Root System | Research & Encyclopedia Articles

Root System

The root system, an organ of higher plants that is usually found underground, has several functions, including anchoring of a plant in the soil and absorption and transportation of nutrients and water into the plant. Roots may also store food and function in asexual reproduction.

The first root that forms is known as the radicle. It elongates during germination of the seed and forms the primary root. In many plants, root systems comprise a branching mass of similar-sized roots leading off into the soil in all directions. These fibrous roots (i.e., the roots of grasses) provide a large surface area for absorption of nutrients and water. Fibrous roots hold soil in place and prevent erosion. Some plants, however, (i.e., beets and carrots), have a primary food storage root, which is called a taproot because it is much larger than the secondary roots and penetrates deeper into the soil. Taproots may be fleshy, like that of a carrot, or woody, like a tree root. Some plants with a taproot cannot be transplanted, because if the taproot is broken, the plant may die. Adventitious roots (roots that occur in an unusual position), may arise from nodes, or junctions, on the stem, such as at the base of a corn stem. Adventitious roots may also form high on a stem. These roots, called aerial stems or prop roots, form above ground level and go down into the soil. Adventitious roots serve to support the stem and are found on the banyon and the mangrove. Aerial roots (i.e., roots of epiphytic orchids and mistletoe) are not in contact with the ground at all but absorb water from sources available above ground (i.e., from the air or from other plants). Underground plant parts such as tubers (i.e., potatoes), bulbs (i.e., onions), and corms (i.e., gladioli) are not roots but modified stems that store food.

A root comprises three types of tissue: the epidermis, or surface layer; the cortex, or root wall where water and food are stored; and the vascular core, located at the center of the root, which carries food and water into the stem. Root hairs are modified long, fine, tube-like epidermis cells that act as the absorptive surface of the root and also anchor the root to soil particles. Once absorbed, water and nutrients pass through the cortex to the vascular core where they are transported upward by xylem and phloem, which are complex tissues in the vascular system of higher plants. In a stem, xylem and phloem are grouped together in vascular bundles; but in a root, there is a central core of xylem with radial bands that extend outward toward the cortex. In between the bands are strands of phloem. In underground roots, the xylem core is solid; aerial roots can have a central zone of pith (spongy tissue, used to store nutrients).

Roots grow downward into the soil under the influence of gravity and the presence of water. However, if more water is available at the surface, downward growth will be inhibited. The tip of the root, which is protected from hard soil grains by a protective cap, penetrates into the soil through cell division and by the elongation of cells. In addition to the primary growth in length, a secondary growth occurs, in which xylem is added to the inside of the root, and phloem to the outside. This phloem helps form bark, which covers adult roots.

Roots help plants adapt to yearly cold seasons. In perennial plants, the whole plant survives the winter season (i.e., conifers), though in deciduous perennials, the foliage is shed and the plants use food reserves accumulated during other seasons that are stored in the stems and roots (i.e., flowering trees in the temperate zone). However, in herbaceous perennials (i.e., asparagus and dandelions), only the roots and a small piece of underground stem survive the winter, using reserve food stored in the underground parts to survive and to start growth in the spring. In biennial plants (i.e., the carrot), leaves die off during the first winter after they have manufactured extensive food reserves that are stored in bulky roots. In the spring, a new plant develops from the roots and portions of stems that survived the winter. The second year, the plant flowers and forms seeds. By the second winter, the whole plant, including the roots, dies, while only the seeds survive to initiate a new two-year cycle. In annual plants, the plant flowers and produces seeds every year. The whole plant dies in the fall, and its seeds give rise to a new generation the following spring.

Roots of many plants are edible and contain food materials, especially starch. Important root crops include beets, turnips, sweet potatoes, carrots, parsnips, and cassava.