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the system of tissues that ensures a plant’s sturdiness, that is, its capacity to withstand such static and dynamic stresses as the force of gravity and gusts of wind. Mechanical tissues include collenchyma, sclerenchyma, brachysclereids, bast fibers (in the secondary cortex), and libriform fibers (in wood). Also sometimes considered to be mechanical tissues are certain integumentary tissues and thick-walled tracheids located in the late annual layers of conifers that, along with their principal function, play a mechanical role. Thin-walled, delicate tissues serve mechanical functions when they are in a state of turgor; they fill the space between mechanical tissues, thereby increasing the sturdiness of the plant.
Performance of the principal functions of mechanical tissues is ensured by substantial thickening of cell membranes, firm bonds between cells, and elasticity of membranes, as well as by the particular distribution of mechanical tissues in the plant. The elasticity and tensile strength of mechanical tissue, for example, sclerenchyma, is close to that of steel. The tissue’s elasticity is slightly less than that of rubber, and its ability to withstand dynamic stress without deformation is considerably greater than that of steel.
The systematic study of mechanical tissues was begun by the German botanist S. Schwendener in 1874. In Russia mechanical tissues were first studied in 1912 by V. F. Razdorskii, who came up with the theory recognizing structural-mechanical principles in plants. Razdorskii regards the plant and its organs not as a construction that statically resists external mechanical effects (as Schwendener supposed) but as a dynamic system of a living organism that changes according to external conditions.
The mechanical tissues of herbaceous plants form a grid, yet some of their strands are arranged at an angle. The interweaving of tissues, the septa in the nodes of hollow stems, and the epidermis and its peripheral elements ensure special sturdiness of the stem. In the secondary cortex of woody plants the skeletal grid consists of strands and membranes of bast mechanical fibers and sclereids. In wood the libriform strands form a framework for the basic mass of vessels and tracheids.
Environmental conditions influence the mechanical tissues of plants. For example, in aquatic plants the tissues are poorly developed. The thickness of mechanical tissue increases with an increase in soil moisture and in the intensity of illumination, as well as with a decrease in atmospheric humidity.
REFERENCESRazdorskii, V. F. Anatomiia rastenii. Moscow, 1949.
Razdorskii, V. F. Arkhitektonika rastenii. Moscow, 1955.
O. N. CHISTIAKOVA