Growing The Private Life of Plants

High in the canopy of the South American rainforest a fruit is falling.

It has come from a plant sitting on a branch of one of the giant trees.

Now it will rot and release a thousand seeds.

To survive, the seedlings must gain a position like their parent's.

Somehow, they've got to get up into the canopy and the sunshine.

The shoots that come from the seeds, like all shoots, can sense the light.

They can see.

Each, as you might expect, sprouts upwards.

But now these infant plants behave very strangely.

They DON'T head for the brightest light. They seek the densest shade.

And THAT usually lies around the trunk of the nearest tree.

Each seedling is fuelled entirely by the store of food its parents deposited within the seed.

That enables it to travel six feet.

If it doesn't find what it's looking for within that distance, it will die of starvation.

These have made it to first base.

They've reached a vertical surface

a tree trunk.

As soon as one touches it, its behaviour changes dramatically.

It starts growing upwards.

As it does, it puts out its first leaves.

Now, for the first time, it can manufacture food for itself.

With each additional leaf, the young plant increases in strength.

It holds these small circular leaves flat against the bark.

As it gains height it produces bigger ones.

And now, 50ft above the forest floor,

and many months since it emerged as a slim green shoot from its seed,

this extraordinary, active plant has changed the shape of its leaves once again.

They've developed the slits and holes that give it and its relations the name of cheese-plants.

The small, round, green leaves that were pressed up against this trunk,

and the stem that bore them,

have now shrivelled and died.

The cheese-plant has reached its true home the forest canopy.

And THESE are its adult leaves.

Cheese-plant leaves unfurl from pointed spikes like rolled umbrellas.

But there are many ways of unpacking the green sheets to catch the sunlight.

These are ferns.

A tropical Alocasia.

The needle-shaped leaves of a larch.

The broad, five-fingered hand of a chestnut.

Sycamore.

Leaves are the factories in which plants make their food.

They're powered by the sunshine, and use the simplest of raw materials

air, water, and a few minerals.

The process is the unique talent of plants.

No animals can do such a thing.

So all animals too depend, first- or second-hand, on food produced here.

This is the very basis of life.

Air seeps into the leaves through pores on their surface.

It circulates within, and reaches granules containing a green substance chlorophyll.

It is the key facilitator that uses the sun's energy to bond carbon dioxide to hydrogen from water.

And produces carbohydrate sugars and starches.

These, dissolved in sap, are then carried from the leaf into the body of the plant,

even in the night, when the leaf factory has shut down.

Come the dawn, the sun reappears and the process starts up again.

BIRDSONG

In open country in a hedgerow, perhaps

there is so much light that as the sun climbs higher and higher

the plant easily gets all it needs.

In thick forest, it's not so easy.

A plant growing beneath the canopy has to continually move its leaves

to catch the shifting shafts of sunlight.

Above, the trees position their leaves with such accuracy they form a close-fitting mosaic.

The canopy is so efficient at gathering light that little filters down.

There ARE leaves, of course.

This is a sapling of a canopy tree, but it is growing hardly at all.

It's waiting for one of the adult trees to fall, releasing a flood of light.

Then it CAN grow, and it'll race upwards to claim the vacant space.

It can wait 20 years for that chance.

But until it comes there's not enough light for it to grow further.

For most, of course, that chance will never come.

Most will die as saplings.

But some plants spend their whole lives on the dim forest floor.

This begonia, for example.

It produces big leaves, flowers, and sets seeds, all in this dim light.

How?

The secret is in the leaves.

To start with, they have red undersides.

That means light falling on the leaf surface and going through it,

is reflected back into the leaf.

So when sunlight does for a short time fall on the leaf, the plant is able to take maximum advantage of it.

This species of begonia gathers light differently.

These patches on their leaves are transparent,

and act as lenses, gathering the light and focusing it onto the chlorophyll within.

But plants need something else to make food for themselves.

They need water and the nutrients dissolved in it.

And that, of course, they suck up from the ground.

The roots with which they do so probe downwards, seeking moisture.

To get that, they place themselves with just as much accuracy as the leaves do when finding light.

