Meristematic Tissues

Unlike animals, plants have permanent regions of growth called meristems, or meristematic tissues, where cells actively divide (Fig. 4.1). As new cells are produced, they typically are small, six-sided boxlike structures, each with a proportionately large nucleus, usually near the center, and with tiny vacuoles or no vacuoles at all. As the cells mature, however, they assume many different shapes and sizes, each related to the cell's ultimate function; the vacuoles increase in size, often occupying more than 90% of the volume of the cell.

Apical Meristems

Apical meristems are meristematic tissues found at, or near, the tips of roots and shoots, which increase in length as the apical meristems produce new cells. This type of growth is known as primary growth. Three primary meristems, as well as embryo leaves and buds, develop from apical meristems. These primary meristems are called protoderm, ground meristem, and procambium. The tissues they produce are called primary tissues. Note their locations in Figure 4.1; they are discussed in Chapters 5 and 6.

Lateral Meristems

The vascular cambium and cork cambium, discussed next, are lateral meristems, which produce tissues that increase the girth of roots and stems. Such growth is termed secondary growth.

Vascular Cambium

The vascular cambium, often referred to simply as the cambium, produces secondary tissues that function primarily in support and conduction. The cambium, which extends throughout the length of roots and stems in perennial and many annual plants, is in the form of a thin cylinder of mostly brick-shaped cells. The cambial cylinder often branches, except at the tips, and the tissues it produces are responsible for most of the increase in a plant's girth as it grows. The individual remaining cells of the cambium are referred to as initials, while their sister cells are called derivatives. The cambium and its cells and tissues are discussed in Chapters 5 and 6.

Tissues 55

apical meristem ground meristem protoderm procambium shoot vascular cambium cork cambium root apical meristem ground meristem protoderm procambium vascular cambium cork cambium cork cambium shoot root

Cork Cambium

vascular cambium root hairs ground meristem protoderm procambium root cap

Figure 4.1 A diagram of the longitudinal axis of a plant, showing the location of meristems.

cork cambium vascular cambium root hairs ground meristem protoderm procambium root cap

Figure 4.1 A diagram of the longitudinal axis of a plant, showing the location of meristems.

Cork Cambium

The cork cambium, like the vascular cambium, is in the form of a thin cylinder that runs the length of roots and stems of woody plants. It lies outside of the vascular cambium, just inside the outer bark, which it produces. The cork cambium is discussed in Chapters 5 and 6. The tissues laid down by the vascular cambium and the cork cambium are called secondary tissues, since they are produced after the primary tissues have matured.

Intercalary Meristems

Grasses and related plants have neither a vascular cambium nor a cork cambium. They do, however, have apical meristems, and, in the vicinity of nodes (leaf attachment areas), they have other meristematic tissues called intercalary meristems. The intercalary meristems develop at intervals along stems, where, like the tissues produced by apical meristems, their tissues add to stem length.

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Responses

  • Anette
    What tissues are produced by protoderm?
    8 years ago
  • Jan
    Do root hairs come from cork cambium?
    8 years ago

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