Early land plant aging
deutsche Version
Ag agingAglaophyton is the most common fossil plant in the Lower Devonian Rhynie chert but in rare cases like this one it offers surprising aspects which may give rise to contemplations concerning details not thoroughly considered before. The most conspicuous detail in this image is the white ring around the central xylem. It simply indicates that, for whichever reason, the chalcedony had recrystallized there into silica grains of such size that they reflect the incident light like snow. (A similar ring is seen in Rhynie Chert News 70. As a very rare phenomenon, tiny white dots around the xylem are seen in Rhynie Chert News 2.) It is not known whether or not this white ring can provide relevant information on silicification processes.

Fig.: Cross-section of Aglaophyton with several competing phenomena of selective decay and preservation, recorded in progress by silicification. Size 5mm.

The ring of well-preserved tissue consisting of distinctly visible cells below the poorly visible epidermis is doubtless related to silicification in a non-trivial way. The preservation of the tissue up to a certain depth is not simply due to a finite penetration depth of dissolved silica into the dead plant by diffusion as claimed in [1] but had been governed by the live plant instead, as emphasized in Rhynie Chert News 181. The present cross-section is particularly interesting as it shows an intermediate stage on the way towards a final "hollow straw" stage. The peripheral layer of well-preserved
cortex cells is that part of the tissue which had been made decay-resistant. It has persisted while other areas of the cortex tissue are seen in a more or less decayed state.
Black spots, loosely arranged at a distance from the epidermis, are cortex cells filled with a tangle of tiny hyphae (arbuscules) of the
symbiotic fungus Glomites rhyniensis [2,3], partially collapsed.
Obviously this cross-section offers a rare sight on details while most other sections of Aglaophyton, though abundantly seen on cut faces or fracture faces of the chert layer, do less so. 
The two thick-walled black capsules are probably "acaulosporoid glomeromycotan spores" [3] (resting spores) of Glomites. Their symmetrical arrangement may be misleading: It is purely incidental, as can be guessed from Rhynie Chert News 55, 60, 97, 142, 187. 

Sample: Rh15/81.1 (0.32kg) obtained from Barron in 2014.

H.-J. Weiss     2021      revised
version 2022

[1]  www.abdn.ac.uk/rhynie, Chapter Taphonomy.
[2] T.N. Taylor et al.: Fossil arbuscular mycorrhizae from the Early Devonian,  Mycologia 87(1995), 560-73.
[3] T.N. Taylor et al.: Fossil Fungi. Elsevier Inc. 2015, p.122.
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