The velamen radicum, best known from Orchidaceae, but also found in many other families (Zotz et al. 2017) is the name of a spongy, multiple root epidermis with dead cells at maturity. It is a textbook example of an "adaptation" to the epiphytic life style. There are numerous allusions to its function in the literature since its original description in the 19th century, but at close inspection solid information is very limited. We have already evaluated a hypothesis put forward by Went (Ann Jard Bot Buitenzorg, 1940), who had suggested that the primary function of the velamen is the capture and immobilization of the first solutions arriving at the onset of a rainstorm, which are the most heavily charged with nutrients. We found that all necessary prerequisites for this function were given, i.e. rapid uptake of solutions into the velamen, retention of charged particles within the velamen and fast uptake into living tissue (Zotz and Winkler 2013).
Cross-section of aerial roots from Caularthron bilamellatum (courtesy Alischa Stäbner).
Work funded in part by the DFG, in part in the framework of a DAAD fellowship, will finally create the data for an unambiguous evaluation of the function of the velamen in epiphytic orchids and aroids. Specifically, we will verify results with a larger set of species, will address other possible functions such a mechanical protection and protection against overheating and will put variation of velamen structure among orchids and aroids in an ecological context.
Einzmann HJR, Schickenberg N & Zotz G. 2020. Variation in root morphology of epiphytic orchids along small-scale and large-scale moisture gradients. Acta Botanica Brasílica 34: 66-73.
Joca TAC, Coelho de Oliveira D, Zotz G, Cardoso JCF & Moreira ASP. 2020. Chemical composition of cell walls in velamentous roots of epiphytic Orchidaceae. Protoplasma 257: 103–118.
Joca TAC, Coelho de Oliveira D, Zotz G, Winkler U & Moreira ASP. 2017. The velamen of epiphytic orchids: variation in structure and correlations with nutrient absorption. Flora 230: 66-74.
Zotz G & Winkler U. 2013. Aerial roots of epiphytic orchids: The velamen radicum and its role in water and nutrient uptake. Oecologia 171: 733-741.
Zotz G, Schickenberg N & Albach DC. 2017. The velamen radicum is common among terrestrial monocotyledons. Annals of Botany 120: 625-632.