MECHANICAL CONSTRUCTION OF PLANT-BODY 179 



The first column shows the maximum burden per unit of transverse 

 section of the fibrous strand, or of the metal wire respectively, under 

 which the limit of elastic recovery is not overstepped. The fibre of 

 Nolina (25) is actually superior to steel (24.6). Other fibres compare 

 favourably with silver and wrought iron. The second column shows 

 the burden per unit of transverse section which causes rupture ; that 

 is, it states the limit of teyiacity. It is seen that in this metals are 

 distinctly superior. But the table brings out a very important feature 

 of plant-fibres, that their limits of elastic recovery and of tenacity 

 are very nearly coincident, while those of metals are widely apart. 

 Metals are ductile, fibres are not. The importance of this in the plant- 

 body lies in the provision thus made for perfect recovery after strain. 

 Great breaking strength would be of no value if the plant subjected 

 to the strain were permanently deformed. 1 The third column shows 

 the elongation which the strand or wire suffers at the limit of elastic 

 recovery, stated in terms of units of length per 1000 of the strand as 

 a whole. Here the difference between the fibres and the wires is 

 strongly marked, the fibres yielding in much higher degree than the 

 metal wires. This again meets the requirements of a plant exposed 

 to forces, such as wind. For the tissues while resisting the force 

 very efficiently, yield to it, but recover very perfectly when the force is 

 removed. 



In late years metal straps have been used largely in concrete con- 

 struction, reinforcing the concrete in which they are embedded. 

 Ordinary herbaceous plants are constructed on the same principle. 

 The sclerotic strands correspond to the metal straps, the surrounding 

 parenchyma with its turgescent cells corresponds mechanically to the 

 concrete. The office of the latter in either case is to keep the resistant 

 straps in place, while the straps resist the tensions which would 

 produce loss of form. In the reinforced concrete a high degree of 



said to be under tensile stress when pulled, and struts under compressive 

 stress when pushed. The term strain is used to signify the change of length 

 or other dimensions or the change of form which occurs in a material under 

 stress. 



1 Collenchyma is, however, exceptional. While its absolute strength is 

 little inferior to that of bast fibres, its limit of elasticity is much lower : and 

 it is thus liable to permanent elongation. But this is an advantage in growing 

 stems in which it usually occurs ; for it offers no rigid barrier to growth, while 

 it is sufficiently resistant to meet sudden stresses. It is like the Second 

 Chamber in the Constitution of a State, which resists a new initiative ; but if 

 the initiative be continuously pressed, it yields. Meantime stability is main- 

 tained. 



