Black and white stains in silicified wood
deutsche Version
Small fragments of coniferous-type fossil wood are of little interest in palaeobotany unless they provide evidence of processes or structures other than mere silicification of plain wood. The central pith, which clearly differs from the "proper" wood, is seldom preserved and therefore deserves attention. As a remarkable fact also seen here, the pith cell size decreases near the boundary towards the wood. In the course of silicification, part of the silica gel had formed crystallites with sizes exceeding the light wavelength so that they reflect the light and, if in large numbers, appear snow-white. In this way, parts of the pith and the wood, including the boundary region between pith and wood, had become surprisingly well visible.
The dark spot of destroyed tissue between the white cells in Fig.2 suggests microbial activity as a possible cause of this crystallisation. Not understood is the cause of the squeezed aspect of part of the cells along the boundary, notably in Fig.1.   

This sample reveals also another phenomenon which is quite the opposite of whitening: It is the blackening of cell walls, which seems to be another indication of the involvement of microbes. Individual cells may differ clearly from their surroundings by blackened walls.
pith and wood
pith and wood
Coniferous-type wood with pith, whitened cells, and blackened cell walls;
Lower Permian.
Height of the images 1mm.

Pictures taken from the two cut halves of Sample Kc/24 found in the 90s at Kleincarsdorf
near Kreischa,
Doehlen basin, Saxony.

From the observational fact that the black stain occasionally does not cover the whole cell wall but only part of it as seen with some cells in Fig.2, one may conclude that it is due to the spreading growth of some microbe. 

Cell walls with black coatings can provide the illusion of cells with dark fill, as in Fossil Wood News 21 , for example. There can be more than one cause for the dark appearence of cross-sections of individual cells. Varying shades of gray along the tracheids in Fossil Wood News 35 (there Fig.2), seem to be due to microbes which had lived in the cell lumina before silicification. 
The conspicuous lonely cell with white lumen and black wall in Fig.1 serves as evidence that the processes of blackening and whitening did not mutually interfere. The same applies to the white spot spreading across a few black cell walls in Fig.2.
Thin black coatings on surfaces and cell walls of the early land plants in the Rhynie chert are also due to microbial activity.
(See Rhynie Chert News 83, 87.)

H.-J. Weiss  2019
quartz crystal with wood inside
Fossil Wood News  36
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