Phanerogams worldwide with emphasis on tropical and North America, especially rich in collections from Mexico, Guatemala, Costa Rica, Colombia, Venezuela, Ecuador, and Peru; pteridophytes worldwide with emphasis on Guatemala, Costa Rica, Ecuador, and Peru; bryophytes worldwide; mosses of North America, Central America, Andean South America, and Australasia; hepatics of north temperate, South America, and south temperate; all groups of fungi, especially basidiomycetes with emphasis on New World and lichenized fungi of north temperate and Central America; algae worldwide, especially Cyanobacteria; economic botany.