Breezy Love, or the Sacking of the Bees

By OLIVIA JUDSON

Olivia Judson on the influence of science and biology on modern life.

Birds do it. Bees do it. Beetles, bats and light summer breezes do it.

I refer, of course, to that raunchiest of sex acts: the pollination of flowers.

Ruby Washington/The New York Times SEDUCTION Bee and flower meet.

When it comes to sex, plants have more headaches than the rest of us. One problem is that they can’t travel about to find a mate — they are, after all, rooted to the spot — so they have to depend on intermediaries to bring egg and sperm cells together.

For mosses and ferns, the intermediary is water. For conifers like pine trees and cypresses, the intermediary is wind. But for most flowering plants, the intermediaries are animals.

Flowering plants are the largest, most successful group of plants on the planet today. There are thought to be more than quarter of a million different species — nearly 10 times more than all the other types of plants added together. (To put things in perspective, the number of living species of fish, amphibians, reptiles, birds and mammals combined is less than 58,000.) The flowering plants include roses and waterlilies, grasses and oak trees, tulips and orchids. They include, in short, most of the plants that come to mind when one thinks of vegetation.

It was not always thus. Before the mid-Cretaceous, 100 million years ago or so, flowering plants were scarce: conifers and their relations ruled the landscape. But then, for reasons that are not well understood, flowering plants upstaged all others, and the Earth came into bloom.

Flowering plants were not the first to seduce animals into spreading their pollen for them. Fossils suggest that some earlier groups of plants, now extinct, had evolved a dependency on insects like scorpionflies. Nonetheless, the earliest flowers appear to have been pollinated by insects, and the full-scale blossoming of flowering plants coincides with the rise of animals as go-betweens. Bees, for example, buzzed onto the scene with flowering plants; the evolutionary history, and success, of both groups is intimately linked.

Librado Romero/The New York Times

The appearance of flowering plants brought a new flamboyance to the planet. Flowers pollinated by animals tend to be big and colorful; they often smell. (To a human, flowers pollinated by bees typically smell pleasant; flowers pollinated by flies tend to smell foul, like rotting meat.) Often, flowers offer something for the animal to eat — a sip of nectar, perhaps. Sometimes, they provide heat.

(One plant that heats its flower is Philodendron solimoesense, an Arum from the South American tropics. In doing so, it turns itself into an assignation hotel for scarab beetles. The beetles arrive in the evening, spend the night feeding and mating, spend the morning recuperating and head off to a new flower later on — complete with pollen from their host. Sure enough, the heat saves the beetles energy. Beetles in a heated flower don’t have to use as many calories to keep warm as they would if they spent the night outdoors.)

Yet, from time to time, flowering plants abandon their animals, evolving instead to throw pollen to the wind. Wind-pollination — if you’re a vocabulary fiend, the technical term is “anemophily,” meaning lover of wind — has evolved at least 65 times in flowering plants, and around 10 percent of the species do it. Indeed, as I mentioned last week, many grasses are pollinated by the wind.

It’s not clear what causes this transition, though there are several ideas. One is that it happens in plants that, although generally pollinated by insects, already have a small capacity for wind pollination — small, light pollen grains, and flowers that can, in principle, catch pollen if it floats past on a breeze. Then, the balance between insects and wind can easily shift. In a tropical forest, for example, the advantages of insects are great: they provide highly targeted pollen-delivery in a complex milieu. But in big open spaces, the wind may do a better job — especially if the climate is inhospitable, and insects are few. Such circumstances may cause a shift away from traits that lure insects, and enhance those that seduce the wind.

A plant that has sacked bees or other insects can make its flowers smaller, less colorful and more aerodynamic. Liberated from the expense of making nectar, it can make more pollen instead. A bee, after all, can only carry so much pollen at once. The wind is not so limited.

And wind-pollinated plants tend to produce huge quantities of pollen. Whereas animal-pollinated plants produce a median of 3,450 pollen grains for every ovule, wind-pollinated plants produce almost 10 times as much. No wonder wind-pollinated plants are the chief causes of eye-itching, nose-tickling human misery. (It’s not just the anemophilous flowering plants that are to blame, though. Wind-blown cypress pollen is a major cause of allergies in some parts of the world.)

This massive production of pollen is usually put down to the inability of wind to make reliable deliveries.

Librado Romero/The New York Times

Charles Darwin himself suspected the wind of being a fickle and inefficient messenger, and that view has largely held until this day. But there is little actual evidence that wind-pollinated plants have more difficulty getting themselves fertilized than other plants do. (Indeed, plants seem adept at plucking pollen of the right species out of the breeze. How they do this isn’t known.) Moreover, in animals, large numbers of sperm tend to evolve when competition between different males to fertilize a female’s eggs is fierce. In many wind-pollinated species, plants flower all together, and for a brief time. Perhaps wind-pollinated plants face greater competition from their rivals.

But whatever the causes, I’m glad that most plants have not sacked their bees. In a world pollinated only by gusts and breezes, spring would be less beautiful. And, for many of us, it would also be more tortured.

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