The resistant, overwintering stage of the parasitic ascomycetous fungus Claviceps purpurea, a pathogen of cereal rye that transforms the seed of rye into a compact spurlike mass of fungal pseudotissue (the sclerotium) containing five or more optically isomeric pairs of alkaloids. The levorotary isomers induce uterine contractions, control bleeding, and alleviate certain localized vascular disorders (migraine headaches).ergotism. SYN: rye smut. [O. Fr. argot, cock's spur]