Petty spurge, Euphorbia peplus, is a common weed of gardens and waste ground but its tiny green flowers look quite exotic in close up. Those horned glands give it a hint of the extra-terrestial.
The winged capsules are the female parts of the flower. There are three here, the central one much larger than those at top and bottom of the picture. Each capsule has a tuft of stigmas (five on the lower one).
Appropriately, the male flowers are the ones that are sporting the stag’s horns (which are glands). Again, there are three in the photograph and I think that on the tips of the stamens of the top one there’s a hint of yellow pollen.
The the small beak-like fleshy ‘leaves’ that can be seen clasping the top and bottom flowers are a structure that is special to euphorbias called the cyathophyll. I think that must be a botanical term for ‘cup-like leaf’ because phyll means leaf and cyathos is derived from a Greek word meaning ‘A cup or ladle used for drawing wine from a bowl’.
The larger ‘leaves’ clasping each flower are bracts, which look very similar to the leaves in this species. Each umbel of flowers has three rays (primary branches).
The petty spurge is not thought to be a native to the British Isles but is believed to be long-established here, since before 1600.