Aglaophyton major, formerly known as Rhynia major [1], is one of the plants found in the Rhynie chert whose position in the phylogenetic tree is not yet certain. The absence of typical tracheids in the conducting strand led D.S. Edwards [2], after comparison with the anatomy of mosses, to the conclusion that this plant is not a vascular plant and therefore not related to Rhynia gwynne-vaughanii, which he regarded as a sufficiently strong reason to change its name. There remained the intriguing fact that, apart from the tracheid wall structure, the two plants were more similar to each other than to any other Rhynie chert plant discovered so far. Particularly obvious is the similarity of their sporangia: In either species the sporangium wall consists of three layers of tissue. The outer one is easily recognized on sections by its distinct palisade-like array of cells which is slightly spirally arranged so that the sporangium appears twisted in side view. (This twist is not brought about by splitting as considered in [2] but is clearly seen in the non-split sporangium.)

These and other similarities nurture the suspicion that the two plants may not be as unrelated as suggested by the absence of tracheid wall thickenings in Aglaophyton [3]. So it will be interesting to look for more evidence supporting one or the other view. Unexpectedly, one small sample of chert with uncommonly well preserved Aglaophyton axes has provided a rare sight as shown in Fig.1: The "fairy ring" of distinct white spots can hardly be regarded as incidental although it is seen in this regularity on only one section. On other sections, the spots, if there are any, do not form a complete ring or are more irregularly distributed. As a possible explanation, some slight chemical difference between cell types, inherent or due to differential stages of decay, led to different mineralisation resulting in strong optical contrast. (Slightly differing starting conditions leading to vastly differing final stages via complex processes is not uncommon.) The white spots are brought about by strong reflection, probably due to fine-grained quartz, contrasting to the more or less transparent chalcedony.

Fig.1: Aglaophyton cross section with unique "fairy ring".

In the particular case seen in Fig.1 the situation seems to have been so subtly balanced that only a certain type of cells has been affected by the contrast-enhancing process. By comparison with the well-known arrangement of tissues in other simple vascular plants it is suggested that this central strand is made up of xylem with 6 spots of protoxylem. This interpretation would provide additional evidence in favour of the assumption that Aglaophyton does not as much differ from a vascular plant as assumed in [2] but is simply an unusual vascular plant without the usual tracheid wall pattern.

text & photograph: H.-J. WEISS/Rabenau