The ground tissues, the second of the primary developmental tissues, differentiate in the zone of maturation to form tissues called parenchyma, collenchyma, orsclerenchyma. The parenchymous tissues are the primary site of cellular metabolism. The organelles of parenchyma cells in different parts of the plant vary so that they can accommodate differences in metabolic functions. Cells of leaf parenchyma and some stem parenchyma have large numbers of chloroplasts to carry out photosynthesis. Stem and root parenchyma cells have amyloplasts, organelles that store starch. Chromoplasts in the parenchyma of flower petals contribute to the color of the flower petals. Parenchyma cells producing large quantities of protein have more ribosome’s than those specialized for starch storage. Reproductive parenchyma cells may have unusual nuclear characteristics that prevent these tissues from competing with the developing embryos for nutrients or space.
Parenchyma fills the inner parts of leaves, stems, and roots. These cells have large, water-filled vacuoles. The water pressure from these vacuoles provides much of the rigidity of the body of nonwoody plants. When a leaf is limp, its parenchyma cells are usually depleted of water. Many of the chemicals that give plants their unique tastes or pharmaceutical characteristics are produced and stored in parenchyma. For example, the bulk of a carrot root (especially outside the central core), the mass of a potato tuber (which is actually a unique form of stem), and much of a lettuce leaf are all made of parenchyma.
Collenchymas cells are similar to parenchyma cells in many ways. They use water pressure to provide support. However, they are normally found near the surface of stems and leaves. Collenchymas cells have a unique pattern of cell-wall thickening that allows expansion in diameter but not in length. This makes collenchymas especially suited to providing support for softbodied plant parts that have completed much of their longitudinal growth. Collenchyma cells rarely provide bulk to plant structures. Instead, they form thin sheets just below the epidermis and outside much of the parenchyma. Because collenchyma is thin, it has a smaller volume than parenchyma and contributes less to the metabolism of the plant organs. It may nevertheless support some of the photosynthesis of the plant, and it provides textures to the organs as well.
Sclerenchyma cells occur throughout the body of the plant and include three types of cells: elongated fibers; branched sclereids, resembling a three-dimensional jigsaw puzzle piece; and globular stone cells. All three cell types have heavy, secondary cell walls and have lost many organelles. Sclerenchyma is a type of differentiated tissue that functions when its cells are dead.
Fibers support plant organs in the same way as does collenchyma, but because the secondary cell walls of sclerenchyma cells resist longitudinal and latitudinal expansion, they are not common in growing tissues. Their rigidity helps to supply support even when tissues are water-stressed, but it also limits the potential for the organs to expand in girth or length. Fibers, sclereids, and stone cells all provide protection against predation. The gritty texture of a ripe pear, the shell of a nut, and the strings of a coconut husk are all composed of sclerenchyma cells and promote the wear and breakage of predators' teeth and other chewing structures.
See also: Procambium
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