Trichome forms in Ardisia (Myrsinaceae) in relation to the bacterial leaf nodule symbiosis

Symbiotic leaf-nodule bacteria in nodulated members of Rubiaceae live in mucilage secreted by colleters located on stipules within buds. These differ from colleters on most nodule-free species. This study was undertaken to examine buds of Ardisia and the related monotypic Amblyanthus of Myrsinaceae to see if nodulated species had secretory structures dissimilar from those of nodule-free species. Buds removed from herbarium specimens (61 species) and live plants (3 species) were paraffin-sectioned. Diverse trichome forms occur, including dimorphism between adaxial and abaxial trichomes in some species. Species within each subgenus were arranged according to trichome form: peltate scale, irregularly capitate, capitate, sessile capitate, bicellular capitate, and uniseriate. Only seven Ardisia species (all in subgenus Crispardisia, widely assumed to have bacteria in marginal leaf nodules of all 30 species) have short-lived trichomes bearing one or more elongate, swollen, distal cells that appear to be secretory cells. These trichomes are analogous to the dendroid or brushlike colleters of nodulated Rubiaceae. Druses occur in most subgenera and this appears to be by far the most predominant crystal type m Ardisia.