Twisted evidence
deutsche Version
The sporangia most easily recognized as such in the Rhynie chert are those of Aglaophyton. They are often split lengthwise (Fig.1), and sometimes a twist in the texture of the wall
is distinctly seen. This led to the assumption that the twist might have originated as a result of splitting [1], as it is known from the seed pods of several leguminous plants, where mechanical stress built up by drying of the ripe pod is released in a sudden burst scattering the seeds and leaving the halves of the empty pod in a twisted state.
Cross-section of split Aglaophyton sporangium Side view of Aglaophyton sporangium with right-hand thread texture
Fig.1 (far left):  Cross section of empty Aglaophyton sporangium split lengthwise, sides overlapping,  inner wall layers absent owing to decay. Note also the level bands in the cavity.

Fig.2:  Side view of non-split Aglaophyton sporangium with right-hand thread texture. (The wall is cut off above.)

There is, however, ample evidence that the twist of Aglaophyton sporangia is not due to splitting but is an inherent feature. Fig.2 provides a view on the outside of a sporangium lying just below the cut and polished face of the chert sample. Apparently the sporangium is not split but the twist in the texture of the outer wall is clearly there.
(See also Rhynie Chert News 62.) More evidence comes from inclined sections of non-split sporangia (Fig.3): The sporangium wall cells appear wider on one side of the capsule and narrower on the opposite side. This is what is expected from elongated cells arranged nearly parallel to the axis of the lengthy capsule except for a slight twist or spiral texture, since they are more inclined with respect to the cut face on one side and less inclined on the other one so that their width seems to vary correspondingly along the circumference.* Inclined section of Aglaophyton sporangium

Fig.3:  Inclined section of non-split Aglaophyton sporangium whose spiral texture can be inferred from 
the apparent width of the outer wall cells changing along the circumference. Photographs by H. Sahm.

In [1], an epidermis texture is shown which seems to belong to a sporangium with a left-handed twist, while ours in Fig. 2 is clearly right-handed. This is suspicious since Nature often prefers one type of thread, which suggests the idea that the picture in [1] had been obtained with the peel technique which gives the mirror image of the real thing and hence transforms right into left hand thread if one does not take care to reverse it before printing.  
In this connection it is interesting to compare the handedness of other twisted sporangia as they are known from a few more early fossil land plants (Rhynia, Huvenia, Torticaulis) and extant mosses. Sporangia of one species with either handedness are shown in [2] without any comment on the difference.

Apparently the type of twist is not always conserved in the reproduction of pictures in botany but no-one seems to bother. In view of this state of things it will be interesting to find out if the thread of twisted sporangia is equal within one species or among related species so that it could serve as a useful feature in cladistics.

H.-J. Weiss       2005

* Addendum 2014: The aspect of the cells in Fig.3 hints at an additional asymmetry of the wall texture which is not yet understood.
See Rhynie Chert News 65.
[1] David S. Edwards : Aglaophyton ... , Bot. J. Linnean Soc. 93(1986), 173-204.
[2] P. Kenrick, P.R. Crane : The origin and early diversification of land plants, Smithsonian Institution Press, Washington, 1997.
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