Flowers and Pollination

A flower is not for you. Its colour, scent and shape evolved over millions of years to catch the attention of insects, birds and bats, because plants have a problem: they cannot move, yet they need to mate. The flower is the plant's solution, a device for persuading an animal to carry its pollen to another plant of the same species. The bargain it offers, food for delivery, has shaped the living world more than almost any other partnership.

Why plants need help

Reproducing sexually, mixing genes from two parents, produces variety, and variety is what lets a species adapt. For plants that means getting pollen, which carries the male cells, from one flower to the female part of another. Some plants simply release pollen into the wind in enormous quantities and hope; grasses, pines, oaks and birches do this, which is why hay fever exists. Wind is cheap but wasteful. Most pollen falls on nothing.

About 130 million years ago, some plants began recruiting insects instead. Beetles and flies were already visiting plants to eat pollen; a plant that offered a sweet reward and a conspicuous sign got more visits, and visitors that moved from plant to plant carried pollen with them. From that beginning evolved the flowering plants, now about 300,000 species and nearly 90 percent of all plant life.

The anatomy of an advertisement

A flower has four parts. The outer sepals protect the bud. The petals are the billboard, coloured and often scented. Inside stand the stamens, whose tips produce pollen. At the centre is the female part, with a sticky surface to catch pollen at the top and, at the base, the ovary, holding the eggs that will become seeds. Nectar, a sugary liquid, is usually offered deep in the flower, so the visitor must brush past the pollen to reach it.

Every detail is tuned to a customer. Bees see blue, yellow and ultraviolet but not red, so flowers that court bees are blue, yellow and purple, often with ultraviolet patterns, invisible to us, that point like runway lights toward the nectar. Hummingbirds see red well and have no sense of smell, so hummingbird flowers are red, scentless, tubular and hang downward, out of reach of bees. Flowers pollinated by bats and moths open at night and are pale and heavily scented. Those pollinated by flies and beetles may smell of rotting meat or dung; the titan arum, the largest flower spike in the world at up to three metres, stinks so badly that it is known as the corpse flower.

Tricks and tight partnerships

Some flowers cheat. Bee orchids mimic the shape, colour and scent of a female bee so convincingly that males try to mate with them and leave covered in pollen. Others trap their pollinators overnight and release them in the morning, dusted. Some partnerships have become so close that neither side can live without the other: each species of fig has its own species of tiny wasp, which breeds inside the fruit and is the only insect able to pollinate it, and the Madagascar orchid with a 30-centimetre nectar tube is pollinated only by a moth with a 30-centimetre tongue, which Darwin predicted must exist decades before anyone found it.

From flower to fruit

When a pollen grain lands on the right flower, it grows a tube down into the ovary and fertilises an egg. The egg becomes a seed, and the ovary around it swells into a fruit. A fruit is another bargain, this time with a different animal: eat the sweet flesh, carry the seed away in your gut, and drop it somewhere new with a dose of fertiliser. Apples, tomatoes, cucumbers, peppers, peas, nuts and grains are all, botanically, fruits. Many flowers are also food in themselves: broccoli, cauliflower and artichokes are flower buds.

Pollinators and people

About three quarters of the world's crop species need animal pollinators for a full harvest, including almost every fruit, vegetable, nut, coffee bean and cocoa pod. Honeybees are the best known, but wild bees, of which there are 20,000 species, along with hoverflies, butterflies, moths, beetles, birds and bats, do much of the work. Many of them are declining, through habitat loss, pesticides, disease and a changing climate, and farmers in some regions already hire hives by the truckload or pollinate by hand with brushes.

Why it matters

Flowers are the reason the land is colourful, the reason there is fruit, and a large part of the reason there are so many insects. The partnership between plants and pollinators built the modern countryside and feeds humanity. Every apple is a successful advertisement, and every bee on a flower is holding up one end of a deal older than the dinosaurs' extinction.