I suspect it is the seed pods from the flowers of your Bok choy. they have run to seed instead of leafing up.
did they have yellow flowers on them?

Definitely Bok choy that has gone to seed. There has been a lot of problems with brassicas this year. The early heat got them really growing then the cooler weather made them think it was time to set seed... we're seeing it a lot. Obviously their biennial nature has been confused.

Is Bok Choy a brassica. The seed heads look very much like Oil Seed Rape otherwise.

Yes love it is a brassica xxx One you do not like :-)

The plants did have yellow flowers before the seed pods appeared. Thanks to everyone for your help.

I had though that Bok Choy was similar to spinach beet. I have just looked it up under its scientific name of Brassica rapa and - I don't believe it - it is oilseed rape!!!! It is also known as Field Mustard. Seemingly, the leaf vegetable, turnips and oilseed rape are all varieties of the same plant species.
Therefore, Moongrowers comments above would be your answere.

That's what my bok choy turned into, they had yellow flowers for a small time. Should I cut the flower of the others so I get bok choy? and how do I keep and use the seeds off the others? Some of the flowers were hiding in the leaves, maybe thats why you didn't notice it suddenly go to flower?

You need to watch all brassicas and remove flower heads before they actually flower or what you see above is the result. Personally I'd be getting any brassica that had gone to see out - those pods will suddenly open up and let the seed fly everywhere and you'll be weeding the stuff out for years. Otherwise remove all the seed heads apart from one pod and watch that like a hawk.