Nectar is a sweet, pleasant-tasting liquid secreted by nectaries. These are specialized organs located inside flowers, and also outside. For example, the "glands" on leaf stalks of cherry (Prunus) and wattle (Acacia) trees are nectaries. The nectar produced inside flowers attracts crawling or flying insects, birds, or mammals. As a result of feeding on nectar, these visitors may transfer pollen from anther to stigma, within the same flower, or from one flower to another of the same species. Such pollen transfer ensures that fertilization takes place. The nectar produced by Prunus and Acacia nectaries attracts ants, which then protect the host plant from the leaf-eating larvae of other insects.
Nectar is sweet because of its large sugar concentration. Any or all of sucrose, maltose, glucose, and fructose may be present. Their proportions vary with species, as do their concentrations. For some species (e.g. hellebores, Helleborus spp.), the sugar present is entirely or almost entirely sucrose; for others (e.g. blue bells, Campanula spp.), the hexose sugars glucose and fructose are the major sugars present. Total sugar concentrations between 30% and 60% (by weight) are common. Nectar with relatively smaller sugar concentrations is preferred by insects, while larger concentrations are preferred by birds and mammals.
Amino acids are the most abundant of the other nutrients present in nectar. The relative proportions of amino acids versus sugars, and the acidity of nectar can change during the day. This may influence the timing of flower visits by specific nectar-seeking visitors, but the significance of these daily changes is not fully understood.
The production of nectar by nectaries occurs at precise stages of floral development, in flowers coinciding with pollen release, and with the stigmatic surface becoming receptive to pollen. Production is flexible-it stops if nectar is not removed, and resumes to compensate for nectar consumed by visitors. Floral structure can influence how the composition of nectar is controlled following production. Where the petals are fused to form a tube, the secreted nectar can be protected from a concentrating process that might otherwise come about through evaporation of water. This extends the period during which the nectar is most attractive to pollinators.
See also Pollination.