Scandent shrub or liana, climbing to 8 m or more. Stems slender, wiry-woody, glabrous to moderately pubescent with trichomes 0.1–0.2 mm long, these frequently in distinct lines along stem.
Sympodial units plurifoliate.
Leaves simple, the arrangement distichous, the blades 4.4–11 x 1.3–4.5 cm, 2–3 times as long as wide, ovate to elliptical, chartaceous to coriaceous to somewhat fleshy, glabrous to moderately pubescent on the midvein adaxially, glabrous abaxially; venation pinnate, with ca. 5 pairs of secondary veins, the veins sand-punctate; base oblique, sides of the lamina 3–4 mm distant on the petiole, the two sides rounded to truncate to somewhat cordate; margins entire; apex acuminate; petioles 0.2–0.6 cm, moderately pubescent adaxially, sand-punctate. Internodes 0.7–2.8 cm.
Inflorescences 3–55 cm long, unbranched, terminal to extra-axillary to nearly leaf-opposed, with 14–116 flowers (scars), the axes slender, densely pubescent; peduncle 2–5 cm; rachis 2–49 cm; pedicels 5–6 mm in flower, slender, 13–14 mm in fruit, glabrous to minutely pubescent, spaced 1.5–8 mm apart.
Flowers 5-merous, all perfect. Calyx 1.5–2 mm long, membranous, very sparsely pubescent, sand-punctate, the tube ca. 1 mm long, the lobes 0.5–1 x ca. 2 mm, broadly rounded, rounded to shortly acuminate at tips; fruiting calyx not accrescent. Corolla 0.7–1 cm in diameter, 3–5 mm long, stellate, membranous, white to greenish-white, the lobes 3–4.5 x 1.5–2 mm, lanceolate, acute at tips, reflexed at maturity, glabrous adaxially, pubescent along center of petal abaxially, the margins ciliate. Stamens with filaments 0.5-1 mm long, glabrous; anthers ca. 2.5 x 1 mm. Ovary glabrous; style 3–4 x ca. 0.3 mm, clavate, glabrous; stigma capitate.
Fruit a berry (only immature seen), 9–18 x 4–6 cm, 2–3 times as long as wide, spindle-shaped to possibly cordate, pointed at apex, the color unknown, glabrous.
According to the IUCN Red List Categories (IUCN 2010), S. dolichorhachis is classified as B1a+biii (Critically Endangered). This species is only known from five widely scattered locations, two of which are in western Ecuador, an area that continues to experience extreme habitat degradation (Dodson and Gentry 1991). The location at Limoncocha is in the center of an oil field, continued development of which is encroaching on all sides of the 4,600 ha Reserva Biológica Limoncocha (E. J. Tepe, pers. obs.).