Velamen is a spongy, multiple epidermis

Epidermis (botany)

The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...

 that covers the roots of some epiphytic or semi-epiphytic plants, such as orchid and Clivia

Clivia

Clivia is a genus of monocot flowering plants native to southern Africa. They are from the family Amaryllidaceae, subfamily Amaryllidoideae. Common names include Kaffir lily and bush lily....

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 fungi or bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, the latter may fix

Fix

Fix as a noun may refer to:* Fix , a position determined by navigation* Fixed point combinator* Fix , a feature film by Tao Ruspoli* FIX protocol, a communications protocol for financial information exchange...

 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

Vascular tissue

Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...

 tissues

Biological tissue

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

in the root cortex, shielding the root from transpirational water loss, and, in many cases, adhering the plant to the substrate.