Nectar

Floral nectaries

Floral nectaries, also called honey glands, are secretory tissues found in various locations within a flower. They may occur on the receptacle, hypanthium, tepals, sepals, petals, stamens, pistil, pistillodes, or ovaries. Their function is to attract pollinators such as insects, hummingbirds, and bats. Nectaries can be structural—specific areas of tissue that exude nectar—or non-structural, secreting infrequently from undifferentiated tissues.

In Lamiaceae, a nectariferous disc surrounds the ovary base; in Brassicaceae, the nectary sits at the base of the stamen filament; many monocotyledons have septal nectaries at the unfused margins of carpels. Nectar is secreted from epidermal cells with dense cytoplasm, often via trichomes or modified stomata, and adjacent vascular tissue supplies sugars from the phloem. Longer nectaries may contain a vascular strand to assist transport.

Components

The main ingredients in nectar are sugars—sucrose, glucose, and fructose—in varying proportions. Nectars also contain diverse phytochemicals, including carbohydrates, amino acids, and volatiles that attract some species, as well as alkaloids and polyphenols that provide protective functions.

Nicotiana attenuata uses benzylacetone as a strong aroma to attract pollinators, while adding bitter nicotine to discourage prolonged foraging. Neurotoxins such as aesculin occur in some nectars, like that of the California buckeye. Nectar additionally contains water, essential oils, ions, and numerous other compounds.

Similar attractive substances

Not all insect-pollinated plants have nectaries; some use other structures to lure visitors. Elaiophores secrete oils, while osmophores release volatile scents. In orchids, these scents act like pheromones.

Osmophores feature a thick, domed or papillate epidermis with dense cytoplasm. The labellum epidermis of Platanthera bifolia gives off a nighttime scent, and Ophrys labella have dome-shaped, dark-staining epidermal cells that form osmophores. From its corona, Narcissus emits volatiles tailored to specific pollinators.

Did You Know?

• The word 'nectar' derives from Greek νέκταρ, the fabled drink of eternal life, and may mean 'unkillable' or 'immortal'.
• Extrafloral nectaries are also known as extranuptial nectaries.
• In carnivorous plants, extrafloral nectaries are used to attract insect prey.