Ventarura xylem
deutsche Version
The last discovered early land plant in the Rhynie chert, Ventarura [1], which is Latin for "windy field" and refers to the Windyfield farm near Rhynie, is not abundant but also not really rare so that one may wonder why it had been overlooked for decades, all the more so since it can be recognised by a unique feature which remains conspicuous even in stages of decay: It is an apparently rot-resistent fraction of tissue shaped like a tube, 
seen on cross-sections as a mostly dark concentric ring of well-preserved cells. It had been thought to consist of sclerenchyma [1] but does not, as discussed in Rhynie Chert News 58 [2].Ventarura

Part of this peculiar dark ring, named "middle cortex" in [1], is distinctly seen on the raw surface of the sample in Fig.1. An illusion of mechanical strength of the "middle cortex" arises from the weakened or vanished "outer and inner cortex" tissue.
An outer contour deformed by shrinkage before silicification in combination with a persistent circular ring in the middle of the cortex is a characteristic feature of Ventarura if silicified while slightly shrivelled.
The space inside the ring in Fig.1 had undergone several transformations. Judging from the levels at the bottom it had been filled with water where precipitate grains formed suspensions which settled down into not quite distinct horizontal layers. The larger part of the cavity became nearly filled with silica gel grown as a coating along the walls, later turning into chalcedony, partially stained yellow with iron oxides. The white stain, as usual, is brought about by light reflection from Ám-size crystallite grains in the transparent chalcedony.
The innermost part of this stump is of particular interest as it offers a rare sight at a xylem cross-section (Fig.2).
Ventarura xylemFig.1 (right): Ventarura stump protruding from an old fracture face in Rhynie chert, appearing as a cross-section in top view. Note the combination of deformed contour and circular ring which is typical for wilted Ventarura.
Image width 10mm.


Fig.2 (left): Ventarura xylem cross-section, detail of Fig.1. Image width 0.9mm.

Ventarura xylem cross-sections are seldom distinctly seen. Therefore, another example with distinctly seen xylem, Rhynie Chert News 91 , is shown here once more: Fig.3.
VentaruraVentarura xylem
Fig.3 (left): Ventarura cross-section with well preserved xylem and the very seldom seen epidermis on the right. Image width 2.2mm, xylem width 0.45mm.

Fig.4 (right): Ventarura xylem with scalariform tracheid wall pattern.
Image width 1mm, xylem width 0.33mm, same scale as Fig.2.
Photograph by Gerd Schmahl  


Apparently this contribution does not address one of the many problems concerning the early land plants. It is meant to raise interest in the unsolved problems by drawing attention to the beauty of details like the complex stump in Fig.1 and the golden staircase in Fig.4. It can be assumed that pondering about the details will be helpful in solving the real problems, as in this case, the purpose of the persistent tube in Figs.1,3.

 
Samples:  Rh11/94 (0.2kg), found in 2011, Part2: Figs.1,2;   Rh2/143 (0.9kg), obtained from
Shanks in 2008, Part3: Fig.3;   Rh2/29 (? kg), obtained from Shanks in 2001, Part4: Fig.4;

H.-J. Weiss     2021

[1]  C.L. Powell, D. Edwards, N.H. Trewin: A new vascular plant from the Lower Devonian Windyfield chert, Rhynie.
      Trans. Roy. Soc. Edingurgh, Earth Sci. 90(2000 for 1999), 331-349.

[2]  H.-J. Weiss: Ventarura with less-common features, www.chertnews.de,  Rhynie Chert News 58.
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