nectary

nectary, a plant gland in many angiosperms that secretes nectar. Floral nectaries are usually located at the base of the flower stamens (male structures), which draw animal visitors, such as hummingbirds, bees, and butterflies, into contact with the pollen to be transferred for pollination. Nectaries on vegetative plant structures, such as stems and leaves, are known as extrafloral nectaries and often serve to attract animals that defend the plant. Nectar is typically 10 to 50 percent sugar (namely sucrose, fructose, and glucose) with trace amounts of amino acids and may be actively or passively secreted from the secretory cells of each nectary.

Despite their proximity to stamens, most floral nectaries are not modified stamens but usually arise from other floral tissues, namely from the receptacle (which houses the ovaries) or from modified petals. In some cases, however, the nectaries coalesce into a structure known as a nectary or staminal disc. A staminal disc is often formed from a whorl of stamens that has been reduced and modified, though some such discs are derived from nectary-producing tissues of the receptacle. Interestingly, the nectar production of many flowers follows a particular rhythm and may be influenced by the age of the flower, daytime temperatures, sunlight, or the activity of animal visitors. To encourage cross-pollination, most flowers produce only relatively small amounts of nectar at a time so that animals must visit several flowers for a full meal.

Extrafloral nectaries may be associated with a variety of plant structures, such as leaves, petioles (leaf stalks), stems, and bracts. The nectar from such nectaries is generally used to facilitate mutualistic relationships between the plant and insects. In aspen sunflowers (Helianthella quinquenervis), for example, extrafloral nectaries are borne on the involucral bracts that surround the base of the flower. These produce nectar that is highly attractive to a number of ant species, and a high density of ants on the flowers has been shown to reduce the rate of seed predation from flies and moths. Members of the unrelated genus Costus similarly attract nectar-eating ants to extrafloral nectaries on their inflorescences; the ants protect the plants from insects that lay eggs in the seeds. While most extrafloral nectaries are used for mutually beneficial relationships, a few carnivorous plants employ nectar as bait. Many species of pitcher plants, for example, use nectar from extrafloral nectaries along the rims of their traps to attract prey to their deaths.

One of the most famous ant-plant mutualisms involves the extrafloral nectaries of the bullhorn acacia (Vachellia cornigera). Acacia ants (Pseudomyrmex ferruginea) live inside the hollow modified spinous structures of bullhorn thorns and feed on the nectar produced on the leaves and petioles (leaf stalks). In return for this food source, they ferociously attack and can destroy animals of all sizes as well as other plants that contact the acacia plant. In doing so, the ants protect the bullhorn thorn from herbivores and other plants competing for the available space, light, and minerals. Given that neither member can survive successfully without the other, this relationship is considered an obligative mutualism.