Nutlet morphology and anatomy of the genus Lycopus (Lamiaceae: Mentheae)

Nutlet morphology and pericarp structure of 16 species in the genus Lycopus were studied by light microscopy (LM) and scanning electron microscopy (SEM), and a detailed description of nutlet morphological features for all examined taxa is provided. The nutlets of all taxa in the genus Lycopus are well adapted to typical hydrochory (or nautochory) with an air-filled pericarp, and myxocarpy was not at all found. It is noteworthy that the nutlet morphology (in particular the shape of corky crests and corky ring and the distribution of glandular trichomes) and pericarp anatomy of Lycopus are unique and are well distinguished from the other genera in the tribe Mentheae. We also found some groups of the species within the genus Lycopus by the present nutlet morphological and anatomical data that appear to be useful as diagnostic characteristics for delineation purposes at the specific/interspecific levels. The earlier infrageneric classification for the genus Lycopus is, however, not well supported by the present results. In addition, the systematic and biological implications of the nutlet characteristics investigated are briefly discussed.