Plant roots

Overview

The root is typically the part of the plant that grows into the soil, although it can be aerial in waterlogged soil. Roots have two main functions, anchoring the plant to the ground, and absorbing nutrients, water, and minerals for the plant. There are two main types of roots, tap roots and fibrous roots, both of which are explained in this page. Plant root systems can be very extensive, and are harder to study than the above ground biomass. Current methods for studying root systems include: the harvest method, isotopic analysis, root ingrowth, and rhizotrons. Roots can often have symbiotic relationships with ectomychorrhizal fungi and Arbuscular Mycorrhizal Fungi.

Parts of the plant root

Parts of the Root. Source: Biology Junction

Root Cross Section. Source: Encyclopædia Britannica, Inc.

Root Hairs

The root hairs are thin hairlike structures growing from the epidermis. These help with the absorption of moisture and nutrients from the soil, which is then transported to the rest of the plant. The majority of plant water absorption happens with the root hairs. The length and shape allows them to have a large surface area while being able to go between soil particles, both of which helps with water absorption. In legume plants, they are involved in root nodule formation.

Xylem

In vascular plants, xylem transports nutrients and water in a sap from the roots to the stem and leaves. It uses passive transportation, so it does not need an input of energy to operate. The xylem is primarily composed of dead cells, and can only flow upward. This movement is mainly driven by negative pressures.

Phloem

In vascular plants, phloem transports the products of photosynthesis in a sap from the chloroplast down to the roots or storage structures. The sap holds a lot of sucrose, but is water-based. The phloem is primarily composed of living cells and is able to flow in many different directions. It's flow is called translocation, and is mainly caused by positive hydrostatic pressures.

Pericycle

The pericycle is made up of sclerenchyma or parenchyma cells in a cylindrical shape. In dicots, it gives protection to vascular bundles and strengthens the roots. In eudicots, it can create lateral roots, which grow horizontally and help anchor the plant.

Endodermis

The endodermis is the innermost layer of the cortex. The outer ring of the epidermis is deposited with the casparian strip, which helps stop the flow of water from around the cell membranes. This helps to regulate the water that flows into or out of the xylem, and stops gas bubbles from reaching the xylem.

Apical meristem

The apical meristem is full of actively dividing cells. It allows for primary growth, where the plant grows up and down.

Root cap

The root cap protects the growing apical meristem by secreting a mucus that eases the movement of the root through the soil.

Epidermis

The epidermis is the outerlayer of cells on the root. It absorbs water and nutrients, regulates gas exchange, stops water loss, and puts out metabolic compounds. It is covered in stomata, which is a pore that regulates water vapor and gas exchange.

Types of plant roots

Types of Roots. Source: National Gardening Association, Inc.

Tap Roots

Tap roots typically are large roots that grow downward, which other roots laterally sprout from. Some tap roots will persist for the entire plant life, but most plants will replace them with a fibrous system. Dicots are an example of plants that start with a tap root system. In some plants, like carrots, the tap root is later developed into an organ for storage. Most plants replace the tap root with the fibrous root because the tap roots grows from the radicle of the plant, which often dies after germination, forcing it to switch.

Fibrous Roots

Fibrous roots grow out of the stem and are made up of many thin branching roots. This can spread widely in the soil, sometimes becoming larger than the above ground plant. This type of root system can be seen in monocots, such as grasses.

Adventitious Roots

Although tap roots and fibrous roots are the primary root system seen, adventitious roots are sometimes seen. These roots arise from the stem or a leaf, often being seen on plants that have an underground stem. Some of these roots are aerial, being completely or partially above the soil. Examples of plants that have these roots are mangroves, bamboo, and corn.

Other Specialized Roots Systems

There are various other specialized root systems that are not as common to see. For example, some parasitic plants have root projections called haustoria, which penetrate the hosts tissues and absorb its nutrients. Another example of a specialized root system is pneumatophores, which can be seen on swamp plants. These roots are aerial and allow for gas exchange in the water-logged soil.

Citations

Gyssels, G., et al. “Impact of Plant Roots on the Resistance of Soils to Erosion by Water: a Review.” Progress in Physical Geography, vol. 29, no. 2, 2005, pp. 189–217.
Cannon, William Austin. “A Tentative Classification of Root Systems.” Ecology, vol. 30, no. 4, 1 Oct. 1949, pp. 542–548. 
Glinski, J. Soil Physical Conditions and Plant Roots. CRC Press, 2018.
Russell, Robert Scott. Plant Root Systems: Their Function and Interaction with the Soil. English Language Book Society and McGraw-Hill, 1982.
Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.
Beeckman, Tom; De Smet, Ive (2014). "Pericycle". Current Biology. 24 (10): R378–9
Britannica, The Editors of Encyclopaedia. “Root.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 13 Apr. 2018