Morphology, anatomy, and taxonomic position of Plocaniophyllon Brandegee (Rubiaceae), a monospecific genus endemic to Mesoamerica

The monospecific genus Plocaniophyllon occurs in mixed forests of Chiapas (Mexico) and San Marcos (Guatemala). Despite the fact that it was described in 1914, it has been poorly collected and consequently it is poorly understood. Here we describe the morphological and anatomical features that characterize Plocaniophyllon flavum Brandegee with the aim to re‐evaluate its generic circumscription and tribal placement. We compare the characteristics of this genus with Deppea , with which it has been confused, but also with other genera of the tribe Hamelieae, where both have been classified. Plocaniophyllon shares with the other members of Hamelieae the presence of raphides in all organs; endodermis in the stem; palisade parenchyma formed by one layer of cells, and open spongy parenchyma in the leaf blade; stamens adnate near the corolla base; and seeds with tuberculate micro‐ornamentation seed coat. Moreover, veins in Plocaniophyllon have fibers sheathing the phloem cells, with the quaternary veins forming an admedial ramified pattern; flowers are tetramerous with alternative aestivation; fruits are capsular with loculicidal and septicidal dehiscence. This combination of characters allows the distinction of Plocaniophyllon from the rest of the genera in the tribe Hamelieae. Furthermore, the fibrous vein sheathing and the admedial pattern of the quaternary veins are unique for Plocaniophyllon. The morphological and anatomical attributes of Plocaniophyllon not only justify its segregation at generic level, but also support its inclusion in the tribe Hamelieae.