STRUCTURAL VARIATIONS IN HOMOLOGOUS CELLS 167 



Confirmation of the presence of flatter spirals in these outer layers 

 comes from the observation that spiral markings can sometimes be 

 seen on these outer walls where inclination to cell length is closely 

 similar to the inclination of the cellulose chains as predicted from the 

 above relation. There can therefore be no doubt but that the observed 

 variation in /7^" with length is largely attributable to a change in the net 

 orientation of cellulose chains. Reorientation alone will not, however, 

 explain the decrease in n^ with increasing length; for a moment's 

 reflection will show that simple change in orientation implies a rotation 

 of the index ellipsoid about the «„ direction so that n^ (=«„) should 

 be invariate. The observed change in «„ must therefore be associated 

 with variation in other subsidiary factors. One possibility is that the 

 angular dispersion of the cellulose chains is less for longer fibres, i.e. 

 the "micelles" are more nearly parallel to each other in longer fibres. 

 This would clearly involve a small decrease in n^ coupled with a small 

 increase in 77^". An increase in the crystalline/non-crystalline ratio in 

 longer fibres would also give the same effect and it is therefore perhaps 

 of significance to note that there is some evidence that this may indeed 

 be a factor involved here (48(«)). The first observation of importance 

 made was that longer fibres have a higher density (Table XVI). Densities 



TABLE XVI 



were determined on fibres dried in vacuo over phosphorus pentoxide by 

 flotation in mixtures of carbon tetrachloride and nitrobenzene, both 

 bone dry, and the fibres were given a preliminary purification to Cross 

 and Bevan cellulose. The figures for the density refer therefore only to 



