We describe the ultrastructure of the essential oil producing peltate glandular trichomes of Mentha x piperita L. During oil secretion, the cuticle covering the glandular cells separates from the cells to form an oil storage cavity. Filling of the cavity is correlated with the enlargement of gland cell plastids, and the development of an extensive smooth endoplasmic reticulum. Immunocytochemical localizations, and the occurrence of plastidial targeting sequences indicate that the enzymes responsible for the early steps of menthol biosynthesis (to limonene) occur in gland cell leucoplasts, while subsequent hydroxylation and reductions occur elsewhere.

Key words: glandular trichome, Mentha x piperita, monoterpene, peppermint