Kasrod

Fiddleheads or fiddlehead greens are the furled fronds from a fledgling fern, harvested for use as a vegetable. Left on the plant, each fiddlehead would unroll into a new frond . As fiddleheads are harvested early in the season, before the frond has opened and reached its full height, they are cut fairly close to the ground. Fiddleheads from brackens contain ptaquiloside, a compound associated with bracken toxicity, and thiaminase. Not all species contain ptaquiloside, such as Diplazium esculentum, a fern with fiddleheads regularly consumed in parts of East Asia, which differs from bracken (Pteridium aquilinum).

Source: Wikipedia