Aerial roots of orchids: the velamen radicum as a porous material for efficient imbibition of water

Abstract

Aerial roots of orchids are able to absorb atmospheric water (e.g. rain, mist, dew) and to conduct it to the root interior. The water is absorbed by imbibition into a biological porous material, the velamen radicum, which envelops the aerial root and comprises one or two to several layers. The velamen radicum consists of walls of dead cells and shows an intricate structure. In this contribution, the imbibition process was studied in more detail using mathematical principles and by experimental observation of eight orchid species. The process of absorption can be subdivided into two stages. The initial stage is the attachment of a drop to the rough surface. The drop is then imbibed into the layers of the velamen radicum. During imbibition, the contact angle decreases, because the drop remains firmly pinned to the surface. Imbibition times vary widely, due to differences in the porous structure of the considered species. It could, however, be demonstrated that a detailed theoretical study of imbibition for this complex biological material is possible using structural parameters obtained from microscope images. A third aspect which was considered was evaporation out of the velamen radicum. Also here, observations were consistent with modeling results. The velamen radicum, and other tissues involved in water absorption by aerial roots of orchids, represents an intriguing biological material for studying imbibition and flow in porous materials and has probably a high potential for discovering technically interesting features.