184 



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

 between impeding obstacles, the skeletal system is central, where 

 without interfering with the bending, it can more effectually sustain 

 strains upon the longitudinal rigidity of the organ by being thus 



massed together (Fig. 

 f> 177). The stereome is 

 more highly developed 

 in those roots which 

 mainly serve to attach the 

 plant to the substratum 

 than in those the main 

 duty of which is to absorb 

 food material ( r '). 



Where, however, pres- 

 sure must be guarded 

 against (as in Plum-stones, 

 and in Hazel-nuts and Walnuts), and also in some subterranean or 

 submerged parts of plants, the mechanical resistance is maintained by 

 an arching mass of sclerotic cells or sclerenchymatous fibres. 



Stems of trees which have to support heavy and frequently large 

 crowns, must, like pillars, be constructed to withstand 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 



Fin. 177. 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 resist lateral 

 pressure, in addition to the central strand. 



FIG. 178. Leaf of Phonniumtenax. 1. Transverse section ; Sc, plates and strands of sclerenchyma ; 

 Aj green assimilatory parenchyma ; H, hypoderma serving for water-storage ; W, colourless 

 mesophyll (internal water-storage tissue). 2. Edge of the same leaf ; E, thick brown epidermis ; 

 7,', marginal strand of sclerenchyma fibres. 



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 (Phormium tenax, Fig. 178, 1). 

 Protection against tearing forces is mainly afforded 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 strands of stereome (Fig. 178, 2). Where, 

 as in the Banana, both these means of protection are wanting, the 

 lamina is regularly and easily torn by the wind. 



