82 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



Other properties of cellulose that there can be no doubt but that the 



more modern view is in essence correct. Thus the tensile strength of 



cellulose compares very favourably with that of metals (Table IV) and 



this could hardly be so if the substance were composed of isolated 



micelles. 



TABLE IV 



Tensile Strength of Cellulosic and other Materials 



Cellulose occurring in plant cell walls* can therefore be thought of 

 rather loosely as a two-phase system. In one phase the chains are 

 arranged in a regular crystal lattice into which water cannot penetrate, 

 and other substances penetrate only with difficulty, and in the other 

 the assembly of chains is non-crystalline. Into the latter, water can 

 penetrate easily (causing swelling) and other substances rather easily 

 depending on their molecular size, electric charge and so on. It should 

 be obvious, therefore, that while the anisotropy of physical properties 

 depends on the degree of ahgnment of the cellulose chains (and there- 

 fore on the orientation, angular dispersion and so on of the crystalline 

 fractions) the precise value of any property in any particular direction 

 in a piece of cellulose will depend to a large extent on the non-crystaUine 

 fraction. Thus such properties as swelling, moisture absorption, density, 

 extensibility, rigidity and time effects on mechanical properties will 

 depend to a large extent both on the degree of alignment in the non- 

 crystalline fraction and on the relative amount of such non-crystalline 

 material present. The importance of this has perhaps not been stressed 

 in the botanical literature as it has in the technological, but it must 

 equally be of importance. 



Such a division of the cellulose matrix into two phases is, however, 

 quite arbitrary; and it is certain that between the rigidly spaced chains 

 within a micelle and the most randomly oriented chains which may 

 occur in the larger spaces, there are all degrees of order. It is therefore 



