There are several ways the plants approach attracting pollinators: visual cues, fragrance, ultraviolet hints, and nectar tweaking.
There are many visual cues flowers present which attract pollinators, cues they can spot from the air. These are shape, size, showy petals and sepals, and color.
Shape is an important visual cue because it enhances their access to the nectar. For example, irises produce large flowers to attract bumblebees; the bell-shaped flower of the azalea attracts hummingbirds; open, flat flowers like the purple coneflower are more pleasing to beetle and other non-flying pollinators.
Fragrance is connected to color in the pollinator syndrome. Connecting visual and olfactory channels, the signal the flowers send is stronger. If the fragrance is dissipated by wind, the color is still in effect. The connection between these two elements was determined by an international research team that reported its finding in Nature Ecology & Evolution.
It seems that certain plants are capable of an optical effect that attracts bees, which are able to see what has been termed the “blue halo.” Certain species of flowering plants have tiny ridges on their petals. These ridges, which vary in height and spacing, scatter light to cast this bluish color, which is not visible by humans. A study by Cambridge University in 2017 suggests that these ridges evolved independently, possibly as a way to attract pollinators by tapping into the ability of bees to see blue and UV light.
Insects can see ultraviolet light, although some colors are not apparent to certain insects. Ants, for example, cannot see red. Because of this ability, flowers appear differently to insects in their structure and color, such as the black-eyed Susan. Yellow or violet flowers show a greater tendency to reflect ultraviolet light than do flowers of other visible colors. Many flowers which appear to have uniform structure in the visual spectrum reveal a more complex structure under ultraviolet light.
Nectar is primarily sugar water and is sort of a quid pro quo between the flower and the pollinator. Research has shown that the makeup of nectar varies among flowers. Butterflies are attracted to plants whose nectar is high in amino acids since butterflies lack protein in their diet. Hummingbirds prefer highly-concentrated nectar because of their energy needs. Bees, it was determined, are capable of discerning differences in sugar concentration as small as 1%-3%.
Plants are clever, however. Apparently they seem to know not to produce too much nectar or the pollinators will be sated quickly, which interferes with the pollination process.
On a side note, did you know that some flowers produce two kinds of pollen? One is the pollen involved in pollination; the other is a sterile pollen that serves as a protein food source for pollinators — yet another attracting element.
Symbiotic relationships are when two participants mutually benefit from the relationship. Of course, pollination in general is symbiotic or mutualistic. Sometimes, however, it goes beyond simple pollination.
The Monarch butterfly is the pollinator for the milkweed plant. Beyond the nectar, the butterfly needs the milkweed to host its eggs and larvae. The caterpillars rely solely on milkweed for nourishment. This is not the end of the benefits Monarchs derive from the milkweed.
The sap that leaks out from the stem or leaves of the milkweed contains toxins. However, the Monarch is immune to its effects. Instead, the toxin serves to ward off predators, especially on the Monarch’s long migration, by making it deadly to vertebrates. The most common vertebrate predators of the Monarch butterfly are lizards, frogs, and birds. Feeding on a Monarch will make them sick and vomit, which tends to avoid viewing Monarchs as a food source.
HOW PLANTS KNOW TO PREPARE FOR POLLINATORS
A researcher reasoned that since plants and pollinators have obviously co-evolved over time, it would not be reasonable to believe that sound would be ignored as part of the plants’ pollinator syndrome arsenal. Considering the level at which sound saturates our planet, Prof. Lilach Hadany, Tel Aviv University, set out to prove that plants can differentiate between sounds. Once exposing the flower to various sounds, she discovered that the ultra-low frequency and the bee wingbeats caused the plant to spend the next three minutes increasing the sugar content in its nectar by 17-20%!
A benefit of this reaction to the sound of wingbeats, the plant was nine times more likely to be visited by a second pollinator within six minutes, a definite bonus for reproduction.
A new field of study, phytoacoustics, has been opened up by this study and one other, which played the sound of caterpillars munching on leaves next to an Arabidopsis plant. They discovered that in response to the sound, the plant flooded its leaves with a chemical deterrent.
Through some means which scientists are researching, plants are able to retain information and make decisions about their survival based on past experiences. They are able to respond to threats such as when the leaves of a plant are being invaded by unwelcome insects and the plant releases airborne biochemical substances to other parts of the same plant as a defense mechanism.
Are they truly conscious? Not perhaps in terms of consciousness as defined for humans, which involves self-awareness. Heidi Appel, professor at the University of Toledo takes this stance: “Some have suggested that since plants can form ‘memories’ they are therefore conscious beings. Plants do retain information about what they experience, in that their response to changes in their environment can depend on what they’ve experienced previously. Even offspring may exhibit some traits that are influenced by what their parents experienced.
“Retaining information within and between generations of organisms is a trait of all living things, with an increasingly well-understood genetic basis. Whether these constitute ‘memories’ depends on whether you define ‘memory’ as ‘recall’ or something more. If we return to common definitions in the English language, memory as commonly defined does not require self-awareness.”
Regardless, if we are going to save our pollinators and our planet, we need to have a better understanding of their world, and perhaps the new fields of neurobiology and phytoacoustics can help.