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Permanently embryonic tissue involved in cell division at the apices of plant roots and stems, and forming dynamic regions of growth. These apical meristems, usually consisting of small, densely cytoplasmic cells, become established during embryo development. Thereafter they divide, producing the primary plant body of root and shoot. Below the apical meristems, tissue differentiation begins: the protoderm gives rise to the epidermal system, the procambium to the primary vascular system, and the ground meristem to the pith and cortex (see illustration). Plant apical meristems have been the object of experiments on development similar to those carried out on animal embryos.
Root apical meristem is covered by a root cap, a region of parenchymatous, cells which has a protective function and is responsible for perceiving gravitational changes. Root tips have been shown to possess a central region, usually hemispherical, which consists of cells which rarely divide or synthesize deoxyribonucleic acid (DNA), and have less ribonucleic acid (RNA) and protein than adjacent cells; this region is known as the quiescent center. The cells which divide and give rise to root tissues lie around the periphery of this region. Cells in the quiescent center are regarded as cells that are mitotically young and genetically sound; they can renew the initial cells from time to time.
Shoot apices vary greatly in size and shape. The diameter can vary from about 50 micrometers to 0.14 in. (3.5 mm); the shape may be elongated and conical, dome-shaped, flat, or even slightly concave. The distance from the center of the apex to the youngest leaf primordium also varies considerably. Apices increase in size during the development of a single plant; for example, the apical meristem of flax (Linum usitatissimum) increases in area 20-fold from the seedling up to the time of flowering. Apices may also change in size during the time between the formation of one leaf primordium and the next. A single apical cell is present in shoot apices of bryophytes and vascular cryptogams; however, surrounding cells are also mitotically active, and these plants have multicellular apical meristems. In flowering plants, the outer layer or layers of cells (tunica) may divide predominantly by walls at right angles to the surface; the inner tissue (corpus), in less regular planes. Regions of the apical meristem may react differently to certain stains, reflecting so-called cytohistological zonation.
Cells in the central terminal region of the vegetative shoot apex divide less actively than those on the flanks or at the periphery, where leaf and bud primordia are formed. Various surgical experiments, involving incision of the apical meristem, have shown that new apices can be regenerated from portions of the flank. Excised apical meristems, devoid of leaf primordia, can be successfully grown on agar nutrient medium, in the presence of auxin, and will eventually yield new plants. See Bud, Leaf