Aerial roots of orchids: the velamen radicum as a porous material for efficient imbibition of water
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T.C. BIOLOGICAL AND BIOMIMETIC MATERIALS
Aerial roots of orchids: the velamen radicum as a porous material for efficient imbibition of water Frank Hauber1 · Wilfried Konrad1,2 · Anita Roth‑Nebelsick3 Received: 27 June 2020 / Accepted: 1 October 2020 © The Author(s) 2020
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. Keywords Imbibition · Contact angle · Porous material · Porous surface · Evaporation · Aerial root · Velamen radicum
1 Aerial roots—structure and functions
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00339-020-04047-7) contains supplementary material, which is available to authorized users. * Wilfried Konrad wilfried.konrad@uni‑tuebingen.de Frank Hauber [email protected]‑tuebingen.de Anita Roth‑Nebelsick anita.rothnebelsick@smns‑bw.de 1
Department of Geosciences, University of Tübingen, Hölderlinstrasse 12, 72074 Tübingen, Germany
2
Institute of Biology, Technical University of Dresden, Zellescher Weg 20b, 01217 Dresden, Germany
3
State Museum of Natural History Stuttgart, Rosenstein 1, 70191 Stuttgart, Germany
This contribution deals with an intricate and multifunctional porous material that has essential and vital functions in certain plants. The investigated biological material is in fact a quite common sight: it envelops the thick and fleshy roots of orchids which are also popular as houseplants. Like in “normal” roots, these orchid roots absorb water but in a remarkable way, by imbibition into a complex multilayered porous tissue. Wetting
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