Enigmatic hollow straws of early land plants
deutsche Version

AglaophytonHollow straws of early land plants are occasionally seen on sections of Rhynie chert as circular rings with well preserved cells while most of the tissue is missing (Fig.1). An explanation as a result of limited diffusion depth of silica in the dead plant lying in the swamp water, as proposed in [1,2], is rejected here. Not the epidermis as the outermost layer but the tissue immediately below is well preserved, usually with cell walls stained dark.

Fig.1: Cortex tissue well-preserved immediately below the epidermis but decayed and vanished elsewhere. Width 2.5mm.

This could not have been brought about by mere diffusion. Apparently the living plant had prepared, by unknown means, part of the tissue so that it served as a protection against some intruder, with the side effect that the tissue thus prepared persisted while the majority of the tissue decayed and vanished before becoming silicified.
Such an interpretation involving something more complex than mere diffusion could be compatible with the sudden onset (or ending) of the strip of black-walled cells seen on the inclined cut in Fig.2. The central strand had not been affected since it has retained its light-brown stain. This applies also to the poorly visible cells on the inclined cut face of another sample in Fig.3.

Fig.2 (below): Sudden onset (or ending) of a strip of well-preserved cortex cells below the epidermis.
Image width 2mm, same scale as above.
Aglaophyton stained black

The cut face of the chert in Fig.3 unexpectedly reveals a rare phenomenon providing information concerning the processes involved here. A hole in the layer of black-walled cells inside the hollow straw is now covered with a dome-shaped layer of cells of the same type. This dome could not have grown in the empty straw. It must represent, like the black layer of tissue below the epidermis, the remaining cortex spared from decay after it had been made decay-resistant by the live plant. (The cortex tissue had formerly filled the whole space between epidermis and central strand.) Undoubtedly the transformation of cortex tissue into the persistent dome covering the damaged spot had been governed by the plant with the aim to prevent further damage. The combination of persistent dark dome and persistent dark peripheral layer suggests that the latter, too, had been prepared by the live plant to keep out intruders.

Fig.3 (below): Protective dome formed from cortex cells formerly present throughout before part of the cortex cells was made persistent and became black-coated while the others decayed and vanished, thus leaving a hollow straw. Image width 10mm.repair cap

As a side effect, the persistence of the tissue favoured the formation of the black coatings before all turned into silica gel and chert.
The interpretation of these hollow straws as Aglaophyton seems justified by the observation that typical details of this plant, as the "palisade wall" of its sporangia, can be found nearby.
Samples: Rh12/91.3+5 (2006) Figs.1+2;   Rh12/162.2 (2007) Fig.3.

H.-J. Weiss   2021  

[1]  C.L. Powell, N.H. Trewin, D. Edwards: Palaeoecology and plant succession
            in a borehole through the Rhynie cherts, ...
      Geological Society, London, Special Publications 180 (2000), 439-457.
[2]  www.abdn.ac.uk/rhynie, Chapter Taphonomy.
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