Pollination
Will the wind ever remember? ~ American musician Jimi Hendrix in the song “The Wind Cries Mary” (1967)
In most places, the wind is an unreliable partner. Animals may be more dependable. But enticing animals willing to work for wages requires advertising.
However free the breeze, plants had been long used to producing compounds and altering their forms to deal with animals, particularly putting off herbivores. Putting energy into attracting animals to aid a plant’s prolific propagation seemed a fair exchange.
Pollination systems reflect a biological market. Potential pollinators may choose their visits based upon advertised allures and rewards.
Plants are careful to honestly advertise their rewards; which is why they signal that a pollination visit has recently occurred, and so landing upon a certain flower is not worthwhile in the moment, as the flower is being restocked. Because attracting pollinators is a competitive exercise, floral deceivers that rely upon pollination do not exist, as plants understand the social dynamic involved.
Insects are the most common pollinators. Angiosperm-insect coevolution occurred among many species of both plants and animals.
Having suffered their herbivory, plants already knew insects intimately, so coopting them was not so difficult. Pollinating bees evolved from wasps during the rapid diversification of flowering plants during the early Cretaceous.
Such luring was a trick learned much earlier. Primitive mosses emit smells that attract tiny arthropods – springtails – that inadvertently pick up sperm and cart it to another moss.
At one time, pollen itself was an attractive enough food. But then other plants did the same, demonstrating the limit of pollen packing as the only ploy to attract pollinators.
Competition being what it is, signage became more flagrant and fragrant. Flowers began to ostentatiously advertise: by their shapes, patterns, colors, and scents. This explains the enormous diversity in the florid displays that angiosperms put on to bewitch pollinators.
Nectar was a logical next step: a sweet treat to attract bees on the beat. Soon, thanks to a competitive market, something more than sheer sugar-water was needed to keep pollinators coming back. Remembering floral traits is difficult when flickering from flower to flower at a fast pace. A memory aid would help an angiosperm’s cause.
Many plants produce alkaloids to ward off herbivores by their bitter taste and psychoactive effects. Caffeine and nicotine are exemplary.
Caffeine evolved independently in tea, coffee, and cocoa plants; put in leaves or seeds to discourage herbivory. But carefully applied, caffeine has a power beyond dissuasion.
Some flowering plants, including coffee and citrus, lace their nectar with caffeine. The bitterness should be off-putting, but just enough is added by plants to have the intended effect while preserving good taste.
Caffeinated nectar provides a tremendous memory boost to a visiting pollinating insect. A little caffeine buzz has a bee remember a specific sweet spot by its floral scent for days. By enhancing a pollinator’s memory, plants reap reproductive benefits through pollinator fidelity.
The effect of caffeine is akin to drugging, where bees are tricked into valuing the forage as a higher quality than it really is. ~ Swiss ethologist Roger Schürch
Plants further help their pollinators by putting in metabolites which prevent parasites and pathogens. Plants take prodigious care of those who take care of them, even if inadvertently.
Competition for pollinator attention cannot be understated. Many bees collect honeydew produced by scale insects found on shrubs. Though relatively unadvertised, the wafting scent of sugar, and social cues from other foraging bees, is enough to attract insects with a taste for sweets.
Plants made the acquaintance of birds as both benefactor – by eating plant pests – and as seed stealers. Induced by florid flowers and fruits, birds got into the pollination business.
For fruit eaters, plants have a follow-on trick for quick seed dispersal: adding a laxative well-suited to its consumers. For plants, sugars are easy to produce. But let’s not have potential offspring sit in the guts of nasty animals, where fermentation might doom the next generation; hence the laxative in fruit.
The best way to profit is to secure a monopoly. For plant pollination, this involves picking a species that you can count on and courting it until it goes to no other. Or, at least, making yourself a favorite.
Bee-pollinated flowers are designed specifically to attract bees: yellow, blue, and violet colors, as well as ultraviolet patterns that only bees can see. Bee flowers typically offer a landing platform, with petals that form a tube specifically configured to allow nectar access only to insects with specialized mouth parts.
Pollination is not the only benefit that bees bring to plants. The buzz of bees about flowers discourages caterpillars, who fear the sound may be a predatory wasp.
Beetle-pollinated flower plants tend to have an open structure, with short mouth parts that beetles can readily access. Some have specialized food structures, such as clusters of cells with a flower that a beetle can eat.
Fly and beetle-pollinated flowers are often strongly scented, smelling of other fare preferred by these insects, such as dung and rotting meat. Orchids are common cultivators of stink-loving pollinators. The blunt-leaved orchid depends exclusively on mosquitoes for pollination. This orchid attracts its clientele by smelling like human body odor.
One benefit of a select pollinator clientele, besides a reliable relationship, is a steady transfer of pollen between flowers of the same species. It’s no good to give up pollen that has little chance of landing on a potential mate. Insect pollinators such as bees and butterflies do engage in flower constancy: transferring pollen to other conspecific plants.
This pollination specialization can be taken to extremes. Some plants have exclusive pollinators: a single-species symbiosis. Fig plants are absolutely dependent upon pollination by minute fig wasps.
The purple-throated carib hummingbird is the sole pollinator of 2 Heliconia species. Each hummingbird sex feeds at the flower species that matches the size of its bill. Sexual dimorphism met coevolution between plant and animal. Heliconia speciated to meet the specific needs of just 1 gender of hummingbird. Meanwhile, female and male hummingbirds adapted to a certain flower.
Plants are judicious retail mavens in their pollination practices: giving up only enough to keep the pollinators coming back and restocking at a speed expected to keep up with demand.
Biotic pollination is a tremendous benefit to plants widely distributed in small populations. Without it, populations would likely perish. With animal-aided pollination, plants can succeed in places which would otherwise be perilous.
Old-school angiosperms that depend upon wind pollination, now only 10% of flowering plants, run some risk from an unreliable ally, but need not bother with sizzling signage, and so can conserve their resources by not resorting to consorting with animals.
As in all things, there are always tradeoffs. Plants have a long history of unsurpassed adaptability, coupled to chemical proficiency and genetic manipulation savvy, that has let them make intelligent choices well beyond the reach of other macrobial life. Microbes have comparable sophistications in these realms, but scale presents unique challenges.
Plants can precisely tune their gene expressions to get the desired flower shapes, colors, patterns, and scents, as well as producing specific secondary metabolites that serve specific needs. How it is that plants understand their target audience’s nervous and digestive systems is outside the realm of empirical investigation, as Nature is an exposition involving sources unobservable.
