84 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



on p. 56 in such a way that the matrix was optically isotropic only in a 

 hquid whose refractive index was the same as that of silica. Here, then, 

 must have been a clear-cut topographical separation between cellulose 

 and sihca. Similarly the same worker showed more recently that in 

 fibres wherein silver particles are induced to grow, the silver particles 

 range in size from very small (say 10-15 A. wide) up to 100 A. or more, 

 and since these spaces are filled with silver they can hardly be con- 

 sidered to contain any cellulose chains at all. It would seem, however, 

 quite certain that in this latter example the growth of the larger silver 

 particles at any rate had involved the displacement of cellulose chains, 

 so that the size of the particles is no guarantee that empty spaces of this 

 size occurred in the original untreated cellulose. 



The distribution of the incrusting substances 



From the botanical point of view it is of interest to notice the dis- 

 tribution of the incrusting (i.e. non-cellulosic) substances in the wall in 

 terms of this roughly two-phase conception of cellulose organization. 

 Since the lattice energy of the crystalline fraction is so high that even 

 water does not penetrate, then it might be expected that non-cellulosic 

 substances in cell walls would be found outside the crystalline regions, 

 unless they happened to be such as to form, as it were, "mixed crystals" 

 with the cellulose. This is found, in fact, to be the case. 



Just as the removal of water from native cellulose has no effect on its 

 X-ray diagram (and therefore no effect on the crystalhne lattice) so the 

 removal of lignin from lignified material has no appreciable eff"ect. This 

 is illustrated in Plate II, Fig. 3, for hemp fibres. The lignin must there- 

 fore occur outside the crystalline regions (35). Pectin is in somewhat 

 the same position; in fact this substance even occurs in, for instance, 

 collenchyma cells in separate wall layers which are almost free from 

 cellulose (36). This positioning of lignin in particular around the 

 crystalline fraction calls to mind the fact that in lignified material, such 

 as wood, it is difficuU to obtain a positive (blue) reaction for cellulose, 

 on treating with iodine and sulphuric acid, until the lignin is removed. 

 On the hypothesis of discrete micelles with real surfaces this was readily 

 understandable in terms of a complete covering of the micellar surface 

 by Hgnin. On the more recent modification of this hypothesis, a similar 

 interpretation is, however, still possible if we assume that the chains of 

 cellulose responsible for the reaction occur somewhere in the transition 

 region between the crystalhne (in the X-ray sense) and the non-crystal- 

 line fractions.* This particular location would also appear to be 

 * i.e. possibly where the chains are still parallel but not spaced regularly. 



