Nectar is a sugary fluid produced by plants that serves as a reward to attract animals. This sweet liquid incentivizes various creatures, such as insects, birds, and bats, to visit flowers, thereby facilitating pollination. It plays a significant role in the reproductive process of many flowering plants.
The Nectary Gland
Nectar is created in specialized plant structures called nectaries. These glandular tissues actively secrete nectar. The process involves the plant’s vascular tissue, the phloem, transporting sugars into the nectary cells. Water then follows by osmosis, creating the fluid necessary for secretion.
Nectaries can vary in appearance and can be found in diverse locations on a plant, not just within flowers. While floral nectaries are directly involved in attracting pollinators, extrafloral nectaries, located on parts like leaves or stems, produce nectar to attract beneficial insects that defend the plant from herbivores.
Common Nectar Locations
The placement of nectar within a flower is highly variable among different plant species. They can be found at the base of petals, sepals, stamens, or even around the ovary. For example, many flowers have nectaries at the base of their petals or in the center of the flower, making them accessible to insects.
In some cases, nectar is held within specialized structures like nectar spurs, which are hollow extensions of floral parts, often petals or sepals. Only pollinators with tongues or proboscises long enough to reach the nectar in these spurs can access the reward. For instance, in the Brassicaceae family, the nectary is typically found at the base of the stamen filament. Many monocotyledons, on the other hand, often possess septal nectaries, which exude nectar from small pores on the surface of the gynoecium.
Why Nectar is Placed There
The specific positioning of nectar within a flower is a result of co-evolution between plants and their pollinators. This strategic placement ensures that a visiting animal, while attempting to access the nectar, comes into contact with the flower’s anthers, which produce pollen, and the stigma, which receives pollen. This contact facilitates the transfer of pollen, leading to successful fertilization and seed production.
Flowers often produce only small amounts of nectar at a time, encouraging pollinators to visit multiple flowers for a full meal. This behavior promotes cross-pollination, where pollen is transferred between different individual plants, enhancing genetic diversity. The varying lengths of nectar spurs, for example, have co-evolved with the differing proboscis lengths of pollinators, ensuring that only specific animals can efficiently access the nectar, thereby influencing which pollinators are most effective. This intricate relationship between nectar placement and pollinator behavior is a key mechanism for plant reproduction and diversification.