Root Structures

Root structures are adapted for several functions. Study Figure 29-5. Notice that the root tip is covered by a protective root cap, which covers the apical meristem. The root cap produces a slimy substance that functions like lubricating oil, allowing the root to move more easily through the soil as it grows. Cells that are crushed or knocked off the root cap as the root moves through the soil are replaced by new cells produced in the apical meristem, where cells are continuously dividing.

Root hairs, which are extensions of epidermal cells, increase the surface area of the root and thus increase the plant's ability to absorb water and minerals from the soil. Root hairs are shown in Figure 29-5. Most roots form symbiotic relationships with fungi to form a mycorrhiza. The threadlike hyphae of a mycorrhiza also increase the surface area for absorption. The spreading, usually highly branched root system increases the amount of soil that the plant can "mine" for water and mineral nutrients and aids in anchoring the plant. The large amount of root parenchyma usually functions in storage and general metabolism. Roots are dependent on stems and leaves for their energy, so they must store starch to use as an energy source during periods of little or no photosynthesis, such as winter.

Primary Growth in Roots

Roots increase in length through cell division, elongation, and maturation in the apical meristem in the root tip (shown in Figure 29-5 on the previous page). Dermal tissue matures to form the epidermis, which is the outermost boundary of the root. Ground tissue in roots matures into two different, specialized regions: the cortex and the endodermis. The cortex is located just inside the epidermis, as you can see below in Figure 29-6. This largest region of the primary root is made of loosely packed parenchyma cells.

The innermost boundary of the cortex is the endodermis (EN-doh-DUHR-mis), also shown in Figure 29-6. Endodermal cell walls contain a narrow band of a waterproof substance that stops the movement of water beyond the endodermal cells. To enter farther into the root than the endodermis, water and dissolved substances must pass through a selectively permeable membrane. Once past this membrane, dissolved substances can move from cell to cell via small channels in cell walls that interconnect the cytoplasm of adjoining cells, including those of the endodermis. The endodermis thus controls the flow of dissolved substances into the vascular tissue of the root. The endodermis also prevents dissolved substances from backing into the cortex.

Vascular tissue in roots matures into the innermost core of the root. In most dicots and gymnosperms, xylem makes up the central core of the root, as shown in Figure 29-6a. Dicot root xylem usually forms an X-shaped structure with pockets of phloem between the xylem lobes. In monocots, the center of the root usually contains a pith of parenchyma cells, as Figure 29-6b shows. Monocot root xylem occurs in many patches that circle the pith. Small areas of phloem occur between the xylem patches.

www.scilinks.org Topic: Root Structures Keyword: HM61330

Maintained by the ,i j National Science I. \ J\ .3. Teachers Association

www.scilinks.org Topic: Root Structures Keyword: HM61330

Maintained by the ,i j National Science I. \ J\ .3. Teachers Association figure 29-6

(a) This cross section of a dicot root shows the arrangement of vascular tissue and ground tissue. Note how the xylem tissue forms an "X." The cortex and the endodermis, which are composed of ground tissue, surround the vascular tissue. (b) This cross section of a monocot root shows a prominent endodermis, the innermost boundary of the cortex. The center of the root, called the pith, is made up of parenchyma cells.

Li--Pericycle

(a) DICOT ROOT

Epidermis

Cortex

Endodermis

Li--Pericycle

Pith

Xylem Phloem

Epidermis

Cortex

Endodermis

Pith

Xylem Phloem

(b) MONOCOT ROOT

(a) DICOT ROOT

(b) MONOCOT ROOT

figure 29-7

The vascular tissues of a primary root are surrounded by the pericycle, a tissue that produces lateral roots. The pericycle is also shown in Figure 29-6 on the previous page.

Epidermis Cortex

Endodermis

Lateral root Vascular tissues Pericycle

Epidermis Cortex

Endodermis

Lateral root Vascular tissues Pericycle

Observing Roots

Materials wilted radish seedlings, hand lens, Petri dish, water, pipet Procedure

1. Place the wilted radish seedlings in a Petri dish. Observe them with the hand lens. Record your observations.

2. Using a pipet, cover only the roots with water. Observe the seedlings with the hand lens every 5 minutes for 15 minutes. Record each of your observations.

3. Use the hand lens to observe the roots. Draw and label what you see through the lens.

Analysis What happened to the wilted seedlings when you put them in water? How can you explain what happened? Describe two functions of a root.

The outermost layer or layers of the central vascular tissues is termed the pericycle (PER-i-siE-kuhl). Lateral roots are formed by the division of pericycle cells. The developing lateral root connects its vascular tissues and endodermis to those of the parent root. Figure 29-7 shows how a lateral root grows out through the parent root's endodermis and cortex, finally emerging from the epidermis.

Secondary Growth in Roots

Dicot and gymnosperm roots often experience secondary growth. Secondary growth begins when a pericycle and other cells form a vascular cambium between primary xylem and primary phloem. The vascular cambium produces secondary xylem toward the inside of the root and secondary phloem toward the outside. The expansion of the vascular tissues in the center of the root crushes all the tissues external to the phloem, including the endodermis, cortex, and epidermis. A cork cambium develops in the pericycle, replacing the crushed cells with cork.

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