XVI] ON STRESS AND STRAIN 689 



Such experiments have been amply confirmed, but so far as 

 I am aware, we do not know much more about the matter: we 

 do not know, for instance, how far the change is accompanied by 

 increase in number of the bast-fibres, through transformation of 

 other tissues; or how far it is due to increase in size of these 

 fibres; or whether it be not simply due to strengthening of the 

 original fibres by some molecular change. But I should be much 

 inclined to suspect that the latter had a good deal to do with the 

 phenomenon. We know nowadays that a railway axle, or any 

 other piece of steel, is weakened by a constant succession of 

 frequently interrupted strains; it is said to be ''fatigued," and 

 its strength is restored by a period of rest. The converse effect 

 of continued strain in a uniform direction may be illustrated by 

 a homely example. The confectioner takes a mass of boiled 

 sugar or treacle (in a particular molecular condition determined 

 by the temperature to which it has been exposed), and draws the 

 soft sticky mass out into a rope ; and then, folding it up lengthways, 

 he repeats the process again and again. At first the rope is pulled 

 out of the ductile mass without difficulty; but as the work goes 

 on it gets harder to do, until all the man's force is used to stretch 

 the rope. Here we have the phenomenon of increasing strength, 

 following mechanically on a rearrangement of molecules, as the 

 original isotropic condition is transmuted more and more into 

 molecular asymmetry or anisotropy ; and the rope apparently 

 "adapts itself" to the increased strain which it is called on to bear, 

 all after a fashion which at least suggests a parallel to the increasing 

 strength of the stretched and weighted fibre in the plant. For 

 increase of strength by rearrangement of the particles we have 

 already a rough illustration in our lock of wool or hank of tow. 

 The piece of tow will carry but little weight while its fibres are 

 tangled and awry : but as soon as we have carded it out, and 

 brought all its long fibres parallel and side by side, we may at once 

 make of it a strong and useful cord. 



In some such ways as these, then, it would seem that we may 

 co-ordinate, or hope to co-ordinate, the phenomenon of growth 

 with certain of the beautiful structural phenomena which present 

 themselves to our eyes as "provisions," or mechanical adaptations, 

 for the display of strength where strength is most required. 

 T. G. 44 



