TREE BARK: CHARACTERISTICS, COMPOSITION, FUNCTIONS - BIOLOGY - 2025

The bark of a tree is composed of all the tissues external to the vascular cambium. In it two parts are differentiated; the outer cortex, which is made up of dead cells, and the inner cortex, made up of secondary phloem, peridermis, and remains of the cortex. The bark of the tree is much thinner than its woody portion.

This structure, also known as rhytidome, is essential for the life of the plant, because while the living portion (secondary phloem) transports the sap produced from the leaves to the rest of the plant, the dead cells prevent excessive loss of water and they protect the plant from predation and attack by pathogens.

Woody tree outer bark. Taken and edited from: Pedro Camilo Márquez Vallarta.

The bark has multiple uses, ranging from simply decorative, to its use in the kitchen for gastronomic purposes or in the pharmaceutical industry for the manufacture of medicines. Quinine, for example, is a medicine extracted from the cinchona tree that for many years was the only one used to treat malaria.

Characteristics (botanical description)

The bark of the tree is separated from the xylem by the vascular cambium, which is responsible for producing both the secondary phloem and the secondary xylem.

It contains both living and dead cells. Three layers can be distinguished: peridermis, cortex and phloem.

-Peridermis

The peridermis is the outermost tissue that replaces the epidermis as a protective structure in plants with secondary growth. It originates from the cambium suberoso and a layer is usually formed annually towards the interior of the old peridermis. It is formed by cambium suberoso, feloma and felodermis.

Cambium suberoso

This tissue, also called phellogen, is a secondary meristematic tissue that will give rise to the new dermal tissue. It is a lateral meristem that can grow seasonally in the form of continuous or discontinuous bands under the epidermis.

Súber

Also called feloma or cork, it is a tissue that is formed towards the outside of the feloma. It is characterized because its cells have a fatty substance with insulating properties called suberin, which permeates the internal part of the primary walls.

Felodermis

It is a thin layer of living parenchymal cellular tissue that comes from the different layers of cambium suberoso. These cells lack suberin and may have chloroplasts.

-Cortex

It is a fundamental tissue that is formed between the vascular and the dermal tissue and is mainly made up of parenchyma.

-Floem

It is a vascular tissue responsible for the transport of food (processed sap) from the leaves to the rest of the plant. It is made up of living cells called elements of the sieve tubes.

Cross section of a willow tree. Taken and edited from: Keith Edkins / felled willow cross section.

Chemical composition

Although the bark of different tree species has basically the same constituents in different species, the proportion of these can vary enormously. Among the elements that constitute it are:

-Polysaccharides

The main polysaccharide found in tree bark is cellulose, representing about 30% of these substances. Hemicellulose follows, with 15% or less of the total carbohydrates.

Other polysaccharides are available to a much lesser extent, such as D-galactose, D-mannose, L-arabinose, and starch.

-Lignin

Lignin is a cross-linked phenolic polymer found in the cell walls of wood and bark. It is the second most abundant constituent in the bark after cellulose.

This compound is the natural polymer with the most complex structure and heterogeneity of all known molecules. It is insoluble in acids and soluble in strong alkaline substances.

-Suberin

Suberin is a biological polymer made up of fatty acid hydroxides and epoxides linked by ester bonds. This polymer is produced by the cell walls of some cells, mainly those of súber or cork, and is responsible for the impermeability of these cells.

-Tannins

Tannic acid, or tannin, is a compound that plants use to repel insects. Its amount in the bark varies depending on the species, for example in the Cuban pine it is higher than 10%, while in the Caribbean pine it does not exceed 8.5%.

Tannins can be extracted from the bark in the form of salts with diluted alkaline solutions, and the use of these is one of the main uses that is given to the bark.

-Other components

In addition to the components already mentioned, the bark presents a great variety of other substances, among which we can mention: flavofen and other phenols, terpenes, waxes, fats, aliphatic alcohols, proteins and soluble carbohydrates, vitamins, etc.

Among the minerals, the most abundant are calcium and potassium, while other minerals such as boron, copper and manganese are only found in trace amounts. The total of minerals, expressed as ash, can represent between 1 and 5% of the total dry mass of the crust.

Tree functions

One of the main functions of the bark of trees is to transport the sap produced from the leaves to the rest of the plant; for this it uses phloem cells.

That is why when removing the bark in a continuous cross section the plant dies, since it cannot transport the elaborated sap to its roots.

