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The Great Barrier Reef, Australia, at night.

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I'm surrounded by corals.

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They do look extraordinarily like plants,

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branching into fans and twigs and bushes.

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At night, the similarity is particularly marked.

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All over their stony surface, tiny buds open into what look like flowers.

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But these structures don't behave in a flower-like way.

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They seize and eat any edible particle that drifts by. They are clearly animals.

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But even so, they look like plants. Why?

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It was only comparatively recently that we understood the answer in full detail,

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and it only becomes evident when the sun comes up,

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for then the corals change their behaviour in a radical way.

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Corals, like plants, must have light.

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They can't grow if the water is cloudy or the depths so great that the rays of the sun can't reach them.

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These resemblances are not just coincidences.

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If I go back underwater now, now that it's day and the sun is up,

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I shall see that many of these corals are feeding in a way that is not like animals at all.

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Now the plant-like form of the coral is even more obvious.

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The rosettes of groping arms have withdrawn into their stony sockets on the surface of the coral skeleton.

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But they're still within the reach of sunlight.

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And within their tiny bodies are microscopic green plants, algae,

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and they are feeding by making starches and sugars.

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But the corals are feeding too.

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They have partly digested the walls of these captive plants

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and 80% of the food the algae make leaks out of them and is consumed by the coral.

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Having dined on meat all night, the corals are now getting their vegetables.

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The corals provide their internal gardens with the best possible light by growing into these shapes,

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which is just what bushes do for their food factories, their leaves, when they grow in the same way.

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The coral algae do get some benefit from this arrangement.

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These glassy waters are very poor in nitrates and phosphates which algae need.

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Those substances are in the coral's waste.

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So the algae can absorb their fertiliser directly

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and live in waters that otherwise could not support them.

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Other animals on the reef also cultivate similar gardens.

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Giant clams keep their algae not inside their cells,

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but in special compartments just beneath the surface of the mantle that form long, brown lines.

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To give them the light they need, the clam has to open its shell wide, so exposing itself to danger,

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but the blue spots are sensitive to light and warn of unexpected shadows

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that might indicate an approaching threat.

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A few jellyfish maintain algal populations as well.

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These, in a lake on the Pacific island of Palau,

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pamper theirs in an extraordinary way.

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This lake is cut off from the sea by ramparts of coral limestone and there are very few fish here.

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So these jellyfish can't live, like most of their relations, by catching animal prey

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and their tentacles no longer carry stings for hunting.

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Instead, they have been converted into allotments for algae.

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The lake is surrounded by a tall forest growing on the limestone wall.

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The sun doesn't rise above the trees

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until several hours after dawn.

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But, at last, its rays strike the water at one end of the lake

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and there, several million jellyfish have assembled awaiting the sunlight.

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As the sun moves across the sky,

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so the vast fleet travels slowly towards the other side of the lake,

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keeping always in the sunshine.

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So reluctant are the jellyfish to leave the light that, on the edge of the shadow,

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they crowd together in a tightly-packed shoal.

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But without stings, the jellyfish are defenceless.

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Now, if they blunder into the arms of a sea anemone,

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they have no way of repelling the tentacles. They're eaten.

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The daytime voyage across the lake is not the only action the jellyfish take to nurture their algae.

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Come the evening, they swim down to the bottom.

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There the water is murky with decaying vegetable matter

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and there, in the night, the algae absorb the fertiliser they need.

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That animals should sometimes kidnap plants is not surprising.

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All animals, including ourselves, have always exploited plants in one way or another,

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directly or indirectly.

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It's more surprising that sometimes things are the other way round.

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Sometimes it's plants that keep animals for the plants' benefit.

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Here in the forests of Borneo, the rattan cane does just that.

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No plant benefits from being eaten, but most can't do much to stop it.

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Not so the rattan. Watch and listen.

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Out of a nest around the stem of the rattan, close to its tip,

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come angry ants.

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RHYTHMIC HISSING

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They're making this throbbing hiss

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by banging their heads synchronously against the rattan stem.

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These ants have  oh! a particularly vicious bite,

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as I well know. Ow!

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And I certainly try to keep clear of them when I'm in the forest.

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I'm sure plant-eating animals do too.

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So when I, or they,

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hear this alarming noise, we try to steer clear of what's making it,

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and the rattan's tip, its most vulnerable part, remains undamaged.

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In Africa, there are a great number of very determined plant-eaters.

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Acacias protect themselves with spines, but they're by no means a total defence.

