SYSTEMATICS AND Ricardo M. Rueda2EVOLUTION OF THE GENUSPETREA (VERBENACEAE)'ABSTRACT Fourteen of the 35 previously accepted species of Petrea L. (Verbenaceae) are accepted in the present revision,three known only as fossils. Morphological, anatomical, and ecological characteristics of the species of Petrea arepresented, along with descriptions, illustrations, and distribution maps. A cladistic analysis of the tribe Petreeae Briq.,using Citharexylum L. and Duranta L. for outgroup comparison, revealed that Petrea is most closely related toXolocotzia Miranda. Xolocotzia was then used as an outgroup to polarize the character states within Petrea, resultingin a cladogram with a consistency index of 0.74 and a retention index of 0.86, after three rounds of successiveweighting.RESUMEN El ninmero de aceptadas especies en el g�nero Petrea L. (Verbenaceae) fue reducido de 35 a 14; tres son soloconocidas de f�siles. Caracteristicas morfol�gicas, anatomicas, y ecol�gicas de las especies de Petrea son presentadas,junto con descripciones, ilustraciones, y mapas de distribuciones. En el an�lisis cladistico de la tribu Petreeae Briq.,se us6 como grupo externo Citharexylum L. y Duranta L.; este an�lisis revel� que Xolocotzia Miranda es el mascercano evolutivamente al g�nero Petrea. Xolocotzia fue usado como grupo externo para polarizar el estado de loscaracteres dentro de Petrea, resultando un cladograma con un indice de consistencia de 0.74 y un indice de retenci�de 0.86, despu�s de tres corridas de revalorizaci�n de los caracteres. Petrea L. is a poorly known genus of the Ver-collections and new species described since 1938 benaceae. The most recent taxonomic treatment have rendered the keys obsolete. was written by Moldenke (1938). This monograph Moldenke (1938) included 29 species in Petrea, has long been outdated, making identification of and six additional species were described later, one the species dificult. A considerable number of dis-as a preliminary part of the present study (Rued, crepancies have been found between the characters 1992). Petrea, as interpreted here, contains 14 used in the keys and those given in the individual species, three of which are known only from fosslS. descriptions of the species. Moreover, additional The extant species are exclusively neotropical, 'This study was completed as a partial fulfillment of a Ph.D. degree in biology at the University of Misso Louis. I ar grateful to the late Alwyn H. Gentry and the other members of my committee for guidance and iti connents on the manuscript. I thank Richard Keating for the use of his laboratory facilities at Southern I University for the anatomical studies. SEM facilities at Washington University were made available by Michael Vei Jorge Crisci and Peter Hoch provided invaluable help with the cladistic analysis. I also thank John Myers and Pry Bick for their skillful illustrations. A Peter Raven fellowship and a teaching assistantship from the U niversit Missouri-St. Louis and the Compton Foundation provided financial support for my studies. The Missori Garden provided facilities and research space. Fieldwork was made possible by NSF grant No. INT-924094.-L support was provided by the following institutions during my travels in South America: Jardin Botanico "Gu Piferes"; Jardin Botanico "Joaquin Antonio Uribe"; Instituto de Cieneias Naturales de La Universidad Nacional Colombia; Instituto Nacional de Pesquisas da Amaz�nica, Manaus; Mobil Oi, Bolivia; and Fundacibn Jatun S Ecuador. I am grateful to the curators and staff of the following herbaria for loans of material: AAU, B, BM, C, CAS, CAY. CHAPA, COL, CR, ENCB, F, G, GH, HBG, IAN, K, L, LE, M, MAD, MEXU, MG, MICH,M NY, P PORT, PR, R, S, SP, TEFH, TEX, U, UB, UC, UCLA, UEC, UPS , US VEN, W, WAG, and WIS. SUniversidad Nacional Aut�noma de Nicaragua-Le�n, Facultad de Ciencias, Departamento de Biologi, L Nicaragua, C. A. ANN. MIssoURI BOT. GARD. 81: 610-652. 1994.