Calamite fragment with peculiar details
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silicified calamite fragment with pith cavity
The calamites of the Carboniferous and Permian swamps and floodplains are ancient relatives of the extant horsetails (Equisetum). Except for their big size, most of them resemble the horsetails with their articulated habit of stem and branches and their material-saving construction: strong tubes, originally filled with pith, later empty. The pith cavity extending along the wooden members becomes much narrower and eventually vanishes at the ends, which makes sense since this strengthens the joint between the members. The silicified fragment presented here shows just such an end of the pith cavity (Fig.1), however with unexpected additions. 

Fig.1: Fragment of a silicified calamite branch, Lower Permian, Saar-Nahe Basin, Germany.  Width 5.5cm.

The striations converging towards the end of the cavity reflect the principal structure of calamite wood [1], with growth beginning as narrow wooden wedges, radially arranged with primary pith rays in between, which eventually fuse into homogeneous wood not much differing from conifer wood, depending on the calamite species.
An innermost layer of the wood of about 1mm or less, with the characteristic feature of the calamites, the carinal canals, seems to have decayed before silicification. The striations are not clearly seen everywhere so that their number can only be estimated as about 70.  
What incidentally looks like a weathered statuette in a vault defies an easy explanation.
It is a hard conglomerate of various components glued together by amber-coloured chalcedony. Unexpectedly, it is related to the surrounding wood in intricate ways:
First, it is partially undercut, with the gap seen here as dark shadows probably indicating the lost inner wood layer. If so, it must have become silicified earlier than the wood. This assumption is substantiated by the presence of a few imprints complementary to the lost tips of the wood wedges (Fig.2) and by some remaining wood in the depth of the gap. What remains unexplained is which kind of process could cause the innermost wood
to decay so evenly that the pith cavity wall looks as nicely striated as it does.
Second, at least two details of the conglomerate are outgrowths of the wood protruding into the cavity, as indicated by irregularities of the striation pattern. The "head" is a wooden knob around which the striations are curved as if they were streamlines (Fig.3). The knob has got curved wood cells aligned along its surface. There is another connection between the wood and the conglomerate below right in Fig.1, also with deviations in the wall pattern. Concerning the meaning of such structures in the pith cavity, no explanation is attempted here.
calamite detailcalamite with pith cavity, detail
Fig.2: Lost calamite wood wedge tips preserved als imprints in the chalcedony above.

Fig.3 (right): Calamite wood, irregularity in the striation pattern due to a wooden knob grown into the cavity. 

There is no bark left on the specimen, as usual. The wood is rather dense, resembling coniferous wood in cross-section, tracheid diameters 30-40Ám. The secondary pith rays are numerous and narrow (Fig.4). Slightly broader pith rays tend to be there where in an early growth stage the primary rays had been.
calamite wood with pith rays
Fig.4: Calamite wood fracture face (above, dark) and wall face of the empty
pith cavity (below), textured according to the cellular structure.

chalcedony in calamite pith cavity with root, AstromyelonFig.5: Chalcedony fill of the cavity on the back of the calamite fragment, with root cross-section (right) and quartz with black mineral deposit, probably hematite (left).
calamite root cross-section, Astromyelon
Fig.6: Cross-section of calamite root preserved in chalcedony,
width 1.2mm.

The depth of the present fragment is only about 2cm. Within this distance the cavity diameter reduces from about 2.5cm at the front of the fragment as seen in Fig.1 to about 1cm on the back. There the cavity is filled with coloured chalcedony. Enclosed is an object with well-preserved tissue seen on its circular cross-section (Figs.5,6). With large pith cells in the centre surrounded by a wooden ring, its structure agrees with that of a tiny calamite root pictured in [2], named Astromyelon. One could think up ways of how it got into the cavity: grown there while the stem lay broken on the ground, washed into the cavity together with the debris seen in Fig.1, or being another unexpected outgrowth of the calamite wood. Perhaps the matter must remain undecided. 
Sample: Surprise find at a shallow pit dug on a field near Schallodenbach, Saar-Nahe basin, Germany, on the occasion of a field trip guided by H. Kerp in connection with the IOPC VIII conference,  Bonn 2008.
As an unexpected side effect, Fig.1 can be turned into an optical illusion, as demonstrated here.  
H.-J. Weiss     2012

[2]  T.N. Taylor et al.: Paleobotany. Elsevier 2009. Fig. 10.52.
quartz crystal with wood inside
Fossil Wood News  13

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