Another important function is to protect the plant against the attack of herbivorous animals, mainly insects, as well as infections by fungi and microorganisms.

Due to the presence of insulating substances such as suberin, the bark also protects the plant against the loss of water by evapotranspiration.

In some plants, living cells in the bark can contain chloroplasts, so this tissue can also participate in the production of food through photosynthesis.

Use for humans

As a coating in constructions

The bark provides protection and impermeability to the tree, man has taken advantage of these properties for many years in the development of tiles and coatings to protect roofs from rain and inclement weather.

Native Americans have used poplar bark to clad their homes for more than 500 years. It is currently used not only in exterior cladding but also in interior applications, porch railings, furniture, and as accent pieces.

This bark can last more than 75 years without the need to apply any type of sealer or paint. It also does not require maintenance.

Birch is also used as a cladding and because its bark is more flexible and more decorative, it has been used more in interior decoration. The bark of this tree is also used to make different utensils.

Culinary

The bark of many trees, dried and pulverized, becomes flour and is used to make bread. Among these trees are the American beech, birch and different species of pines. The inner bark of these species, cut into strips, is also used to make a kind of noodle.

Cinnamon is a plant from Sri Lanka whose inner bark (cinnamon) is used worldwide for making desserts, drinks, infusions, as well as condiments. The inner bark is obtained mainly by peeling and rubbing the branches of the plant.

Obtaining cork

The suber or cork is a part of the peridermis of the plants. The extruded suber of the cork oak is industrially exploited as natural cork, which is used mainly for the production of stoppers for glass bottles. It is the best stopper for bottles of wine and other spirits.

It is also used for the production of decorative panels and billboards, as an acoustic or thermal insulation, as a closure for wind musical instruments, in the manufacture of footwear, among others.

Natural coke has a series of qualities for which the industry continues to use it extensively today, among which are its lightness, flexibility, high coefficient of friction, impermeability, caloric power, low water content and resistance to passage of water. hot.

The world production of natural cork is close to 350,000 tons per year, of which more than 95% come from Portugal, Spain and Italy.

Diversity of caps or cork stoppers for wine bottles. Taken and edited from: Lesekreis.

Obtaining tannins

Tannins are secondary metabolites of plants that are very abundant in the bark. They are heterogeneous polymers of phenolic acids and sugars or of anthocyanidin. Plants use them as a mechanism to prevent herbivorous animals from eating them or from microorganisms attacking them.

The industry uses tannins for leather tanning because they react with the collagen present in them, resulting in a greater resistance of the leather to heat and decomposition due to the effect of water and microbes.

In medicine

Many trees produce and concentrate bioactive compounds in their bark that are used by humans as natural remedies and / or for the industrial production of medicines.

Quinine, for example, is a compound produced by the cinchona tree. For many years it was the only drug used by doctors worldwide to treat malaria. Cinnamon, in addition to its organoleptic properties, also has healing and calming power.

Traditional medicine uses the bark of the ceibo as a healing, analgesic, antidiarrheal, anticonvulsant and disinfectant. Holm oak is used to treat intestinal inflammations, diarrhea, angina, pharyngitis and also as a healing, disinfectant, digestive, among other uses.

The bark of some trees also appears to have properties that inhibit the growth of cancer cells. Examples of this are: the bark of fir, mahogany, yew, bluewood, gaviola or catuche and many other species.

Other uses

The bark cloth is a material made from the bark of plants of the Moraceae family and is used for the manufacture of clothing. The artisanal elaboration of these fabrics was very popular in some countries of Asia, Africa and Eastern Europe. Today it is a very restricted practice.

The bark of the trees is also useful in making compost and paper, obtaining insecticides, making canoes, making ornamental items, and a host of other uses.

References

1. MW Nabors (2004). Introduction to Botany. Pearson Education, Inc.
2. Lira. Peridermis. Recovered from lifeder.com.
3. Bark (botany). Recovered from en.wikipedia.org.
4. WC Dickison (2000). Integrative Plant Anatomy, Academic Press.
5. Use of the bark of the tree. Recovered from monografias.com
6. Tree Bark. Recovered from biologydictionary.net.
7. Cinnamomum verum. Recovered from es.wikipedia.org.
8. KB Sandved, TP Ghillean & AE Prance (1993). Bark: the formation, characteristics, and uses of bark around the world.