Trichostema L., a North American genus comprising five sections and 17 species, has long been recognized as a natural group based on its morphology. Since relationships among the species remain unclear, cladistic analyses of morphological and molecular data for Trichostema are needed to verify its monophyletic status and to assess the sectional classification within the genus. Both ITS and ndhF data indicate that Trichostema is monophyletic and sister to the Asian Caryopteris s.s. When it includes T. brachiatum, an eastern US species sometimes placed in its own genus, section Orthopodium is polyphyletic, the remainder of the species occurring in the western US. The ndhF data support sections Paniculatum and Rhodanthum as sister to one another. Vegetative structures, floral development, and fruit morphology have been examined in much detail in combination with characters available from the literature (pollen morphology, base chromosome, and breeding system, etc.). In particular, the characteristic trichomes (glandular and nonglandular) of Trichostema have been studied and assessed for their phylogenetic informativeness. The results of the morphological study support the monophyly of Trichostema, and are mainly congruent with relationships inferred from DNA sequence data, with the notable exception of T. brachiatum, which is placed in section Orthopodium.

Key words: ITS, Lamiaceae, morphology, ndhF, systematics, Trichostema