CHAPTER IV 



Physical Methods for the Investigation of Structure in 



Plant Cell Walls 



IT SHOULD be clear from the preceding chapters that the cell walls of 

 plants owe their peculiar properties to the presence of cellulose, so 

 that investigations of structure in these walls is to a large extent an 

 investigation of the structure of this substance. That is not to say, of 

 course, that the incrusting substances can be ignored. Far from it, for 

 they do modify to a considerable extent the features of cell walls which 

 would, in the absence of these substances, be determined only by the 

 cellulose itself. Nevertheless, since cellulose forms the framework of 

 the wall, so that the incrusting substances can be removed without 

 causing any loss of form in the cell, then it must receive first attention. 

 Fortunately, therefore, cellulose is so constituted that it is, in a molecular 

 sense, crystalline and confers upon cell walls some crystalline properties. 

 This is at first sight rather surprising, since cell walls never show the 

 beautiful external features, such as crystal faces, which we normally 

 associate with the crystalline state. It is nowadays, however, almost 

 axiomatic that a substance can possess internal crystallinity without 

 showing the external form of crystals. 



Since cellulose occurs in such a crystalline state, it becomes imperative 

 to consider for a little while just what is implied by the term crystalline, 

 and the methods whereby the nature of crystalline material can be 

 investigated. Attention will be confined here to two methods only — the 

 method of X-ray crystallography and the method of polarization optics. 

 These are undoubtedly the two most important tools available for wall 

 studies and, used under properly controlled conditions, they can 

 together present a rather complete picture of the organization of these 

 walls. It is not possible in the space available to give anything but a 

 brief elementary discussion of these methods, and those readers who 

 wish to go further into these matters should consult any one of a range 

 of text-books (12). In particular, attention is here confined largely to 

 the crystal system to which cellulose is thought to belong, to the 

 exclusion of all other crystal types, and the examples used are, for 



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