The Anatomy of a Tree Root Explained

A picture of tree roots above ground
Photo by engy ibrahim / Unsplash

Understanding the anatomy of trees is important for any Arborist. By understanding the anatomy, an Arborist can diognose problems easier.

All tree roots have the same basic structure. So, you will only need to learn the anatomy of a root once. Once learned, you can apply your knowledge to all species of trees.

In this article, I will describe the anatomy of a tree root. I will highlight each part of the root and the function of that part.

Parts of a Tree Root and Their Functions

The different parts that make up a root are:

  • Epidermis
  • Root Hairs
  • Root Cap
  • Quiescent Center
  • Cortical Cells
  • Endodermis
  • Pericycle
  • Lateral Root Primordium
  • Mature Vessels

Epidermis

The epidermis is the skin of the root. The epidermis protects the center of the root and helps in the absorption of water.

Root Hairs

For a definition of root hairs, let's look to Alex L. Shigo from his book, Modern Arboriculture:

Root hairs are extensions of single epidermal cells on nonwoody roots. The root hairs extend greatly the absorptive surface of the root. They are like heat radiation grills on a radiator. The hairs and epidermal cells of small nonwoody roots, and the soil and water and microorganisms surrounding them, make up the rhizosphere. The rhizosphere is a unique area where root, water, elements, and microorganisms all seem to "melt" together in a gel or "soup". The rhizosphere is a niche that serves the survival requirements of tree and associates. The root hairs may live for a few weeks or longer; but they soon become a part of the "soup". - Pg. 250

So, root hairs originate from the cells in the epidermis layer of a root. These hairs extend out increasing the roots surface area. By creating a bigger surface area, the roots can absorbs more matter from the soil.

The root hairs assist the tree in absorbing water and nutrients from the soil. Eventually, the root hairs die and become part of the surrounding soil.

Root Cap

Roots push through soil in search of nutrients and water. Soil can contain a range of particles that can damage roots. Thus, root tips need protecting as they travel through the soil.

The root cap will protect the tip and sensitive parts of a tree root. The cap forms a hard surface on the end of the root. This hard surface will allow the root to push through soil.

The root cap has other functions too:

  • Assists in gravity perception
  • Secretes a mucilage to help the root move through the soil
  • Can communicate with the soil microbiota

Another term for the root cap is calyptra.

Quiescent Center

The quiescent center was discovered in the 1950's. However, some details are still a mystery to us.

The general theory is the quiescent center contains stem cells. These stem cells have the ability to regenerate damaged parts of the root.

Here is what Britannica say:

The quiescent centre is a group of cells, up to 1000 in number, in the form of a hemisphere, with the flat face toward the root tip; it  lies at the centre of the meristem, in much the same position, in fact, as the tetrahedral apical cell in certain lower plants. The cells of the quiescent centre are unusual in that their division rate is lower than that in the surrounding meristem.

Furthermore:

When roots are damaged mechanically or by radiation, the cells of the centre can resume a rapid division rate, and then participate in regeneration.

Cortical Cells

The cortical cells make up the cortex of a tree root. The cortex is the section of a root between the skin and inner vascular system.

The cortical cells are loosely connected and can store different nutrients and substances. Some of these substances may include:

  • carbohydrates
  • resins
  • latex
  • essential oils
  • tannins

Endodermis

The endodermis is the inner skin of the tree root. The endodermis protects the vascular system from the outside world.

Here, in Applied Tree Biology, Andrew Hirons and Peter Thomas describe the function of the endodermis:

Water can pass between the cells of the cortex but, to get into the centre of the root, water must go through the cells of the endodermis; in this way the tree can control what gets into the root. - Pg. 142

So, the endodermis acts like a filter, controlling how much water enters the inner root. The Casparian strip is what helps make this layer impenetrable.

The endodermis contains suberin, which forms the Casparian strip. Suberin is a corky like substance.

Pericycle

The pericycle is the layer that sits beneath the endodermis. This layer is responsible for growing lateral roots.

Lateral roots will form from pericycle cells. The lateral roots will then penetrate through the outer layers of the tree root, growing outwards. Once the lateral root pushes through the epidermis, the lateral root will grow out into the soil.

Some fine lateral roots may not have root caps.

Lateral Root Primordium

The lateral root primordium is a mound of cells in the pericycle. This mound of cells becomes a lateral root.

Lalit M. Srivastava, in Plant Growth and Development: Hormones and Environment, describes:

Lateral root primordia arise by localized cell divisions in the pericycle, resulting in a mound of tissue. Further oriented divisions and cell enlargement in the mound give rise to an organized structure, the root primordium, which acquires its own root apex and root cap and grows through the parent root cortex, finally emerging as a lateral rootlet.

Mature Vessels

In the centre of a root, you will find the vascular system. The vascular system consists of:

  • Xylem
  • Phloem

The xylem and phloem transport nutrients and water throughout the tree. The vascular system begins in the roots and travels all the way up to the leaves.

The xylem is responsible for transporting water. And, the phloem is responsible for transporting glucose.

For a much deeper look into all aspect of tree roots, check out this article.

After learning the anatomy of a tree root, you may be interested in other parts of a tree. Check out this article to learn about the anatomy of a leaf: Leaf Anatomy 101: Everything You Need to Know

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