Velamen

Velamen is a spongy, multiple epidermis that covers the roots of some epiphytic or semi-epiphytic plants, such as orchid and "Clivia" species.

The velamen of an orchid is the white or gray covering of aerial roots (when dry, and usually more green when wet as a result of the appearance of underlying photosynthetic structures). It is many cell layers thick and capable of absorbing atmospheric moisture and nutrients. Often, the roots of orchids are associated with symbiotic bacteria or fungi that may fix nutrients from the air. This functionality allows the orchid to exist in locations that provide a reproductive or vegetative advantage such as improved exposure or reduced competition from other plant species.

The velamen also serves a mechanical function, protecting the vascular tissues in the root cortex, shielding the root from transpirational water loss, and, in many cases, adhering the plant to the substrate.