Psaronius conducting strands with peculiar aspects
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The tracheids making up the conducting strands in the centre and in the aerial roots of Psaronius tree ferns are known to show scalariform wall patterns if suitably preserved, as in Fig.1. There, a crack had taken a path along two tracheid walls, seen right and left of a tracheid with bluish fill showing a faint opalescence. The picture plane has been chosen such that it essentially coincides with the crack face on the natural surface of this chert sample.
Chert surface with Psaronius tracheid walls
Fig.1:  Two Psaronius tracheid walls on the raw surface of a natural chert fragment, with "scalariform" pattern, steps 10Ám. Width of the picture 0.87mm.

Psaronius tracheid walls of quite different aspect are seen in another chert sample in Fig.2. They appear as an arrangement of surprisingly straight and uniform thin planks, resembling an artistic design rather than plant remains.
Psaronius tracheids
Fig.2 (left):
Psaronius tracheid walls
seen here as if emerging from the depth below the cut face of the transparent chalzedony.
Width of the picture 1mm.


With some effort one may see the scalariform pattern on a few walls in Fig.2, as in the lower left quarter of the picture.   Owing to both foreshortening and slightly lower magnification, the patterned walls are less well visible than those on Fig.1.
The triple points or lines where 3 tracheid walls meet are prone to decomposition before silicification so that the tissue appears disintegrated into a bundle of straight walls here. The thickness of some of them seems to be as low as 3Ám or less. Hence, the visible pattern must be only an imprint of the scalariform structure seen in Fig.3, after the larger part had broken off.

Psaronius tracheids, scalariform
Fig.3: Psaronius tracheid walls with larger parts of the scalariform structure still adhering, seen at the cut face and below in transparent chalzedony. Spacing 10-11Ám.
Width of the picture 1mm.
Same scale, same sample as Fig.2.





In this sample, most conducting strands with well visible tracheid walls similar as in Fig.2 show a peculiar feature: The tracheids positioned around the cirumference of the bundle look like empty thin-walled boxes with one or two walls missing. There are more of them seen as cross-sections in Fig.4. The place where the outer wall must have been can be guessed from the colouring in some cases. Why the outer walls of the outer tracheids are missing while the inner walls are conspicuously distinctive, remains an open question here. There may be no walls missing on conducting strands in other Psaronius samples, as in
Permian Chert News 12, for example.
The tracheids are of various shapes: Their cross-sections vary between triangle (below) and septagon (above right). The walls appear much thicker than those in Fig.2. (Note the factor 2 in magnification.)
Psaronius tracheidsPsaronius tracheids

Fig.4: Psaronius conducting strand cross-section with apparently separate or missing tracheid walls. Width of the picture 1.5mm.

Fig5 (far right):  Psaronius conducting strand cross-section with cohering tracheid walls. Width of the picture 1.5mm.

There is a conducting strand of less-common aspect in the same sample: Fig.5. Here, either of two adjoining tracheids contributes to the common wall apparently consisting of three layers, with the thin dark middle layer probably representing the 3Ám-walls as seen in Fig.2. Considering that the middle layer had been formed from two cell walls, it is a double layer, although not visible as such in the present images.
Apparently the composite walls can disintegrate, beginning at the triple points of the network. Similar thin lines along the walls between tracheids of conducting strands have also been seen on other Psaronius cross-sections, though less distinct:
Permian Chert News 12 , (there Fig.6).
The thickness of the composite walls in Fig.5, 20...25Ám, is about the same as that of the walls in Fig.3 with a row of tubes on either side, as expected. The wall pattern elements, seen here as straight tubes, are usually thought to be a means of stiffening, which seems questionable.

Small pale spots on part of the raw outside of this sample which is deep red inside throughout, offer a surprisingly different aspect of the conducting strands: Fig.6. Apparently the chalzedony had recrystallized in a thin surface layer of this trunk fragment and thereby turned whitish, as it is known from flintstone, for example. Other than the transparent chalzedony in the above pictures, the whitish aspect is due to light reflection at crystals whose sizes exceed the wavelength of light. The coarser crystalline structure also reveals itself by slowly soaking up water, thereby turning dark.  

Psaronius tracheids, white chalzedony
Fig.6:  Psaronius conducting strand broken across, of different aspect resulting from recrystallization at the surface, same trunk fragment as the polished cut faces in Figs.2-5. Width of the picture 2.5mm, same magnification as Figs.4,5.

The tracheid fills, conspicuous in Fig.6, less so in Figs.4,5, and nothing except clear chalzedony in Fig.2, seem to be the result of locally differing conditions during silicification. The largely different aspects as represented by Figs.2,4,5,6 may be found even within one conducting strand, with smooth transitions between them. They may also be found in the central strands of the aerial roots. (Related pictures will be shown in a forthcoming contribution.)
As mentioned above, the colour of Fig.6 is an exception since it has formed after fragmentation of the petrified Psaronius trunk, which could have been as late as in the Cretaceous, judging from tiny remains of sandstone sticking tightly to the surface of the trunk and to the old fracture faces.  

This sample is a fragment of a Psaronius tree trunk apparently collapsed from circular into flat cross section when the parenchyma between the conducting strands decayed. Apparently there had not been  more squeezing involved since the conducting strands and their cells do not look squeezed. Often the centre with the conducting strands is more squeezed, as in Permian Chert News 6
.
This sample is one from the rare "wine-red" variety of the "red", or rather ocre, fossiliferous cherts in the Lower Permian Doehlen basin in Saxony, Germany. It had been found by S. Weiss in about 1991 as the only piece of red chert in a glacial stream deposit cast open on the area of the Wilmsdorf golf course near Dresden.
The samples are kept in the own collection under the labels W/19 (Fig.1) and W/20 (Figs.2-6).

H.-J. Weiss      2019
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