Common St. Johnswort (Hypericum perforatum):
This is a pretty five-petaled flower of fields and waste areas. It is the ingredient in St. Johnswort health supplement. It supposedly improves your memory. It also enhances your ability to sunburn. In fact cows and goats with white, thin fur can sunburn after eating St. Johnswort so badly that they will slough these patches of skin exposing raw flesh. This won’t happen with one or two plants but if they are turned out into a field of St. Johnswort with little else to eat they will graze on it as a last resort. In spite of this hazard, it may be the reason that some cows and goats have such good memories.