WALL STRUCTURE IN THICK CELL WALLS 



141 



now seems reasonably certain that the wall structure is essentially the 

 same as in the fibres described here and, fortunately, the course of 

 lignification has been followed rather carefully. It has been observed 

 that the thickness of the outer layer in the wall increases very con- 

 siderably as Hgnification proceeds (49) (Table X). This can hardly be 



TABLE X 



Increase in thickness of the outer layer in the walls of jute fibres during the 



deposition of inner layers 



* Under the ordinary microscope. 



t Under the polarizing microscope (dark). 



X Under the polarizing microscope (bright). 



due to deposition of new cellulose within the outer layer, since at the 

 time when this increase in thickness occurs the central layer is already 

 present, so that the protoplasm is far removed from the outer layer. 

 It seems far more probable that the development of lignin from some 

 precursor already present involves a "swelhng". That this swelling 

 would lead to an increase in angular dispersion is indicated by some 

 observations on the highly Hgnified coconut fibres. When lignin is 

 removed from these fibres, then the angular dispersion decreases con- 

 siderably (Plate IX, Fig. 1). It follows as a natural corollary that as the 

 developing fibre lignified, the angular dispersion increased. 



Such dispersion would then materially reduce the contribution the 

 outer layer would make to the diagram; and remembering that the 

 chains here are in a flat spiral anyway, so that the arcs are already 

 widely spread, this might cause a virtual disappearance of the diagram 

 of the outer layer except as a contribution to the circular "halo" which 

 accompanies the arcs in all these imperfectly crystalline bodies. This 

 explanation must be accepted with some reserve at the moment, since 

 the sisal fibres used here had been retted, so that we have no informa- 

 tion as regards the lignin content in the naturally occurring fibre, and 



