Anatomy of recent and peatified Calluna vulgaris stems: implications for coal maceral formation

Abstract Anatomical characteristics of peatified Calluna vulgaris stems isolated from a selection of Northwest European raised bog peat deposits were compared with intact stems in order to reveal anatomical modifications caused by the peatification process. Four main decomposition processed were discerned: (1) loss of structural integrity of cell walls and cell inclusions; (2) gelification and swelling of cell walls; (3) discolouration of cell walls; and (4) deposition of matter in cell lumina. Fibre-tracheids and wood parenchyma appear to be the most affected cell types. The two main decomposition trends discerned in these were related to different fungal types. Precipitation of decay products was mainly observed in vessels. Bark tissues, primary xylem and medullary parenchyma were characterized by an excellent anatomical preservation. In the first stages of peatification a rapid decrease in cell contents was observed in bark tissues. Swelling and gelification was mainly seen in cell walls of the cambium and vessels. We conclude that different types of cell walls respond differently to decay conditions. This is probably related to differences in the original cell wall chemistry. A high intra- and intracellular variability in degree of preservation was sometimes observed in identical tissues. The microscopic variability arising during peatification of wood and bark testifies to the heterogeneity of coal macerals derived from these tissues.