XVI International Botanical Congess

One of the apomorphies characterizing the core Malvales (Bombacaceae, Malvaceae, Sterculiaceae, Tiliaceae) is the possession of trichomatous floral nectaries, a type absent in the relatives of this group, where only other kinds of nectaries occur, or none at all. Malvalean floral nectaries consist of multicellular clavate hairs, which release nectar only from the top cell and are usually aggregated in cushions. They presumably have evolved from hydathodes, e.e. dispersed trichomes, active in the bud stage and ubiquitous in the green parts. The extrafloral nectaries, likewise trichomatous and homologous, show various steps of hair aggregation, which may reflect the evolutionary pathway whereby similar aggregates became established in the flower for nuptial function. Replacing disk nectaries, which are basically absent or lost in this clade, they formed the precondition of a broad adaptive radiation including nectarivorous animals as pollinators. The mostly calyx-borne position of the nectaries has strongly influenced flower construction. Different modes of nectar exposition and the repeated occurrence of taxa with nectarless deceptive or pollen flowers are discussed on a tribal basis, relative to new concepts of classification suggested by molecular data.