A pattern of questionable origin

A conspicuous pattern of radially oriented oblong voids is occasionally found on cross-sections of plants in the Lower Devonian Rhynie chert (Fig.1). The pattern is well known, for example, from the most abundant Rhynie plant, Aglaophyton, and has also been found in the rarest one, Ventarura [1], which belongs to a quite different phylum. This suggests the existence of a common cause not inherent in the nature of the plant. In [2] it is assumed that the void pattern is due to some decay process resulting in the formation of shrinkage cracks. This assumption was seemingly confirmed by the observation that the pattern had been found in decaying axes but not in fresh ones.

Now it appears that the assumption can be refuted by evidence from only one sample (Fig.1). Although the plant tissue is not completely preserved on the whole cross section it is seen that the cells near the alleged shrinkage crack tip are neither decayed nor torn, they look rather healthy instead (Fig.2). The same can be said of the cross section in Fig.3. So the assumption seems justified that the voids had been there in the living plant, which requires a more involved explanation.

						Fig.1: Pattern of large voids in a cross section of ?Aglaophyton.
						Fig.2: Cells at the apex of the void.
						Fig.3: Pattern of voids in a cross section of ?Rhynia.

The two chalcedony spherulites faintly seen in one of the holes in Fig.1 could not have been involved in the void formation process: They must have formed in the already existing void. There is a most likely explanation based on the following facts:

(1) Fungus infection can cause abnormal growth in extant plants.
(2) Fungus infection had been identified as a cause of bloating in Rhynie chert plants [3].
(3) Often Rhynie chert plants are heavily infected with various fungus species [4].

Hence it can be tentatively assumed that the void pattern was formed by abnormal growth under the influence of substances released by some fungus hidden in the tissue of the living plant. There is room for wild speculation concerning the effect or purpose of this phenomenon. The voids do not seem occupied by the ubiquitous hyphae normally seen in the Rhynie chert. If some fungus created them in the living plant, one might expect that it profited from them in some obscure way. The voids could enable the plant to grow a larger diameter with a given amount of tissue, and thus to become mechanically more stable. This would be a bizarre type of interaction for mutual benefit.

Supplement:

Fig.4 shows two neighbouring cross-sections of Aglaophyton with void pattern and kidney-shaped outline. This duplicity in the combination of features could hardly arise incidentally by decay and shrinkage. It suggests the explanation that some infection in a forking aerial axis caused the same abnormal growth in either prong.

Fig.4: Aglaophyton sections

text and photographs: H.-J. WEISS/Rabenau, photographs: H. SAHM/Dresden