MECHANICAL STRENGTH AND RIGIDITY 53 



water-plants, the tensile strength is dependent solely upon the character 

 of the material and its sectional area. Hence we find that the vascular 

 bundles, and the mechanical tissues as well, occupy a more central position. 

 A similar tendency is shown in rhizomes as compared with the upright 

 stems of the same plant. 



More especially in the herbaceous stems of monocotyledons, as 

 Schwendener has shown, are the mechanical tissues arranged as close to 

 the periphery as possible, that is, in the position where they give the 

 greatest rigidity to the plant. Similar purposeful arrangements are ex- 

 hibited in leaves and petioles, as well as in the herbaceous stems of 

 dicotyledons. In the latter, peripheral layers of collenchyma or sclerenchyma 

 are often present, and these, together with the somewhat more centrally 

 placed ring of vascular bundles, form the main strengthening tissues. As 

 the subsequent secondary growth progresses, the wood- cylinder acquires 

 an increasing importance, and ultimately performs practically the entire 

 mechanical function. In any case the ultimate rupture of the bark would 

 render it incapable of yielding the required support. Frequently peripheral 

 strands of mechanical tissue alternate with chlorophyllous parenchyma, and 

 even when a continuous peripheral layer of sclerenchyma is present, as in 

 the leathery leaves of conifers, it is interrupted beneath the stomata 1 . 

 Not only in leaves, but also in young stems, the arrangement of the 

 mechanical tissues is such as to interfere as little as possible with other 

 functions such as absorption and photosynthesis, and indeed the plant 

 frequently sacrifices some of its strength or wastes a certain amount of 

 constructive materials in order to favour the performance of important 

 functions. 



When necessary, special structures may be developed to protect the 

 softer peripheral tissues from external pressure. Thus, the epidermis is 

 frequently strengthened by secondary thickening of its walls, or by 

 radiating strands of sclerenchyma, as in the acicular leaves of Hakea 

 brachyrhyncka 2 , in which strands of sclerenchyma extend between the 

 central cylinder and the epidermis, leaving spaces filled with loose thin- 

 walled parenchyma. In the case of soft-walled parenchyma cells, the 

 smaller their diameter the greater is the total pressure they can withstand. 

 Hence, in the extremely large cells of Caulerpa, transverse bars of cellulose 

 are formed, which aid in stiffening the outer walls 3 . Transverse cell-walls 

 and the cellular partitions formed across the large air-spaces of aquatic 

 plants act in the same manner, and the interposition of solid nodes at 

 regular intervals in the haulms of grasses and bamboos confers greater 



1 Cf. de Bary, Comp. Anat., Clar. Press Trans., 1884, p. 418 sq. 



2 A figure is given in Mohl's Vermischte Schriften, 1845, PI. vn, Fig. 2. 



3 Klemm, Flora, 1893, p. 463 ; Reinke, Ueber Caulerpa, 1899, and the literature quoted 

 in those works. 



