Root Structure

Close examination of developing young roots usually reveals four regions or zones. Three of the regions are not sharply defined at their boundaries. The cells of each region gradually develop the form of those of the next region, and the extent of each region varies considerably, depending on the species involved. These regions are called (1) the root cap, (2) the region of cell division, (3) the region of elongation, and (4) the region of maturation (Fig. 5.2).

The Root Cap

The root cap is composed of a thimble-shaped mass of parenchyma cells covering the tip of each root. It is quite large and obvious in some plants, while in others, it is nearly invisible. One of its functions is to protect from damage the delicate tissues behind it as the young root tip pushes through often angular and abrasive soil particles. The root cap has no equivalent in stems. The dictyosomes of the root cap's outer cells secrete and release a slimy substance that lodges in the walls and eventually passes to the outside. The cells, which are replaced from the inside, constantly slough off, forming a slimy lubricant that facilitates the root tip's movement through the soil. This mucilaginous lubricant also provides a medium favorable to the growth of beneficial bacteria that add to the nitrogen supplies available to the plant (see the discussion of the nitrogen cycle in Chapter 25).

The root cap, whose cells have an average life of less than a week, can be slipped off or cut from a living root, and when this is done, a new root cap is produced. Until the root cap has been renewed, however, the root seems to grow randomly instead of more or less downward, suggesting that the root cap also functions in the perception of gravity (see gravitropism in Chapter 11). It is known that amyloplasts (plastids containing starch grains) act as gravity sensors, collecting on the sides of root-cap cells facing the direction of gravitational force. When a root that has been growing vertically is artificially tipped horizontally, the amyloplasts tumble or float down to the "bottom" of the cells in which they occur. The root begins growing downward again within 30 minutes to a few hours. The exact nature of this gravitational response is not known, but there is some evidence that calcium ions known to be present in the amyloplasts influence the distribution of growth hormones in the cells.

The Region of Cell Division

Cells in the region of cell division, which is composed of an apical meristem (a tissue of actively dividing cells) in the center of the root tip, produce the surrounding root cap. Most of the cell divisions take place next to the root cap at

Chapter 5

root cap root cap

Zone Elongation Maturation

region of maturation region of elongation apical meristem (region of cell division)

region of maturation region of elongation apical meristem (region of cell division)

root cap

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Responses

  • ella
    Why the response is taken place in root zone region and not root cap?
    8 years ago
  • simone
    Where in the root tip is there cell division'?
    7 years ago
  • stephan
    What protects the tip of the root as it pushes through the abbrasive soil particles?
    4 years ago

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