Orchids are a diverse family of plants that includes species adapted to life high above the ground in humid rainforests. These orchids are epiphytes that grow on other plants. To collect water, some epiphytic orchids dangle their roots in the air and absorb moisture directly from the atmosphere, from rain, and from water that drips off vegetation above it. Others spread their roots over the surfaces of tree branches and collect water as it trickles over the tree’s surfaces.
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“The velamen radicum, a spongy, usually multiple epidermis of the roots, which at maturity consists of dead cells, is frequently described as an important adaptation of epiphytic orchids…We tested the notion originally put forward by Went in 1940 that the velamen allows plants to capture and immobilize the first solutions arriving in a rainfall, which are the most heavily charged with nutrients…First, we show that the velamen of a large number of orchid species takes up solutions within seconds, while evaporation from the velamen takes several hours. Charged ions are retained in the velamen probably due to positive and negative charges in the cell walls, while uncharged compounds are lost to the external medium. Finally, we demonstrate that nutrient uptake follows biphasic kinetics with a highly efficient, active transport system at low external concentrations. Thus, our results lend strong support to Went’s hypothesis: the velamen fulfills an important function in nutrient uptake in the epiphytic habitat.” (Zotz and Winkler 2013:733)
Aerial roots of epiphytic orchids: the velamen radicum and its role in water and nutrient uptakeOecologia, 171(3): 733–741March 1, 2013
“Orchids of many kinds have also adopted this high life. They lack the ponds that sustain the bromeliads, so they must collect their nourishment in other ways. Some dangle their roots in the air, absorbing moisture from the humid atmosphere and rely on the tiny amount of nutriments it might have dissolved on its descent through the forest vegetation. Others spread their roots over the surface of the branches and collect the water that has trickled through the leaves and dripped from branch to branch, gathering a little nutriment on the way.” (Attenborough 1995:166)