Ventarura with enigmatic tube inside
deutsche Version
Ventarura cross-sectionConspicuous dark rings, miraculously well preserved amidst largely decayed tissue are occasionally seen on cross-sections of the early land plant Ventarura  (Fig.1). All the time while the tissue inside the ring (or tube in 3-D) decayed and vanished and the tissue outside the ring decayed and shrunk or vanished so that the originally circular outer contour crumpled, the ring itself remained unaffected, which had led to its questionable interpretation as consisting of sclerenchymatic tissue [1].
However, the rare phenomenon of apparently thick dark cell walls locally disintegrating into dark flakes and thin walls reveals that, despite of its persistence and solid aspect, the tissue of the ring is not sclerenchymatic. Also it appears that the persistence of the ring is not coupled to some dark and solid aspect, as seen in Fig.2. Also it may be concluded from Figs.1,2 that the ring had essentially kept its shape while the tissue outside became deformed or torn off.

Figs.1,2: Ventarura cross-sections, largely degraded and deformed before silicification except for a tube of well preserved tissue. Image width 10mm.

Ventarura cross-sectionThe persistent rings (or tubes in 3-D) pose a veritable problem. Their persistence among decaying tissue would not make sense unless related to some purpose in the live plant:  Rhynie Chert News 58The absence of sclerenchyma cells implies that the tubes were not meant to stiffen the plant. In view of the various inventions by early land plants to deter herbivores, a coaxial tube of prepared tissue might have served the same purpose. A tube consisting of poisonous tissue could prevent creatures from getting at the innermost sap ducts.
As a secondary effect, the poison could possibly prevent decay so that the tissue persisted. As another side-effect, the persistent tissue offered a substrate for microbes to form dark coatings on cell walls as in Fig.1.
The problem remains how the plant managed to prepare a definite part of the cortex tissue shaped like a cylindrical tube with a substance providing persistence.
  Rh15/28 (0.038kg),
obtained from Barron in 2009, Part1: Fig.1.
     Incidentally, Part2 of this sample has provided the first epidermis of Ventarura ever seen:
Rhynie Chert News 61 .
  Rh4/66 (0.16kg), found in 2009, Part3: Fig.2.

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.
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