Fiddleheads are the immature leaf stalk and fronds of ferns, named for the resemblance of their furled tips to the scroll of a violin. Long a traditional springtime delicacy, harvested as the fern fronds begin to elongate and unroll but before they toughen, fiddleheads are now cause for some caution. A common species especially enjoyed in Japan and Korea, the bracken fern Pteridium aequilinum, has been found to contain a potent DNA-damaging chemical. It should be avoided. Stalks of the ostrich fern, various species of Matteuccia, are thought to be safer to eat.