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Some animals are put off by them,

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but others, like the giraffe, seem able to ignore them.

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But a few acacias, like the rattan, have recruited ants as guards

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and provide them with special barracks

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the swollen bases of their thorns.

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One nibble from the giraffe is enough to bring out the defenders.

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They attack the animal's tongue and lips.

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Eventually, the irritation becomes too much.

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Even though there are a lot of good leaves left, the giraffe moves away.

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Several different acacias employ ants as defenders.

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As well as providing accommodation,

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the trees pay their security staff with a sugary nectar that wells up from little glands on their stems.

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This South American species rewards its ants even more extravagantly.

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It not only provides nectar for them, but packets of protein,

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little beads that grow on the tip of its leaflets.

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But these are not for the adults.

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They're special baby-food which the workers take back to their larvae.

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These infants are housed in the swollen bases of the thorns.

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The worker tucks the bead into a special pouch, just beneath the larva's jaws.

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Whenever the youngster wants a meal,

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it just bends its head down and takes a nibble.

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In return for these lavish provisions and amenities,

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the ants mount an energetic defence

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of the acacia, rushing to attack intruders.

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Any insect that lands on a tree, hoping to nibble a leaf or two, is soon dealt with.

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The ants even defend their tree against rival plants.

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Patrols go down the trunk and range for a long way over the earth.

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Seedlings that sprout within this area, so threatening to take some of the acacia's sustenance, are mauled.

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The ants aren't eating this plant.

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They're chewing it to death.

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The tendrils of any plant that reach over and try to climb onto the acacia

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get similar treatment.

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It's well worth the acacia's while to provide food and lodging

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for such a valiant and dedicated defence force.

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This plant is even more accommodating.

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It has inflated most of its stem into an ant mansion.

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It grows in New Guinea,

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clinging to the branches of other trees, and it's called, with good reason, an ant plant.

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Ants are continually running about on its surface on their way to, or returning from, a hunt for insects.

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The accommodation the plant provides for the ants is truly spacious

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and suited to their requirements.

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Immediately within its walls,

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a network of corridors ensures that the structure is air-conditioned,

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an essential for any well-appointed residence in the tropics.

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Farther inside, 

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there are the nurseries

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smooth-walled chambers where the larvae are reared.

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And there are also special refuse tips.

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The workers dump the droppings of the colony.

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These chambers are not only middens,

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they are mortuaries

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the last resting place of members of the colony that die within the mansion.

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The chambers in which these bodies lie

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have walls covered with warts.

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These absorb nutrients from the rotting piles.

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This is how the plant collects its rent.

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Fungi may seem unlikely, even dangerous,

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organisms with which to form a partnership.

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After all, they do feed on plants.

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Fungi are neither animals nor plants.

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They're fundamentally different from either.

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They can dissolve all kinds of substances

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rock, metal, even plastic

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but most notably, they consume the bodies of plants,

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and these bracket fungi eat trees.

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We tend to notice them only when they provide spectacular structures like these  their fruiting bodies.

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Spores fall from their underside in astronomical numbers millions a minute.

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Fungal spores exist pretty well everywhere.

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They may enter a tree through a wound in the bark.

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They then develop into threads that slowly move inwards and start to digest the wood.

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The tree now, as we would see it, has a rotten core.

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Eventually, after tens or even hundreds of years,

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a tree may have its interior completely eaten away by fungal threads, as has happened here.

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It's not as disastrous as it sounds. The fungus only consumes dead tissue.

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It leaves the living tissue untouched,

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and it survives as an outer cylinder from which all new growth comes

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and that's all that the tree needs.

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So although this 800-year-old oak in Windsor Great Park is completely hollow,

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it's still thriving.

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Every year it puts out a fresh crown of green leaves

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and I guess it's got many more years of life in it yet.

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The change of form brings a positive advantage to the old tree.

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A hollow cylinder is better able to absorb great shocks than a solid pillar.

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Trees standing out in the open, as they do in parks, can get severely buffeted by stormy winds,

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and it's not unusual after a gale to see young oaks uprooted,

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whereas older ones, with the age and the girth to become hollow, are still standing.

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The surgery performed by the fungus brings other advantages too.

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It enables the oak to reclaim some of its lifetime's savings.

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Roots develop on the inside of the hollow trunk.

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They grow down and collect nutriment that the fungus has released from the wood as it digested it.

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That is not the only goodness here.

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Animals have come to live in the hollow tree.

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Owls may be roosting in its upper parts,

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bats hanging from its walls.

