Carbohydrates chemical degradation can occur at two distinct levels. Degradations occurring at the fibres weak spots and fibre ends (in red) and degradations affecting the whole of the fibre which can be discriminated at the ultrastructure level (in blue). Crossed green dots represent degradations mostly confined to external layers (P, S1 and S3) and purple dots illustrate more homogeneous degradations affecting the wide S2 layer as well.