Some Meristems Arise during Postembryonic Development

The root and shoot apical meristems formed during embryogenesis are called primary meristems. After germination, the activity of these primary meristems generates the primary tissues and organs that constitute the primary plant body.

Most plants also develop a variety of secondary meristems during postembryonic development. Secondary meristems can have a structure similar to that of primary meristems, but some secondary meristems have a quite different structure. These include axillary meristems, inflorescence meristems, floral meristems, intercalary meristems, and lateral meristems (the vascular cambium and cork cambium). (Inflorescence and floral meristems will be discussed in Chapter 24.):

(A) Leaf primordia

(A) Leaf primordia

L3, with randomly L1 and L2, with anticlinal oriented cell divisions cell divisions

FIGURE 16.13 The shoot apical meristem generates the aerial organs of the plant. (A) This longitudinal section through the center of the shoot apex of Coleus blumei shows the layered appearance of the shoot apical meristem. Most cell divisions are anticlinal in the outer L1 and L2 layers, while the planes of cell divisions are more randomly oriented in the L3 layer. The outermost (L1) layer generates the shoot epidermis; the L2 and L3 layers generate internal tissues. (B) The shoot apical meristem also has cytohistolog-

L3, with randomly L1 and L2, with anticlinal oriented cell divisions cell divisions

FIGURE 16.13 The shoot apical meristem generates the aerial organs of the plant. (A) This longitudinal section through the center of the shoot apex of Coleus blumei shows the layered appearance of the shoot apical meristem. Most cell divisions are anticlinal in the outer L1 and L2 layers, while the planes of cell divisions are more randomly oriented in the L3 layer. The outermost (L1) layer generates the shoot epidermis; the L2 and L3 layers generate internal tissues. (B) The shoot apical meristem also has cytohistolog-

ical zones, which represent regions with different identities and functions. The central zone contains the stem cells, which divide slowly but are the ultimate source of the tissues that make up the plant body. The peripheral zone, in which cells divide rapidly, surrounds the central zone and produces the leaf primordia. A rib zone lies below the central zone and generates the central tissues of the stem. (A ©J. N. A. Lott/Biological Photo Service.)

• Axillary meristems are formed in the axils of leaves and are derived from the shoot apical meristem. The growth and development of axillary meristems produces branches from the main axis of the plant.

• Intercalary meristems are found within organs, often near their bases. The intercalary meristems of grass leaves and stems enables them to continue to grow despite mowing or grazing by cows.

• Branch root meristems have the structure of the primary root meristem, but they form from pericycle cells in mature regions of the root. Adventitious roots also can be produced from lateral root meristems that develop on stems, as when stem cuttings are rooted to propagate a plant.

• The vascular cambium (plural cambia) is a secondary meristem that differentiates along with the primary vascular tissue from the procambium within the vascular cylinder. It does not produce lateral organs, but only the woody tissues of stems and roots. The vascular cambium contains two types of meristematic cells: fusiform stem cells and ray stem cells. Fusiform stem cells are highly elongated, vacuolate cells that divide longitudinally to regenerate themselves, and whose derivatives differentiate into the conducting cells of the secondary xylem and phloem. Ray stem cells are small cells whose derivatives include the radially oriented files of parenchyma cells within wood known as rays.

• The cork cambium is a meristematic layer that develops within mature cells of the cortex and the secondary phloem. Derivatives of the cork cambium differentiate as cork cells that make up a protective layer called the periderm, or bark. The periderm forms the protective outer surface of the secondary plant body, replacing the epidermis in woody stems and roots.

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