SECT. II 



PHYSIOLOGY 



269 



meclianical tissues, the individual ribs of stereome are easily bent (Fig. 213, 2). 

 In erect steins and flower-stalks where rigidity is an essential requirement, the 

 , mechanical tissue is situated at the periphery, and often takes the form of 

 jjrojecting ridges (Fig. 213, 1, 2). 



In roots, and in many rhizomes and stolons, which must push between 

 impeding obstacles, the skeletal system is central, where, without interfering with 

 tlie bending, it can more effectually 

 sustain strain upon the longitudinal 

 rigidity of tlie organ by being thus 

 massed together (Fig. 215). The 

 stereome is more liighly developed 

 in those roots wliicli mainly serve 

 to attacli the plant to the substratum 

 tlian in those the chief duty of wliicli 

 is to absorb food material. 



Where, however, pressure must be 

 guarded against (as in Plum-stones, 

 and in Hazel-nuts and Walnuts), 

 and also in some subterranean or sub- 

 merged parts of plants, the mechani- 

 cal resistance is maintained by an arching mass of sclerotic cells or sclerenchymatous 

 fibres. 



Stems of trees which have to support heavy and frequently large croAvns must, 

 like pillars, be constructed to witlistand the longitudinal pressure as well as the 

 bending. 



Leaf-blades must be strengthened not only against bending forces, but against 

 tearing of their expanded surface by the action of wind or currents of water. The 

 former requirement is met by the vascular bundles which often project as ribs on 

 the lower surface, and by girder-like arrangements of the stereome {Fhormin.m ienax, 

 Fig. 216, 1). Protection against tearing forces is mainly alForded by the firmness 

 of the margin of the leaf; this is partly attained by the vascular bundles forming 

 marginal arches or running parallel to the margin, in part by special marginal 



Fio. 2l5. — Mechanical tissue of roots. 1, Centrally 

 placed to resist longitudinal pulling strains ; 2, a 

 prop root with a peripheral layer of mechanical 

 tissue (P) to resLst lateral pressure, in addition 

 to the central strand. 



Fro. 216. —^jt'zi of Phormiumtr liar. 1. Transverse section ; .SV, plates and strands of sclerenchyma ; 

 A, green assimilatory parenchyma ; H, hypoderma serving for water-storage ; li', colourless 

 niesophyll (internal water-storage tisstie). 2. Edge of the same leaf ; E, thiclc brown epidermis ; 

 R, marginal strand of sclerenchyma fibres. 



strands of stereome (Fig. 216, 2). Where, as in the Banana, both these means of 

 protection are wanting, the lamina is regularly and easily torn by the wind (p. 35). 

 Since, as has already been pointed out, the resistance of the mechanical elements 

 to flexure is greater the farther they are removed from the centre of an organ, it will 

 be readily seen that, while a fl ittened, outspread organ, can be easily bent, its 

 power of resisting a deflecting force would be increased if it were folded or rolled 