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Its lodgers,

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having fed out in the woodland,

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drop their dung within the hollow.

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So the tree receives food from places that otherwise would be far beyond its reach.

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So thanks to its fungal partner,

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an oak often has an old age that is both robust and well-fed.

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But fungi bring food to many plants throughout their lives,

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and that is particularly so in forests such as this one on the northwest coast of America.

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Even the tallest of these giant spruces, totally healthy and in the prime of its life,

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is dependent for its health and strength on a fungus.

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Its partner is down here.

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This is a rootlet through which the tree absorbs its nourishment,

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but wrapped round it are a mass of tiny white threads.

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They belong to the fungus and are part of a dense mesh increasing the surface area

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through which the tree can absorb water and nutrients.

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The partnership starts at the very beginning of a tree's life,

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when a fungus entwines itself around the seedling's infant roots.

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Seedlings which germinate in the soil without fungi are likely to starve to death.

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If there's a fungus to convey food, the seedling will get a good start.

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And that connection is never broken.

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An adult tree is able to collect nutriment-laden moisture from fungal threads,

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suck it along its roots, up its trunk,

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into its leaves and combine it with that other essential raw material, carbon dioxide gas, to make food.

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So trees, including giants like this one, can't grow without the help of tiny organisms within the soil

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organisms that we don't even notice until they fruit,

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and that may not happen more than two or three days in twenty years.

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This is how the fly agaric uses its share of the profits from the partnership.

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About a quarter of the sugars and starches produced by the tree in its leaves

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travel back down the trunk and into the ground

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to feed its multitude of fungal partners.

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Fungi fruit so briefly and often so rarely,

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it's difficult to appreciate how widespread they are, and how varied.

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There are over a thousand different species in the coniferous forests.

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Although trees do have preferences, any one individual may have links with up to 200 different partners.

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And it is not only limited to trees.

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Many small plants are also dependent,

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and none more so than those most glamorous of plants  orchids.

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It seems paradoxical that such opulent and flamboyant blooms should be totally dependent upon the help

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of drab, thread-like organisms wrapped around their roots.

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Most plants provision their seeds with stores of food

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to fuel germination and the first stages of growth.

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But not these orchids. This is an orchid seed capsule,

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and here...is orchid seed

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so fine it's blowing away in the air.

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Minute seeds like this have always been difficult to get to germinate.

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And, infuriatingly, the seed from some of the most dazzling and rare of orchids wouldn't germinate at all.

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Then scientists tackled the problem.

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They found that many orchids have their own special fungal partner.

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They found methods of isolating that fungus and then culturing it with the orchid seed.

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Under the right conditions, the two strike up their partnership immediately.

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The fungus extracts nutriment from the culture medium in a way that the orchid can't do for itself

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and supplies it to the young plant.

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Within a month,

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the fungus invades the seed and conveys nutriment to it

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and the seedling is on its way to becoming a vigorous plant.

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You could argue that it is the orchid which is the dominant member of this partnership.

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It is, after all, the one we can see with our naked eye.

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There are plant-fungus relationships where the balance is the other way.

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The fungus determines the shape into which that partnership grows.

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One of those shapes is flat and plate-like,

240
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but in order to see the two partners, you have to look at it through very high magnification.

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This is a section

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through one of those plate-like partnerships.

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The top is formed by the fungus.

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These threads are part of the fungus and this sphere here is the plant.

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To see just how intimate their relationship is, you have to look at them in greater magnification.

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This is magnified 10,000 times.

247
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Here are the fungal threads and this is the plant, the algae,

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from which they're getting their sustenance.

249
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Together, the different organisms form one of the most widely distributed of living structures

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lichens.

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The partners operate so closely together that each pairing is given a single name

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and there are over 13,000 of them.

253
00:29:02,600 --> 00:29:07,200
They not only form these hard skins and curling crusts.

254
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Some lichens grow into little branched bushes.

255
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And very successful organisms they are too.

256
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They come into their own in the harshest of conditions.

257
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No grass can grow on these arid slopes here on the edge of the Namib Desert in southern Africa.

258
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This extraordinary orange colour is produced entirely by a carpet of lichen.

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It can get so hot here that it's painful to put your hand on rock.

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And there's no relief with a shower of rain, for it hardly ever falls.

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Yet 29 species of lichen flourish here.

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The red one is particularly successful.

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One of the functions of the fungus

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is to absorb moisture and deliver it to the algae.

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If there's no moisture, the organism shrivels and becomes brittle.

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And that's what's happened to this here.

