Spirally splitting sporangia   -  Aglaophyton 
deutsche Version

Aglaophyton sporangium, empty As a well known fact, the sporangia of Aglaophyton, a common plant in the Lower Devonian Rhynie chert, are borne upright at the end of shoots. Although their shape is axially symmetric, the cellular pattern of the outer wall is not. This is seen in the lengthwise section in Fig.1, where a close look at the wall reveals that all cells, where visible, are leaning to the right, indicating a twist of the whole pattern.
Splitting of the capsule (Fig.2) is an easy and apparently adequate way of shedding the spores. It is not obvious why the more advanced contemporaneous plants had developed special means of spore release, as holes at the top of every compartment of the branched capsule (Horneophyton), or valves opening along pre-formed weak lines in the capsule walls. (Nothia, Trichopherophyton, Ventarura, Asteroxylon).
One may wonder why the capsule in Fig.1 is empty but not split. Either the fissure is small and not seen on this section or there is none and the spores were completely consumed by some spore eater going in and out through a hole gnawed into the wall above left. (Although sporangia with a hole in the wall are not rare, they were not known in the scientific literature and have been described first in
Rhynie Chert News 7.)

Fig.1 (far right): Aglaophyton sporangium, empty, with the cells of the dark outer wall in a lopsided array. Width of the capsule 4.7mm.Aglaophyton sporangium, split, empty

Fig.2: Cross-section of split and empty Aglaophyton sporangium

        Width of the capsule 3.2 mm.

The split and apparently twisted capsules had led to the idea that the twist came with the splitting [1,2], similar as with numerous seed plants whose pods become mechanically stressed in maturation, then release the stored elastic energy while bursting and scattering the seeds with a jerk. No contrary evidence had been available, judging from the statement in [2] that "... the alignment of the epidermal and other wall layers in intact sporangia is not known."
The alignment of the epidermal wall layer in non-split sporangia is known now.
Evidence is provided by the rare cases of sporangia seen from outside, as in Rhynie Chert News 5 and in Figs.3,4 below. The latter show a slight growth anomaly which incidentally offers a unique view on the spiral texture of the cell array. Unawares of the sporangium and its shape and orientation, the cut plane has been chosen such that it nearly coincides with the tangential plane of a saddle point of the sporangium curved like a cucumber. (Note that the normal sporangium, spindle-shaped as in Fig.1 or more cylindrical, does not have saddle points, which are characterized by concave curvature in one direction and convex curvature in another one.)
The sporangium is not split open but the prospective splitting paths are clearly seen, arranged not at random but with roughly equal spacing. Apparently they do not or not always extend over the length of the capsule. One may wonder why they look conspicuously straight although they are supposed to be part of 3D-spirals or screw lines. The straight aspect is due to an interesting mathematical fact: Every screw line, or every turn of a thread, if looked at straight on, is seen as a curve which changes its sign of apparent curvature at the point nearest to the observer. This means the apparent curvature is zero there and nearly zero for some distance on either side, and if the more curved ends are hidden in the depth as in the below pictures, the visible parts of the screw lines appear straight.
Aglaophyton sporangium, twisted patternAglaophyton sporangium, twisted pattern

Figs.3,4 (left):  Aglaophyton sporangium misshapen as a cucumber, offering a rare view on the wall texture with prospective splitting paths.

Fig.5: Aglaophyton sporangium, about 7mm across before splitting.          
                         big empty Aglaophyton sporangium

Not closely related to the present problem of spiral splitting but nevertheless interesting are the following facts:
An uncommonly big sporangium (Fig.5) with estimated original diameter of 7mm is incompatible with the (slightly contradictory) size data in [2]: "Sporangia ... < 12mm long and < 4mm wide, ..." and, on the next page, "... up to 12mm long and 5mm wide, ...".  Wider sporangia are not quite rare: see
Rhynie Chert News 11The separation of the capsule parts in Fig.5 may be due to later deformation.
The inner cavity of hollow sporangia, like that of hollow shoots, often looks distinctly different from the surrounding chert matrix. Even split sporangia show such difference indicating a different sequence of silicification stages: See the blue agate in Fig.2 and coarse quartz crystals in Figs.3,4,5. No satisfactory explanation can be proposed here at present but one observation on Fig.5 may eventually give a clue: Tiny dark dots in white circles are cross-sections of aquatic fungus hyphae which throve in the water-filled cavity before they became coated with silica gel turned into whitish chalcedony, and the remaining space eventually became filled with coarse quartz. The same has repeatedly been observed with former gas bubbles which had formed and got stuck in the waterlogged habitat, got fixed by silicification of the water, later became water-filled as the gas gradually escaped by diffusion through the silica gel while water and dissolved silica entered likewise. Hence, similar processes might have been going on within empty sporangia with hyphae seen inside.
No gap is seen in Fig.2 despite of the split. Certainly the dry capsule had a gap for spore release which possibly closed after inundation.
As another remarkabe fact, the thread of the cell pattern is always right-handed. It would be interesting to know whether or not there is a deep relation to the right-handedness of other
objects: sporangia of other plant species, charophyte whorls, and the enigmatic Nematoplexus, for example.

Despite of some remaining uncertainty in the interpretation of Aglaophyton fossils, the observations reported here seem to justify the following assumptions:
(1) The spiral texture of the sporangium wall had been there before splitting.
(2) The position of the split in the wall is not pre-determined.
(3) There are several prospective split lines, spaced apart by about 20-30 cell files.
(4) Not every prospective split line runs along the whole sporangium.
(5) Which one of the prospective split lines becomes active is left to chance.

H.-J. Weiss     2014

[1]  W. Remy: Der Dehiszenzmechanismus der Sporangien von Rhynia*,  
      Argumenta Palaeobotanica 5(1978), 23-30.    * re-named  Aglaophyton in [2]
[2]  David S. Edwards, Aglaophyton major, a non-vascular** land-plant from the Devonian Rhynie Chert,
      Bot. J. Linn. Soc. 93(1986), 173-204.            ** doubted in [3]
[3]  Dianne Edwards : A review of the sporophytes of embryophytes in the cherts at Rhynie,
      Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 397-410.
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