What is Tree Bark?
Bark, often called “tree bark” in botany, means the outer covering of wood plants. In contrast to herbaceous plants, woody plants form a complex framework of cells and fibers that offer considerable support and protection.
The bark is the woody exterior of this structure. Technically, the bark isn’t exactly a scientific term. The bark is considered to be a collection of several different outer layers of a wood plant.
It consists of tissues outside the vascular cambium or the central bundle of vascular cells. Unlike bark, these tissues are completely alive and transfer fluids from the roots to the leaves.
Function of Tree Bark
The bark, which includes everything from the vascular cambium to the outside of the plant, is much thicker than most people assume. The inner bark is made up of living tissues that help move the sugar produced in the leaves to other parts of the plant.
This happens in the secondary phloem. Outside the secondary phloem, the cells began to die and the layers began to compress. These layers are responsible for protection. The outermost layer, the periderm, consists of several layers of more compressed cells. Some of them are cork cells that are covered with a special type of wax and do not collapse when they die.
Trees use their outer bark for a variety of purposes, but mostly to protect against water loss and predators. Insects and herbivores want to eat the leaves of woody plants. These plants are often protected by thick bark that local herbivores can reach.
The outer bark that the compressed cork layers is also waterproof. This prevents the inner bark from drying out and ensures that the plant can continue to transport sugar from the leaves to where it is needed.
Structure of Tree Bark
As woody plants grow, they grow by adding cells to the inner layers. When the layers are pushed outward, they compress and the cells die. Bark forms as part of this process and is sometimes thought of as the entire outside of the vascular cambium.
The vascular cambium is the main growth layer in woody plants. When layers are added to the inner ring of the vascular cambium, the outside is pushed outward. The cells die and the fibrous matrix of cellulose and lignin molecules remains. This tough structure forms the bark and protects the tree or plant from many types of damage.
While the cortex is sometimes recognized as any tissue outside of the vascular cambium, others identify the rhytidoma as the cortex. The rhytidome is just the outermost layer of the plant. If you were to peel off part of the tree, the rhytidome would come loose first. This is what most non-scientists would call bark.
However, the cells underneath eventually turn to the cortex, and anatomically, there isn’t much of a difference. However, the rhytidome is completely dead.
Scientists have therefore called the rhytidome the outer cortex, while the secondary phloem and cortex are considered the inner cortex because they still have living cells and play a role in metabolite transport. Remember that all of these layers are outside the vascular cambium.
The following picture shows only the living tissue in a wood plant, excluding the outermost layer of rhytidome. As can be seen below, the periderm is also part of the bark and has further subdivisions within which different layers are represented. These layers create a barrier against bacteria and insects and prevent water and nutrients from leaking out of the plant.
The bark is thickest on the trunk of the plant. This is not only where plants are oldest, but also where they can suffer the greatest harm from herbivores, predators of plants. In fact, bark is 10-20% the weight of most woody plants. The bark not only resists damage from animals, but also prevents dehydration and disease, and withstands extreme temperatures.
Uses of Tree Bark
Bark has many commercial uses and it is often stripped from the heartwood being processed. Shingles and sidings can be made from the dead outer bark. The outer bark is also known as cork and can be ground to make cork products like bulletin boards, cork floors, and even specialty items like yoga mats.
Throughout history, bark has been used to make everything from boats to clapboards, as its waterproof nature remains until it decays. In the past, the inner rind was even used to make flour and bread, although the nutritional capacity pales in comparison to regular grain.
Some plant species also contain special substances in their bark that are good for making spices, sunblock, and insect repellants. The inner bark is an important commercial resource for resins, tannins, and even the precursors of products like latex gloves.
In agriculture, there is a technique in which the bark is peeled off under-ripe fruit. This keeps the sugar concentrated in the fruit and the harvest improves. This technique is known as girdling and is sometimes used to produce exceptionally large fruit.
When a branch is girded and all but one fruit on that branch is picked, the plant will incorporate all of the sugars and metabolites from the leaves on that branch into the one remaining fruit.