shamrock, a plant with leaves composed of three leaflets. According to legend it was used by St. Patrick in explaining the doctrine of the Trinity; it is now used as the emblem of Ireland. An artificial or real shamrock leaf is customarily worn on St. Patrick's Day. The actual species of the true shamrock has long been debated, but the plants most often favored and used are the white clover ( Trifolium repens ), the black medic ( Medicago lupulina ), the wood sorrel ( Oxalis acetosella ), and a hop clover ( Trifolium minus ). All are classified in the division Magnoliophyta, class Magnoliopsida. Trifolium and Medicago are in the order Rosales, family Leguminosae; Oxalis is in the order Geraniales, family Oxalidaceae.