XVI] ON STRESS AND STRAIN 985 



of the physical causation of what at first sight strikes us as a purely 

 functional adaptation: as a phenomenon, in other words, whose 

 physical cause is as obscure as its final cause or end is apparently 

 manifest. 



Partly associated with the same phenomenon, and partly to be 

 looked upon (meanwhile at least) as a fact apart, is the very im- 

 portant physiological truth that a condition of strain, the result 

 of a stress, is a direct stimulus to growth itself. This indeed is no 

 less than one of the cardinal facts of theoretical biology. The soles 

 of our boots wear thin, but the soles of our feet grow thick, the 

 more we walk upon them: for it would seem that the living cells 

 are "stimulated" by pressure, or by what we call "exercise," -to 

 increase and multiply. The surgeon knows, when he bandages a 

 broken limb, that his bandage is doing something more ^an merely 

 keeping the parts together: and that the even, constant • pressure 

 which he skilfully applies is a direct encouragement of growth and 

 an active agent in the process of repair. In the classical experiments 

 of Sedillot*, the greater part ef the shaft of the tibia was excised 

 in some young puppies, leaving the whole weight of the body to 

 rest upon the fibula. The latter bone is normally about one-fifth 

 or sixth of the diameter of the tibia ; but under the new conditions, 

 and under the "stimulus" of the increased load, it grew till it was 

 as thick or even thicker than the normal bulk of the larger bone. 

 Among plant tissues this phenomenon is very apparent, and in 

 a somewhat remarkable way; for a strain caused by a constant or 

 increasing weight (such as that in the stalk of a pear while the pear 

 is growing and ripening) produces a very marked increase of strength 

 without any necessary increase of bulk, but rather by some histo- 

 logical, or molecular, alteration of the tissues. Hegler, Pfeifer, and 

 others have investigated this subject, by loading the young shoot 

 of a plant nearly to its breaking point, and then redetermining the 

 breaking-strength after a few days. Some young shoots of the 

 sunflower were found to break with a strain of 160 gm.; but when 

 loaded with 150 gm., and retested after two days, they were able 

 to support 250 gm. ; and being again loaded with something short 



* Sedillot, De I'influence des fonctions sur la structure et la forme des organes, 

 C.R. Lix, p. 539, 1864; ef. lx, p. 97, 1865; Lxvm, p. 1444, 1869. 



