XVI] OF STRESS AND STRAIN 973 



stem, which Gahleo was the first to call attention to. Schwendener* 

 shewed that its strength was concentrated in the little bundles of 

 "bast-tissue," but that these bast-fibres had a tensile strength per 

 square mm. of section not less, up to the limit of elasticity, than 

 that of steel-wire of such quality as was in use in his day. 



For instance, we see in the following table the load which various 

 fibres, and various wires, were found capable of. sustaining, not 

 up to the breaking-point but up to the "elastic Hmit," or point 

 beyond which complete recovery to the original length took place 

 no longer after release of the load. 



* This hgure should be considerably higher for the best modern steel. 



In other respects, it is true, the plant-fibres were inferior to the 

 wires; for the former broke asunder very soon after the limit of 

 elasticity was passed, while the iron- wire could stand, before snapping, 

 about twice the load which was measured by its limit of elasticity : 

 in the language of a modern engineer, the bast-fibres had a low 

 "yield-point," little above the elastic limit. Nature seems content, 

 as Schwendener puts it, if the strength of the fibre be ensured up 

 to the elastic limit; for the equihbrium of the structure is lost as 

 soon as that limit is passed, and it then matters httle how far off 

 the actual breaking-point may bef. But nevertheless, within cer- 

 tain limits, plant-fibre and wire were just as good and strong one 



* S. Schwendener, Das mechanische Princip im anatomischen Bau der Monocotyleen, 

 Leipzig, 1874; Zur Lehre von der Festigkeit der Gewachse, Sb. Berlin. Akad. 1884, 

 pp. 1045-1070. 



t The great extensibility of the plant-fibre is due to the spiral arrangement of 

 the ultramicroscopic micellae of which the bast-fibre is built up : the spiral untwisting 

 as the fibre stretches, in a right or left-hand spiral according to the species. Cf. 

 C. Stein bruck, Die Micellartheorie auf botanischem Gebiete, Biol. Centralbl. 1925, 

 p. 1. 



