A pattern of questionable origin
deutsche Version
Voids in the central region of a Rhynie chert plant
A conspicuous pattern of radially arranged oblong voids is occasionally found on plant cross-sections  in the Rhynie chert (Fig. 1). The pattern is well known, for example, from the most abundant Rhynie plant, Aglaophyton, and has also been found in the rarest one, Ventarura [1], which belongs to a quite different phylum. This suggests the existence of a common cause not inherent in the nature of the plant.

Fig.1:  Pattern of voids in a cross section with otherwise healthy aspect, probably Rhynia.


Fig.2:  Pattern of large voids in a cross section, probably of Aglaophyton.  Photographs: H. Sahm
Voids as fungus-induced growth anomalies in a Rhynie chert plant
Detail of tissue with fungus-induced growth anomaly
Fig.3: Detail from Fig.2, no indication of decay or rupture at the apex of the void.

In [2] it is assumed that the void pattern is due to some decay process resulting in the formation of shrinkage cracks. This assumption was seemingly confirmed by the observation that the pattern had been found in decaying axes but not in fresh ones.

Now it appears that the assumption can be refuted by evidence from only one sample (Fig.2). Although the plant tissue is not completely preserved on the whole cross section it is seen that the cells near the alleged shrinkage crack tip are neither decayed nor torn, they look rather healthy instead (Figs. 2,3).  So the assumption seems justified that the voids had been there in the living plant *, which requires a not so simple explanation.
The two chalcedony spherulites faintly seen in one of the holes in Fig. 2 could not have been involved in the void formation process: They must have formed in the already existing void. There is a most likely explanation based on the following facts:
   (1) Fungus infection can cause abnormal growth in extant plants.
   (2) Fungus infection had been identified as a cause of bloating in Rhynie chert plants [3].
   (3) Often Rhynie chert plants are heavily infected with various fungus species [4].
Hence it can be assumed that the void pattern was formed in the living plant, with abnormal growth under the influence of substances released by some fungus hidden in the tissue.
There is room for wild speculation concerning the effect or purpose of these large voids. They do not seem occupied by the ubiquitous fungus hyphae normally seen in the Rhynie chert. If some fungus created them in the living plant, one might expect that it profited from them in some obscure way. Also the plant could have profited from the voids as it could make a larger diameter with a given amount of tissue and thus become mechanically more stable. This would be a bizarre type of interaction for mutual benefit.

H.-J. Weiss
    2005

[1]  C.L. Powell, D. Edwards, N.H. Trewin : A new vascular plant from the Lower Devonian Windyfield chert, Rhynie, NE Scotland.
      Trans. Roy. Soc. Edinburgh, Earth Sci. 90(2000 for 1999), 331-349.
[2]  www.abdn.ac.uk/rhynie
[3] T.N. Taylor, W. Remy, H. Hass : Parasitism in a 400-million-year-old green alga,
      Nature 357(1992), 493-494.
[4] T.N. Taylor, E.L. Taylor : The Rhynie chert ecosystem: A model for understanding fungal interactions,
     in: Microbial Endophytes, eds. Ch.W. Bacon, J.F. White Jr.; Marcel Dekker Inc. 2000.

 * Annotation 2014: For related evidence see Rhynie Chert News 21, 54.
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