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Bacteria grows on the tips of the Blockthorn spines. It is possible to have a bad reaction if you ge stuck by those. I spent one very unpleasant night with a swollen throbbing hand having stuck myself whilst hedgelaying!
The problem here is, I think, a commonplace one although rarely considered in floras (although the latest Flora of Hertfordshire does indeed have a good summary). In addition to P. cerasifera, P. domestica (Wild Plum) also often has spines and can resemble P. spinosa. It is now thought that it arose as a stablised hybrid between P. spinosa and P. cerasifera. There is in fact a wide range of variation between these three species (which probably cross and back-cross as well to add to the confusion). I suspect that much planted "P. cerasifera" is in fact hybrid, or if not has been subject to nursery selection. A lot may even come in from abroad! There is a tendency (quite understandably) to record anything with large thorns as P.spinosa and anything without as P. cerasifera or P. domestica. Unfortunately the Plant Crib is not much help - merely saying that the position is complex! If anyone knows of any recent work on this problem it would be good to know. I have about six different floras in front of me as I write this and I'm not sure I'm much the wiser!
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Lat/Lng: 52.3, -0.1
OS grid ref: TL3173