The secret of extrafloral nectaries

A nectary is a specialized group of cells designed to secrete sugary or protein-rich liquids. The purpose of a nectary depends on where in the plant it occurs.

Nectaries can be divided into two types according to their evolutionary function. Floral nectaries occur inside of a flower and are designed to attract visitors that will collect the nectar and inadvertently pollinate the plant. Usually the pollinator brushes against the stamens in pursuit of the nectar, and the pollen from the anther sticks to its body. When the insect travels to the next flower, some of the pollen is transferred to the stigma of the new plant.

Extrafloral nectaries offer protection

Extrafloral nectaries occur on the outside of the flower or on the stems and leaves. They too attract insect visitors, but not for pollination. Instead, these obscure nectaries attract visitors that provide protection for the plant. For example, extrafloral nectaries often attract ants, and a bevy of ants is off-putting to caterpillars or other creatures that may be interested in eating the plant. Both ants and wasps have been seen to actively defend this food source.

Nectaries can take many different forms and occur in a variety of places. Some nectaries are simply a group of cells that look much like the neighboring cells, making them difficult to see. Other nectaries are highly visible, having a unique structure and color. Some plants may have both floral and extrafloral nectaries.

Honey from rubber trees

Although extrafloral nectaries are not part of the pollination system of a plant, they are still attractive to nectar-eaters. Bees, wasps, flies and other insects often collect from these sources. In some cases, the extrafloral nectary can produce an entire crop of honey, such as that made from the rubber tree, Hevea brasiliensis.

According to the Rubber Board, “The rubber tree is a prolific source of honey which is obtained from the extra floral nectaries at the tip of the petiole, where the leaflets join.” In India, the Rubber Board is encouraging beekeeping as an alternate income source for rubber growers.

ExtrafloralNectaries.org (really!) lists 3797 angiosperm (flowering plant) species that have extrafloral nectaries, 22.5% of which are in the Fabaceae (the legume or “pea” family).

Nectar flow is variable

Nectar is largely composed of various sugars, including sucrose, glucose, and fructose, and a variety of trace components that may include amino acids, lipids, alkaloids, saponins, as well as compounds that impart flavor and odor.

In any given species, the amount of nectar produced is highly variable and dependent on factors including soil type, water supply, air temperature and humidity, hours of daylight, and time of day. In addition, nectar flow decreases as the flower ages and after it has been fertilized. Because the vegetative parts of a plant frequently persist after flowering, extrafloral nectaries may produce over a longer period of time.

Rusty
Honey Bee Suite