthesis, the sterile staminate portion to 2.5 cm long, 1.9-2.5 cm in diam., the sterile flowers scarcely distinguishable from the fertile flowers; pistils 5 mm long, narrowly ovoid, stigma depressed- globose, 0.4 mm high, 0.8 mm in diam.; ovary 1.9-2.1 mm long, 1-1.3 mm in diam., 8-9-locular, each with a gelatinous envelope in which are embedded 10-12 long-funiculate ovules; ovules 0.2 mm long, about half as long as the funicles, with axile placentation, the funicles all arising from an area encompassing the median portion of the axil wall, arising from one side of the gelatinous envelope near the middle, the gelatinous envelope attached to the ovary wall in the middle.
The species is a member of Philodendron subg. Philodendron sect. Philodendron subsect. Philodendron ser. Philodendron and is most closely allied with P. acutatum, which is also distributed in French Guiana.
Philodendron acutatum has blades with a similar texture as our new species but is typically a much smaller plant with more slender stems (2.5-4 cm versus usually 4-7.2 cm in diam. for P. scottmorianum), typically longer internodes in flowering plants (usually3-4 cm or longer between each inflorescence produced). P. acutatum has typically 1-2 inflorescences per axil, whereas P. scottmorianum has rarely one and more frequently up to 5 inflorescences per axil, borne near the apex of the stem where the internodes are very short and typically much broader than long (Fig. 2C). In addition, P. acutatum has more slender, terete petioles (oval in P. scottmorianum), narrowly ovate, moderately glossy, usually greenish drying blades only rarely up to 63 cm long (in contrast, to mostly more than 60 cm long in P. scottmorianum) with a yellow-white, very contrasting margin (c. 1 mm wide). In contrast, the blades of P. scottmorianum are ovate-elliptic and more brownish drying with the upper surface only weakly glossy to matte and with the margins whitish green and not markedly contrasting. Other differences in the two species are reflected in the number of primary lateral veins, usually 9 pairs in P. scottmorianum and 5-6 pairs in P. acutatum. The ovules in P. scottmorianum have pistils with a single gelatinous envelope in each locule, within which are embedded 10-12 long-funiculate ovules with axile placentation and with the funicles all arising from an area encompassing the median portion of the axile wall, arising from one side of the gelatinous envelope near the middle. In contrast, the ovules of P. acutatum have 2-9 ovules per locule with subbasal placentation.
Philodendron scottmorianum is also similar in the shape and even the texture of the blade with P. speciosum Endl., but that species from southern Brazil, ranging to at least as far north as Minas Gerais (Almeida & al. 2005), is a member of P. subg. Meconostigma, which differs in having a more robust stem with conspicuous interpetiolar squamulae, a spadix with the sterile staminate portion equal to or longer than the fertile staminate portion, and staminate flowers much longer than broad, typical of all the members of that subgenus. In contrast, P. scottmorianum has stems moderately smooth in comparison, with interpetiolar squamulae lacking or inconspicuous and with staminate flowers only slightly longer than wide. In addition, the stamens of P. speciosum are at least 3 times longer than wide whereas those of P. speciosum are much less elongate.
Known only from French Guiana in the region between Cayenne and Montsinéry to the north at about 50 m elevation in what is probably Tropical wet forest (Holdridge 1967). The area around the Scholderman residence where the type was collected (4°53'N, 52°32'W) is endangered but P. scottmorianum is rather common along the RN 1 highway between Montsinéry and Cayenne.