Same symbiotic fungus in Aglaophyton and Rhynia ?
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Aglaophyton (former Rhynia major), the most common plant in the Rhynie chert, is often inflicted with an anomaly showing on cross-sections as a concentric ring of cells with dark content scattered among the normal cells, positioned at a depth of a few cell diameters below the epidermis (Figs.1,2). High magnification reveals that the dark matter in the cells consists of a dense tangle of tiny branched fungus hyphae, much the same as in extant cases of symbiosis known as arbuscular mycorrhiza [1]. The phenomenon has been described in detail and explained as being due to the fungus Glomites rhyniensis [1]. As a remarkable fact, much thicker hyphae, and even thicker strands of combined hyphae of this fungus, are often seen in the chert between the plant matter even with low magnification. They must have grown in the mud and water and developed the tiny variety of hyphae when entering into the cells of the living plants.
Aglaophyton with Glomites rhyniensisAglaophyton with Glomites rhyniensis
Figs.1,2: Aglaophyton sections (4mm) with dark cells affected by the fungus Glomites rhyniensis, loosely arranged in a layer below the surface.

According to [2], this fungus species seems to be restricted to Aglaophyton (*). So it is worth mentioning that cross-sections with the same aspect, namely dark cells loosely arranged as a concentric ring, are also seen in the similar but smaller plant in the Rhynie chert, Rhynia gwynne-vaughanii (Fig.3 below, same scale as Figs.1,2).

This may not seem important in itself but taken together with other recent observations it may serve as another argument in a scientific dispute unfortunately started by a publication by David S. Edwards [3]. 
Rhynia with fungus, Glomites rhyniensis ?There the plant then known as Rhynia major [4] had been declared a non-vascular plant, unrelated to the rhyniophytes but somehow related to the mosses, and re-named Aglaophyton major. This name and interpretation are presently officially valid although contrary evidence has recently accumulated. (See, for example, Rhynie Chert News 2.) The present observation, too, seems to indicate a closer relationship between the two plants, as originally thought [4] and apparently also assumed by Dianne Edwards [8].

Fig.3 (right): Rhynia section (2mm) with dark cells whose aspect strongly resembles Aglaophyton sections with Glomites rhyniensis.

H.-J. Weiss ;      2009,  2014

(*) Annotation:  The phenomenon of arbuscular mycorrhiza has been mentioned in [5,6] for Rhynia, too, without explicitly naming the fungus. In [1,5,7] there is a
 rare photograph of a tiny hypha of Glomites rhyniensis penetrating a cell wall of Aglaophyton. (This picture is also seen here.)

[1]  T.N. Taylor et al.: Fossil arbuscular mycorrhizae from the Early Devonian,
     Mycologia 87(1995), 560-73.
[2]  T.N. Taylor et al.: Fungi from the Rhynie chert,
     Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 457-73.
[3]  David S. Edwards , Aglaophyton major ...,  Bot. J. Linn. Soc. 93(1986), 173-204.
[4]  R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure from the Rhynie Chert bed,
     Part II, Trans. Roy. Soc. Edinburgh 52(1920), 603-27.
[5]  H.Kerp: De Onder-Devonische Rhynie Chert,
     Grondboor & Hamer 58(2004), 33-51, Fig.19.
[6]  T.N. Taylor, E.L. Taylor: The Rhynie chert ecosystem: a model for understanding fungal interactions,
     in: Microbial Endophytes, eds.:  Ch.W. Bacon, J.F. White Jr., Marcel Dekker Inc., New York 2000.
[7]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009, Fig.3.96.
[8]  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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