1. Outer layer of cork cells. 2. Cortical parenchyma cells. 3. Stone cells. 4. Phellogen. 5. Cortical parenchyma cells.] ORIGIN OF CORK CELLS The cork cells are formed by the meristimatic phellogen cells, which originate from cortical parenchyma. These cells divide into two cells, the outer changing into a cork cell, while the inner cell remains meristimatic. In other instances the outer cell remains meristimatic, while the inner cell changes into a cortical parenchyma cell. The development of a cortical parenchyma cell from a divided phellogen cell is shown in Plate 101, Fig. 6. Both the primary and secondary cork cells originate from the phellogen or cork cambrium layer. Cork cells do not contain living-cell contents; in fact, in the majority of medicinal barks the cork cells contain only air. The walls of typical cork cells are composed, at least in part, of suberin, a substance which is impervious to water and gases. In certain cases layers of cellulose, lignin, and suberin have been identified. Suberin, however, is present in all cork cells, and in some cases all of the walls of cork cells are composed of suberin. Suberized cork cells are colored yellow with strong sodium hydroxide solutions and by chlorzinciodide. CHAPTER III MECHANICAL TISSUES The =mechanical tissues= of the plant form the framework around which the plant body is built up. These tissues are constructed and placed in such a manner in the different organs of the plant as to meet the mechanical needs of the organ. Many underground stems and roots which are subjected to radial pressure have the hypodermal and endodermal cells arranged in the form of a non-compressible cylinder. Such an arrangement is seen in sarsaparilla root (Plate 38, Fig. 4). The mechanical tissue of the stem is arranged in the form of solid or hollow columns in order to sustain the enormous weight of the branches. In roots the mechanical tissue is combined in ropelike strands, thereby effectively resisting pulling stresses. The epidermis of leaves subjected to the tearing force of the wind has epidermal cells with greatly thickened walls, particularly at the margin of the leaf. The epidermal cells of most seeds have very thick and lignified cell walls, which effectively resist crushing forces. The cells forming mechanical tissues are: bast fibres, wood fibres, collenchyma cells, stone cells, testa epidermal cells, and hypodermal and endodermal cells of certain plants. The walls of the cells forming mechanical tissues are thick and lignified, with the exception of the collenchyma cells and a few of the fibres. Lignified cells are as resistive to pulling and other stresses as similar sized fragments of steel. The hardness of their wall and their resistance to crushing explain the fact that they usually retain their form in powdered drugs and foods. BAST FIBRES