268 



BOTANY 



PART I 



well adajDted to the various ends in view, according as they may.be 

 required to withstand the strain of flexure, traction, or pressure. 



To withstand bending, iind to otter the utmost possible resistance to it, a 

 peripheral disposition of the rigid mechanical tissue is the most favourable. 



a i i' a' 



I I 



Fuj. 213. — 1. Longitmlimil section of an elastic cylinder, before bending (dotted outline) and after 

 bending (heavy outline). After bending the convex side (a) is stretclied and the concave side 

 ((() compressed. F, Connecting tissue. 



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

 indeijendently and remain unaltered in length. 



When a straight rod (Fig. 213) is bent, the convex side elongates and the concave 

 side contracts, that is, the outer edges («, a and iil , a) are exposed to the greatest 

 variations in length, while, nearer the centre {%, i and i', i') the deilection and 

 consequent variations in length are less. Accordingly, if the supporting skeleton 

 of a plant stem be placed near the centre {i, i'), a considerable degree of curva- 



1 S, 



Fio. 214. — Rigidity against bending. 1. Transverse section of a young twig of .SKm/ranis ; c, collen- 

 chynta. 2. Part of the tiansverso section of a haulm of grass {Mulinia coernlea); Sc, ribs of 

 sclerenchyma ; Nc It, sclerenchyniatous ring connecting them laterally ; A, green assiinilatory 

 tissue ; Mil, pith-cavity. 



tare is possi1)]e with but little flexure of the mechanical tissue. Nearer the 

 periphery this would be subject to greater strain, and so oifer a greater resistance to 

 the deflecting 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 clastic. In the absence of such connecting tissue, the place of wliich 

 in hollow members may, however, be taken by firm tangential connection of the 