On finding water they put out rootlets, and from them a fur of tiny hairs

so multiplying many thousands of times the surface area through which water can be sucked in.

So the soil in a woodland is a tangle of precisely-placed rootlets from many different kinds of plants,

each individual doing its best to ensure it gets its fair share of moisture.

If the rainfall is reasonably good for much of the year,

and if the water in the ground is able to dissolve an adequate amount of nutrients from the soil,

then some plants will become very big indeed.

Growing 70ft tall, like this sycamore, brings great advantages

like overtopping its neighbours so it can get all the sunshine it needs,

and spreading out a huge surface area of leaves.

Through their pores it sucks in carbon dioxide.

It also brings considerable problems.

As well as carbon dioxide, the leaves need water to make food.

And water in the leaf can easily evaporate through the pores.

Indeed, 90% of the water sucked in by the roots

is lost through the surface of the leaves at the top of the tree.

But pumping water up here, to this height, can cause considerable problems.

To pump this jet of water 70ft up in the air here,

it takes that huge, noisy engine down there.

But this tree pumps up about a hundred gallons every hour,

and manages to do so in total silence.

How?

The answer is to be found in the tree's trunk.

The central part of this is wood.

Around the outside of this pillar there are ranks of hair-thin pipes.

Those immediately beneath the bark carry the food-laden sap down from the leaves.

Farther inside the trunk there's another set of tubes.

These are the ones that carry the water up.

They are continuous pipes that extend the whole length of the trunk.

As water evaporates in the leaves above, the threads of it are pulled up the tubes into the branches,

and, ultimately, into the leaves themselves.

Some of it is used in the food-making process.

The rest evaporates through the leaf pores as vapour.

Of course, leaves can't absorb water directly.

Water lying on their surface can cause problems as it clogs up the pores.

So some leaves have shapes which help to reduce that problem.

Plants in the tropical rainforests have particular difficulties.

For here the rain drenches down in torrents.

They have to be tough to withstand the pounding.

They must have gutters to carry away the water.

Many have pointed tips at the end,

ensuring water doesn't linger on the leaf

and doesn't obstruct air passing through the pores.

Others use dense hairs to keep their pores free.

But rainfall is the least of the dangers that threaten leaves.

Leaves are breakfast, lunch, supper for the proboscis monkeys in Borneo.

They eat pretty well nothing else.

Maybe a few flower petals now and then, perhaps a little fruit, otherwise entirely leaves.

But leaves have a drawback as food. They're not very nutritious.

So these monkeys have to spend hours and hours and hours every day stripping the trees of their leaves.

The leaf sap, loaded with starch and sugars, is certainly nutritious.

The problem comes from the walls of the cells enclosing the sap. They are made of cellulose.

The digestive juices of mammals can't deal with it. But bacteria can.

And those animals that eat a lot of leaves

have to sit around after feeding to give time for the bacterial colonies in their stomachs to work.

Despite these drawbacks, lots of mammals, and even some birds and reptiles, have taken to this diet.

But in fact, such big leaf-eaters are in the minority.

The plants' most numerous attackers by far are insects.

Around me in this Borneo rainforest there are millions of tiny mouths munching away invisibly.

To give you some idea of the lengths to which an insect will go in order to get a vegetarian meal in safety,

look at this.

It's a damaged leaf, but where's the creature that's doing the damage?

This is it a tiny caterpillar.

It's soft. It's defenceless.

It's an excellent mouthful for many a bird.

To survive, it must take steps to protect itself.

It starts by making a semi-circular cut into the leaf from the margin.

When the cut is only half complete, it starts from the other end.

It spins silk across the hinge.

That, as it dries, contracts,

helping the caterpillar pull it over to form a roof.

To make its tent more commodious it cuts a pleat, pulls it across, and now it's got a little wigwam.

The whole process only takes a few hours and is usually done at night when there are no birds around.

The caterpillar can feed in safety,

shaving off the soft surface layers of the leaf out of sight of hungry birds.

And at significant cost to the plant.

The damage and loss inflicted on plants by animals both large and small is huge and never-ending.

Plants do what they can to defend themselves.

Some develop long, ferocious, needle-sharp spines.

These APPEAR sufficient to deter anything.

