Pith rays in Permian wood
deutsche Version

Looking for pith rays in fragments of silicified wood is an easy means of selecting the less-common specimens from a large majority of the usually less interesting samples summarized as Dadoxylon. Three wood samples from the Lower Permian Doehlen basin with quite different aspect are shown here. One of them, although most probably Dadoxylon, is interesting for several reasons nevertheless. 
torn Permian woodstained Permian wood

Figs.1,2: Coniferous wood with narrow pith rays, one or two cells wide, among wood tracheids.
Picture widths 2.5mm,
same scale. Sample W/55.

Fig.1: Stem cross-section, slightly inclined, therefore pith rays cut not quite lengthwise.

Fig.2: Lengthwise section of deformed degraded wood, pith rays visible as cross-sections, some of them torn asunder.

It took some effort to refute the favourite interpretation of this black petrified wood as fossil charcoal. Careful inspection of the sample revealed wood tracheids bent like limp tubes as in Fig.2. This and other evidence shows that the wood had become degraded and torn while still soft, then silicified. As a remarkable fact seen in Fig.2, the cracks chose always a path through the pith rays and their cells, then between the tracheids but never within.
Concerning crack propagation, there seem to have been subtle differences in fracture toughness of the degraded wood before silicification: The uncommonly well seen narrow crack in Fig.1 avoided the pith rays but took a long multiply kinked path between the tracheids. 

Permian pith raysPermian pith rays
Wood which had got squeezed before silicification is seen here:

Figs.3,4: Wood of uncertain affiliation, pith rays 1-3-(5) cells wide, among collapsed wood tracheids.
Picture widths 1.7mm, same scale as above. Sample W/45.

The pith ray cross-sections in Fig.3 may be compared to the much smaller ones in Fig.2. Other than in Fig.2, the tracheids are collapsed in Fig.3, as seen on the wood cross-section in Fig.4. Here the pith ray cells, too, had become compressed but less conspicuously deformed. Their size makes them appear more similar to calamites rather than to conifers.

Despite of the random arrangement of wood tracheids in Fig.5, there seems to be no deformation involved. Perpendicular to this tangential cut plane, on the cross-section of the trunk (Fig.6), the radial files of wood and pith extend straight towards the edge of the sample.
Faintly seen is a "brick wall aspect" of the pith in Fig.6, similar as in [1]. The big pith rays indicate a calamite affiliation. No significant divergence of the radial files is apparent on the available cross-section, hence it must be a fragment of a rather big trunk.
calamite woodcalamie wood
Wood similar as Arthropitys, pith rays up to about eight cells wide, among wood tracheids.
Picture widths 3mm, 1.5mm,
same scale as above. Sample Bu7/20.

Fig.6: Cross-section of the trunk with tracheids and lengthwise cut pith rays with faintly seen "brick wall aspect".
Fig.5 (far left): Tangential section of the trunk showing cross-cut pith rays and lengthwise cut wood tracheids seen as randomly curved tubes. Detail of a picture taken by H. Sahm.

The find of this sample at the "maggot stone" locality in the Döhlen basin in 2000 had led R. Kretzschmar (Chemnitz) to the assumption that there were not yet unearthed big tree trunks with large pith rays. His assumption, made public at www.kieseltorf.de, really led to the discovery of the world's biggest calamite tree trunk in Chemnitz. The story of that discovery and subsequent developments have been told in Fossil Wood News 15. 

Samples: W/55(ca.0.6kg), W/45(ca.40g): found in 1991 at Wilmsdorf, Possendorf near Dresden; Bu7/20(ca.0.8kg), found in 2000 at Burgk/Kleinnaundorf
near Dresden.
H.-J. Weiss  2019

[1]  R. Rößler, R. Noll: Der permische versteinerte Wald von Araguaina /Brasilien.  Veröff. Mus. Naturk. Chemnitz 25(2002), 5-44. Figs.46, 49, 70, 77.
quartz crystal with wood inside
Fossil Wood News  37
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