Velamen radicum micromorphology and classification of Orchidaceae

Roots of 344 species of 262 representative genera of Orchidaceae have been examined under systematic aspects by light and scanning electron microscopy; the paper is illustrated by 25 SEM-photos and schematic drawings. In particular the stratification and micromorphology of the rhizodermis (velamen radicum) exhibits a high diversity which can be applied taxonomically. Combinations of root characters are classified into 12 syndromes ("types"). A particularly remarkable cortex structure (tracheoidal cortex) is described and the term "pseudovelamen" introduced. Additional features relevant for taxonomic work are exodermis structure and occurence of "Stabkorper" (tilosomes). The distribution of these characters shows a high conformity with existing classifications of the family. However, in certain cases discussed in detail (e.g. Genyorchis ) the characters are discordant with some taxonomic views.