PHYSIOLOGY 



169 



provided for by special groups of cells with thickened and hardened 

 walls, which thus constitute a firm framework for the other tissues 

 similar to the bony skeleton of the higher animals. 



Meehanieal Tissues (Stereome). — The supporting framework of 

 plants is provided by the thick-walled elements of the wood, the 

 thickened sclerenchymatous fibres of the fundamental tissue and the 

 bast, and more rarely by groups of stone-cells. The resistance which 

 these forms of tissue offer when the attempt is made to cut or break 

 them affords sufficient evidence of their hardness, tenacity, and rigidity. 

 Moreover, Schwendener has been able to determine their mechanical 

 value by means of exact physical experiments and investiga- 

 tions. According to such estimates, the sustaining strength of 

 sclerenchymatous fibres is, in general, equal to the best wrought-iron 

 or hammered steel, while at the same time their ductility is ten or 

 fifteen times as great as that of iron. Just as the mechanical tissues 

 of the internal framework of plants exhibit the physical properties 

 most essential for their purpose, their arrangement, as Schwendener 

 showed, will also be found equally well adapted to the various ends in 

 view, according as they may be required to withstand the strain of 

 flexure, traction, or pressure. To withstand bending, and to offer the 

 utmost possible resistance to it, a peripheral disposition of the rigid 

 mechanical tissue is the most favourable. 



When a straight rod (Fig. 168) is bent, the convex side elongates 

 and the concave side contracts, that is, the 

 outer edges (a, a and a', a) are exposed to 

 the greatest variations in length, while, 

 nearer the centre (i, i and % , %') the deflec- 

 tion and consequent variations in length 

 are less. Accordingly, if the supporting 

 skeleton of a plant stem be placed near 

 the centre (i, i'), then a considerable degree 

 of curvature is possible with but little 

 flexure of the mechanical tissue. Nearer 

 the periphery it would be subject to 

 greater strain, and so offer a greater re- 

 sistance to the deflecting force. In erect 

 stems and flower-stalks, where rigidity is 

 an essential requirement, the mechanical 

 tissue is situated at the periphery, and 

 often takes the form of projecting ridges 

 (Fig. 169, 1, 2). In roots, and in many 

 rhizomes and stolons, as they must push 

 circuitously between impeding obstacles, 

 the skeleton system is central, as by this 

 arrangement it is subject to less deflection, and can more effectually 

 sustain strains upon its longitudinal elasticity (Fig. 169, 4). Fig. 169, 3 



ft it tt 



Fig. 16S. — Longitudinal section of an 

 elastic cylinder before and after 

 curvature. Before curvature the 

 peripheral (a, a') and central (i, i') 

 vertical lines are of the same length 

 (31 '4 mm.). After curvature the 

 peripheral line a' is 6-2 mm. longer ; 

 the other peripheral line a 6'3 mm. 

 shorter. At the same time the 

 central lines undergo but little 

 change of dimensions ; i' is length- 

 ened 1-2 mm., i 1-3 mm. shortened. 



