The velamen of an orchid is the white or silvery cover on old roots. This is a multilayered structure (multiple epidermis) of dead cells with special thickenings in the cell walls. The special thickenings seem to prevent cellular collapse and provide the root with some protection from mechanical injury. When the root is wet, the velamen fills passively with water, aided by perforations or tears in the walls from when the roots went through a drying cycle. While dry, the velamen may provide a barrier to water loss via transpiration from the wet, internal cells of the root.

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