These are the seeds of hemlock water dropwort (Oenanthe crocata), probably the UK’s most poisonous plant.
It is a tall, impressive perennial that grows thickly on either side of the Lambrok tributary. In the spring, it has bright green, ferny foliage and throughout the summer many large white umbelliferous flowers that attract beetles and flying insects. Every part of the plant is poisonous, including its seeds.
The seeds are black when they are ripe, laid out enticingly in rounded umbellifers. The temptation to grab a handful as you pass is strong: they crumble off the seed head and into your hand like worry beads. Take care; don’t put them into your pocket where they will stick to your dog treats; don’t put them into your mouth to check if they are fennel or caraway. They aren’t.
The dried stems of O.crocata are hollow, like hogweed stems, and while they aren’t as poisonous now as they were in the spring, they don’t make good peashooters or throwing-sticks for dogs. Treat the whole plant with well deserved suspicion. Go carefully in the wild places.
More poisons to worry about from our archive.
Southwick Country Park Nature Reserve 
How does it differ from ‘cow parsley’?
Ever so much! Cow parsley is edible for one thing.
It’s a taller, bigger plant and much more robust than cow parsley and it always grows near water.