Observations on fungus resting spores

assemblage of resting sporesResting spores, often called by the not precisely defined term "chlamydospores" are formed separately at the end of fungus hyphae, with the aim to persist when the hyphae decay. Since they are not comparable with the plant spores formed closely together in capsules, the question arises why they can be found in some places as densely spaced as in this picture, where their  volume fraction exceeds 1/2. From the aspect of similar assemblages of silicified resting spores in the Rhynie chert (Lower Devonian) it can be concluded that they had not been washed onto the spot where they are seen now. They must have grown on the very spot, and the hyphae as well as the decaying plant tissue which the fungus had lived on must have vanished before silicification.

Assemblage of resting spores (chlamydospores) of some fungus feeding on plant matter in the Devonian habitat preserved as Rhynie chert. Width of the image 1.4mm.

How the fungus did manage to produce this amount of organic substance would be less problematic if the spheres consisted mainly of water, which would make sense as a precaution against possible draught.
Most peculiar is the highly different aspect of the spheres, considering that probably they all belong to the same fungus species. Such phenomena are known from neighbouring small cavities in volcanic rock or in chert, also in the hollow aerial roots of Psaronius, where tiny incidental differences in the chemical composition of the enclosed or entering substances may cause widely differing developments of the agate-like fills.
The separate ways of mineralisation inside the spheres, resulting in different colours, are favoured by the possible presence of a diffusion barrier around the sphere, which can be deduced from Fig.2. The half sphere protruding from the raw surface of the sample indicates that a crack running right against the sphere had been deflected along its surface. Apparently the surface of the fungus spheres is less well silicified so that the crack may use the easy path around instead of right through. (The same is known from terrestrial plants with their cuticle on the surface.)
Apparently the crack deflection effect is a subtle one: More often the crack keeps its plane path cutting through the sphere, as seen in Fig.2 on the right.

Fungus resting spore 3-D
Fig.2: Rhynie chert,
raw surface with two resting spores bigger than those in Fig.1, the right one cut through and the left one carved out by the running crack which had separated the sample from the chert layer. Width of the image 1.4mm.

It is not known why the spheres in one half of the assemblage in Fig.1 have got an apparently thick dark wall in the silicified state but are pale and translucent in the other half, above left and beyond.
The thick dark walls suggest strength but three spheres left of the middle indicate that things are more involved.
One sphere with thick dark wall has got pear-shaped by contact while the two contacted spheres have not become deformed at all. So it can be concluded that (1) the mechanical stiffness of the dark-walled spheres could vary and (2) could equal that of the pale ones. Possibly the thick-looking wall is not really thick but clad with a microbial layer which did not contribute to stiffness.

This essay covers only a small part of the questions related to the several different resting spores corresponding to the several fungus species found in the Rhynie chert, which "cannot yet be assigned to a particular clade with certainty, as important parts of their life cycles have not yet been discovered [1]."  See also "Funny fossil fungus formations".

H.-J. Weiss      2016

[1]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009, 76.

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