Teeny-tiny toadstools - or are they?

Yesterday I was pointed towards a log cut from a fallen tree in my in-laws' garden. On it was a fairly dried-out growth of moss and on the leaves, what looked like some tiny (1mm) fungal caps. Now, I'm no mycologist (or bryologist), but I do find microfungi intriguing and happen to have a copy of Ellis & Ellis (1998) which is the standard (only?) work covering British microfungi on substrates other than vascular plants - for those you need Ellis & Ellis (1997).

The specimen - the caps are about 1mm in diameter.

A slightly closer view showing the wrinkled stalks.

The moss is fairly dry and a bit of a miserable specimen without capsules, but looking at the excellent Atherton et al. (2010), I think that it is the common and widespread species Brachythecium rutabulum (sometimes known as the Rough-stalked Feather-moss). The fungus itself is white with a split cup-like cap and a wrinkled stem that widens towards the base. The cap is granular beneath and bears hairs in the upper/outer surface.

Two of the fruiting bodies showing details of stems and caps.

Side view of a fruiting body showing the split cap and granular spore-bearing structures.

Some moss-epiphytic microfungi have clear fringes of hairs around the edge of the cap. This doesn't appear to be the case here - instead there appears to be an irregular tufting of hairs on the top of the cap with a few at the edge (I wondered if they were the hyphae of another even smaller fungus, but I don't think so).

Side view of the cap showing tufts of hairs.

High-power image of the cap showing branching hairs.

Lastly, spores are an important feature used in the identification of microfungi. I collected some of these on a slide and they are clearly almost spherical and reddish-brown in colour, and each is about 10um in diameter.

Spores (x100 magnification)

So, the overall size, colour, spore colour/shape/size, habitat, hairs and other features lead me to tentatively identify this as Chromocyphella muscicola, a species with caps up to about 3mm diameter which is associated with mosses on bark. However, having consulted with a mycologist from the Hampshire Fungus Recording Group, it appears that C. muscicola doesn't produce stalks like this. Other superficially similar genera such as Leptoglossum also differ from this specimen in one or more ways e.g. spore shape, cap hairs. And so, it was suggested that this might not be a true fungus at all, but instead a myxomycete or 'slime mould' where the globular sporangium had broken open to reveal the network of spore-bearing hairs known as the capillitium.

I have posted about myxomycetes before (e.g. here and here) but it still a group of organisms I know little about, fascinating as they are - taxonomically protozoans but traditionally treated as 'honorary' Fungi. So, swapping books to look at Ing (1999), it soon became clear that this was indeed a 'myxo'. It also happened to be one that was relatively easy to identify from the keys and descriptions - the pale colour, size, stalk, open split 'cap' with the hairs of the capillitium, and spores (shape, size, colour, texture). This process also taught me that an important identification feature can be the shapes, colours and sizes of nodes of the capillitium i.e. structures about the size of a spore where hairs meet and branch.

These features all combined to indicate that this is the myxomycete Physarum nutans - a common and widespread species, though rarely found on mosses (it is usually on dead wood or the bark of live trees). It is very similar in appearance to Didymium squamulosum but they can be separated by features such as the spores (in D. squamulosum they are dark brown in transmitted light whereas in P. nutans, as here, they are pale brown). So, an interesting process for me, looking at an unfamiliar group - the only way to learn more about species identification!

References

Atherton, I., Bosanquet, S. & Lawley, M. (eds.) (2010). Mosses and Liverworts of Britain and Ireland: A Field Guide. British Bryological Society.
Ellis, M.B. & Ellis, J.P. (1997). Microfungi on Land Plants: An Identification Handbook. Richmond, Slough.
Ellis, M.B. & Ellis, J.P. (1998). Microfungi on Miscellaneous Substrates: An Identification Handbook. Richmond, Slough.
Ing, B. (1999). The Myxomycetes of Britain and Ireland: An Identification Handbook. Richmond, Slough.