182 BOTANY PART i 



assist, as in the leaves and stems, in maintaining the rigidity of the plant body. 

 Longitudinal and transverse tensions occur, particularly when, through secondary 

 growth, newly formed growing tissues have to overcome the resistance of other 

 tissues. In this way the primary and later the secondary cortex of trees become 

 greatly stretched by the new cambial growth, so much so, that if a ring of bark 

 be removed from a stem and then placed round it, a force of ten atmospheres is 

 needed to make the edges meet ; this was shown by an experiment of KUABBK. 



In the meristematic tissues of growing points there is scarcely any perceptible 

 tension, while, on the other hand, in regions which are in a state of elongation the 

 tension of the tissues attains its highest limit. After an organ has completed its 

 growth the elasticity of the cell walls and the turgoscence of the cells decrease ; and 

 the tension of the tissues is therefore also diminished. The requisite rigidity is, 

 however, now 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. 



Mechanical Tissues (Stereome) ( 4 ). 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 firm thick walls 

 of these tissues are not infrequently further hardened by deposits of 

 mineral substances. The resistance which these forms of tissue offer 

 when the attempt is made to cut, tear, or break them affords 

 sufficient evidence of their hardness, tenacity, and rigidity. More- 

 over, SCHWENDENER has been able to determine their mechanical 

 value by means of exact physical experiments and investigations. 

 According to such estimates, the sustaining strength of sclerenchy- 

 matous fibres is, within the limits of their elasticity, 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. 

 It is true that soon after exceeding its limit of elasticity the 

 stereome of the plant becomes ruptured, while the modulus of 

 rigidity for iron is not reached until the load is increased threefold. 

 It is, however, of value for the needs of the plant that its limit of 

 elasticity extends almost to the limit of its rigidity. 



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. 175) 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 i', i') the deflection 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 



