Living Together The Private Life of Plants

The Great Barrier Reef, Australia, at night.

I'm surrounded by corals.

They do look extraordinarily like plants,

branching into fans and twigs and bushes.

At night, the similarity is particularly marked.

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

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

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

But even so, they look like plants. Why?

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

and it only becomes evident when the sun comes up,

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

Corals, like plants, must have light.

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

These resemblances are not just coincidences.

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

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

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

The rosettes of groping arms have withdrawn into their stony sockets on the surface of the coral skeleton.

But they're still within the reach of sunlight.

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

and they are feeding by making starches and sugars.

But the corals are feeding too.

They have partly digested the walls of these captive plants

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

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

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

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

The coral algae do get some benefit from this arrangement.

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

Those substances are in the coral's waste.

So the algae can absorb their fertiliser directly

and live in waters that otherwise could not support them.

Other animals on the reef also cultivate similar gardens.

Giant clams keep their algae not inside their cells,

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

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

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

that might indicate an approaching threat.

A few jellyfish maintain algal populations as well.

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

pamper theirs in an extraordinary way.

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

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

and their tentacles no longer carry stings for hunting.

Instead, they have been converted into allotments for algae.

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

The sun doesn't rise above the trees

until several hours after dawn.

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

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

As the sun moves across the sky,

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

keeping always in the sunshine.

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

they crowd together in a tightly-packed shoal.

But without stings, the jellyfish are defenceless.

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

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

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

Come the evening, they swim down to the bottom.

There the water is murky with decaying vegetable matter

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

That animals should sometimes kidnap plants is not surprising.

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

directly or indirectly.

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

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

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

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

Not so the rattan. Watch and listen.

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

come angry ants.

RHYTHMIC HISSING

They're making this throbbing hiss

by banging their heads synchronously against the rattan stem.

These ants have oh! a particularly vicious bite,

as I well know. Ow!

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

I'm sure plant-eating animals do too.

So when I, or they,

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

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

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

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

Some animals are put off by them,

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

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

and provide them with special barracks

the swollen bases of their thorns.

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

They attack the animal's tongue and lips.

Eventually, the irritation becomes too much.

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

Several different acacias employ ants as defenders.

As well as providing accommodation,

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

This South American species rewards its ants even more extravagantly.

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

little beads that grow on the tip of its leaflets.

But these are not for the adults.

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

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

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

Whenever the youngster wants a meal,

it just bends its head down and takes a nibble.

In return for these lavish provisions and amenities,

the ants mount an energetic defence

of the acacia, rushing to attack intruders.

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

The ants even defend their tree against rival plants.

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

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

The ants aren't eating this plant.

They're chewing it to death.

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

get similar treatment.

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

for such a valiant and dedicated defence force.

This plant is even more accommodating.

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

It grows in New Guinea,

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

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

The accommodation the plant provides for the ants is truly spacious

and suited to their requirements.

Immediately within its walls,

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

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

Farther inside,

there are the nurseries

smooth-walled chambers where the larvae are reared.

And there are also special refuse tips.

The workers dump the droppings of the colony.

These chambers are not only middens,

they are mortuaries

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

The chambers in which these bodies lie

have walls covered with warts.

These absorb nutrients from the rotting piles.

This is how the plant collects its rent.

Fungi may seem unlikely, even dangerous,

organisms with which to form a partnership.

After all, they do feed on plants.

Fungi are neither animals nor plants.

They're fundamentally different from either.

They can dissolve all kinds of substances

rock, metal, even plastic

but most notably, they consume the bodies of plants,

and these bracket fungi eat trees.

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

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

Fungal spores exist pretty well everywhere.

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

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

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

Eventually, after tens or even hundreds of years,

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

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

It leaves the living tissue untouched,

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

and that's all that the tree needs.

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

it's still thriving.

Every year it puts out a fresh crown of green leaves

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

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

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

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

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

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

The surgery performed by the fungus brings other advantages too.

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

Roots develop on the inside of the hollow trunk.

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

That is not the only goodness here.

Animals have come to live in the hollow tree.

Owls may be roosting in its upper parts,

bats hanging from its walls.

Its lodgers,

having fed out in the woodland,

drop their dung within the hollow.

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

So thanks to its fungal partner,

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

But fungi bring food to many plants throughout their lives,

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

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

is dependent for its health and strength on a fungus.

Its partner is down here.

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

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

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

through which the tree can absorb water and nutrients.

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

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

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

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

And that connection is never broken.

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

suck it along its roots, up its trunk,

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

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

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

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

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

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

travel back down the trunk and into the ground

to feed its multitude of fungal partners.

Fungi fruit so briefly and often so rarely,

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

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

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

And it is not only limited to trees.

Many small plants are also dependent,

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

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

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

Most plants provision their seeds with stores of food

to fuel germination and the first stages of growth.

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

and here...is orchid seed

so fine it's blowing away in the air.

