Root, the principal organ for the absorption of water and dissolved nutrients in most seed plants and in ferns. The root also anchors the plant body, and can store reserve foods. Roots usually grow beneath the surface of the soil and extend from the base of the stem. Some roots, however, grow above the ground from branches or leaves. These roots either penetrate the soil or become attached to another plant or object as they grow.Roots anchor a plant in the soil, absorb water and minerals, and store food.
The single root that develops from the seed (or from the base of a stem, in the case of ferns) is called the primary root. In some species, the primary root thickens and becomes the principal root of the plant. This kind of root system (found, for example, in carrot plants) is known as a taproot system. Tuberous roots bear clusters of tubers (thickened storage organs) that contain buds or eyes from which new plants grow. This type of root system is found in the dahlia and ranunculus buttercup.Taproot systems have a primary root that thickens and becomes the principal root of the plant.
If the primary root branches, the branching roots are called secondary roots. This kind of root system, usually found in grain crops such as wheat or rye, is known as a fibrous root system.Fibrous root systems hold topsoil in place and prevent soil erosion.
Roots growing aboveground from parts of the plant other than the stem base are called adventitious roots. There are two kinds of adventitious rootsprop roots and aerial roots. Prop roots, and many aerial roots, penetrate the soil. For example, prop roots of corn grow from upper parts of the stem down to the soil, and the aerial roots of poison ivy grow from horizontal branches down to the soil. Some aerial roots do not penetrate the soil, but become attached to other plants or objects instead. Plants with such roots are called epiphytes. Many orchids are epiphytes.Prop roots grow from upper parts of the stem down to the soil.Aerial roots grow aboveground from parts of the plant other than the stem base.
Root tissues can be classified in the order in which they are found within the root. As shown in the illustration Cross Section of a Root, a root has three layers:
the outermost layer of the root. It is composed of epidermal tissue and is one cell-layer thick. The epidermis serves as the protective covering of the root.
This is primarily a storage area for food and water. It is composed of storage tissues, air spaces, and the endodermis. The endodermis is the inner boundary of the cortex and is one cell-layer thick. The cells of this layer prevent water from moving into the cortex from the next layer.
The innermost area of the root contains vascular tissue, which is made up of xylem cells and phloem cells. Xylem cells transport dissolved minerals and water, and sometimes stored food, into the stem. Phloem cells carry food made in photosynthesis from the stem into the root. The pericycle is a single layer of cells surrounding the vascular tissue. Branch roots develop from cells of the pericycle.
The root is composed of several regions that give rise to the specialized cells needed in the formation of plant tissues. These regions, unlike the layers of the root, are not distinctly separated from each other but blend from one into the next. The regions, as shown in the illustration Root Tip, are, from bottom to top:
where in a growing root rapid cell division takes place. At the bottom of this region a cup-shaped root cap covers the apex (tip) and protects the young cells from injury as they are pushed down through the soil.
the region of cell growth. Here cells increase in size, but are not differentiated into specific tissues. The elongating cells exert a mechanical force on the root apex and push the root tip farther down into the soil.
The root hair zone is the area where root hairs grow. A root hair is a tubular outgrowth of an epidermal cell. Root hairs provide the plant with additional surface area for the absorption of water and minerals from the soil. Within the root hair zone is the region of maturation, where the cells differentiate. During differentiation the cells become specialized to perform various functions.
where epidermal, cortical, and vascular tissues form.
Many plants are economically important root crops. Taproots, such as the beet, carrot, parsnip, radish, turnip, sweet potato, salsify, and cassava (used in making tapioca), are used as food. Many fibrous and tap roots are used for flavorings or in making medicines. Goldenseal, jalap, licorice, and mandrake roots are used in flavoring medicines. Common food flavorings are obtained from the roots of such plants as ginger, horseradish, angelica, and sarsaparilla. Some of the roots used in making drugs are colchicum and ipecac. Fibrous root systems are also agriculturally important because they hold topsoil in place and prevent soil erosion.