forms. The reserve foods consist chiefly of starch, inulin, cellulose, sugars, oils, and proteid materials of divers kinds. The proteids are stored in the form of granules. (See Aleurone.) Starch is also stored in granular form, each grain being organized as a sphere-crystal by a leucoplast (q.v.). Sugars are accumulated in solution in the cell-sap, which may contain 5 to 20 per cent. Oils are stored usually in liquid form, as droplets in the protoplasm.
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a, a glandular lobe of the leaf of Drosera, with the clear secretion in situ. In the centre of lobe a water-conducting bundle. b, a glandular hair from the leaf of tobacco. The secreting cells are shaded.
In a few cases tissues are devoted to water storage. Water-storing tissues are found in plants which inhabit regions where they must provide against excessive water loss. The leaves of Begonia and the stems of Caetaceæ furnish illustrations.
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A bit of the section of a potato tuber, showing parenchyma cells, with starch grains in place.
(8) The Mechanical System. The mechanical system consists of cells called stereids, sometimes short, sometimes elongated, whose walls have become much thickened. The living contents of mechanical cells usually disappear when the walls become excessively thick. Short (isodiametric) stereids, developed in mass, afford resistance to crushing, as in fruits and seeds. Elongated stereids are called fibres. They impart elasticity, extensibility, flexibility, and tensile strength to the body. All these qualities depend solely upon the cell walls and the mode of union of the elements. The mechanical tissues are disposed in the plant body in such a way as to be highly efficient, conforming in their distribution to the best modes of mechanical construction for imparting rigidity and strength. In cylindrical organs like the stem they are placed near the periphery, on the principle of the hollow column, or the built-up column of latticed girders. In bilateral organs, like leaves, where tearing is possible from wind strains, the mechanical tissues are so developed as to afford protection to the edge against tearing, and in the ribs they are so placed as to maintain the softer tissues in the expanded position.
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Diagrams of cross-sections of stems to show the distribution of mechanical tissues (black), a, shows the I-beam principle; b, the hollow column; c, the built-up girder.
The development of mechanical tissues depends, in large measure, upon the influence of external agents, particularly stresses acting in various directions. Thus it comes about that the different organs attain the strength necessary to maintain their position, or to hold the attached parts as they increase in size and weight.
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Elaioplasts from a plant with the milky sap, showing o, oil droplets, which sometimes accumulate at one side of the elaioplast.
For a description of the different kinds of tissues involved in the foregoing systems, see Histology.
Bibliography. For elementary works, consult: Barnes, Plant Life (New York, 1898); Bessey, Botany (New York, 1892). For more advanced works: De Bary, Comparative Anatomy of the Vegetative Organs of the Phanerogams and Ferns, translated by Bower and Scott (Oxford,
