Kingdom of Plants: Solving the Secrets
Plants hunt. They count. They signal to insects in colours we can't see and scents we can't smell, and a giant flower heats itself to the temperature of your body. With time-lapse, ultraviolet and heat cameras, David Attenborough uncovers the hidden lives of plants through a year at Kew.
Can a Plant Count?
No. One touch is not enough. Plenty of things brush a hair by accident. But a fly wandering about sipping nectar will probably touch two, and if two touches come within about 20 seconds, the trap slams shut. In other words, the plant counts to two. Scientists think the hairs work by sending a tiny electrical signal.
The bladderwort, an underwater plant. Its little bladders aren't floats: each one holds a partial vacuum behind a one-way trapdoor. When a tiny animal such as a mosquito larva touches the trigger, the door flies open and the water rushes in, sweeping the prey inside, in less than a thousandth of a second.
Put Your Instincts to the Test
Plants live in a world of signals we can't sense. Pick an answer for each question, then see if your instincts were right.
Patterns that point the way to the nectar. Insects can see ultraviolet light, which we can't. Photographed in ultraviolet, many plain-looking flowers show bullseyes, rings and lines that lead straight to the nectar and pollen, like landing lights on a runway.
A buzz at exactly the right note. Only one kind of bee buzzes at the right pitch to shake the pollen loose. At Kew, David Attenborough fools the flower with a tuning fork. The bee gets food no other insect can reach, and the plant gets a courier that goes straight to another Gustavia.
To spread its smell far and wide. At night a layer of still, cold air sits under the rainforest canopy. Pulses of warm air from the spike punch through it like smoke from a chimney, carrying the smell of rotting flesh out over the trees. The warmth may also mimic a freshly dead animal, which the pollinating beetles and bees are looking for.
Understanding the Science
Tap each card to uncover the hidden ways plants move, sense, signal and team up with other living things.
Key Concepts
Plants That Hunt
Tap to learn moreIn bogs and swamps the soil has hardly any minerals or nutrients, so some plants get them from animals. A sundew's tentacles are tipped with drops that look like nectar but are glue. An insect that lands is stuck fast and can't breathe; the other tentacles bend towards it, the leaf folds round it and juices dissolve its body so the plant can absorb it.
A Plant That Counts
Tap to learn moreA Venus flytrap ignores a single touch on its trigger hairs, because raindrops and bits of leaf touch them all the time. Two touches within about 20 seconds mean something is moving about, so the trap snaps shut. The prey's struggles touch more hairs, the trap seals tighter and it fills with acid digestive juices. A meal takes about ten days to digest.
The Fastest Trap
Tap to learn moreBladderwort lives in ponds and lakes. Each tiny bladder has had water pumped out of it, so it holds a partial vacuum behind a one-way door. When a water flea or mosquito larva brushes the trigger, the door swings inwards and the prey is sucked inside in less than a millisecond. Slowed down 240 times, the cameras can just catch it.
Timing the Seasons
Tap to learn moreMolecules in plant cells that respond to light and warmth tell a plant when to sprout and when to flower. Timing is everything: bloom too early and frost can kill you; too late and your rivals have taken the light. In spring the bulbs flower first, then trees such as cherries and magnolias, while bluebells rush to catch the sunshine before the leaves above them close over.
Seeing in Ultraviolet
Tap to learn moreWe see colours from red through to violet. Insects can also see ultraviolet, light with an even shorter wavelength. Special cameras show that many flowers carry ultraviolet patterns we never notice: bullseyes, rings and lines leading to the nectar. On a foxglove they act like landing lights. The four o'clock flower even has glowing pollen to attract moths at dusk.
Messages in Scent and Sound
Tap to learn moreA garden is full of scents, but your nose picks up only about 5 per cent of them. Some insects can sense just a few parts per billion of a scent and smell a flower from more than a kilometre away. Messages travel the other way too: Gustavia's flowers only release their pollen when a bee buzzes at exactly the right note, a trick called buzz pollination.
Pollen: A Key for One Lock
Tap to learn moreUnder an electron microscope every kind of pollen has its own shape, and only pollen of the right shape and the right DNA can fertilise its own species. Plants post their pollen in different ways: mountain ebony's sticks to bats' fur, eelgrass pollen floats in the water and pine pollen flies on tiny air sacs. On arrival, a grain grows a tube down into the flower to reach the egg.
Fungi: The Hidden Partners
Tap to learn moreFungi aren't plants; they're more closely related to animals. A mushroom is only the fruit of a fungus. Its body is a web of fine threads that can spread through a whole wood. Fungi break down dead leaves and wood and return their nutrients to the soil. Many also team up with living trees: on the roots of Kew's Lucombe oak a fungus passes nitrogen to the tree and takes sugar in return. Around nine in ten kinds of plant depend on fungi.
Try It: Snap or Wait?
You are a Venus flytrap. Snap shut when there's dinner inside, but not for every raindrop. Then switch to the plant's own rule and compare the scores.
