Pith in petrified wood from the Kyffhäuser mountains
deutsche Version

The central pith in extant trees, with diameters of no more than a few millimeters, seems to be relevant in early growth stages only. With increasing diameter of the wooden stem, it becomes more and more irrelevant. This tendency is also seen in paleozoic wood. Most often, petrified wood of the coniferous type is found as fragments. In rare cases a fragment includes the centre of the trunk with well preserved pith. This differs clearly from the surrounding wood (xylem) with its orderly radial cell files but the transition area between pith and wood is confusing. The wood bordering to the pith, with cells much smaller cells than those of both pith and "normal" wood, are usually called protoxylem, which does not explain anything.
petrified wood

The fragment seen here mimicks the cross-section of a calamite stem with broad radial streaks but it is coniferous-type wood with a conspicuous central pith area. By lucky incidence it can serve as a suitable object for demonstrating the contact region between pith and wood. A red spot of about 2mm with heightened contrast due to thin layers of hematite deposited on the cell walls provides an unexpectedly clear image of the tissue (Fig.2).

Fig.1: Fragment of petrified conifer trunk with large pith and broad radial streaks of unknown cause in the wood, with well preserved cellular structure, locally with deposits of red hematite on the cell walls, notably in a red spot at the boundary of the pith below left.
Width 9cm.

Fig.2: Boundary of the pith area in Fig.1 with small-cell (15-25µm) protoxylem (?) between large-cell pith (100µm) and wood (30-50µm), here particularly clearly seen owing to hematite on the cell walls, 
nevertheless difficult to understand.  Width of the picture 2mm.

Fig.3 below right: Typical coniferous wood from Fig.1, with radial cell files and pith rays, cells 40-50µm.
Width of the picture 2mm.
pith and wood


When magnified slightly as in Fg.1, not only the alternating radial streaks but also the apparent "wood wedges" pointing into the pith seem to mimick calamite wood.
In the middle of Fig.2, the pith seems to force its way between two wood wedges but does not continue as a wide primary pith ray as it would do with calamites. It simply is replaced with wood (near the lower boundary of Fig.2).
Well away from the confusing transition area between
pith and wood with its varying cell sizes, there is the plain coniferous-type wood (Fig.3) with the well-known narrow secondary pith rays.
A central pith was absent in the first land plants, quite large in the calamites, and much smaller in the tree trunks with coniferous-type wood. In the present case the pith is as large as with the cordaites, the close relatives of the conifers, but the wood is that of the "common" paleozoic conifers, summarily called Dadoxylon
This uncommon sample contains the hematite not only as a red stain made of thin translucent platelets but also in a massive form as kidney iron ore, 5mm wide, on the back.
W.+ G. Etzrodt have found this sample in their gravel pit at Borxleben, recognized it as something special, and provided it for investigation. It is kept under the label KyB/120.

H.-J. Weiss   2017

[1] K. Mägdefrau: Die Kieselhölzer im obersten Oberkarbon des Kyffhäuser-Gebirges. Ber. Dt. Bot. Ges. 71(1958), 133-142.
quartz crystal with wood inside
Fossil Wood News 27

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