MECHANICAL PROPERTIES OF STEMS 289 



subject to bending stresses, though at the same time it acts 

 as a pillar. In trees, where the wood occupies nearly the 

 whole bulk of the axis, no special arrangement of the 

 mechanical tissues exists, unless we count as such the series 

 of concentric cylinders formed by the closer textured and 

 mechanically stronger summer wood, separated by the 

 alternating cylinders of spring wood. We may note, 

 however, that the base of the trunk is often markedly 

 thicker than the top, and that this helps to resist longitudinal 

 compression as well as bending stress. 



In herbaceous plants, the mechanical tissues occupy 

 only a fraction of the total bulk of the stem, and it becomes 

 of interest to see how they are arranged with regard to the 

 function they must perform. If a bar of material is bent 

 the substance on the convex side is elongated, that on the 

 concave side compressed ; inwards these changes become 

 less and less towards the centre, where they vanish. Stress 

 and strain, therefore, are felt most at the edges, and much 

 less towards the centre, and on the flanks. So it comes 

 about that in constructing a beam which has to resist 

 bending it is important to distribute the material so that it 

 will lie mostly on the two edges and least in the middle. 

 Resistance to bending depends not only on the area of the 

 cross section, but on the distribution of the material. The 

 common shape of a steel beam is a T or X in cross-section, 

 the two flanges united by sufficient material to join them 

 rigidly, often in the form of a lattice work. Or the con- 

 struction may take the form of a hollow tube where all the 

 material is on the periphery and is distributed so that a 

 stress in any direction can be met. 



The distribution of mechanical tissue in a stem follows 

 the same plan. Schwendener and Haberlandt have described 

 a great many types, especially among monocotyledons, where 

 the absence of secondary thickening makes the arrangement 

 of sclerenchyma important. Two examples may, however, 

 suffice here (Fig. 43). In the square stems of the.Labiatae, 

 e.g. in Lamium album, there runs down each angle a thick 

 strand of collenchyma, and these four strands, connected 



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