"sourgrass" a pesky widespread, but edible weed with a tart acidic bite due to the presence of oxalic acid. Unfortunately, oxalic acid is poisonous in large concentrations so sourgrass must be foraged sparingly. However, oxalic acid naturally occurs in regularly consumed produce including: spinach, rhubarb, sorrel(unrelated to oxalis), and black tea. Along these lines in a lecture by Dan Barber at Harvard Food and Science, it was explained that plants, including herbs/greens we consume(basil and thyme), produce aromas as defense mechanism meant to increase the toxins which counter-intuitively, we humans, find delicious in minute quantities. Take a taste, you'll be pleasantly surprised.