STRENC4TH AND ELASTICITY OF MECHANICAL CELLS 163 



A consideration of the above figures leads to conclusions which 

 are no less interesting than surprising. The tensile strength of bast 

 below the limit of elasticity is quite remarkable. It lies in general 

 between 15 and 20 kg. per sq. mm., and is thus equal to that of 

 wrought iron ; the bast of Nolina recurvata vies even with steel in 

 this respect. Bast, however, differs from metals in two important 

 particulars. For one thing it is far more extensible than any metal ; 

 the extension at the limit of elasticity varies from 1 per cent.- 1 '5 per 

 cent, for bast, while for metals the corresponding value is on an average 

 less than *1 per cent. Further, there is in the case of bast only a very 

 small difference between the limits of elasticity and cohesion : in other 

 words, a very slight increase of tension over that which just suffices to 

 produce a permanent elongation will cause the fibres to break. In the 

 case of metals the difference is very much larger ; the breaking strength 

 of a rod of wrought-iron, for example, is very nearly three times as great 

 as its modulus of elasticity. Schwendener remarks in this connection 

 that " Nature has evidently concentrated her attention upon the tensile 

 strength [of mechanical cells], and quite properly so ; for great breaking 

 strength would be of no value in the case of structures which cannot 

 be stretched beyond the elastic limit without suffering injury." 



There appears to be no definite correlation between the strength 

 of bast-fibres and the degree of lignification of their walls. According 

 to Sonntag, however, there does seem to be some connection between 



