THE STRUCTURAL FEATURES OF CELLULOSE 85 



required by the dichroism of fibres dyed in this way, for failing this the 

 assumption has to be made that the lignin actually occupies the place 

 otherwise taken by the iodine. In view of the resulting dichroism of 

 fibres stained with iodine and with, for instance, congo red (where it 

 has been shown by an X-ray method that the elongated crystals of dye 

 lie parallel to the micelle direction) this leads to the suggestion that the 

 chains in the intermicellar region are still more or less parallel to each 

 other. The same suggestion has already been put forward for sisal 

 fibres following interpretation of their birefringence (37). 



The position of the cellulosans is somewhat more difficult to define. 

 Some years ago Norman (38) concluded on chemical grounds that the 

 association between cellulose and xylan was much closer than that 

 between cellulose and lignin, and this seemed to receive full support 

 from the later discovery that removal of xylan from high-xylan fibres 

 apparently rendered the cellulose more crystalline(35(fl)). Again, 

 removal of xylan from Angiosperm wood or of mannan from Gymno- 

 sperm wood has been shown to result in a decrease in the crystallinity 

 of wood as judged by the X-ray diagram. It could therefore reasonably 

 be concluded, since xylan is built up from xylose in much the same 

 way as cellulose is built up from glucose, that the chains of xylan oc- 

 curred inside the cellulose micelles and caused there little lattice distor- 

 tion. More recently, however, other evidence has appeared which makes 

 this interpretation less attractive (39). Thus it has been shown by Brims 

 that if fibres containing xylan are treated even with very dilute caustic 

 soda to remove some of the xylan, then the cellulose in the residue has 

 a shorter average chain length. Since it seems very unlikely that the 

 caustic soda could penetrate the crystalline cellulose to any marked 

 extent, it seems most likely that some, at any rate, of the xylan is 

 associated with the non-crystalline fraction even though it does appear 

 to be associated with the cellulose in the same molecular chains. Even 

 more critical, however, is the statement by Hirst that carefully prepared 

 samples of xylan are composed of molecular chains which are branched. 

 If this is substantiated, then it would be certain that xylan does not 

 partake in the structure of the crystaUine fraction of cellulose, and the 

 earlier evidence would need reinterpretation. The last word has not been 

 said here. It will be seen later that the relationship between cellulose and 

 xylan may be vital to an understanding of some cell wall phenomena. 



Micelle aggregates. The electron microscope 



During the past 100 years or so, repeated attempts have been made 

 to demonstrate in cell walls the presence of particles much larger than 



