82 SCIENCE PROGRESS 



have a trick of converging to some unlooked-for discover} 7 . The 

 conclusion here sketched is a good instance of such convergence, 

 none of the contributory investigations being such as were likely 

 to lead to results having any bearing on the technique of a 

 spinning mill ; nor, indeed, was any such conclusion desired. 

 It may be summarised as follows. 



The familiar hairs of cotton lint are each a single dead cell, 

 having a delicate but resistant cuticle and a primary wall, with 

 a secondary thickening of cellulose. For most purposes these 

 cells may be regarded simply as cylinders, the final living 

 dimensions being from about iomm. to 50 mm. in length, and 

 from 0'022 to 0*013 mm. in diameter. In the secondary thickness 

 of the wall there are numerous oblique simple pits ; consequently, 

 when the internal osmotic pressure falls, just before the opening 

 of the capsule-fruit and the death of the lint-hair cell, the pits 

 close up, and the cylinder undergoes torsion. Further reduction 

 of pressure through desiccation allows the cylinder wall to 

 collapse, and this collapse follows a spiral line, forming convo- 

 luted ribbons with thickened edges, which are the characteristic 

 cotton lint-hairs of commerce. 



The breaking-strain of the normal lint hair-cells depends 

 almost entirely on the thickness of the secondary deposits of 

 cellulose, which is itself determined by inheritance and by en- 

 vironment, just as in the case of the length of the same cells. 

 Incidentally this thickness will obviously influence the width 

 of the ribbon, and also the amount and pitch of the convolutions. 

 The breaking-strain, or lint hair-strength, has long been known 

 to have no appreciable effect on the yarn-strength, since only 

 about 20 per cent, of the total available tensile strength in any 

 cross-section of yarn is actually obtained in that yarn, showing 

 that the strength of yarn depends principally upon the grip of 

 adjacent fibres on one another. Yarn breaks primarily by slip, 

 and not by rupture of individual hairs. 



The results obtained by introducing pure-strain lint to the 

 spinner have been most instructive. In order to control the 

 seed supply of Egyptian cotton, four existing pure strains which 

 had been bred out or isolated from the commercial varieties were 

 propagated into bulk. The lint obtained from such pure-strain 

 populations is necessarily more regular in length and breaking- 

 strain than that from impure varieties, but nothing was known 

 about the spinning properties of the lint of these strains when 



