Nuytsia@Tas also has an excellent written accompaniment to this photograph of field cow-wheat (an annual hemiparasite), so I’ll start off by quoting:
“A schedule 8 rarity of the British Isles, this plant is now known from only two or three sites. ”
“The level of parasitism in this genus is high with most species failing to get past the cotyledonary stage if grown without a host. The seeds are amongst the largest in the Orobanchaceae, so much so that the capsule only holds a pair of seeds. Peculiarly the capsule splits to release the seeds wet, which essentially just drop out.”
“The seeds themselves have a fatty elaiosome at one end. It is this elaiosome that enables dispersal of the seeds. Ants collect the seeds eat the elaiosome and then discard the seeds. The seeds have a rather complex dormancy and germinate during a following winter/spring period. ”
One of the reasons for its rarity in the British Isles is that it was purposefully extirpated (PDF). Once a common weed of fields, people were employed to hand-pull it. This animosity was directed towards the plant because the presence of its seeds in grain to be milled would contaminate and sour the flour. The Species Action Plan (PDF) for cow-wheat cites some local threats to existing populations in the UK, including erosion into a lake.
The species is distributed throughout Eurasia; the Species Action Plan link above points out that it is considered rare throughout the western European part of its range, but no mention is made of its relative rarity in the east.