The floral nectar of angiosperms is primarily a solution of simple sugars, but contains detectable amounts of other solutes, particularly amino acids. These have been regarded as diagnostic, for phylogenetic and taxonomic purposes, and their mean concentrations may be significant in relation to the pollination syndrome of a plant species. However, in several temperate flowers the amino acid concentration varied by an order of magnitude when measured at intervals through a single day. In open, cup-shaped flowers, this was partly due to post-secretory equilibration with the varying humidity of the air. But the effects of floral visitation by insects on nectar amino acids were also important, both in open flowers and in those with long corollas and more enclosed nectar. Visitors could add amino acids to nectar by direct contact, by salivation, by damaging the neighbouring tissues causing cell leakage, and by dislodging pollen into the nectar; the importance of each of these effects varied according to corolla morphology and the spectrum of visitors.