Selective decay resistance of soft tissue in early land plants (4) 

As shown before, the "hollow straw" phenomenon often observed with early land plants fossilized in the Rhynie chert requires a more complex explanation than the one proposed in [1,2]. (See also Rhynie Chert News 60, 66, 97.)  It cannot be reduced to a silicification front moving into the plant for a short distance only while the larger part of the tissue is left to decay. Such front would preserve the epidermis first but there is ample evidence that the epidermis is less well or not at all preserved in plant parts with hollow straw aspect than in others.
Aglaophyton hollow straw
Another detail supports the idea that the hollow straw aspect had not been brought about by non-biological processes like diffusion and silicification but had been purposeful prepared by the living plant: It is the presence of gaps or chambers in the wall of the straw. There must be quite a number of them in some straws, judging from 5 of them seen on the cross-section in Fig.1 on the right. Note the well preserved seam of cortex tissue making up the
wall, which contrasts with the degraded epidermis and the decayed cortex tissue within.
Aglaophyton hollow straws
Figs.1,2:  Aglaophyton preserved as hollow straws with enigmatic gaps or chambers in the wall. Widths of the pictures 6.9mm (right), 2.8mm (left).
Aglaophyton hollow straw detail
Fig.3: Detail of Fig.1: Empty chamber in the wall of the straw connected to a gap in the epidermis. Width of the picture 1.7mm.

Usually the cells in the wall of the straw are filled with bluish pale chalzedony but the dark cell walls make the straw wall appear dark. The straw in Figs.1-3 is darker than usual because the chalzedony inside the cells is partially clear so that one looks into the dark interior. One should be careful not to mistake the dark aspect for a black fill.
Specimens like this one can be particularly misleading since there are really black fills in plant cells in other samples of Rhynie chert.
An erronous interpretation is suggested by a few cells in Fig.3 with a horizontal boundary indicating a deposit within the cell. At first sight it may look like a dark deposit with whitish chalcedony above but it is not. It appears that Figs.1-3 are shown here upside down, hence what had gone on in these cells was silica clusters forming in the water and settling into a suspension at the bottom which finally turned into whitish chalcedony while the water above turned into transparent chalcedony later.
A quite unrelated observation is worth mentioning here, although it does not contribute to understanding: None of the various cracks in Figs.1,3 had been deflected along the surface of the plant where the waxy cuticle on the epidermis usually precludes thorough silicification, thus providing an
 easy way for crack propagation. It must be concluded that, for reasons unknown, there had been no easy crack path.

The enigmatic chambers in the wall of the straws deserve closer consideration. There are samples where none of the hollow straws has gotAglaphyton capsule and straw walls chambers of this size. Conspicuous chambers are never seen on sections of plants with well-preserved tissue, as in Rhynie Chert News 2, 85. Sub-stomatal chambers are so small that they are usually not seen on sections. Nevertheless they could possibly be the sites where the large chambers formed later, for whichever reason.
A few more of the large chambers are shown in Rhynie Chert News 97 and in the pictures below.
Aglaophyton hollow straw detail
Fig.4 (left):
Aglaophyton hollow straw, chamber in the wall with access hole in the epidermis and special arrangement of cells .

Fig.5 (right): 
Aglaophyton cross-sections, sporangium wall (left) and hollow straw with chamber in the wall, bulging out at the bottom (right).
Note also the remains of the destroyed epidermis.

Fig.6 (below left): Aglaophyton hollow straw, epidermis decayed, apparent gap in the dark wall filled with faintly seen cells, see drawing Fig.7. (The cuticle seen as a dark line must have been shifted since the vanished epidermis had been

Aglaophyton hollow straw detail much thinner than the spacing seen here.)

Width of Figs.4,5,6: 1.4mm, 1.7mm, 1.4mm. Equal scale for Figs.3-6.

A clue to an explanation of the empty chambers or gaps occasionally present in the wall of hollow straws may be hidden in Fig.6. It shows a rare case of an apparent gap in the dark wall filled with faintly seen cells (Fig.7) which have not got the black stain usually present on the cell walls of the hollow straw. The faintly seen cells fit so neatly to the other ones that an explanation as newly grown cells filling a hole gnawn by a creature can be excluded. Hence, the faint cells must have been there all the time, and when the living plant prepared a few circumferential layers of cells by unknown means to become rot resistant in order to form a persistent tube, apparently several clusters of cells did not get that rot resistance and thus were left to decay or dissolved by themselves, thus leaving holes in the straw. The plant must have done this on purpose but it is not known to which purpose.
straw gap drawing Judging from Figs.1-4, the epidermis seems to be involved, which only makes the problem more confusing.    

Fig.7: Detail of Fig.6 possibly leading the way to a partial explanation of the intriguing phenomenon of gaps or holes only in those specimens of  Aglaophyton which appear as hollow straws. Width of the drawing 0.4mm.

This drawing ...
   ... obviously contradicts the assumption in [1,2] that the hollow straw aspect is brought about by silicification,
   ... shows that a couple of cells is conspicuously spared from getting a thick dark coating,
   ... suggests that the occasional absence of the dark coating is not incidental but had been controlled by
unknown means.
Comparison with other gaps in the wall of the straw seems to indicate that the cells without coating had been destined to decay or even to dissolve by themselves so that an empty chamber is left whose purpose is by no means obvious.

The phenomenon of cell walls vanishing in a controlled way for a particular purpose is not quite exotic among plants. Euphorbias, for example, make long tubes for poisonous liquid in this way. The same can be suspected of Nothia, also from the Rhynie chert. (See Rhynie Chert News 57.)

In addition to the open questions already mentioned, the formation of chambers, gaps, or holes in the wall is all the more peculiar since the plant would grow a dome-shaped cover over a hole eaten into the wall so that it would become closed, as seen in Rhynie Chert News 60.  Incidentally, the latter phenomenon serves as a proof that the hollow straw aspect is not an attribute of the dead plant subject to partial silicification but had been there in the live plant. Obviously Aglaophyton was able to live as a hollow straw with the central strand still functional but the cortex in between dead and more or less decayed.

H.-J. Weiss       2017   

[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], Chapter Taphonomy.

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