The velamen radicum is common among terrestrial monocotyledons.

Background The velamen radicum, a spongy and usually multiple-layered root epidermis composed of dead cells at maturity, is a textbook example of an adaptation that is typically associated with epiphytic orchids. Scope Based on an extensive literature review, the occurrence of this structure was documented in approximately 240 genera of terrestrial monocotyledons (162 orchid and 74 non-orchid genera) and in one genus of dicotyledons. Mapping this character on a phylogenetic tree reveals that a velamen is found throughout the monocotyledons, ranging from members of early-branching Araceae to derived Cyperaceae. Conclusion The current depiction of the velamen radicum in angiosperms and its almost exclusive association with the epiphytic lifestyle in orchids is misleading, which inevitably leads to a biased view of its evolution and function. The findings propose a re-evaluation of the occurrence, evolution and function of the velamen radicum in both terrestrial and epiphytic angiosperms. [ABSTRACT FROM AUTHOR]