But not so.

This tongue is so mobile it can pick the soft leaves BETWEEN the spines.

This hide is so tough even the sharpest spines don't puncture it easily.

And these rubbery lips seem able to survive the most prickly of mouthfuls.

The attacker is a giraffe.

It can reach 15ft above ground. It's the tallest of all living animals.

Such intensive grazing means it's difficult for plants to grow bigger than stunted bushes.

Thanks to their thorny defences some acacias manage to grow to maturity.

Then they develop the umbrella shape so characteristic of the East African grasslands.

Now, at last, the acacia has some parts even a giraffe can't reach.

The branches up at the top, in the centre.

There the acacia can save precious energy and reduce the scale of its thorny armaments.

On the outside, the thorns are as long and dense as anywhere.

But in the middle of the crown there are no thorns whatsoever.

The techniques employed by plants to defend themselves are very varied.

Some involve very refined armaments.

This is one of the commonest plants of the European countryside.

In summer, many might think it TOO abundant.

Beneath its leaves, it produces sprays of tiny flowers.

We all recognise nettles, and have been able to since our youth,

for the very good reason they have painful stings.

But this sting is actually quite a complex weapon. Watch.

Ow.

It's a hollow hair made from silica, the mineral from which we make glass.

And it's filled with poison.

Its tip is so sharp a mere touch cuts our skin,

and so fragile, it then breaks releasing poison into the wound, resulting in a painful swelling.

Young humans learn to avoid nettles. So do young rabbits.

This one knows leaves are edible.

It has yet to learn that SOME can defend themselves.

The nose has a little protective fur. And that hurt!

It's better to stick to grass!

With such an effective armoury, nettles grow unmolested, and rapidly establish themselves in thickets.

But there are two kinds of nettles growing here. The kind on the right is slightly different.

Its leaves look like those of a stinging nettle,

but its white tubular flowers look quite different from those small brown ones of the true nettle.

In fact, this is a relative of mint and thyme. This is the dead-nettle.

And it has no sting of any kind.

But even an adult rabbit doesn't apparently know the difference.

It certainly doesn't risk a sting.

The dead-nettle, without the trouble of producing poisoned hypodermics,

has found protection in mimicry.

And this is another mimic.

A tortoise in the southern African desert looks for a juicy mouthful.

But it walks over as good a one as it might find all day, feeding instead on a few shrivelled leaves.

The pebble plant mimics surroundings so accurately it even varies its colour to match that of the gravel.

Few animals even notice it.

The passion flower uses mimicry to defend itself in perhaps the most extraordinary way of all.

It's pestered by heliconias butterflies

because its leaves are the favourite food of heliconias caterpillars.

So the female butterflies always lay their eggs on the plants

in order that their youngsters when they hatch will find their favourite food immediately in front of them.

The egg is a bright yellow globe.

There's another one.

The caterpillars are particularly voracious.

They'll tackle leaves, stems, shoots and buds pretty well every part of the passion flower.

Because her young need so much food a female heliconias won't lay where there are eggs already.

Before she starts she makes a survey.

This female has decided NOT to lay here.

Hardly surprising the leaves are already covered with "eggs".

Except they're NOT eggs. These yellow spots are imitations, fakes, produced by the plant as a deterrent.

This species of passion flower produces even more convincing "eggs" on the leaf stalks.

Surely one of the subtlest of strategies based on mimicry.

Bracken has adopted a rather more straightforward defence.

You might think a nutritious-looking carpet of leaves like this would show signs of damage by grazers.

I can see none.

The fact is that bracken is full of a cocktail of toxins so powerful

that any mammal that eats it, such as rabbit or cattle,

is liable to go blind or get cancer.

When they're young, the leaves are packed with cyanide which deters most things, including insects.

As the plant matures it starts to synthesise more complex poisons that deter almost every living creature.

And as a result, the plant sprawls unchecked and covers vast areas of European hillsides.

Ferocious spines, painful stings, poisonous sap, near-perfect disguise.

Plants seem to have evolved every conceivable defence for their leaves,

which have to spread wide to catch the light, and so are very visible.

