Every so often, a botanical garden somewhere in the world puts out an urgent announcement: the corpse flower is about to bloom.

Within hours, crowds begin to gather. People queue around the block, sometimes late into the night. Live webcams stream the bud as it swells. And when it finally opens, the visitors file past, gasp, pull faces — and hold their noses.

The titan arum smells like a dead body. On purpose.

The giant of Sumatra

The titan arum grows wild only in the rainforests of Sumatra, in Indonesia. It was described for Western science in 1878 by the Italian botanist Odoardo Beccari, who sent seeds back to Europe. A plant grown from them flowered at the Royal Botanic Gardens at Kew, near London, in 1889 — the first time it had bloomed in cultivation.

Its scientific name, Amorphophallus titanum, means roughly "giant misshapen phallus", which hints at the bloom's shape. When David Attenborough featured it in a television series in the 1990s, he reportedly felt the name was too rude for broadcast and called it the "titan arum" instead. The name stuck.

A strange anatomy

What looks like one huge flower is actually an inflorescence: a cluster of many small flowers. A tall central spike, called the spadix, rises from the middle. Around its base, hidden inside a huge frilled, cup-like leaf called the spathe, are rings of tiny male and female flowers. The spathe is green on the outside and a deep, meaty red-purple on the inside.

The whole structure can grow taller than a person. The tallest recorded blooms have been more than three metres high, making it one of the largest flowering structures in the plant kingdom.

The plant grows from an enormous underground tuber, which can weigh as much as an adult human. For most of its life, it sends up a single giant leaf, shaped like a small tree, to gather energy. Only after years of storing energy does it put everything into a bloom.

Smell and heat

When the bloom opens, usually in the afternoon or evening, the smell begins. Visitors have compared it to rotting fish, dead animals, sweaty socks and rubbish bins in summer. Chemists have identified compounds in the scent including some of the same molecules found in decaying flesh.

The stench is an advertisement. It attracts carrion beetles and flesh flies, which normally lay their eggs in dead animals. As they crawl around inside the bloom, they carry pollen from one plant to another.

Two days of glory

After a day or two, the bloom collapses. If it has been pollinated, it may produce bright orange-red fruit eaten by birds, which spread the seeds. Then the plant goes back to growing leaves, sometimes for years, before it blooms again.

Because blooms are so rare and short-lived, each one is an event. Botanic gardens around the world share pollen and seeds to help keep the species' genetic diversity healthy in cultivation.

Why make a smell like that?

Most flowers attract bees and butterflies with sweet scents and nectar. The titan arum takes a different route. In the dense rainforest, where plants of the same species may grow far apart, it needs a pollinator that will travel a long way to find it. Insects that feed on dead animals are good at following a smell over long distances, because a carcass is a valuable and short-lived resource.

So the titan arum imitates a carcass. It produces the smell, the warmth and even the dark red colour of rotting flesh. The insects arrive expecting a meal or a place to lay their eggs, and leave with nothing except a dusting of pollen.

Famous blooms

Some individual plants have become celebrities. Botanical gardens give them names, track their growth online and announce blooms on social media. In 2016, a bloom at the New York Botanical Garden drew tens of thousands of visitors, and live-streamed blooms have attracted audiences around the world. For many people, it is the only chance they will ever have to see, and smell, a plant from the Sumatran rainforest.

The giant leaf, which appears between blooms, is impressive too. It can reach several metres tall and looks like a small tree. Visitors often walk past it without realising it is the same plant.