Broken symmetry of Aglaophyton sporangia
deutsche Version

The sporangia of the Lower Devonian plant Aglaophyton, formerly known as Rhynia major, are borne at the end of upright stalks and seem to be axially symmetric at first sight. Closer inspection reveals that the axial symmetry applies to the overall shape only. The cells of the capsule wall are arranged with a right-hand twist so that the whole surface pattern comes close to the symmetry of a screw, which becomes apparent when the capsule splits along a screw line (of very large pitch). The idea of axial symmetry seems to have been so suggestive that the shape of the split had been interpreted as the result of twisting when splitting [1] but there is ample contrary evidence. (See
Rhynie Chert News 5.)  Also there are other Devonian plants whose capsules show an axially symmetric overall shape but a twisted surface pattern.
Another structural feature violating the apparent axial symmetry of spore capsules is possibly a particular feature of Aglaophyton: It is seen on capsule cross-sections as a skew orientation of the narrow cells of the capsule wall. Along the circumference the orientation may vary from radial to inclined, with inclination up to about 45, then back to radial again.
Aglaophyton sporangium cross-sectionAglaophyton sporangium cross-section drawing
Figs.: Aglaophyton sporangium cross-section with a peculiar asymmetry in the array of the capsule wall cells. (The cells are not so well seen on the photograph as on the drawing after microscopic observation.)

Since the same type of asymmetry is found repeatedly and is possibly there with every sporangium, it is obviously not due to some growth anomaly but an inherent feature. Apparently it has not been noticed and described before, for several reasons: Most sporangium sections are tilted with respect to the surface or cut face so that the spiral texture of the capsule wall adds another asymmetry to the pattern. Also the array of the capsule wall cells is most often not clearly seen all around the circumference, and the capsule wall is often deformed, broken, or dark, with the individual cells hardly seen.
All this may produce a confusing picture of cell orientation but evidence for the peculiar asymmetry described here can be found more often than expected if one knows what to look for. It has not yet been checked whether the skew varies with vertical position.

H.-J. Weiss       2014

[1]  David S. Edwards, Aglaophyton major, a non-vascular land-plant from the Devonian Rhynie Chert,
      Bot. J. Linn. Soc. 93(1986), 173-204.
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