Arcytophyllum is a genus of flowering plants in the family Rubiaceae. The genus contains 18 species, distributed from New Mexico to Bolivia. Description Members of this genus are shrubs or dwarf shrubs that occupy mountainous habitats. Their leaves are oppositely arranged, small, sessile or sub-sessile, and possess inconspicuous secondary venation. Flowers are tetra- or pentamerous, with the stamens adnate to the white or pink funnelform or salverform corolla. The fruit is a capsule with septicidal dehiscence. Species Arcytophyllum aristatum Standl. - Colombia, Ecuador Arcytophyllum cachirense (H.Karst.) K.Schum. - Colombia Arcytophyllum capitatum (Benth.) K.Schum. - Colombia, Ecuador Arcytophyllum ciliolatum Standl. - Ecuador, Perú Arcytophyllum ericoides (Willd. ex Roem. & Schult.) Standl. - Ecuador, Perú Arcytophyllum fasciculatum (A.Gray) Terrell & H.Rob. - New Mexico, Texas, Coahuila Arcytophyllum filiforme (Ruiz & Pav.) Standl. - Bolivia, Perú, Ecuador, Colombia Arcytophyllum lavarum K.Schum. - Costa Rica, Panamá Arcytophyllum macbridei Standl. - Amazonas (of Perú) Arcytophyllum muticum (Wedd.) Standl. - Costa Rica, Panamá, Colombia, Ecuador, Venezuela Arcytophyllum nitidum (Kunth) Schltdl. - Colombia, Venezuela Arcytophyllum peruvianum (Wernham) J.H.Kirkbr. - Perú Arcytophyllum rivetii Danguy & Cherm. - Ecuador, Perú Arcytophyllum serpyllaceum (Schltdl.) Terrell - Guatemala, Chiapas, Veracruz, Oaxaca Arcytophyllum setosum (Ruiz & Pav.) Schltdl. - Bolivia, Perú, Ecuador, Colombia Arcytophyllum thymifolium (Ruiz & Pav.) Standl. - Colombia, Ecuador, Perú Arcytophyllum venezuelanum Steyerm. - Venezuela Arcytophyllum vernicosum Standl. - Ecuador, Peru Phylogeny and biogeography Arcytophyllum is a monophyletic genus when A. serpyllaceum is excluded, as this species has been shown to be more closely related to Bouvardia but has not been formally transferred to another genus. The remaining species are most closely related to the North American and Caribbean genus Houstonia (including Stenaria) and one species of the polyphyletic genus Oldenlandia (Oldenlandia microtheca) distributed in Mexico and Central America, indicating that Arcytophyllum may be one of the few cases in the family Rubiaceae of a southward migration to reach the Andes. The center of diversity is in South America; namely in Ecuador, where 10 species are documented. Floral morphology and evolution Flowers of Arcytophyllum are distylous, meaning that two morphs exist differing in the vertical orientation of anthers and stigmas. Pin morphs have stigmas positioned above the anthers, while thrum morphs have stigmas positioned below the anthers. A study of 11 species showed significantly higher seed production in thrum than pin morphs of A. aristatum, A. lavarum, and A. vernicosum, and a similar tendency in all other sampled species.