The velamen or velamen radicum ("covering of the roots" in Latin) is a spongy, multiple

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

that covers the roots of some

epiphytic An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphyt ...

or semi-epiphytic plants, such as

orchid Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants that are found in almost every habitat on Eart ...

and ''

Clivia ''Clivia'' is a genus of monocot flowering plants native to southern Africa. They are from the family Amaryllidaceae, subfamily Amaryllidoideae. Common names are Natal lily or bush lily. They are herbaceous or evergreen perennial plants, with ...

'' 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, but its main function may lie in protecting the underlying cells against damaging UV rays (Chomicki et al., 2015). Often, the roots of orchids are associated with

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

; the latter 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 Vascular can refer to: * blood vessels, the vascular system in animals * vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...

tissues in the root cortex, shielding the root from transpirational water loss, and, in many cases, adhering the plant to the substrate. The typical orchid root has a

stele A stele ( ) or stela ( )The plural in English is sometimes stelai ( ) based on direct transliteration of the Greek, sometimes stelae or stelæ ( ) based on the inflection of Greek nouns in Latin, and sometimes anglicized to steles ( ) or stela ...

of comparatively small diameter. It is surrounded by a cortex which is further enveloped by a highly specialized exodermis, most of which at maturity do not contain protoplasm. A few cells, however, are living and allow the passage of water through them. The exodermis is surrounded by velamen, consisting of one to several layers of cells, which can develop root hairs under proper environmental conditions. The velamen arises from the root tip by division of a special tissue. 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.

References

* Chomicki, G., L. P. R. Bidel, F. Ming, M. Coiro, X. Zhang, Y. Wang, Y. Baissac, C. Jay-Allemand, and S. S. Renner. 2015. The velamen protects photosynthetic orchid roots against UV-B damage, and a large dated phylogeny implies multiple gains and losses of this function during the Cenozoic. New Phytologist 205(3): 1330–1341. Plant morphology {{Plant-morphology-stub