Corn Root Cortex

Figure 5.2 A longitudinal section through a dicot root tip. A. Regions of the root. B. Locations of the primary meristems of the root.

the edges of this inverted cup-shaped zone, located a short distance behind the actual base of the meristem. Here the cells divide every 12 to 36 hours, while at the base of the meristem, they may divide only once in every 200 to 500 hours. The divisions are often rhythmic, reaching a peak once or twice each day, usually toward noon and midnight, with relatively quiescent intermediate periods. Cells in this region are mostly cubical, with relatively large, more or less centrally located nuclei and a few very small vacuoles.

In both roots and stems, the apical meristem soon subdivides into three meristematic areas: (1) the protoderm gives rise to an outer layer of cells, the epidermis; (2) the ground meristem, to the inside of the protoderm, produces parenchyma cells of the cortex; (3) the procambium, which appears as a solid cylinder in the center of the root, produces primary xylem and primary phloem (Fig. 5.3). Pith (parenchyma) tissue, which originates from the ground meristem, is generally present in stems but is absent in most dicot roots. Grass roots and those of most other monocots, however, do have pith tissue.

The Region of Elongation

The region of elongation, which merges with the apical meristem, usually extends about 1 centimeter (0.4 inch) or less from the tip of the root. Here the cells become several times their original length and also somewhat wider. At the same time, the tiny vacuoles merge and grow until one or two large vacuoles, occupying up to 90% or more of the volume of each cell, have been formed. Only the root cap and apical meristem are actually pushing through the soil, since no further increase in cell size takes place above the region of elongation. The usually extensive remainder of each root remains stationary for the life of the plant. If a cambium is present, however, there normally is a gradual increase in girth through the addition of secondary tissues produced by the cambium.

The Region of Maturation

Most of the cells mature, or differentiate, into the various distinctive cell types of the primary tissues in this region, which is sometimes called the region of differentiation, or root-hair zone. The large numbers of hairlike, delicate protuberances that develop from many of the epidermal cells give the root-hair zone its name. The protuberances, called root hairs, which absorb water and minerals, adhere tightly to soil particles (Fig. 5.4) with the aid of microscopic fibers they produce and greatly increase the total absorptive surface of the root. Differentiation is discussed further in Chapter 11.

The root hairs are not separate cells; rather, they are tubular extensions of specialized epidermal cells. In fact, the nucleus of the epidermal cell to which each is attached often moves out into the protuberance. They are so numerous that they appear as fine down to the naked eye, typically numbering more than 38,000 per square centimeter (250,000 per square inch) of surface area in roots of plants such as corn;

Primary Xylem

epidermis cortex endodermis pericycle primary phloem primary xylem patch pith

Figure 5.3 Across section of a portion of a root of greenbrier (Smilax), a monocot, x500. (Photomicrograph by G. S. Ellmore)

epidermis cortex endodermis pericycle primary phloem primary xylem patch pith

Figure 5.3 Across section of a portion of a root of greenbrier (Smilax), a monocot, x500. (Photomicrograph by G. S. Ellmore)

Root Hair Cell Photomicrograph

developing root hair root hair air space epidermal cell cell of cortex developing root hair root hair air space epidermal cell cell of cortex

Hair Root Pictures
Figure 5.4 A. A radish (Raphanus) seedling shortly after germination, showing the root hair zone. B. A diagram of an enlargement of a longitudinal section through a small portion of a root hair zone, showing root hairs in contact with soil particles.

they seldom exceed 1 centimeter (0.4 inch) in length. A single ryegrass plant occupying less than 0.6 cubic meter (20 cubic feet) of soil was found to have more than 14 billion root hairs, with a total surface area almost the size of a football field.

When a seedling or plant is moved, many of the delicate root hairs are torn off or die within seconds if exposed to the sun, thereby greatly reducing the plant's capacity to absorb water and minerals in solution. This is why plants should be watered, shaded, and pruned after transplanting until new root hairs have formed. In any growing root, the extent of the root-hair zone remains fairly constant, with new root-hairs being formed toward the root cap and older root-hairs dying back in the more mature regions. The life of the average root-hair is usually not more than a few days, although a few live for a maximum of perhaps 3 weeks.

The cuticle (see Fig. 4.11), which may be relatively thick on the epidermal cells of stems and leaves, is thin enough on the root hairs and epidermal cells of roots in the region of maturation (Fig. 5.5) to allow water to be absorbed but still sufficient to protect against invasion by bacteria and fungi.

70 Chapter 5

Greenbrier Root Diagram

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Responses

  • clifton
    Where is the pith in a greenbrier cell?
    8 years ago
  • Hending
    How hyaluronic acid protects against bacterial invasion?
    8 years ago
  • giovanna
    Does primary xylem or pith occupy the center of root corn?
    8 years ago
  • patricia
    Where is the endodermis in the corn root?
    7 years ago
  • leah sanger
    Why pith is absent or reduced in the primary dicot root?
    6 years ago
  • jaana
    Where is the mature xulem and phloem on the root tip of corn?
    2 years ago

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