Glandular trichomes on the leaves and flowers of Plectranthus ornatus:: Morphology, distribution and histochemistry

The types of glandular trichomes and their distribution on leaves and flowers of Plectranthus ornatus were investigated at different stages of their development. Five morphological types of glandular trichomes are described. Peltate trichomes, confined to the leaf abaxial surface, have, in vivo. an uncommon but characteristic orange to brownish colour. Capitate trichomes, uniformly distributed on both leaf surfaces, are divided into two types according to their structure and secretory processes. In long-stalked capitate trichomes, a heterogeneous secretion (a gumresin) is stored temporarily in a large subcuticular space, being released by cuticle rupture, whereas, in the short-stalked capitate trichomes, the secretion, mainly polysaccharidic, is probably exuded via micropores. On the leaves, digitiform trichomes, which do not show a clear distinction between the apical glandular cell and the subsidiary cells, occur with a similar distribution to the capitate trichomes. They do not develop a subcuticular space, and secrete small amounts of essential oils in association with polysaccharides. The reproductive organs, particularly the calyx and corolla, exhibit, in addition to the reported trichomes, unusual conoidal trichomes with long unicellular conical heads. A large apical pore, formed by tip disruption, releases the secretion (a gumresin) stored in a rostrum-like projection. On the stamens and carpels, digitiform, capitate and conoidal trichomes are absent, bur peltate trichomes are numerous. They occur between the two anther lobes, on the basal portion of the style, and between the four lobes of the ovary. The results presented are compared with those of other studies on Lamiaceae glandular trichomes. (C) 1999 Annals of Botany Company.