Morphology and Ultrastructure of Secreting and Nonsecreting Foliar Trichomes of Helichrysum aureonitens (Asteraceae)

Electron microscopical examination of foliar appendages of Helichrysum aureonitens Sch. Bip. has revealed two types of trichomes. The nonsecreting type consisting of four cells form a dense mass of long, fibrous hairs, covering the entire surface of the shoot. The glandular secreting type are few in number, club shaped, and consist of several cells. Glandular trichomes consist of secretory cells with dense cytoplasm, free ribosomes, numerous cisternae of endoplasmic reticulum, and scattered mitochondria. Secretions accumulate in a cavity beneath a sheath derived from separation of the cuticularized outer wall surface of the top tier of secretory cells and are released to the outside when the cavity ruptures. Only the apical pair of cells participate in secretion. Cells of both types have plastids without thylakoids. We hypothesize that the active therapeutic compounds (probably flavonoids) secreted by H. aureonitens are produced by the endoplasmic reticulum in the glandular trichomes.