apical meristem

Also found in: Dictionary, Thesaurus, Medical.
Related to apical meristem: intercalary meristem, Root Apical Meristem, Shoot apical meristem

Apical meristem

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.

Diagram of a root apical meristemenlarge picture
Diagram of a root apical meristem

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

apical meristem

[′ap·i·kəl ′mer·ə‚stem]
A region of embryonic tissue occurring at the tips of roots and stems. Also known as promeristem.
References in periodicals archive ?
These observations establish the presence of cytokinins in sporophytes and gametophytes of leptosporangiate ferns, the influence of light on cytokinin response by gametophytes, and that, like seed plants, cytokinins are associated with the maintenance of an apical meristem and female gender expression of gametophytes.
This observation indicates the lack of centralized hormonal regulation of cell structure changes in root apical meristems.
The leaf base becomes sheathing, protecting the younger leaves and the apical meristem (Fig.
1998) who found that cytokinin is directly responsible for reprogramming the embryonic apical meristem axes of cotton towards the multiplication of buds.
Antheridiate prothalli were elongated and did not form well-defined apical meristem.
Nonetheless, in laboratory experiments both Pemberton (1980) and Markham (1985) reported burrowing by chironomid larvae into undamaged hydrilla, with destruction of the apical meristem (Pemberton 1980).
7), it is clear that the apical meristem conforms to a typical Gymnosperm root tip structure (Fahn 1982 Fig.
A mechanism similar to the one that determine whether the apical meristem of the inflorescence branch becomes a spikelet es responsible for the IM not developing axillary meristems (or BM, SPM, or SM) and acting only as SM generating the terminal spikelet.
Plumule Shoot apical meristem and leaf primordia in the embryo.
Egg-guarding females sitting on top of egg masses oriented themselves toward the apical meristem of the branch (n = 18) (Fig.
As the plant shifts from vegetative growth to the reproductive phase, the vegetative apical meristem ("shoot apex") of a plant usually changes in a variety of ways, reflecting physiological changes.
Vegetative structures included complex leaves (or parts thereof), distichous phyllotaxy, internode elongation, and apical meristem indeterminacy, as did first-order inflorescence stems.