velamen

thumb|right|Transverse section through an aerial root of an orchid. The outer most layer is the velamen, comprising several cell layers of dead cells. The velamen is a specialized root covering that acts like a natural sponge, found on orchids and many other plants. This thick, mesh-like layer replaces the normal root skin and is made up of dead cells that form tiny air pockets. While commonly associated with air-growing orchids that live on trees, this root covering is actually found in nearly 240 types of ground-dwelling plants as well, mostly within the monocot group. The velamen serves multiple functions including rapidly absorbing rainwater, temporarily storing moisture and nutrients, and protecting roots from damage when soil dries out and shrinks away. Laboratory studies have shown that it works as both a water reservoir and a selective filter, capturing nutrient-rich water from rainstorms and passing key minerals like phosphate and potassium to the living root tissues underneath.

==Structure and composition==