PERIDERMIS: CHARACTERISTICS, FUNCTIONS, TRAINING - BIOLOGY - 2025

The peridermis is a protective tissue that replaces the epidermis as an external protective covering in plants that present secondary growth. Secondary growth is an increase in the thickness of the stem and roots, originating from the secondary meristems in conifers and dicotyledonous plants.

The peridermis originates from the suberous cambium and is composed of the phelema or suber, the felodermis and the felogen. Generally each year a new layer of peridermis forms towards the interior of the old peridermis.

Peridermis. Taken and edited from KatarinaS97.

The bark is made up of both peridermis and secondary phloem. It has various uses; súber, for example, also known as cork, is used as a container stopper. Commercial cork, which is still considered the ideal stopper for wine bottles, comes mainly from the cork oak.

characteristics

It is formed by secondary growth, as it occurs from cells of the parenchyma, collenchyma or the epidermis that have dedifferentiated to become meristematic cells again.

It is produced by periclinal divisions of the felodermis, which are parallel to the surface.

It is present only in stems and roots and absent in leaves, flowers and fruits. It can appear several times during the life of the plant.

The peridermis is impervious to both water and gases and resistant to the action of acids.

Parts

Source: pixabay.com

Phelogen

Also called cambium suberoso, it is a secondary meristematic tissue responsible for the production of new dermal tissue. It is a lateral meristem that can grow seasonally in the form of continuous or discontinuous bands under the epidermis.

Súber

This fabric is also known as feloma or cork. It is formed towards the outside of the phellogen and is made up of cells that lose protoplasm and die when they mature.

The primary walls of these cells are covered on the inside by a relatively thick layer of a greasy substance that is impermeable to water and gases called suberin.

Felodermis

It is a thin layer composed of living parenchymal cells, originating from the different layers of suberous cambium. These cells are characterized by not presenting suberified walls and may eventually have chloroplasts.

Features

The main function of the peridermis is to protect the trunk and roots against predation by various organisms, mainly insects, and from infections by fungi and other pathogens, by replacing the epidermis of plants with secondary growth.

Secondary growth causes the trunk and root to widen. This growth causes the cortex and the epidermis, which have stopped growing, to elongate and break, for which they must be replaced.

The development of the peridermis ends up causing the death of the epidermis by isolating it from the cortical parenchyma and preventing it from exchanging substances with it.

The peridermis also prevents water loss from the plant through the stem due to evapotranspiration. This is thanks to the suberin, which also prevents the loss of phloem sap, which is rich in sugars, and its presence outside the plant makes it more attractive to different types of animals.

To allow gas exchange between the cells of the internal tissues and the external environment, small openings, called lenticels, appear in the suberous layer.

Cork oak (Quercus suber), tree from which commercial cork is extracted. Taken and edited by Javier García Diz.

Growth in plants

Plants that live for several years have two types of growth, a primary and a secondary growth.

Primary growth

It is the growth produced by the apical meristem, it allows the increase in length of the roots and shoots. In many seedless plants the apical meristem consists of a single cell that will divide successively to allow the plant to grow.

In plants with seeds, the apical meristem consists of more than a hundred cells located at the apex of the root and stem, and whose successive divisions will allow the longitudinal growth of the plant, lengthening from the apex.

This apical meristem also gives rise to the primary meristems, which are called protodermis, procambium and fundamental meristem. The first will produce the epidermis, the procambium will give rise to the xylem and phloem, while the fundamental meristem will produce the fundamental tissue.

Secondary growth

It is produced from the secondary meristem, named in this way because it is made up of cells that were originally parenchyma, but which were dedifferentiated to become meristematic cells again.

The secondary or lateral meristem is a single cell thick layer that completely surrounds the stem and root. The divisions of this meristem are periclinal and cause growth both outward and inward of stems and roots. It is the growth that allows the increase in thickness of these structures.

Training

The first phellogen originates when primary growth no longer occurs. This develops from cells of the parenchyma of the outer layer of the cortex, below the epidermis. These cells dedifferentiate and become meristematic cells again. The first pericline division of these cells will give rise to two cells.

The outermost cell will give rise to the suber, while the innermost cell will continue to divide. Of these new cells, the innermost will form the felodermis, while those located between this layer and the suber, will continue to be cells of the felogen.

In this way, the first peridermis is formed that will replace the epidermis, which will die because the suber will prevent any exchange of water and nutrients between its cells and those of the adjacent tissues.

Periodically a new layer of peridermis will be produced towards the interior of it, which can have a complete growth or in bands. For the formation of these new layers of peridermis, the layers of phellogen originate from cells of the secondary meristem coming from the parenchyma of the secondary phloem.

The formation of a new layer of subber will isolate the living elements from the outermost layers, causing their death as happened with the epidermis. This outer layer of dead tissue forms what is the outer cortex. This is the bark that trees lose during their growth.

Lenticels

The súber layer waterproofs the plant and prevents the exchange of gases from the stem and root cells with the outside. In order to carry out the gas exchange, small openings appear in the outer cortex, in the area where the suberous layer is thinner and the cells are further apart.

These openings are called lenticels and allow gas exchange. With the emergence of new layers of suberous cambium, new lenticels will also appear, which will be aligned with the older ones to allow the continuity of this exchange.

References

1. MW Nabors (2004). Introduction to Botany. Pearson Education, Inc.
2. Bark (botany). On wikipedia. Recovered from en.wikipedia.org
3. WC Dickison (2000). Integrative Plant Anatomy, Academic Press, San Diego
4. Peridermis: bark and derivatives. Recovered from www.biologia.edu.ar
5. Topic 19. Cesundaria structure of the stem II. 19.3. Peridermis. Recovered from www.biologia.edu.ar
6. Vegetable tissues: Protection. Recovered from mmegias.webs.uvigo.es