Symbiotic fungus in Trichopherophyton ?
deutsche Version

Cells with dark fill, seen on cross-sections of Aglaophyton (former Rhynia major)
as a dark ring at some distance below the epidermis, is a common sight in the Lower Devonian Rhynie chert. Quite similar rings on cross-sections of the smaller Rhynia are not rare. The phenomenon has been described in detail and explained as being due to the fungus Glomites rhyniensis engaged in a type of symbiosis known as arbuscular mycorrhiza [1]. It would be interesting to know, of course, whether or not such symbiosis had been more widespread among the Lower Devonian plant species.
Evidence is provided here of the occasional occurrence of quite similar arrangements of cells with dark fill in the "hair-bearing plant" Trichopherophyton, which is not related to Aglaophyton. It is one of the less abundant plants in the chert, and it is easily recognized only if the less often preserved upper parts with the bristles are seen, as in these pictures.
(The big bristle below left in Fig.1, length 1.55mm, might be one of the longest ever seen on this plant. It can easily be mistaken for a crack.)

Fig.1 (left):  Three bristly cross-sections of Trichopherophyton in full view and about five more partially seen on this small area of 7mm width. Note also the big bristle below left.

Trichopherophyton sections
Figs.2,3: Cells with dark fill loosely arranged as a dark ring on Trichopherophyton cross-sections.
Width of the pictures 4mm. Same scale for Figs.1-3.
Trichopherophyton with ring of dark cellsTrichpherophyton with ring of dark cells
Fig.4 (below): Detail of Fig.3. Cells with dark fill and
definite shape contrasting to the degraded tissue around.
Width of the picture 1.3mm.
Trichopherophyton cells with dark fill
Apparently the plant parts with dark cells as those in Figs.2,3 and others had not been less vigorous than those without. Thus one may conclude that the dark fill did not do any harm and perhaps had even been beneficial as previously assumed in connection with Aglaophyton [1].
As seen in Fig.4, the dark fill is confined to individual cells which are in a better state of preservation than the largely decayed tissue harbouring them. The hyphae vaguely seen in Fig.4 as thin dark lines could as well belong to some rot fungus which entered into the dead plant. 

All pictures taken from Sample Rh14/18, obtained from Barron in 2007, Part 4 (slab).

What has to be done with higher resolution is to show that the dark fill is made up of fungus arbuscules. This would be another piece of evidence of this type of symbiosis between fungi and early land plants.

H.-J. Weiss       2015,

[1]  T.N. Taylor et al.: Fossil arbuscular mycorrhizae from the Early Devonian,
      Mycologia 87(1995), 560-73.

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