Science Fair Project Encyclopedia
The collenchyma is the typical supporting tissue of the primary plant body and growing plant parts, though it is kept with unaltered structure and function even in outgrown organs like stems, petioles, laminae or roots. In cross-sections of stems, the collenchyma commonly appears as discrete strands or as a peripheral cylinder that lies, depending on the species, either directly beneath the epidermis or is separated from it by several layers of parenchyma. The cylinder is usually composed of several layers. Collenchyma is also found bordering the veins of dicot leaves. It forms fibres in edgy stems that run along the edges or ribs. Often either phloem or xylem of the vascular bundles is associated with collenchyma cells.
Many transitions prove the collenchyma's origin from the parenchyma. The differentiation is reversible, a degeneration to meristematic states has often been observed. The walls of collenchyma cells are strengthened by the deposit of cellulose and the coating with pectin. These strengthenings are often restricted to single parts or edges of the cell. The walls of parenchyma cells are opened by pits that are often arranged in special areas.
The unevenly thickened cell walls led the German botanist C. MÜLLER (1890) to distinguished between different collenchyma types:
1. Angular Collenchyma. A thickening of the cell's edges can be seen in cross-section. Longitudinal sections show the elongated shape of both cell and thickening. A cross-section through the stem of Begonia rex or related species is the typical specimen used in botanical microscopic courses. Angular collenchyma occurs also in species of the following genera: Ficus, Vitis, Ampelopsis, Polygonium, Beta, Rumex, Boehmeria, Morus, Cannabis, Pelargonium and others.
2. Tangential Collenchyma. The tangential walls of this collenchyma type are thicker than the radial walls. Examples: Sambucus nigra, species of the genera Sanguisorba, Rhoeo, Eupatoria.
3. Lacunar Collenchyma. While hardly any intercellular spaces exist in the two types above, are those of this type very large. Clear gaps can be recognized between the cells. Occurrence: species of the genera Lactuca, Salvia, Prunella and the Composite-family.
The cell walls of collenchyma cells are distortable when stretched. Shape and arrangement of the cells cause a high mechanic stability with a capacity of 10-12 kg/mm2. This quality is especially advantageous in growing plant organs. It enables the collenchyma cells to stretch in synchrony with the other cells without spoiling the toughness of the tissue. The new state is stabilized by the simultaneous working-in of additional wall material.
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