THE DANCE OF POLLINATION

A hundred million years ago, when dinosaurs roamed Earth, the plant world was dominated by cone-bearing conifers such as pine and redwood trees, which spread their pollen by the wind; when the pollen reaches a plant, fertilization of an egg can occur. But a new type of pollination was evolving at this time, one that would forever change the terrestrial landscape: pollination by insects.

With the arrival of pollinating insects, including bees, on the evolutionary scene, flowering plants blossomed, diversified, and radiated around the globe. Their great success owes everything to the reproductive advantage of relying on insects, rather than wind, to deliver pollen.

Wind pollination is like junk mail: you need to send thousands of letters to hook just one receptive customer–or in this case, to fertilize just one egg. All those wasted pollen grains represent a huge energy loss to the plant. Pollinating insects, on the other hand, are like FedEx: they deliver a pollen package directly to the appropriate recipient, contributing to the reproductive success of the plants.

Of the 250,000 species of flowering plants, or angiosperms, that exist worldwide, more than 75% are dependent on insect pollinators to reproduce. And of the many types of insect pollinators, bees are by far the most important ecologically. In fact, in many natural environments, bees are keystone species–those that play a central role in holding the community together.

You can think of a keystone species as analogous to the keystone in an archway–it doesn’t support as much weight as the other stones, but if it is removed, the doorway collapses (INFOGRAPHIC 22.1).

The majority of beekeepers in the United States and Europe cultivate the bee species Apis mellifera, the Western honey bee. It is hard to overestimate the importance of this tiny pollinator to modern agriculture. In the United States alone, more than 100 different crops–worth an estimated $15 billion annually–are dependent on honey bee pollination, including apples, oranges, blueberries, melons, pears, pumpkins, cucumbers, cherries, raspberries, broccoli, avocados, asparagus, clover, alfalfa, and almonds. A 2007 study published in the British journal Proceedings of the Royal Society B found that 87 of the leading global food crops, accounting for 35% of global crop production, are dependent upon pollinators, the most important of which are honey bees.

“One in every three bites of food we eat is pollinated directly or indirectly by honey bees,” says Dennis vanEngelsdorp, State Apiarist for Pennsylvania’s Department of Agriculture. Without honey bees, he says, we wouldn’t starve–we would still have wheat, rice, corn, and other crops that are either wind- or self-pollinated–but many of our favorite foods might no longer grace our tables (INFOGRAPHIC 22.2).

For bees, flowers are food: they contain the protein-rich pollen and sugary nectar that bees need to nourish themselves and their hives. With their long tonguelike proboscis, bees are able to reach deep into a flower to draw out the nectar. Being fuzzy and having a slight electrical charge, bees attract pollen as they snuggle up to a flower the way warm socks attract other clothes as they come out of the dryer. The bees can then transfer this pollen to other plants as they continue their hunt for food.

Honey bees are more efficient at pollination than many other types of pollinators, which is why farmers have come to depend on them. The average honey bee will make 12 or more foraging trips a day, visiting several thousand flowers. On each trip, she (the foragers are all female) will confine herself to flowers from a single plant species, thus ensuring delivery of the proper pollen. Because they come and go from a central home base–their hive–honey bees can be counted on to stay in a fixed area around a crop. And with roughly 40,000 individual bees per hive, this is a versatile workforce that’s also easy for beekeepers to transport from crop to crop.

Of course, pollination isn’t just about feeding bees and humans; it’s also how flowering plants have sex. The insect pollinators that travel among them play an unwitting yet crucial role in the courtship. Masters of seduction, flowers have evolved countless colorful and fragrant adaptations that lure their pollinators to the blossom. In the words of poet Kahlil Gibran, “For to the bee a flower is a fountain of life, and to the flower a bee is a messenger of love.”

A flower is the reproductive hub of an angiosperm–it is where its reproductive organs are located, male and female in the same flower in some species, separate flowers in others. In order for a flowering plant to reproduce, male pollen–containing sperm–must find a way to deliver the sperm to the female eggs of a plant of the same species. The male reproductive organ, called a stamen, consists of a stemlike filament topped with a pollen-saturated anther. When a bee lands on or brushes against an anther during her pursuit of nectar and pollen, her furry body picks up pollen grains.

As the bee continues to forage, she carries the pollen to the female reproductive organ of another flower–the pistil. The pistil is topped with a sticky “landing pad” called a stigma. When a bee lands on the stigma, pollen grains are deposited. The pollen grain creates a tube through which the sperm can travel down the length of the style–a tubelike passage into the ovary. At this point, the sperm fertilizes the egg. A fertilized egg will eventually develop into an embryo-containing seed, while the surrounding ovary eventually becomes the fruit (INFOGRAPHIC 22.3).