Enigmatic voids in the tissue of early land plants
deutsche Version

Conspicuous patterns of radially arranged voids on plant cross-sections
had been explained as shrinkage cracks of the decaying tissue [1] before own samples (Figs.1-5) gave rise to the suspicion that some fungus present in the growing plant might somehow have affected growth. (See Rhynie Chert News 54.)
void patternvoid patternvoid patternvoid patternvoid pattern

Figs.1-5:  Void patterns on Aglaophyton cross-sections with degraded or well-preserved tissue. All same scale.
6mm square.

Extremely shrunken and collapsed tissue is obviously present between the large voids seen in Figs.1,2,3,5 but this does not mean that the patterns of this kind are merely a result of degradation. An early stage of void formation (Fig.4) suggests that more complex processes must have been at work. The tissue beside the voids does not look decayed but deformed as a result of the expanding voids. Apparently there had been tissue throughout before, and it became squeezed by the expanding voids while alive. Well-preserved tissue near the edges of the voids is clearly seen in Rhynie Chert News 4
It is remarkable but confusing that the void pattern may vary between chaotic as in Fig.5 and ordered as in Fig.4.

Other related information comes from enigmatic "twin patterns" where the two prongs of a forking shoot show essentially the same pattern of voids. (See Rhynie Chert News 21, 54.) Apparently the patterns in the prongs have been inherited from the base of the fork.  
different holes in Aglaophyton
The idea of some fungus being involved in the formation of the above patterns is not immediately supported by fossil evidence. The ubiquitous fungus Glomites rhyniensis, whose tiny arbuscular hyphae are seen as dark clots inside cells in Figs.3,4, is most probably not involved. Possibly the symmetrical hole in Fig.6 with affected tissue is due to misguided growth caused by another fungus.

Fig.6: Aglaophyton cross-section with two types of damage in the live plant: one hole caused by some unknown herbivore, the other one probably by fungus activity. Picture 4mm square.

Since abnormal growth in plants is often due to fungus infection, it can be assumed that a fungus had been involved here, too. Disregarding the different aspect of the expanding cavities in Fig.6 and Fig.4, one may suspect that both are a result of fungus activity.
Parasitic Chytrid fungi, which had been present in the Devonian habitat at Rhynie, are known to be able to bloat plant cells enormously [2]. Hence, the idea suggests itself that all of the peculiar voids in Figs.1-6 represent hypertrophy due to fungus action. What remains enigmatic is the way in which the fungus governs the complex process. 

H.-J. Weiss    2017, 2018

[1]  www.abdn.ac.uk/rhynie
[2] T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi, Elsevier 2015, p.64.


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