How Do Flowers Attract Pollinators?

Flowering plants propagate through sexual reproduction. Pollination is a key stage in their life cycle; they must transfer pollen from the anthers of one flower to the stigma of another in order to reproduce. Not all flowers require the assistance of pollinators, but ones that do have evolved a variety of ways to lure pollinators and keep them coming back for more.

Pollinators typically come seeking nectar, a sweet, sugary fluid secreted by the flower. Some species of birds, for example, depend on flower nectar for food, and bees use nectar to make honey. As it drinks nectar from the flower, the pollinator brushes against the anthers, collecting pollen it will inadvertently transfer to another flower. Making the nectar is expensive for the plant in terms of the amount of energy it consumes, but plants that can attract efficient pollinators produce more offspring, so for many flowers this trait has been favored by natural selection.

Flowers pollinated by insects usually exude a sweet scent, especially ones pollinated by moths, butterflies and bees. The scent serves to attract the insects so they feed on the flower--and pollinate it in the process. Flowers pollinated by birds, by contrast, are seldom strongly scented, because most birds have a poor sense of smell, so scent will not attract this particular pollinator. Bat-pollinated flowers are rare but aromatic; the agave and cactus of the southwestern United States are one example. Flowers pollinated by flies and beetles exude scents like rotting carrion or dung; while humans consider these odors unpleasant, the smell attracts flies that mistake the flower for rotting flesh and lay their eggs, pollinating the flower in the process. The world's largest flower, Rafflesia arnoldii, is one example.

Bright colors mark flowers apart from their surroundings so pollinators can find them. Flowers pollinated by nocturnal animals like moths or bats are usually white or yellow, since these colors are more visible at night. Flowers pollinated by birds are usually yellow or red. Flowers pollinated by bees may be yellow, blue or white; they are seldom red, however, because red looks dull to bees. Flowers may also have markings that absorb or reflect ultraviolet light; this kind of mark stands out if you're a bee, since bees--unlike humans--can see in ultraviolet.

Some flowers don't need to attract pollinators at all. Roughly 20 percent or so of all flowering plants rely on the wind. Unlike flowers pollinated by animals, wind-pollinated flowers are generally dull and inconspicuous and lack scent; their goal is to avoid being seen and release their pollen to the wind. Some aquatic species of plants are pollinated by water; these flowers also have no need to court pollinators.