The orchid root with the velamen radicum: a multifunctional organ

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True epiphytism is associated with a rather precarious supply of water and nutrients. The velamen radicum of orchid roots, i.e. a multiple epidermis with dead cells at maturity, which borders internally on an exodermis that features 1) thick-walled cells and 2) so-called passage cells, is a textbook example of an ´adaptation´ to the epiphytic life style. There are numerous allusions to its function in the literature, but at close inspection solid experimental evidence is extremely limited. We will test the most important of the suggested functions. To this end, our work has physiological components, e.g. we will study uptake kinetics of important nutrient elements, water uptake and loss, quantify the ion exchange capacity of the velamen, and investigate the alleged role of the velamen in preventing overheating of the living cortex. These physiological data will be put in an ecological context by studying in situ root demography and variation in root anatomy and morphology at the community level along environmental gradients. Systematically comparing anatomy / morphology with physiology will also allow us to link form with function in a phylogenetic framework. In summary, our work will finally create the data for an unambiguous evaluation of the function of the velamen in epiphytic orchids. However, there are also numerous terrestrial plants with a velamen (e.g. in Orchidaceae or Asparagaceae), and there are numerous other epiphytes with a velamen, particularly in the Araceae. Thus, the current study is just a first step. In a second phase the proposed project intends to go beyond the topics ´epiphytes´ and ´orchids´ by addressing the major ecological and evolutionary ´dimensions´ of velamentous roots in general.