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For this lichen, salvation is going to come from a surprising source.

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The sea lies only a mile or so away.

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A cold current sweeps up the coast from the south.

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The hot air rising from the desert

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pulls in cold air from the sea and the mixture produces fog.

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The moisture condenses as droplets on the lichen's branches.

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It's swiftly absorbed by the fungal skin

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and conveyed to the alga within 

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and suddenly and miraculously, the desiccated branches turn green.

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But even in the best circumstances, lichen grow only very slowly

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often only a millimetre or so a year.

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One place shows vividly and accurately just how slowly that is  a churchyard.

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The lichens, with their ability to live on bare rock,

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flourish on the tombstones.

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The dates of the inscriptions can tell us exactly when the bare stone surface was exposed to the elements

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and was available for colonisation.

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Some of these blotches,

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only an inch or so across, may be centuries old.

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Lichens also grow in undisturbed ancient forests such as those on the Pacific coast of North America.

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Trees here may live five or six hundred years,

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but well before they reach such an advanced age,

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they have usually been colonised by different kinds of lichens

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that hang in great tufts and blankets from their branches.

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So plants form intimate partnerships with members of the other great kingdoms of life

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in tropical forests, with members of the animal kingdom particularly ants and other insects.

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In the forests of North America, partnerships with fungi are common,

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ranging from those that produced these lichens,

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dangling from the boughs of this great spruce tree,

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down to the tangle of tiny threads meshed around the roots of the tree 250 feet below me.

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There are also partnerships within the plant kingdom between plant and plant.

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Some are just simple these mosses and ferns

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which use the spruce tree simply as a perch,

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but some partnerships are much more intimate.

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This is a mistletoe.

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It exists in partnership with a tree, for it has no roots of its own.

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It's a very one-sided relationship. 

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The mistletoe has leaves, so it can manufacture food,

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but it draws all the liquid it needs from the tree to which it's fastened.

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The tree gets nothing from the arrangement.

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The mistletoe, in short, is a parasite.

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The mistletoe family has over 1,000 species.

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Here in Australia alone, there are 75.

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Somewhere there is always one in fruit.

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And that makes it possible for one bird to eat almost nothing else.

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The mistletoe bird knows exactly how to extract the fruit.

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The bird digests the fleshy coating of the seed with extraordinary speed.

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It takes less than half an hour to travel from beak to bottom.

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The seed when it emerges is still phenomenally sticky

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and has to be wiped off, which suits the mistletoe.

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The seed, when it comes out,

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remains attached to the bird's behind by a long sticky thread.

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The bird has a technique for breaking it.

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Every time it needs to detach a seed, it has to perform this little dance.

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It's this stickiness that is the key to the mistletoe's success

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 in getting from one tree to another.

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Once parked on a living branch, the seed quickly plugs itself in.

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With a connection to its host's liquid supply, it can build leaves and start making food for itself.

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This is another mistletoe.

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It grows only in Western Australia and it flowers in December,

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which is why it's known locally as the Christmas tree.

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I know it's a mistletoe because of the character of its flowers and its green, fleshy leaves.

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But from other points of view, it's very unlike other mistletoes.

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It's a free-standing tree that does not seem to be parasitising anything.

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But it gives us a very good idea as to how parasitism might have started in this family.

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Have a look at its roots.

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This is the root that belongs to the Christmas tree,

333
00:38:26,400 --> 00:38:31,440
and this root belongs to another completely different bush nearby.

334
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And the Christmas Tree has encircled this other root with a white ring.

335
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It's plugged itself in to the root system of another plant,

336
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and it gets all its water and minerals in that way.

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00:38:45,640 --> 00:38:52,480
And it's not at all fussy about what kind of plant it parasitises grasses, sedges,

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small bushes, big trees, gumtrees, cycads  it will go for the lot.

339
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At least the mistletoes have leaves for making some food for themselves.

340
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A few parasitic plants don't even have that.

341
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These are the germinating seeds of dodder.

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They have to find their host within a few days or they will die.

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A favourite target is the nettle.

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Well-armed with stings it may be, but they are no defence against dodder.

345
00:39:40,640 --> 00:39:47,200
The seedlings can detect whether a nettle stem is feeble or well-nourished

346
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and they pick their victim with care.

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This is a strong, healthy one good to feed on.

348
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In goes a nozzle.

349
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The dodder sucks the nettle's sap, which then fuels its growth

350
00:40:13,520 --> 00:40:16,080
and its hunt for another victim.

