The velamen radicum in Orchidaceae, a spongy, multiple epidermis of roots with dead cells at maturity, is a textbook example of an "adaptation" to the epiphytic life style. There are numerous allusions to its function in the literature since the original description in the 19th century, but at close inspection solid information is extremely 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 notion 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).
On-going and planned work funded by the DFG will finally create the data for an unambiguous evaluation of the function of the velamen in epiphytic orchids. 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.
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