Velamen tissue is a non-living, multi-layered epidermis made up of compact cells. It is designed to live in the absence of soil. Velamen tissues are observable in aerial roots.
Aerial roots are those that grow above the earth and are mostly adventitious. They can be found in a wide range of plant species, including epiphytes like orchids (Orchidaceae), banyan tree, figs (Ficus subg. Urostigma), tropical coastal swamp trees like mangroves, New Zealand’s pohutukawa trees (M. excelsa), and warm-temperate rainforest rata (Metrosideros robusta). Velamen tissue is found in epiphytes such as orchids.
Table of Contents
- Definition of Velamen
- Aerial Roots of Orchids
- Velamen Radicum—Effective Imbibition with Lesser Evaporation
- Frequently Asked Questions – FAQs
Definition of Velamen
Velamen, also known as velamen radicum, is a porous, multilayered epidermis covering the roots of epiphytic and semi-epiphytic plants, including orchids and Clivia species.
The white or grey coating of an orchid’s aerial roots is called the velamen when dry and generally greener when wet resulting in the appearance of underlying photosynthetic features. It comprises multiple cell layers and can absorb ambient moisture and nutrients, but its primary role might be to safeguard the underlying cells from UV radiation.
Aerial Roots of Orchids
Epiphytic orchids have roots that do not touch the ground and hang in the air freely. Orchid aerial roots have unique outer layers that segregate the living root cells from the air, mainly from the cortex.
Orchid aerial roots may absorb atmospheric water (rain, dew, and mist) and transmit it to the root core. The water is absorbed via imbibition into the velamen radicum, a porous biological material that envelops the aerial root and consists of two or many layers.
The velamen radicum entirely encloses the root and is made up of numerous layers of dead cells (in some orchids, there are normally one or two layers of velamen radicum cells).
Velamen Radicum – Effective Imbibition with Lesser Evaporation
The structure of the velamen radicum is complex. Because the velamen radicum cells no longer contain any living content, they are simply made up of their cell walls, which are perforated in a complex and species-specific pattern.
As a result, the velamen radicum is a porous medium with different levels of porosity: larger voids in the cell interior and lesser pores perforating the cell walls. The velamen cells’ length, shape, and arrangement contribute spatial properties to the total porous structure.
It has the ability to absorb atmospheric water (along with dissolved nutrients), making it available for absorption into the living cortex. Due to total reflexion induced by air filling the dead cells, the velamen radicum appears whitish-silver in the dry state. When orchid aerial roots are wet, they turn green, exposing the presence of chloroplasts within the live cortical cells.
The water that fills the velamen radicum does not flow directly into the cortex. The exodermis is a layer that separates the velamen radicum from the cortex.
The velamen radicum is made up of dead cell walls and has a complex structure. The imbibition process was examined in greater depth utilising mathematical principles and practical observations of eight different orchid species in research works.
Evaporation from the velamen radicum is particularly important for the plant since less water is available for absorption when water escapes from this “intermediate water storage.” As a result, the velamen radicum should be able to efficiently imbibe water while also preventing evaporation.
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Frequently Asked Questions – FAQs
Give an example of velamen.
Velamen is a sponge-like tissue that can be found in epiphytic roots. Since these plants do not make direct contact with the soil, it aids in the absorption and storage of atmospheric moisture. Dendrobium, Vanda, and Aerides are examples of plants with velamen.
Where is Velamen found?
Velamen can be found in orchids and Clivia species’ aerial roots. Many epiphytic orchids have hygroscopic velamen tissue covering their aerial roots. They are made up of non-living dense cells with lignified secondary wall strips. They do a good job of absorbing water from the air. The velamen is the grey or white coating of aerial roots in orchids.
Which are the external protective tissues of a plant?
The epidermis and cork are a plant’s protective outer tissues. The epidermis is a layer of cells generated from protoderm that covers the root, stem, flower, leaf, and seed components of a plant. The epidermis and its waxy cuticle protect the epidermis from mechanical damage, water loss, and infection. A secondary meristem strip replaces the epidermis of the stem. It becomes multiple layers of tough cork (bark).
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