351
00:40:32,920 --> 00:40:37,480
The dodder is a relative of the bindweed, convolvulus,

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and it climbs in the same sort of way.

353
00:41:03,840 --> 00:41:06,720
Wherever the feeding seems good,

354
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the parasite inserts a tube and draws off the nettle's sap.

355
00:41:22,320 --> 00:41:24,880
Once it's fully established,

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drinking from the nettle through hundreds of connections,

357
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the dodder is siphoning off enough nourishment from its victim to enable it to flower.

358
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Eventually, the whole bed of nettles is overwhelmed by writhing dodder stems.

359
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The dodder is completely parasitic,

360
00:42:50,480 --> 00:42:54,520
getting all it needs from another plant.

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But the relationship between parasite and host can be even closer.

362
00:42:59,560 --> 00:43:06,920
Here in the forests of Borneo is an enormous parasite whose relationship with its host is so intimate

363
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that the parasite is invisible for most of the year.

364
00:43:37,360 --> 00:43:41,920
This is the first that anyone or anything sees of it.

365
00:43:41,920 --> 00:43:47,800
The bud is coming from this root, but the root doesn't belong to this.

366
00:43:47,800 --> 00:43:51,640
The root is part of this great vine.

367
00:43:58,640 --> 00:44:05,680
Inside the massive trunk of this vine, there's a multitude of hair-like filaments.

368
00:44:05,680 --> 00:44:10,720
They don't belong to the vine but to a parasite called Rafflesia.

369
00:44:10,720 --> 00:44:15,280
Rafflesia has no stem, no leaves, and never will have.

370
00:44:15,280 --> 00:44:19,840
It feeds entirely on the sap produced by the vine.

371
00:44:19,840 --> 00:44:24,880
The only time Rafflesia emerges into the outside world is to flower.

372
00:44:24,880 --> 00:44:31,800
That bud was weeks old. If I follow the root of the vine, maybe I'll find more.

373
00:44:41,000 --> 00:44:43,640
Two more, but still small.

374
00:44:48,800 --> 00:44:51,360
A bigger one.

375
00:44:56,880 --> 00:45:01,800
And this one looks as though it might well open tonight.

376
00:45:37,800 --> 00:45:44,360
By the time dawn comes and the first rays of the sun filter down into the forest,

377
00:45:44,360 --> 00:45:47,240
the flower is almost fully open.

378
00:45:56,720 --> 00:46:01,600
Rafflesia produces the largest single flower on earth

379
00:46:01,600 --> 00:46:04,920
a big one can be three feet across.

380
00:46:11,720 --> 00:46:18,280
The surface of the warty petals look a little like that of a putrefying corpse.

381
00:46:18,280 --> 00:46:22,600
There is a faint smell of rotten fish

382
00:46:22,600 --> 00:46:30,160
and the huge flower quickly attracts those that find much of their food in carrion  blowflies.

383
00:46:35,440 --> 00:46:42,000
In the bottom of the cup, a great disc covered in spikes stands on a pedestal.

384
00:46:42,000 --> 00:46:46,920
The flies go in to investigate and crawl all over it.

385
00:46:55,320 --> 00:47:00,360
Hanging from the underside of the disc are droplets of liquid pollen.

386
00:47:03,040 --> 00:47:06,360
As the flies explore,

387
00:47:06,360 --> 00:47:11,520
they touch the droplets and get saddled with a dab of pollen.

388
00:47:16,000 --> 00:47:21,040
This will only benefit Rafflesia if the fly is able to find

389
00:47:21,040 --> 00:47:28,080
another of these very rare flowers fully open in the forest to which it can deliver its load.

390
00:47:28,080 --> 00:47:33,120
Rafflesia produces the biggest single flower in the world.

391
00:47:33,120 --> 00:47:37,520
But why, when all it needs to attract are flies?

392
00:47:37,520 --> 00:47:45,080
Plants, like other living organisms, can only afford to spend a limited amount of food on reproduction.

393
00:47:45,080 --> 00:47:48,120
But Rafflesia does not earn its food.

394
00:47:48,120 --> 00:47:51,320
It takes it straight from the vine.

395
00:47:51,320 --> 00:47:59,360
Provided the vine is not fatally injured, there seems to be no limit to the amount Rafflesia may extract.

396
00:47:59,360 --> 00:48:07,000
Maybe an unearned income in the plant world, as elsewhere, can lead to extravagance

397
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on a truly monumental scale.

398
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Subtitles by Sarah Aitken BBC Scotland 1995