The "black nodules" of Hypericum perforatum L. subsp perforatum:: Morphological, anatomical, and histochemical studies during the course of ontogenesis

Hypericum perforatum L., traditionally used in folk medicine as a therapeutic plant, is today investigated for its antidepressant and antiretroviral activities. This species is characterized by the presence of different types of secretory structures: translucent glands or cavities, black nodules, and secretory canals. The aim of the present work is to characterize the black nodules on both the floral and vegetative parts, morphologically, anatomically, and histochemically. Nodules consist of a cluster of irregularly shaped cells surrounded by a single- or double-layered sheath. Histochemical tests show that the nodules are negative for the presence of lipids, essential oils, sesquiterpene lactones, steroids, and proteins and positive for pectic-like substances, tannins, and alkaloids. Our results show that the inflorescences are richest in nodules and are, therefore, the best sites for the extraction of the secondary metabolites.