Snapping shut costs a flytrap a lot of energy, and an empty trap takes about a day to open again, so false alarms are expensive. The real plant never wastes a snap on one raindrop: it waits for two touches within about 20 seconds. Its flowers grow on a tall stalk, well away from the traps, so the insects that pollinate it don't get eaten. Days pass in seconds here.
Wildlife of the Hidden World
Meet the plants, fungi and tiny animals whose secrets David Attenborough reveals in a year at Kew.
Sundew
Tap to learn moreIts leaves glitter with drops that look like nectar but are glue. Speeded-up cameras show tentacles that weren't even touched slowly bending towards a trapped insect and then the whole leaf curling round it. How the plant senses where its prey is, nobody yet knows.
Venus Flytrap
Tap to learn moreCharles Darwin called it one of the most wonderful plants in the world. It grows wild only in a small area of the coastal plains of North and South Carolina in the USA. Sweet nectar near the rim lures insects in; trigger hairs and a counting rule decide when to snap.
Bladderwort
Tap to learn moreA rootless plant drifting in ponds and lakes, with hundreds of tiny bladder traps along its stems. It's the plant world's swiftest killer: a bladder's trapdoor opens and shuts in less than a millisecond, sucking in mosquito larvae and water fleas.
Gustavia
Tap to learn moreA tree of the Amazon rainforest whose flowers open for only a few hours. Its pollen stays locked inside until a particular bee buzzes at just the right musical note. A tuning fork of the same pitch makes the flower shake out a cloud of pollen far too fine to see.
Lacewing Larva
Tap to learn moreCompletely blind, yet a terror to aphids. It finds them by smell, impales them on its hooked jaws and sucks them dry. One larva can eat up to 600 aphids before it becomes an adult, which is why gardeners and Kew's glasshouse team welcome it.
Roridula
Tap to learn moreA sticky-leaved plant from South Africa that catches insects but can't digest them. A particular capsid bug lives only on Roridula, coated in a non-stick substance so it can run about freely. It eats the trapped insects, and its droppings fall to the ground and feed the plant.
Honey Fungus
Tap to learn moreWhat you see is just clusters of mushrooms. Underground, one honey fungus in an American forest spreads across nearly 10 square kilometres, making it one of the largest living things on the planet.
Titan Arum
Tap to learn moreIt produces the biggest flower spike in the world, but only once every several years, after growing for months from a huge underground tuber. It opens for just two days, stinks of rotting meat and heats its spire to 37°C. David Attenborough first saw one in the rainforest of Sumatra.
Apply Your Knowledge
Each of these living things has its own secret way of surviving. Can you match them up?
Match the Organism to Its Secret
Click an item to select it, then click the matching description to place it.
Real-World Challenge
Kew keeps pests down in its glasshouses with living helpers instead of sprays: ladybirds, lacewings, tiny wasps that lay their eggs inside aphids, even water dragons that eat cockroaches. Imagine your school greenhouse is overrun with aphids. Which of these helpers would you bring in and why? What might go wrong if you brought in too many, or if the aphids ran out?
Protecting Hidden Plants Today
Some of the most remarkable plants in this episode are rare in the wild. Here's what's threatening them and what's being done.
Response: Wild Venus flytraps grow only in a small area of North and South Carolina, and poachers have dug up thousands to sell. In 2014 North Carolina made taking them from the wild a felony, a serious crime, and nurseries now grow millions from seed, so there's no reason to buy a wild plant.
Response: Flytraps need open, sunny ground. In their pine savannas, regular fires used to clear away the taller shrubs and grasses that would shade them out. Where people stop fires, the flytraps disappear, so conservation groups now set careful, controlled burns to keep their habitat open.
Response: The titan arum grows wild only in the rainforests of Sumatra, which are being cleared for plantations, and it is now listed as Endangered. Botanic gardens around the world grow it and share pollen with each other, so their plants stay varied enough to be a back-up population.
What Has Changed Since This Documentary Aired
This documentary was first shown in 2012. Here's what scientists have found since.
Updated: Yes. In 2016 scientists in Germany found it keeps counting. Two touches shut the trap; from three it starts making the signals for digestion; from about five it pours out digestive juices and the pumps that take up the prey's salts. The more the prey struggles, the more juice the plant makes.
Updated: Not quite. In 2017 researchers used microscope scans to look inside infected ants. They found the fungus spreads through the ant's whole body and wraps around its muscles, but leaves the brain alone. It seems to control the ant like a puppet by working its muscles from the outside in.
Updated: It has company. In 2022 scientists found that a seagrass meadow in Shark Bay, Australia, covering about 180 square kilometres, is all one single plant that has been spreading for thousands of years. That's far bigger than the honey fungus in the documentary.
Test Your Understanding
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Reflection
David Attenborough says our senses match what matters to us, so we miss most of what matters to plants. If you could borrow one plant or insect sense for a day (ultraviolet sight, a nose for a few parts per billion, or a flytrap's touch-counting hairs), which would you choose, and what do you think you'd notice?
Episode Discussion
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