Sorrel is the sharp, lemony leaf of Rumex acetosa, a hardy perennial of the dock and buckwheat family eaten as a tangy leafy green. It grows as a rosette of long, arrow- or spear-shaped leaves, bright green and thin-textured, that taste vividly sour — a clean, citrus-like acidity quite unlike the bitterness of most wild greens. The sourness comes from oxalic acid in the leaf, which is why the plant is also called sour dock or spinach dock. Young spring leaves are the mildest and best; older summer leaves grow coarser and more acid. Sorrel is used by the handful rather than the plateful — shredded raw into salads, or, most famously, wilted into a pale green purée for the classic French sauce that partners salmon, trout and eggs.