Pilocosta, a New Genus of Tropical American Melastomataceae

Abstract

Pilocosta, here proposed as a new genus of Tibouchineae, is distin- guished by its solitary axillary, ebracteate flowers, 4-merous floral pattern, and quad- rangular, prominently ribbed hypanthia. Delimitation of taxa, based on gross mor- phology and chromosome numbers, is tempered by field observations of variation in natural populations. Data from these sources provide a rationale for recognizing three species-P. erythrophylla, P. nana, and P. oerstedii. The latter comprises subsp. oerstedii and the newly described subsp. campanensis. Counts of n = 7 and 14 for P. erythrophylla, n = 33 for P. nana, and n = 18 for P. oerstedii subsp. oerstedii are reported for the first time. A systematic treatment is presented for the four recognized taxa, with a key, synonymies, typifications, descriptions, discussions, and diagnostic illustrations for representative taxa. The Melastomataceae are the largest family in the Myrtales and form a frequent and characteristic component of vegetation in tropical areas of the New World. Members of the family are generally identified by opposite leaves, poricidal anthers, and a peculiar foliar venation consist- ing of subparallel or arcuate longitudinal nerves. Although the family is easily characterized as a natural phylad, generic and tribal limits in the Melastomataceae have received little attention and most authors have been content to follow the system of classification presented by Cogniaux (1891), the most recent worker to review the family on a world-wide basis. While pursuing monographic studies of Monochaetum (Almeda 1978) and Heterocentron (Whiffin 1972), we discovered vexing problems re- garding relationships among Neotropical capsular-fruited Melastoma- taceae. This prompted us to refine our understanding of generic limits especially as they pertain to Tibouchineae. In so doing, our attention was drawn to a group of three species that has most recently been placed in the large, morphologically diverse, and geographically widespread genus Tibouchina. The opportunity to broaden our field knowledge of this trio was made possible by a collecting program associated with an ongoing study of the Costa Rican Melastomataceae (by FA). Study of natural populations together with an SEM survey of seed morphology by Whiffin and Tomb (1972) has helped us to formulate new ideas on circumscrip-