Black coatings on cell walls of early land plants
deutsche Version
Trichopherophyton, black cell walls
Part of the early land plant specimens in the Rhynie chert may contain cells whose walls had apparently become stained black (Fig.1). Closer inspection reveals that not the walls themselves but coatings on the walls make the black aspect, which is supported by the rare observation that they may flake off.

A few selected examples are discussed here, fitting to the more comprehensive contribution "Black stains, coatings, and linings in the Rhynie chert", Rhynie Chert News 83.

Fig.1: Trichopherophyton section, uncommon specimen: some cell walls stained black.
          Image width 2.8mm.

usually does not show cells with stained walls, so Fig.1 shows a rare case. It has been chosen here because it shows clearly the individual affected cells. Mostly they are coated as a whole but a few cells below in the picture are only partially or weakly coated.
Much more conspicuous than the rare phenomenon in Fig.1 are the "hollow straws" [1] of the most abundant plant in the Rhynie chert, Aglaophyton (Fig.2).
Aglaophyton "hollow straw"

Fig.2: Aglaophyton "hollow straw": Peripheral layer of well-preserved cortex cells mostly with black walls, contrasting to the decayed larger part of cortex tissue. Width of the cross-section 4mm.

The seemingly evident explanation of the hollow straw aspect as a result of dissolved silica penetrating into a limited depth below the epidermis, preserving a layer of peripheral tissue there while the tissue farther below is left to decay [1,2], has been doubted in [3] and is rejected here by referring to Fig.1. There, the loosely arranged black-walled cortex cells are not compatible with the idea of a silicification front.
The situation seems to be more complex, as suggested by the observation that the well-preserved cell walls are not always stained black (Fig.2). Apparently the preservation as "hollow straws" is not governed by silica diffusion in the dead plant but by the live plant providing some kind of decay resistance to a peripheral part of the cortex tissue. With this interpretation the question arises to which purpose the plant had made a decay-resistant tube, seen as a dark ring on cross-sections.
Aglaophyton "hollow straw" with covered hole
Fig.3: Aglaophyton"hollow straw": Hole in the decay-resistant peripheral layer repaired with a dome-shaped cap of 2mm width made from cortex tissue.

By lucky incidence, a rare structure is documented in Fig.3: A hole in the dark peripheral ring of persistent tissue is seen repaired with a cap covering the hole. This must have occurred while all cortex tissue was still there, because the cap was made from cortex cells. Hence, the ability of the plant to make part of the tissue decay-resistant must have existed over some span of time.

The repair of the damage to the persistent tissue indicates that this tissue, though only a peripheral tube a few cells thick, is of some importane to the plant.
One may guess that not only the black stain is a secondary phenomenon but the decay resistance, too. Possibly the main thing is hardly seen here behind the secondary phenomena: The main thing might be a repellent substance against pests applied by the plant to a narrow peripheral region of the cortex tissue. As a side effect, that hypothetical repellant might preserve the cell walls, thus providing substrates for microbes to make coatings with dark aspect.

2 Aglaophyton with different aspect
Fig.4: Aglaophyton sections of quite different aspect near each other: "hollow straw" (left) and "normal" shoot with very faintly seen tissue (right).

It has not been explained here why Aglaophyton sometimes appears as a hollow straw and sometimes with original tissue (Fig.4).
The phenomenon of black coatings on cell walls is widespread in the Rhynie chert. In addition to the "hollow straws" discussed here, black coatings make conspicuous sights when on the cell walls of the persistent tube of Ventarura amidst the decayed cortex tissue.

Rh14/18.5, obtained from 
Barron in 2007: Fig.1; Rh6/38.1, found in 2003: Fig.2; Rh12/162.2, found in 2007: Fig.3; Rh12/91.5, found in 2006: Fig.4;  
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), 4 39-457.
[3]  A. Channing:  Processes and Environments of Vascular Plant Silicification: Thesis, Chapter 6, Cardiff University, 2001.
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