Sporangia of a slime mould, probably Trichia decipiens, growing on a decaying log.
I took the following specimen to look at at home:
I squashed one of the sporangia onto a microscope slide but was unable to find any spores so I left the sample to mature.
After a couple of days the sporangia had turned a dark olive green:
Under the microscope they yielded the following spores (x600):
The field width is about 86um so the spores are about 10 x 11um.
Also present were the long tapered elaters characteristic of Trichia decipiens:
First photo and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2013-12-18.