A well preserved Psaronius fragment from Döhlen Basin
deutsche Version

Stems of the tree fern Psaronius are not rare in the fossiliferous cherts of the Döhlen Basin. They had been silicified along with fragments of the related foliage while lying prostrate in a swamp. Usually they were in a state of partial decay and squeezed flat before accumulating enough silica gel for final stabilisation of the shape. Even with the best preserved stem fragment found so far, only the aerial roots have retained their shape but the centre of the stem, where the stabilizing silica got latest, was not able to withstand the pressure so that
most of the xylem and sclerenchyma strands collapsed. (See Permian Chert News 6.)  So the unexpected find of a fragment of an uncommonly well preserved central part of Psaronius remains a cause of wonder (Fig.1).
Psaronius centre
Fig.1:  Cross-section of a (half) Psaronius centre, well preserved, locally deformed, estimated original diameter about 10cm, width of the picture 4cm. Photograph: M. Barthel.

With its dark aspect and the virtual absence of any colour it differs conspicuously from all specimens found in the chert but rather resembles the abundant fragments of fossil wood. Possibly it has never been a constituent of a chert layer but became silicified along with coniferous-type tree trunks while embedded in volcanic "ash".
Most of the conducting strands with scalariform tracheids have retained their original shape while the parenchyma between the strands collapsed under slight compression in some places. As a result, the strand cross-sections seem to converge
to the right. What is most interesting about this sample is not its overall aspect seen in Fig.1 but the details of the well preserved tissues.
Psaronius centre

Fig.2 (left): Psaronius conducting strands embedded in slightly damaged parenchyma. Width of the picture 11mm.

Fig.3 (below): Several phenomena revealed by magnification of Fig.2: small cells supposed to be phloem next to the big
xylem tracheids, a small sclerenchyma strand stained black, parenchyma locally replaced by cell-size clots.
Width of the picture 3.6mm.

 Psaronius centre tissues

As seen in Figs.2,3, there are places on the cross-section where the tissues are apparently not deformed. 
The small cells near and between the xylem tracheids, supposed to be phloem, are not clearly separated from the larger parenchyma cells.
By comparison with numerous similar phenomena it can be stated that the dark clots in places of damaged tissue (Fig.3),
usually thought to be mite coprolites, also in the case of Psaronius [1], are the result of fungus activity inside cells.
Psaronius centre 
Figs.4,5,6 (left and below): Details of Psaronius conducting strands.
Width of the pictures 1.4mm, 0.44mm (Fig.6).

The images show several features which seem to be characteristic for the conducting strands of Psaronius:
(1) The tracheid cross-sections differ largely in shape and size.
(2) At some spots on the outside of the conducting strands, 
tracheids seem to vary from phloem size to full size (Fig.4).

Psaronius centrePsaronius tracheids(3) The aspect of cross-sections strongly suggests the presence of a gap of 10-15µm between adjacent tracheids (Figs.4,5), which turns out to be an illusion due to the absence of a visible middle lamella in most cases.
(4) A middle lamella is seldom seen as a very thin dark line along the apparent gap (Fig.6).
(5) The dark aspect of the tracheid walls (Figs.2-5) is a secondary phenomenon. The deposition of a dark stain begins at the junctions between the tracheids and may
spread along the colourless cell walls (Fig.5).
(6) Phloem cells are distributed among the tracheids such that nearly every tracheid cross-section is seen to contact one or more phloem cells (Figs.3-6). Hence it can be concluded that
every one of the long and wide tracheids contacts many small phloem cells.
(7) The tracheid walls are covered with a "scalariform" pattern (Fig.7).

Psaronius scalariform tracheid walls

Fig.7 (right):  Psaronius tracheid walls on
a fracture face at the lateral surface of the sample, two of them revealing a faintly seen "scalariform" pattern with steps of 10µm. Note also the faint opalescence in the middle. Width of the picture 1mm.

A few complementary remarks may be appropriate here.
 - The tracheids of Psaronius have often been pictured as components of the well-known "star-shaped" cross-sections of the conducting strand in the aerial roots but rarely in connection with the 
conducting strands in the stem centre as in [2], Plate 35.
 - The middle lamella
along the apparent gaps between tracheids as faintly seen in Fig.6 is also pictured in [2] (The wide gap in Fig.6 above consists of one or two narrow phloem cells.)
 - The pockets of small tracheids as in Fig.4, irregularly distributed at the boundary of the conducting strands, seem to be analogous to the small tracheids at the tips of 
the "star-shaped" strands in the aerial roots [2].
 - Cracks in the chalcedony fill of the tracheids as seen in Figs.3,4,7 are liable to be mistaken for remains of cell walls.
 - The sclerenchyma, shown here only as a small cluster of cells with dark aspect in Figs.2,3, is present in this sample in the form of narrow or wide strands consisting of small cells (as in Fig.1), pale or stained dark.
 - The dark stain occasionally found on cell walls is a widespread but still enigmatic phenomenon which will be separately discussed within other context.

Sample: Found in 1991 on a small heap of small stones cleared away from an area prepared for the smooth green of the golf course at Wilmsdorf near Possendorf, Döhlen Basin. Cut into 3 parts, the middle slab W/19.2 (Fig.1) given to M. Barthel. Stored in the own collection under W/19.1 (Fig.7) and W/19.3 (Figs.2-6).

Annotation 2016: In [3], Abb.130A-D, the present sample is shown in 4 photographs with size data too small by factors 10, 5, 5, 7.

H.-J. Weiss     2014

[1]  M. Barthel, M. Krings, R. Rößler: Die schwarzen Psaronien von Manebach, ihre Epiphyten,
       Parasiten und Pilze.  Semana* 25(2010), 41-60.   *( recently re-named, former name: Veröff. Naturhist. Mus. Schleusingen)
[2]  C.G. Stenzel: Über die Staarsteine.  Breslau, Bonn 1854, p.753-893, Plate 40.
[3]  M. Barthel: Die Rotliegendflora der Döhlen-Formation. Geologica Saxonica 61 (2) 2015, 105-238.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News 12
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