How Roots Develop

When a seed germinates, the tiny, rootlike radicle, a part of the embryo (immature plantlet) within it, grows out and develops into the first root. The radicle may develop into a thick, tapered taproot, from which thinner branch roots arise, or many adventitious roots may arise from the stem, which is attached to the radicle and continuous with it. Adventitious roots are those that do not develop from another root but develop instead from a stem, leaf, or other plant part. A fibrous root system, which may have large numbers of fine roots of similar diameter, then develops from the adventitious roots (Fig. 5.1). Many mature plants have a combination of taproot and fibrous root systems.

The number of roots produced by a single plant may be prodigious. For example, a single mature ryegrass plant may have as many as 15 million individual roots and branch roots, with a combined length of 644 kilometers (400 miles) and a total surface area larger than a volleyball court, all contained within 0.57 cubic meter (20 cubic feet) of soil. Root hairs (discussed in the section "The region of maturation") greatly increase the total surface area of the root.

Most dicotyledonous plants (e.g., peas and carrots, whose seeds have two "seed leaves"—commonly referred to as dicots) have taproot systems with one, or occasionally more, primary roots from which secondary roots develop (see the discussion of primary and secondary tissues in

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