On the occurrence of a velamen radicum in Cyperaceae and Velloziaceae

Roots of tree‐like Cyperaceae ( Afrotrilepis pilosa, Bulbostylis leucostachya, Coleo‐chloa setifera, Microdracoides squamosus ) and Velloziaceae ( Vellozia variegata, Xe‐rophyta pinifolia ) were examined by light and scanning electron microscopy. For the first time a velamen is reported for Bromeliiflorae and Commeliniflorae. All species studied possess a velamen radicum that is either one‐layered ( Microdracoides, Bulbostylis, Coleochloa ) or multi‐layered ( Afrotrilepis, Vellozia, Xerophyta ). Anticlinal walls of the velamen cells of Microdracoides show perforations similar to those found in orchids. A multiple exodermis delimits the velamen towards the cortex. As in epiphytic Orchidaceae, a velamen may be of considerable adaptive value for arborescent Cyperaceae and Velloziaceae. These mainly occur in edaphically and climatologically extreme habitats (e.g. tropical inselbergs) where rapid water uptake is of crucial importance.