Persistent "palisade wall" of Aglaophyton sporangia
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Aglaophyton sporangium wall
Sporangia of the early land plant Aglaophyton are among those objects in the Lower Devonian Rhynie chert which are most easily identified. While the identification of plant sections seen on sample surfaces or cut chert faces is usually more difficult owing to shrivelling, squeezing, and rotting before fossilisation, Aglaophyton sporangia are much less affected by such phenomena. Also they are distinguished from any other plant parts (with exception of the rare Rhynia sporangia) by an epidermis consisting of stacked slab-like cells with rot-resistant walls which often appear as a row of palisades on capsule cross-sections, as seen here. The visual impression of separate posts is also due to the fact that the recurved parts of the cell walls, 
on the surface of the capsule and inside, are most often not preserved, for reasons unknown. (A rare exception is seen in Rhynie Chert News 105.)

Palisade wall fragments of Aglaophyton sporangium in chert, rare combination of parts with different aspect, degraded spores inside the capsule below right. Image width 2mm.

Since incidental tangential cuts of the capsules are much less probable than others, the capsule wall is seldom seen from outside. Hence the fragments in this image offer a rare combination of different aspects of the palisade wall.
Not seen here are two obvious deviations from the symmetry of the outer shape of the capsule:
In Rhynie Chert News 62 the narrow cells are seen to be aligned with a texture winding around the spindle-shaped sporangium in a screw-like way. Another unexpected asymmetry is hidden below the surface but seen on cross-sections of the spore capsule: In Rhynie Chert News 65 the orientation of the cell flanges appearing as palisades is not always radial as one should expect but varies along the circumference in some systematic but not yet understood way.
No explanation can be given here for the combination of the axial symmetry of the spindle-shaped sporangia and the asymmetries of the epidermis tissue on their surface. 
Sample: Rh9/69, 43g, Part 1, found by S.Weiss in 2004.

 H.-J. Weiss     2018
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