A willow herb seed head. These develop from these:
which, in turn, develop from these:
This plant was growing in the long grass and was rather small compared with the great hairy willow herb (Epilobium hirsutum) that grow beside the lakes, so I don't know whether it was the same species. I will just call it Epilobium sp. for the time being.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-09-09.