A small seed fern stem with peculiar structure
deutsche Version 

Medullosean seed ferns have been regarded as some of the most interesting and most enigmatic plant fossils of the Early Permian [1]. Related scientific work, beginning with B. Cotta (1832), was based on finds at Chemnitz [2,3], where earthworks on construction sites yielded, and still yield, seed ferns as a rare component among other petrified tree trunks. Considering that the Döhlen basin is of similar geological setting and mere 50km away from the fossiliferous site at Chemnitz, one wonders why apparently no seed fern stem except the small fragment seen in Fig.1 has been found here, although other plant fossils, including tree fern stems and foliage, are found abundantly in the cherts. The pictured one may not even be from here: Its yellow-brown aspect is not typical for samples from Döhlen basin, and as it had been found among scattered pebbles including flintstones, it may well have been carried along by some Pleistocene stream.

Fig.1, below: Cross-section of a seed fern stem fragment, slightly deformed before silicification, with structured center surrounded by 2cm of continuous secondary wood. Width of the sample 40mm.Seed fern stem fragment
Wood of seed fern stem cross-sectionFig.2: Secondary wood as the main constituent of the seed fern stem in Fig.1. Cell sizes 80-100µm.

Fig.3, below: Sclerotic structure in the center of the seed fern stem, detail of Fig.1. Width of the picture 10mm.

Sclerotic structure in the center of the seed fern stem
Sclerotic structure in the seed fern stem center
Fig.4: Sclerotic loops in the center of the seed fern stem, detail of Fig.1, same scale, orientation of the U-shaped components indicated on the right.

Obviously the center with several dark loops within a seemingly structureless matrix is small compared to the area occupied by the surrounding wood. Near the circumference the wood is degraded and squeezed and possibly partially lost, hence the original radius of the stem must have been larger.

The loops are the peculiar feature to be looked at. The innermost loop is closed (not seen on this cut face) so that one may assume that all the loops are closed and hence are cross-sections of a tubular structure extending along the stem. A discontinuity seen in the outer loop is most probably due to rupture before silicification.
Every loop consists of cross-sections of what seems to be separate strands (Fig.5), neatly arranged side by side forming a chain, about 30 of them for the small innermost loop and about 300 for the largest loop. Cross-sections of sclerotic strands in seed fern

Fig.5:  Cross-sections of sclerotic strands in the seed fern center. Width of the picture 2mm.

Every strand cross-section consists of what looks like sclerotic cells arranged more or less orderly in one to four files of about 4 to 10 cells. The length of the files varies between 0.3 to 0.8mm.
The ends of the strand sections are unequal: One end is more or less rounded, the other one is more edgy as if something had been torn off, hence the strand sections appear more or less U-shaped. All the “U”s of one loop have got the same orientation with respect to the loop: pointing either inward or outward. All the “U”s of the outermost loop, for example, are oriented such that the open end of the U points outward.
Not seen on this face is the changing connectivity of (some of) the loops along the stem. The question arises how to relate the loops to structures known from other seed ferns, and what purpose the loops may have served. A clue may be provided by the observation that largely decayed wood is vaguely seen in one place adjacent to the outside of the outer loop. Very faint indications of the former presence of attached wood are seen inside other loops, too. The vanished wood seems to have been always attached to the upper side of the "U"s.
By imagining the vanished secondary wood being restored, some similarity with the "star rings" and larger components of Medullosa stellata and Medullosa leuckartii appears. (The structure and tissues of palaeozoic seed ferns and their interpretation have been discussed in [4,5], where it appears that the matter is rather complex.)
Considering that the outer wood of the present sample is well preserved and the wooden components of the Medullosa stems are usually so, too, their reduction to a few cell layers forming some kind of sclerotic tubes, seen as loops in the present section, poses a problem.
As one thinkable option, the small amount of secondary wood of the strands within the pith was of no use to the grown-up plant any more and therefore was left to degrade, with the exception of the small initial fraction turned into sklerenchyma in order to obtain a fibrous component with high tensile strength, which would be advantageous for a vine-like growth habit.
Sample: found in 1994 at Pesterwitz near Freital, kept in the own collection under Pe/6.

H.-J. Weiss     2013

[1]  R. Rößler: Der versteinerte Wald von Chemnitz. Museum f. Naturkunde Chemnitz 2001.
[2]  B. Cotta: Die Dendrolithen in Bezug auf ihren inneren Bau. Leipzig und Dresden 1832.
[3]  O. Weber, J.T. Sterzel: Beiträge zur Kenntnis der Medullosen, Ber. Naturwiss. Ges. Chemnitz 13(1896), 44-143.
[4]  R.W. Baxter: Some pteridosperm stems and fructifications with particular reference to the Medullosae,
      Annals Missouri Botanical Garden 36(1949), 287-353.
[5]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Acad. Press 2009.
quartz crystal with wood inside
Fossil Wood News 19

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