Abstract
Many plants produce secretions that serve as animal attractants or food rewards in the form of nectar, oils, and food bodies. Some chemicals provide plants with their only means of defense by discouraging the visitation of harmful insects and herbivores. Human beings use these products as condiments, medicines, fragrances, dyes, and ornaments. Secretory products are composed of a variety of organic molecules and are secreted by plants in specialized secretory structures and secretory cells. Different types of secretory structures often produce chemically identical products. Many plants possess elongated secretory canals, ducts, or cavities that become lined with specialized secreting epithelial cells. External secretory structures include glandular trichomes, colleters, glands, nectaries, and hydathodes. Glandular trichomes vary widely in structure but typically possess a basal cell, stalk, and unicellular or multicellular secretory head. At maturity, the head contains an enlarged subcuticular space that becomes filled with essential oil or resin as secretion proceeds. Stinging hairs represent another trichome type and produce skin-irritating substances. Internal secretory structures encompass oil cells, secretory cavities and canals, and laticifers.
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