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 pres- 

 sure 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 ropeiike 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 



One of the most important characters to be kept in mind in 

 studying bast fibres is the structure of the wall. In fact, the 

 author's classification of bast fibres is based largely on wall 



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