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).
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 exodermis is surrounded by velamen, consisting of one to several layers of cells, which can develop root hairs under proper environmental conditions.
The dead cells of velamen diffuse light, thus giving it a grey appearance—except at the tips, where the chlorophyll become visible.
Upon absorbing water, the dead cells become transparent, and the whole velamen tissue then appears green.