Monographs Details: Arcytophyllum rivetii Danguy & Cherm.
Authority: Mena V., Patricio. 1990. A revision of the genus Arcytophyllum (Rubiaceae: Hedyotideae). Mem. New York Bot. Gard. 60: 1-26.
Description:Species Description - Shrubs erect, to 150 cm tall, sometimes the lower part prostrate and stoloniferous; stems glabrous, branches glabrescent to densely setose. Stipules forming part ofa continuous sheath that surrounds the branches, with a triangular projection, setose. Leaves opposite, appearing whorled because of the short branching pattern, sessile, narrowly elliptic or narrowly lanceolate, (1.3-)1.5-2(-2.5) X 0.25-0.5 mm, acuminate, rigid, sometimes adaxially setose along the margin and adaxially along the midvein. Flowers solitary, sessile; calyx 4(5)-merous, the fifth lobe, if present, smaller than the others, lobes otherwise subequal, 1.4-1.7 x 0.3-0.7 mm, triangular, setose along the margin, setose intercalycine teeth present; corolla whitish or pinkish, lobes four, 2.4-2.7 X 1.3-1.5 mm, glabrous abaxially, covered with short white hairs adaxially; tube 3-4.5(-4.8) mm long, 1.3 mm in diam.; stamens four, protruding ca. 1.5 mm; anthers bluish, 1 x 0.3 mm; filaments 0.5-1 mm long; style included, 2-3 mm long, glabrous; stigma 0.5 mm long, papillose. Capsules globose, 1-1.8 mm in diam.; seeds 2-4 per locule, irregularly cymbiform.

Discussion:Arcytophyllum rivetii Danguy & Cherm., Bull. Mus. Hist. Nat. (Paris) 28: 434. 1922. Type. Ecuador. Azuay: Chillacocha, 3500 m, Jan 1905, P. Rivet 774 (holotype, P; F-photograph!, GH-photograph!, MO-photograph!). This is a very distinctive shrub, easily identified by the cricoid habit and the exceedingly small and needle-like leaves, and by the prominent solitary flowers. The major practical differences from the species most closely resembling it, A. ericoides, are that the inflorescences of the latter are composed of several flowers forming a dense umbel and that the leaves appear in whorls spaced along the branches.