38 



BRITISH FLOWERING PLANTS 



cally called. This need not be so strong as the flanges. 

 It has not the same force to resist, and may be a lattice 

 or open work. 



The applications of these principles in architecture 

 has led to the well-known " girder " (see Fig. 23), in 



which the material is 

 collected in two more 

 or less thin flanges 

 united by a narrow 

 rib. In some sub- 

 stances — in cast iron, 

 for instance — as Mr. 

 Hodgkinson has 

 shown,^ the resistance to compression is much greater 

 than the resistance to extension, and in the strongest 

 girder, therefore, the greater flange may be on the 

 extended side. If the forces to be resisted act in two 

 directions at right angles to one another, two girders 

 must be combined, as in Fig. 24. If the forces come in 

 all directions, a series of girders would be required (Fig. 

 25). This is the case in the stems of trees (see Fig. 26). 

 Now in the stems of plants the strength is given by 

 tissues known as hard bast, libriform cells, or scleren- 

 chyma, which are collectively termed, in ordinary 

 language, woody fibres. The hard bast 

 of some plants equals steel in its power 

 of resistance. Hence its use for string, 

 ropes, etc. The " web " consists of vas- 

 cular bundles and parenchymatous cells. 

 Here strength is not required. In some 

 cases — for instance, in many Grasses — 

 the centre is hollow. If the mechanical 

 or woody tissue formed a pillar in the 

 centre of the stem, there would evidently be a great 

 waste of strength (see Fig. 25). 



An erect stem which is liable to wind pressure, some- 

 times from one side and sometimes from another, must 

 be strengthened in all directions. Hence a series of 



^ Mem, Manchester Phil. Soc. vol. iv. 



FiQ. 25.— Multiple 

 girder. 



