in susceptible people (p 490) Oxalates Oxalates are various salts of oxalic acid, a waste product of plant metabolism found in a number of foods, notably spinach, chard, beets, amaranth, and rhubarb The sodium and potassium salts are soluble, while the calcium salts are insoluble and form crystals that irritate the mouth and digestive system Soluble oxalates can combine with calcium in the human kidney to form painful kidney stones In very large doses — a few grams — oxalic acid is corrosive and can be fatal Bracken-Fern Toxins Bracken-fern toxins cause several blood disorders and cancer in animals that graze on this common fern (Pteridium), which is sometimes collected in the young “fiddlehead” stage for human consumption Ostrich ferns, Matteuccia species, are thought to be a safer source of fiddleheads, but there’s little solid information about the safety of eating ferns It’s prudent to eat fiddleheads in moderation, and to avoid bracken ferns by checking labels and asking produce sellers Psoralens Psoralens are chemicals that damage DNA and cause blistering skin inflammations They’re found occasionally in badly handled celery and celery root, parsley, and parsnips, when these vegetables have been stressed by near-freezing temperatures, intense light, or infection by mold Psoralens are absorbed through the skin during handling, or by being ingested with the vegetable, either raw or cooked They lie dormant in skin cells until they’re struck by ultraviolet rays in sunlight, which causes them to bind to and damage DNA and important cell proteins The psoralen-generating vegetables should be bought as fresh as possible and used quickly In addition to their own chemical defenses, fruits and vegetables can carry other toxins ... are chemicals that damage DNA and cause blistering skin inflammations They’re found occasionally in badly handled celery and celery root, parsley, and parsnips, when these vegetables have been stressed... light, or infection by mold Psoralens are absorbed through the skin during handling, or by being ingested with the vegetable, either raw or cooked They lie dormant in skin cells until they’re struck... which causes them to bind to and damage DNA and important cell proteins The psoralen-generating vegetables should be bought as fresh as possible and used quickly In addition to their own chemical