The majority of flowering plants rely on biotic pollination, a biological partnership engaging animals like insects, birds, and bats to transfer pollen. This strategy requires a system of attraction and reward to incentivize a pollinator’s visit. While nectar is the most famous incentive, its presence is not universal. A flower’s decision to produce nectar depends on the specific mechanism it has evolved to ensure successful genetic transfer.
Defining Nectar and Nectaries
Nectar is a fluid produced by a plant gland called a nectary, serving primarily as a caloric payment to animal visitors. This sweet secretion is fundamentally a watery solution where sugars are the most abundant solutes, typically including fructose, glucose, and sucrose in varying ratios. Trace amounts of amino acids, proteins, and minerals also contribute to the liquid’s nutritional value.
Nectaries are specialized secretory tissues that can be located in various parts of the flower. Floral nectaries are the most common and are often positioned strategically at the base of the petals or stamens. This placement forces a visiting animal to brush against the flower’s reproductive parts to access the reward. Some plants also possess extrafloral nectaries on leaves or stems, which secrete nectar to attract defensive insects, like ants, that protect the plant from herbivores.
Nectar as a Pollination Reward
The production of nectar represents a significant investment of plant energy, but it is a highly effective mechanism for ensuring reproductive success through co-evolution. This sugar-rich liquid acts as a metabolic reward, ensuring the pollinator gains enough energy to continue foraging and repeatedly visits flowers of the same species. Flowers and their primary pollinators have developed highly specific characteristics that fine-tune this relationship, a concept often referred to as a pollination syndrome.
The quantity and concentration of the nectar are precisely tailored to the energy demands of the intended pollinator. For example, flowers pollinated by hummingbirds or hawk moths often produce nectar that is highly concentrated and sucrose-dominant, providing a high-octane fuel source for their rapid flight. Conversely, flowers that rely on bees frequently produce nectar with a higher proportion of hexose sugars like glucose and fructose, matching their different digestive needs. This tailoring ensures efficient and directed pollen transfer while minimizing the energy cost to the plant.
Flowers That Don’t Produce Nectar
The absence of nectar in a flower is a strong indicator that the plant has evolved an alternative, non-caloric strategy to achieve pollination. Many plants forego the energetic expense of nectar production entirely because they rely on non-living environmental forces, a strategy known as abiotic pollination.
Abiotic Pollination
Wind-pollinated plants (anemophilous species), including grasses, oaks, and pines, are typically small, inconspicuous, and lack bright colors or strong scents. They produce vast quantities of lightweight, dry pollen easily carried on air currents to large, feathery stigmas. Hydrophilous plants rely on water currents to transport pollen and also lack nectar, as the liquid reward would be unnecessary in the aquatic environment.
Alternative Biotic Rewards
In other cases, flowers still employ biotic pollinators but offer alternative forms of compensation. Some species, often called pollen flowers (like many roses), provide pollen as the main reward. Pollen is rich in protein and lipids, serving as a direct food source for certain bees.
Other specialized flowers, such as those in the Malpighiaceae family, produce floral oils collected by specific New World bees for larval provisioning. Finally, some plants use deception, offering no reward at all. Many orchids, for instance, use mimicry, evolving structures that look like a mate or food source to trick an insect into visiting and inadvertently transferring pollen.