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Fibrous root systems consist of a mass of similarly sized roots

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Most monocots (including grasses) have a fibrous root system consisting of an extensive mass of similarly sized roots (fig. 7.2). In these plants, the radicle is short-lived and is replaced by a mass of adventitious roots (from the Latin word adventicius, meaning "not belonging to"), which are roots that form on organs other than roots. Fibrous roots of a few plants are edible; for example, sweet potatoes are fleshy parts of fibrous root systems of Ipomoea batatas. Some plants have two

FIGURE 7.1

Taproot system of dandelion (Taraxacum), consisting of a prominent taproot and smaller branch roots.

FIGURE 7.1

Taproot system of dandelion (Taraxacum), consisting of a prominent taproot and smaller branch roots.

kinds of root systems; for example, clover (Trifolium) has a taproot and an extensive fibrous system of roots.

Corn plants have fibrous roots belowground and aboveground adventitious roots that help to support the tall upright stem (fig. 7.3). These adventitious roots, called prop roots, are common in tropical trees in areas where windstorms often topple unsupported trunks. Adventitious roots also grow from underground stems, called rhizomes, of ferns, club mosses (Lycopodium), and horsetails (Equisetum). In many plants, adventitious roots are a pri-

FIGURE 7.2

Fibrous root system of a grass. Fibrous root systems consist of many similarly sized roots that form extensive networks in the soil.

FIGURE 7.2

Fibrous root system of a grass. Fibrous root systems consist of many similarly sized roots that form extensive networks in the soil.

mary means of vegetative, or asexual, reproduction; that is, the formation of offspring without the fusion of sperm and egg but by growth and development of a vegetative organ such as a root, stem, or leaf. For instance, what appear to be many individual trees in a forest of quaking aspen (Populus tremuloides) are often a single large plant with many stems (a clone) spread by adventitious roots. We use adventitious roots on cuttings to propagate many plants (fig. 7.4), including raspberries, apples, cabbage, brussels sprouts, and many kinds of houseplants.

Adventitious roots form in all sorts of places on plants, including leaves, petioles (the thin stalk that attaches a leaf to a stem), and stems. The formation of adventitious roots is controlled by hormones such as auxin (see chapter 6), a form of which is the active ingredient in commercial rooting compounds, such as Rootone, that promote the propagation of cuttings. Although they arise at

FIGURE 7.3

Prop roots on corn (Zea mays) are adventitious roots that arise from the stem. As their name suggests, prop roots help support the plant.

FIGURE 7.3

Prop roots on corn (Zea mays) are adventitious roots that arise from the stem. As their name suggests, prop roots help support the plant.

FIGURE 7.4

Adventitious roots at the base of a plant cutting. Wounding induces parenchyma cells near the cut surface to become meristem-atic cells. These meristematic cells then divide and produce roots on the cutting.

FIGURE 7.5

Root tip (upper right) with its thimble-shaped root cap (lower left) removed (X10).

FIGURE 7.4

Adventitious roots at the base of a plant cutting. Wounding induces parenchyma cells near the cut surface to become meristem-atic cells. These meristematic cells then divide and produce roots on the cutting.

FIGURE 7.5

Root tip (upper right) with its thimble-shaped root cap (lower left) removed (X10).

different locations, primary roots and adventitious roots may have similar structures and functions.

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