Prostrate to acending low shrubs, unarmed, terrestrial, with raphides in the tissues, with internodes short and covered with stipules, often ericoid. Leaves opposite, subsessile, entire, with higher order venation not lineolate, without domatia, often leathery with margins revolute; stipules interpetiolar and fused to bases of petioles or shortly fused around stem, triangular and erose to setose, erect and apparently valvate in bud, persistent and often overlapping the node above. Inflorescences terminal sometimes on short lateral stems, reduced to cymose, 1--several-flowered, bracteate. Flowers pedunculate or pedicellate, bisexual, distylous or perhaps sometimes homostylous, apparently diurnal; calyx limb developed, 4(5)-lobed, without calycophylls, often with well developed colleters in sinuses; corolla funnelform to campanulate, white to pink =, blue or purple, inside papillose to puberulous, lobes 4, triangular, valvate in bud, without appendages; stamens 4, inserted in middle or upper part of corolla tube, included or exserted, anthers narrowly oblong, dorsifixed, dehiscent by linear slits, apparently without appendages; ovary 2-locular, with ovules 4--12 in each locule, axile placentas; stigmas 2, ellipsoid to shortly linear, included or exserted. Fruit capsular, septicidal, rurbinate, to subglobose, chartaceous to woody, with calyx limb persistent, with peduncle or pedicel markedly elongating during development; seeds 4--12 per locule, oblong, planoconvex to concavo-convex or cymbiform, small (0.25--1 mm x 0.25--1 mm), winged, punctate.

Key to Arcytophyllum
by C.M. Taylor, largely based on Mena (1990)

1. Plants with stems creeping, rooting at nodes; flowers solitary.

    2. Leaves suborbicular, 1-3 x 0.8-2.5 mm; Andes of Venezuela (and perhaps adjacent Colombia).....Arcytophyllum venezuelanum

    2'. Leaves lanceolate to ligulate, about 2-10 x 1-3 mm; Central America and Venezuela to Bolivia.

        3. Leaves with long apical arista; western Colombia to southern ecuador.....Arcytophyllum aristatum

        3'. Leaves acute or with short apical projection; Central America and Venezuela to Bolivia.

            4. Leaves generally not falcate; plants often with dense moss-like habit; central Ecuador to Bolivia.....Arcytophyllum filiforme

            4'. Leaves often falcate; plants generally with stems trailing separately; Central America and western Venezuela to southern Colombia.....Arctyophyllum muticum

1'. Plants with stems erect, generally not rooting at nodes; flowers solitary or several in cymes.

    5. Leaves ternate; Cordillera Oriental of Colombia.....Arcytophyllum cachirense

    5'. Leaves opposite, sometimes appearing densely whorled in groups of 5-20.

         6. Stems with internodes regularly developed; leaves with blades flat and different in size, with larger leaves borne on stem and much smaller leaves borne in axils........Arcytophyllum peruvianum

         6'. Stems with internodes developed to reduced; leaves with blades flat to thickened and generally with the stem and axillary leaves all similar in size.

             7. Leaves densely grouped and appearing whorled, so the stems often appear brush-like.

                 8. Flowers solitary; leaves small..

                      9. Stipules triangular with two linear lobes or fimbriae as long as the central triangular portion, with on lobe or fimbria on each side of that central portion; Venezuela and Colombia......Arcytophyllum nitidum

                      9'. Stipules triangular, entire or with margins shortly setose; central Ecuador through northern Peru....Arcytophyllum rivetii

                 8'. Flowers several in cymes; leaves quite small to moderate-stzed (for Arcytophyllum).

                     10. Leaves quite small; calyx limb with extra small teeth between the main teeth.....Arcytophyllum ericoides 

                      10'. Leaves small to moderate-sized; calyx limb without extra teeth.....Arcytophyllum thymifolium

            7'. Leaves evidently opposite even without close examination.

                 11. Plants of Costa Rica and Panama.......Arcytophyllum lavarum

                 11'. Plants of South America.

                      12. Stipules with triangular central portion that is entire or irregular along the margin and also with a well developed linear lobe or tooth or filament on each side, these inserted near the bases of the sides of the triangular central portion and about as long as or longer than the central portion.......Arcytophyllum nitidum

                      12'. Stipules triangular, entire or with one to several linear lobes or fimbriae borne near the middle of the triangular portion.

                          13. Stipules and stems glabrous; southern Ecuador and northern Peru......Arcytophyllum vernicosum

                          13'. Stipules and stems puberulous to pubescent; Veneuezla and Colombia to Bolivia.

                              14. Stipules with several well developed fimbriae or linear lobes 3-5 mm long; northern Peru.....Arcytophyllum macbridei

                              14'.Stipules entire or with fimbriae or lobes 3 mm long or shorter; Veneuzela and Colombia to Bollivia.

                                  15. Stipules triangular, entire or with margins irregular or with a few short (up to 0.5 mm long) fimbriae near apex; inflorescences branched with developed axes......Arcytophyllym ciliolatum

                                  15'. Stipules triangular to ovate or subtruncate, on margins with a few to several fimbriae or linear lobes 1 mm long or longer; inflorescences congested to subcapitate, with axes not evide.

                                       16. Leaves at apex mucronate; stipule fimbriae or linear lobes entire, smooth......Arcytophyllum capitatum

                                       16'. Leaves acute at apex; stipule fimbriate or linear lobes covered with small setae or trichomes....Arcytophyllum setosum