a clonal growth organ formed below-ground (Serebrjakov and Serebrjakova 1965); the rhizome usually grows horizontally at a species specific depth and after some time it becomes orthotropic and forms above ground shoots; the horizontal part of the rhizome bears bracts, a few roots developed at the nodes, and has long internodes; vegetative spreading is often fast, up to several meters . yr-1; persistence of the hypogeogenous rhizome differs considerably among species
