SECT. II 



PHYSIOLOGY 



183 



degree of curvature is possible with but little flexure of the 

 mechanical tissue. Nearer the periphery it would be subject to 

 greater strain, and so offer a greater resistance to the deflecting 



/ I 



l'n;. 175. I. Longitudinal section of an elastic cylinder, before bending (dotted out- 

 line) and after bending (heavy outline). After bending the convex side (a') is 

 stretched and the concave side (a) compressed. F, connecting tissue. 



2. When the connecting tissue (/) is not sufficiently firm, the bands of stereome 

 (a, a') curve independently and remain unaltered in length. 



force. It is essential that the girder- like arrangement of the 

 stereome should be held together by a connecting tissue which is at 

 once resistant and elastic. In the absence of such connecting tissue, 

 the place of which in hollow members may, however, be taken by 



\ 



SoR 



. liii. Rigidity against bending. 1. Transverse section of a young twig of SambiKiis; c, collen- 

 chymn. -'. 1 'art of the transverse section of a haulm of grass (Mullnia enerulea); sc, ribs of 

 sclerenchyma ; sc R, sclerenchymatous ring connecting them laterally ; A, green assimilatory 

 tissue ; MH, pith-cavity. 



firm tangential connection of the mechanical tissues, the individual 

 ribs of stereome are easily bent (Fig. 175, 2). In erect stems and 

 flower-stalks where rigidity is an essential requirement, the mechanical 

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

 projecting ridges (Fig. 176, 1, 2). 



