In most plants, the root system is a below-ground structure that serves primarily to anchor the plant in the soil and take up water and minerals. Roots may be less familiar than the more visible flowers, stems, and leaves, but they are no less important to the plant.
Roots have four regions: a root cap; a zone of division; a zone of elongation; and a zone of maturation (Figure 1). The root cap is a cup-shaped group of cells at the tip of the root which protects the delicate cells behind the cap as it pushes through the soil. The root cap secretes mucigel, a substance that acts as a lubricant to aid in its movement. The root cap also plays a role in a plant’s response to gravity. If a flower pot is placed on its side, the stem would grow upward toward the light, and the root cap would direct the roots to grow down-ward. Above the root cap is the zone of division, and above that is the zone of elongation. The zone of division contains growing and dividing meristematic cells. After each cell division, one daughter cell retains the properties of the meristem cell, while the other daughter cell (in the zone of elongation) elongates sometimes up to as much as 150 times. As a result, the root tip is literally pushed through the soil.
In the zone of maturation, cells differentiate and serve such functions as protection, storage, and conductance. Seen in cross section, the zone of maturation of many roots has an outer layer (the epidermis), a deeper level (the cortex), and a central region that includes the conducting vascular tissue.
The epidermis is usually a single layer of cells at the outer edge of the root, which absorbs water and dissolved minerals, a function greatly facilitated by the presence of root hairs. Root hairs form from the outward growth of epidermal cells and are restricted to a small area near the root tip. A single four-month-old rye plant was estimated to have approximately 14 billion root hairs (Figure 2).
The cortex occupies most of the volume of young roots, and is important for storing substances such as starch.
At the root’s center is the region of vascular tissue which functions in the transport of water up the root and into the stem (in xylem tissue), and in the transport of carbohydrates and other substances from the stem down into the root (in phloem tissue). Cells in the xylem and phloem either attach to each other end-to-end or are tapered, with overlapping walls, facilitating the movement of substances from cell to cell. In many plants, a single cluster of xylem and phloem cells occupies a relatively small area of the root cross section. In other plants, a cylinder of vascular tissue forms a ring around a center of relatively undifferentiated cells, called the pith.
Roots often form symbiotic associations with soil fungi called mycorrhizae. In this association, the plant benefits from phosphorus that is taken up and supplied by the fungus, and the fungus benefits from carbohydrates produced by the plant. Plants grown in the absence of soil mycorrhizae generally do less well than when mycorrhizae are present.
Another symbiotic root association is between plants such as peas and beans (family Leguminosae) and Rhizobium bacteria. The bacteria penetrate the root cells, multiply, and in doing so form nodules where the bacteria have access to carbohydrates synthesized by the plant. In return, the bacteria “fix” nitrogen, converting nitrogen gas from the atmosphere into nitrogen-containing compounds that can be used by plants.
In most trees and wildflowers, one root, the taproot, is more prominent than the other fibrous roots. The taproot is usually relatively large in diameter and extends more deeply than the plant’s other roots, and often has additional lateral roots.
Other plants, particularly grasses, have fibrous root systems formed from many roots of more or less equal size. In general, taproots extend more deeply than fibrous roots, with fibrous roots occupying a greater proportion of the upper soil layers.
Plants may also form other types of roots, such as buttress roots, which form large above-ground support structures such as the lower trunks of plants like the bald cypress and some fig trees. Buttress roots are especially useful in supporting these trees in moist soil. Prop roots arise either from the lower stem (as in corn) or from lower branches (as in red mangrove, banyan, and certain palms), and provide extra stability for these shallow-rooted plants. Climbing plants (such as ivy) produce roots that aid in attaching the plant to other plants, buildings, and walls. Other air roots,
such as those found in mangroves, grow up out of the oxygen-deprived mud in which these plants typically grow and aid in the uptake of oxygen. This growth is unusual for roots, for these roots grow away from the force of gravity, rather than toward it. Perhaps the most unusual root system is that of the flower-pot plant, whose roots grow into a hollow structure formed from the plant’s own modified leaves. This hollow structure collects rainwater, which the roots then absorb.
Carrots, sugar beets, turnips, and cassava are all roots specialized for the storage of carbohydrates. These compounds are stored over winter by the plant for use in the following growing season.
Onions, garlic, potatoes, and ginger grow underground but are not roots; rather, they are stem tissue
Cortex— The root cortex is a relatively soft tissue that occurs between the epidermis and the internal, vascular tissues. Functions primarily in storage and in movement of water into the vascular cylinder.
Epidermis— The outermost and usually single layer of cells in the root. Gives rise to root hairs.
Fibrous root system— A root system comprised of many roots of approximately equal size. Fibrous roots are found primarily in the upper horizons of the soil.
Meristem— A group of cells whose primary function is cell division. Divisions result in one daughter cell that continues to function as a meristem cell and one daughter cell that differentiates into a different cell type.
Mucigel— A polysaccharide produced by roots that aids root penetration, inhibits desiccation, and increases absorption.
Taproot— The dominant root formed by most plants, and from which additional lateral roots arise.
modified to serve a storage function. A root is defined by its structure, rather than its function.
Roots penetrate, bind, and stabilize the soil, helping to prevent soil erosion. Roots also stimulate the growth of soil micro- and macroorganisms, compact the soil, alter soil chemistry through their secretions, and add organic material upon their death.
Gregory, Peter. Plant Roots: Growth, Function, and Interactions with the Soil. Boston: Blackwell Press, May 2006.
Sumner, Judith. American Household Botany: A History of Useful Plants. Portland, OR: Timber Press, September 2004.
Waisel, Yoav, Amram Eshel, and Uzi Kafkafi, eds. Plant Roots: The Hidden Half. 3rd ed. Cambridge, UK: CRC, March 2002.
Steven B. Carroll