2.3: Root Anatomy

The main function of roots is the absorption of water and dissolved nutrients. To understand how water moves from the soil to the root and from there to the stems and leaves, we need to look at the different internal layers (tissues) that are inside of the root. If you take a carrot root and lay it flat on a cutting board, then cut it into thin slices, in each slice, you are able to see different concentric areas or tissues (Figure \(\PageIndex{1}\)). A tissue is defined as a group of specialized cells that have a common function. This type of cut in botany is called a cross-section, and it allows us to have a top view of the tissues inside of a plant; in this case, a top view of the root tissues.

Figure \(\PageIndex{1}\): Cross-sections of carrot roots of different varieties. By DutraElliott is licensed under CC BY-NC-SA 4.0 via Flickr.

Starting from the outermost layer in a root the first thing we find is the cuticle, which is not a tissue, but a waxy substance that covers all external parts of a plant. Its main function is to protect the plant from water loss and bacterial or fungal infection. In the roots, the cuticle is thin to allow water absorption. After the cuticle we find the true tissues of the root, starting on the outside of the cross-section to the inside we find: epidermis, cortex, endodermis with Casparian strips, and the stele or vascular cylinder which is composed of pericycle, xylem and phloem (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Light micrograph of a wheat (Triticum) root cross section. Sclerenchyma cells of the exodermis and xylem cells stain red, and phloem cells stain blue. Other cell types stain black. The stele, or vascular tissue, is the area inside the endodermis (indicated by a green ring). Root hairs are visible outside the epidermis. Biology 2e by Openstax is licensed under CC BY 4.0.

Root tissues

Epidermis (epi = outside; dermis = skin) literally translates as outer skin, and it delimits the root, protecting the inner tissues from the outer environment and physical damage.It usually is one cell layer thick.

Cortex is the tissue just underneath the epidermis and it is several cell layers thick. The function of the cortex is to store food and water.

Endodermis (endo = inner; demis = skin) literally means the inner skin. It delimits the inner cylinder of the root and it is one single layer of cells. The cells of the endodermis are thicker than those in the cortex because the cell walls of the endodermal cells have lignin and suberin (structural plant compounds) that form bands called Casparian trips. These waterproof strips play an important role in the absorption of water in the roots, by not allowing the water and dissolved substances to pass in through the porous cell wall of the endodermal cells, but instead forcing the water to move to the inside of the endodermal cell to be transferred from there to the vascular cylinder. We will review this in more detail in the section below: Absorption of water and dissolved substances.

Pericycle is the tissue found just inside of the endodermis, commonly one cell thick and it is the first one found on the stele or vascular cylinder. Its main function is to produce lateral roots.

Xylem is the tissue in the middle of the root, usually looking as an X in young eudicots, and arranged in a ring around a central pith in monocots (Figure \(\PageIndex{3}\)). The xylem is responsible for water and dissolved nutrient transportation. It transports water upward from the roots to the leaves.

Phloem is the tissue that transports the carbohydrates (sugars) produced in photosynthesis throughout the plant. In roots it is usually found in between the xylem in the vascular cylinder.

Figure \(\PageIndex{3}\): In typical eudicots, the vascular tissue forms an X shape in the center of the root (left). In typical monocots, the phloem cells and the larger xylem cells form a characteristic ring around the central pith (right). Open Stax Biology 2e is licensed under CC BY 4.0.

In Biology all transport tissues are referred to as vascular tissues. For example, when we talk about vascular diseases in humans, we are referring to a disease that affects our vascular tissues: arteries and veins. In plants we also use the term vascular when referring to the plant transportation system. Instead of arteries and veins, plants have xylem and phloem. As we just learned, roots have a vascular cylinder, which in simple terms is a cylinder in the middle of the roots that has xylem and phloem. These two tissues are found in all plant organs: roots, stems, leaves, and reproductive organs.

Absorption of water and dissolved nutrients

Plants need water for several physiological processes required for their survival, such as photosynthesis and transpiration. The water in soil is absorbed by the roots and transferred to the xylem to be transported to other parts of the plant. Water moves from the soil to the root hairs through a process called osmosis. In osmosis, water moves through a plasma membrane from a place with more water to a place with less water. Because of the presence of salts and other minerals inside of the root hairs, water is less abundant inside of the cells than on the outside in the soil, where there are less salts and minerals: therefore the water present in the soil moves to the inside of the root hairs. If you remember, water is transported by the xylem. Do you remember where the xylem is located in a root? It is the innermost tissue, found in the middle of the vascular cylinder. This means that the water that is absorbed by the root hairs has to be moved through every single tissue of the root until it finally reaches the xylem, where it will be transported upward to the plant. After water is absorbed by the root hairs it can move through the root tissues using two different routes: between the porous cell walls of the cells (apoplastic transport) or directly from cell to cell (symplastic transport), through tiny pores that join the cells called plasmodesmata. We can trace the water movement from the root hairs to the cortex and then to the endodermis. But here water finds an impenetrable barrier, the Casparian strips, that impede the water and dissolved substances from moving in between the cell walls, forcing them to go inside of the cell instead. Through this process the plants are able to control which dissolved substances will be transferred to the vascular cylinder for further transportation.

In the process of water absorption, the roots also absorb other substances, like minerals and nutrients, that are dissolved in the water. As with any other organism, plants require certain nutrients to be able to grow and survive. We can divide them into macro and micronutrients. Macronutrients (macro = big) are essential components of plants and they are available in large quantities. Examples of macronutrients are phosphorus (P), nitrogen (N) and Potassium (K). Micronutrients are also needed by plants, but they are available in small quantities. Examples of micronutrients are iron (Fe), copper (Cu) and manganese (Mn).