Volume 23 contains information on 50 genera and 304 species of the family Lamiaceae, recognized as native or naturalized in the region; the largest genera are Callicarpa (55 species), Clerodendrum (54 species), Teijsmanniodendron (54 species) and Vitex (21 species). The introductory part includes a section on wood anatomy (by P. Baas). The genus and species treatments comprise keys, descriptions, drawings, references, synonyms, typifications, distribution, habitat and miscellaneous notes.