Glandular trichomes as further differential characters between Stachys subgenus Betonica (L.) Bhattacharjee and Stachys subgenus Stachys

The genus Stachys L. has always presented great taxonomic and nomenclatural problems; the identification of two different genera, Betonica and Stachys, or of a single genus, Stachys, with the subgenera Betonica and Stachys, is still debated. In this work, different types of glandular trichomes (peltate and capitate) occurring in species belonging to both subgenera Betonica and Stachys are described. Observations were carried out on living plants growing in Italy, using scanning electron microscopy (SEM), to characterise trichome structure and distribution on vegetative and reproductive organs, and light microscopy (LM) to characterise their type of secretion, by means of histochemical methods. Stachys subgenus Betonica presents only peltate hairs, while Stachys subgenus Stachys has different types of large capitate hairs lacking in the subgenus Betonica. Furthermore, the peltate trichomes of the subgenus Betonica show an unusual secretion composed of flavonoids and essential oils. A thorough examination of numerous exsiccata belonging to subgenera Betonica and Stachys, stored in the Herbarium Centrale Italicum of Florence (FI), confirmed observations made on living material. The recorded characters allowed a clear distinction between the species of the subgenus Betonica and those of the subgenus Stachys. Consequently, a higher taxonomic rank than subgenus should be recommended for these taxa.