Fungus

The gills of the oyster mushroom, Pleurotus ostreatus.

The upper surface looks like this:

This yielded the following spores (magnification x600, field width 86um):

These are about 8.2 x 3.3um which agrees well with the 8-10.5 x 3-3.5 µm given for P. ostreatus at MushroomExpert.com.

Specimen taken in Whiteknights Park, Reading, UK, on 2013-12-15.