The root tip. A root grows in length from an area at its apex (tip). This growth area is called the apical meristem. A meristem is any part of a plant where the cells divide rapidly, forming new cells continually. The apical meristem produces the root cap, a thimble-shaped group of cells. The root cap protects the delicate root tip from damage as the root grows in length and the tip pushes through the soil. The cells produced by the apical meristem are all small and nearly identical. In the region of elongation, just behind the apical meristem, the cells rapidly grow longer. Farther back lies the region of maturation.

Root System Modified Roots Function of Roots Characteristics of Roots Pictures of Roots The importance of trees Broadleaf and needleleaf trees

There, the cells differentiate--that is, they take on a different structure and appearance according to their functions in the mature root. The distance from the root cap to the region of maturation is only a few tenths of an inch or a few millimeters.

The outer tissues. The outer layer of cells of a root is called the epidermis. It serves as a sort of skin and protects the tissues beneath. Tiny, hairlike extensions called root hairs grow from the epidermis. The root hairs absorb most of the water and minerals that a plant takes in from the soil. In most kinds of plants, the root hairs live only a few days. They occupy the root hair zone, an area just above the root tip. This area is only a few tenths of an inch or a few millimeters long.

A thick layer of rounded cells called the cortex lies just inside the epidermis. These cells contain stored food and water. The inner layer of cells of the cortex makes up the endodermis.

The core, or stele, is the central portion of the root. Its outer layer of cells is called the pericycle. Branch roots grow from the pericycle. Inside the pericycle are two kinds of tissues, xylem and phloem. Xylem includes rows of dead, tubular cells called vessels, which conduct water and minerals up to the stem and leaves. Phloem consists largely of rows of long, living cells called sieve tubes. These cells transport food down from the leaves for use or storage by the root. In most roots, the xylem forms a pattern shaped like a star or the spokes of a wheel. The phloem lies between the points of the star or between the spokes.

Secondary tissues. All the tissues described so far have been primary tissues. Such tissues differentiate from cells created in the apical meristem. Many plants that live just one year have only primary tissues in their roots. But other plants, especially those that live more than one year, have secondary tissues in their roots in addition to primary tissues. The growth of primary tissue adds to the length of a root.