Philodendron is, after Anthurium, the second largest genus in the Araceae with 700 or more species (Croat, 1979, 1983a, 1988, 1990). This revision is the first major revision of P. subg. Philodendron for Central America since Kurt Krause's 1913 generic treatment in Das Pflanzenreich. Philodendron is one of the most important genera in the neotropics, inhabiting a wide range of mesic habitats from sea level to over 2,000 m and in life zones (Holdridge, 1967) ranging from Tropical moist forest (T-mf) to Premontane rain forest (P-rf). While most species occur in virgin humid forests, the genus is known from freshwater swamps, stream banks, regrowth forest, rock outcrops and roadbanks. It is not only one of the largest genera in the neotropics but often constitutes the most conspicuous element of the vegetation because of its abundance, primarily climbing habit and frequently large, showy leaves. The genus provides a wide variety of choice ornamental plants for horticulture, including most of the species treated here. Unfortunately it is also still very poorly known taxonomically, especially in the South American Andes.
Philodendron has 119 Central American species (Appendix 1, Geographic Distribution of Central American Philodendron), including 128 taxa, distributed in two subgenera of Philodendron. This Central America revision includes only members of P. subg. Philodendron and includes 103 taxa, including 95 species and 8 varieties or subspecies. A total of 65 taxa are new to science. These include 59 species, 6 subspecies or varieties, and 2 combinations. Alternatively, P. subg. Pteromischum, revised separately by M. H. Grayum (1996), also of the Missouri Botanical Garden, contains 24 species (including 25 taxa) for Central America. That revision encompasses all species from Pacific and Caribbean tropical America, ignoring only species from the region of the Guianas and from the Amazon drainage of South America.
The genus is a distinct one, not easily confused with any other, though closest to Homalomena which differs in having a consistently terrestrial habit, frequently spiny petioles, a sap smelling of anise and has staminodia mixed in among the pistillate flowers on the spadix.
Species diversity of P. subg. Philodendron in Central America shows a general diminution from Mexico to Middle America with the lowest totals just north of the San Juan depression, followed by a marked increase approaching the South America continent. Mexico has 21 taxa, Guatemala 15, Belize 9, El Salvador 5, Honduras 13, Nicaragua 18, Costa Rica 49 and Panama 80 taxa. Endemism is high, especially for Panama where 39 taxa are currently considered endemic. Mexico and Costa Rica each have 7 endemic species. With the exception of Belize, which has one endemic, no other country in Middle America has any endemic species.
Most of the Central American species of P. subg. Philodendron (Appendix 2, Technical Data on Pistils) are contained within P. sect. Calostigma (Schott) Engl. with 47 species (50 taxa) and P. sect. Philodendron with 38 species (41 taxa). Other sections represented in Central America include P. sect. Tritomophyllum Schott with six species, P. sect. Polytomium Schott with three subspecies (four taxa) and P. sect. Macrogynium Engl. with one species. Sections not represented in Central America are Schizophyllum Schott, Camptogynium K. Krause, and Philopsammos G.S. Bunting. A key for the sections of P. subg. Philodendron is included under "Taxonomy".