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

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

Then scientists tackled the problem.

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

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

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

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

and supplies it to the young plant.

Within a month,

the fungus invades the seed and conveys nutriment to it

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

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

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

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

The fungus determines the shape into which that partnership grows.

One of those shapes is flat and plate-like,

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

This is a section

through one of those plate-like partnerships.

The top is formed by the fungus.

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

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

This is magnified 10,000 times.

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

from which they're getting their sustenance.

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

lichens.

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

and there are over 13,000 of them.

They not only form these hard skins and curling crusts.

Some lichens grow into little branched bushes.

And very successful organisms they are too.

They come into their own in the harshest of conditions.

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

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

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

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

Yet 29 species of lichen flourish here.

The red one is particularly successful.

One of the functions of the fungus

is to absorb moisture and deliver it to the algae.

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

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

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

The sea lies only a mile or so away.

A cold current sweeps up the coast from the south.

The hot air rising from the desert

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

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

It's swiftly absorbed by the fungal skin

and conveyed to the alga within

and suddenly and miraculously, the desiccated branches turn green.

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

often only a millimetre or so a year.

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

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

flourish on the tombstones.

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

and was available for colonisation.

Some of these blotches,

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

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

Trees here may live five or six hundred years,

but well before they reach such an advanced age,

they have usually been colonised by different kinds of lichens

that hang in great tufts and blankets from their branches.

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

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

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

ranging from those that produced these lichens,

dangling from the boughs of this great spruce tree,

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

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

Some are just simple these mosses and ferns

which use the spruce tree simply as a perch,

but some partnerships are much more intimate.

This is a mistletoe.

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

It's a very one-sided relationship.

The mistletoe has leaves, so it can manufacture food,

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

The tree gets nothing from the arrangement.

The mistletoe, in short, is a parasite.

The mistletoe family has over 1,000 species.

Here in Australia alone, there are 75.

Somewhere there is always one in fruit.

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

The mistletoe bird knows exactly how to extract the fruit.

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

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

The seed when it emerges is still phenomenally sticky

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

The seed, when it comes out,

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

The bird has a technique for breaking it.

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

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

in getting from one tree to another.

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

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

This is another mistletoe.

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

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

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

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

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

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

Have a look at its roots.

This is the root that belongs to the Christmas tree,

and this root belongs to another completely different bush nearby.

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

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

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

And it's not at all fussy about what kind of plant it parasitises grasses, sedges,

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

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

A few parasitic plants don't even have that.

These are the germinating seeds of dodder.

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

A favourite target is the nettle.

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

The seedlings can detect whether a nettle stem is feeble or well-nourished

and they pick their victim with care.

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

In goes a nozzle.

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

and its hunt for another victim.

The dodder is a relative of the bindweed, convolvulus,

and it climbs in the same sort of way.

Wherever the feeding seems good,

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

Once it's fully established,

drinking from the nettle through hundreds of connections,

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

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

The dodder is completely parasitic,

getting all it needs from another plant.

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

Here in the forests of Borneo is an enormous parasite whose relationship with its host is so intimate

that the parasite is invisible for most of the year.

This is the first that anyone or anything sees of it.

The bud is coming from this root, but the root doesn't belong to this.

The root is part of this great vine.

Inside the massive trunk of this vine, there's a multitude of hair-like filaments.

They don't belong to the vine but to a parasite called Rafflesia.

Rafflesia has no stem, no leaves, and never will have.

It feeds entirely on the sap produced by the vine.

The only time Rafflesia emerges into the outside world is to flower.

That bud was weeks old. If I follow the root of the vine, maybe I'll find more.

Two more, but still small.

A bigger one.

And this one looks as though it might well open tonight.

By the time dawn comes and the first rays of the sun filter down into the forest,

the flower is almost fully open.

Rafflesia produces the largest single flower on earth

a big one can be three feet across.

The surface of the warty petals look a little like that of a putrefying corpse.

There is a faint smell of rotten fish

and the huge flower quickly attracts those that find much of their food in carrion blowflies.

In the bottom of the cup, a great disc covered in spikes stands on a pedestal.

The flies go in to investigate and crawl all over it.

Hanging from the underside of the disc are droplets of liquid pollen.

As the flies explore,

they touch the droplets and get saddled with a dab of pollen.

This will only benefit Rafflesia if the fly is able to find

another of these very rare flowers fully open in the forest to which it can deliver its load.

Rafflesia produces the biggest single flower in the world.

But why, when all it needs to attract are flies?

Plants, like other living organisms, can only afford to spend a limited amount of food on reproduction.

But Rafflesia does not earn its food.

It takes it straight from the vine.

Provided the vine is not fatally injured, there seems to be no limit to the amount Rafflesia may extract.

Maybe an unearned income in the plant world, as elsewhere, can lead to extravagance

on a truly monumental scale.

Subtitles by Sarah Aitken BBC Scotland 1995