A broomrape

Not sure which one this is, there are hundreds, but I think it's a broomrape. They are parasitic plants that have no chlorophyll. The seeds germinate when they detect a compatible root nearby, they then send out a root towards it which latches on and steals nutrients from the plant. Eventually they sprout a colourless flowering body like this and then the process repeats.

Apparently some can be so aggressive that if they get established they can write off an entire field of crops. These are growing along the edge of the hippodrome, so they could be parasitising the clover in the grass, or the ivy in the hedge.