Seven New Species of Pilea Lindley (Urticaceae) from Mesoamerica

Abstract

Seven new species of Pilea from Mesoamerica are described and illustrated: Pilea pteridophylla A. K. Monro, P plumulosa A. K. Monro, PR tripartita A. K. Monro, P. rostulata A. K. Monro, P quadrata A. K. Monro, P tutensis A. K. Monro, and P magnicarpa A. K. Monro. Their affinities are discussed and positions within Weddell's and Killip's subdivisions of the genus indicated. Pilea is a genus mainly composed of small shade-loving herbs and is distributed throughout the tropics, subtropics, and temperate regions (with the exception of Australia, New Zealand, and Europe), with the greatest diversity in the Greater Antilles. Pilea can be distinguished from other Mesoamerican Urticaceae by opposite leaves and a single ligulate intrapetiolar stipule in each axil. Cystoliths, found on the stem, leaves, and frequently inflorescence, vary greatly within and between species, but are distinctive in frequently appearing branched, forming V, Y, and X shapes. The last major worldwide taxonomic treatment of the genus Pilea was H. A. Weddell's treatment of 1869, in which 159 species were recognized (24 of which were described as new). Since then, over 500 new species names have been published, and it is likely that there are over 500 to 600 good species worldwide (Adams, 1970; Burger, 1977). Major contributions to the taxonomy of neotropical Pilea have been made by E. P. Killip in his revision of the Andean species (Killip, 1936, 1939), P. C. Standley and J. A. Steyermark (1952) in their treatment for the Flora of Guatemala, C. D. Adams (1972) in his treatment for the Flowering Plants of Jamiaca, and W. C. Burger (1977) in his treatment for Flora Costaricensis. Weddell (1869) subdivided the genus, on the basis of leaf morphology, into three principal species groups: Integrifoliae, with leaves of equal length at each node and entire margins; Heterophyllae, with leaves of unequal length at each node; and Dentatae, with leaves of equal length at each node and toothed margins. Killip (1936) subdivided Pilea into 12 species groups, based largely on those of Weddell's conspecti of 1856 and 1869. In the absence of recent phylogenetic analysis, the new species described here are best placed in three of the subgeneric groups cited in this article: Fallaces, Molles, and Centradenoideae. These three subgeneric groups can be recognized on the following suites of characters: subgenus Fallaces with toothed pinnately nerved leaves, where no pair of the secondary nerves are more prominent than the others; subgenus Molles with pubescent, toothed, 3-nerved or pinnately nerved leaves (where pinnately nerved a basal pair of secondary veins is markedly more prominent than the others), of equal size at each node, averaging more than 2 cm in length and compact staminate inflorescences where the peduncles are longer than the panicle branches; subgenus Centradenoideae with 3-nerved or pinnately nerved leaves (where pinnately nerved a basal pair of secondary veins is much more prominent than the others), of unequal size at each node, the smaller leaf at each node symmetrical, with stipules less than 3 cm long and pedunculate pistillate inflorescenc-