Plant tissue systems

Plant tissue systems

Most plants are composed of coherent masses of cells called tissues. Large units of tissues having some features in common are called tissue systems. In actual usage, however, the terms tissue and tissue system are not strictly separated. A given tissue or a combination of tissues may be continuous throughout the plant or large parts of it.

Plant tissues are primary or secondary in origin. The primary arise from apical meristems, the perennially embryonic tissues at the tips of roots and shoots. The primary tissues include the surface layer, or epidermis; the primary vascular tissues, xylem and phloem, which conduct water and food, respectively; and the ground tissues. The ground tissues are parenchyma (chiefly concerned with manufacture and storage of food) and collenchyma and sclerenchyma (the two supporting tissues). In the stem and root, the vascular tissues and some associated ground tissue are often treated as a unit, the stele. Ground tissue may be present in the center of the stele (pith) and on its periphery (pericycle). The ground tissue system enclosing the stele on the outside is the cortex. It may have a hypodermis peripherally and an endodermis next to the stele.

The secondary tissues arise from lateral meristems, and their formation is mainly responsible for the growth in thickness of stems and roots. They comprise secondary vascular tissues and the protective tissue called periderm. Secondary growth may build up a massive core of wood, but the outer tissue system, the bark, remains relatively thin because its outer or older part becomes compressed and, in many species, is continuously sloughed off.

The production of flowers instead of vegetative shoots results from physiological and morphological changes in the apical meristem, which then becomes the flower meristem. The latter, however, produces tissue systems fundamentally similar to those in the vegetative body of the plant.

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