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 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 
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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