But this sensitive mimosa, common beside tropical roadsides,

has the most radical, and certainly the most dramatic solution, of all.

One touch makes it fold its leaflets.

Another tap and it flops to the ground.

How does that help? Well, watch how a hungry grasshopper gets on.

Obviously, there's a splendid meal ahead(!)

But before it even takes a bite...

the meal vanishes.

This ability to move fast is used by one astonishing plant to turn the tables on animals.

It grows here in this swampy pine forest in northern Carolina.

Animals don't eat IT. IT eats animals. And there's one right here.

Watch.

This is Venus's-flytrap.

Its traps are the ends of its leaves. One or two hairs act as triggers.

Here comes a meal.

Touch the hair, and the trap is sprung.

There's now no escape.

The beetle's struggles stimulate the plant to close the trap more tightly.

It now produces digestive acids from glands on the leaf's inner surface,

which first kill and then dissolve its victim's body.

Growing in the same Carolina swamp there is another carnivorous plant.

These are the trumpet pitchers.

They, like the Venus's-flytrap, find so little nutriment in this impoverished soil

they supplement it with the bodies of animals.

Their traps are also formed from leaves,

but leaves that have been folded lengthways to make a vertical tube which fills with water.

These spectacular trumpets may LOOK like flowers, but, of course, they're not.

Though, in a sense, this bright yellow top to them serves the same purpose as a petal.

It advertises a delicious reward.

The reward itself is under here.

Sweet nectar.

But if an insect comes to collect it and strays into the mouth of the trumpet, it's doomed!

The inside of the throat of the trumpet is covered with microscopic, downward-pointing spines.

As long as it stays on the rim the ant is all right.

But if it strays off it...

it falls into a pond of water and drowns.

The tiny corpse dissolves, and the marsh pitcher absorbs the resulting soup.

And where one ant goes others are likely to follow.

The marsh pitcher attracts other animals too.

This frog hopes to eat some insects before the pitcher, but if it loses its footing the plant will eat IT.

Marsh pitchers have comparatively simple traps.

The pitcher plants proper, producing more elaborate ones,

live on the other side of the world.

The HQ of the pitcher plants are in South-East Asia.

There are 76 different species, 30 of which grow only on the island of Borneo.

They include the biggest of them all, a truly spectacular plant, appropriately called Nepenthes rajah,

that grows only on this great mountain, Kinabalu.

And they're all around me.

I guess...this one...contains oh, two or three pints...of liquid.

It's so big that it catches not just insects, but even small rodents.

And one was recorded that had in it the body of a drowned rat.

So if ever there was a carnivore among plants, this is it.

The traps of this Asian family of pitcher plants are, once again, modified leaves.

But they're not simply folded into a tube. The process is more complex.

A shoot appears that looks just the same as those that turn into normal leaves.

Over a period of several days flanges develop near the end, opening to form a leaf blade.

But then the tip of the midrib continues to grow.

Once it touches the ground it begins to inflate.

The lid opens to expose the plant's lethal pond.

Some of the bigger species may produce half a dozen of these huge elegant traps.

The shape and placing of the pitchers varies between species.

But essentially they're all the same.

They attract their prey with nectar,

they have slippery sides so many of their visitors tumble into them,

and the fluid within contains juices which actively dissolve the bodies.

So leaves, either by catching insects or by absorbing gases and harnessing the energy of sunlight,

manufacture food for a plant.

But leaves are delicate structures.

This plant

the giant arum of Borneo

develops the biggest undivided leaf of all.

It can have a surface area of up to 3 square metres 34 square feet.

The arum keeps these vast leaves outstretched by pumping the cells within them full of water.

If there's not enough water, or if it freezes and bursts the cell walls, the leaf will collapse.

Neither is likely to happen in a tropical rainforest, which is why immense leaves develop.

But elsewhere in the world plants don't have it so easy.

In northern lands where the winters can be very severe,

many trees have to take drastic measures to protect themselves.

As the days grow shorter and colder, and autumn approaches,

the trees prepare to cut their losses and suspend their activities.

They start to shut down their food factories and withdraw the valuable chlorophyll from the leaves.

As the green pigment drains away, waste products that have accumulated over the year are revealed,

and the leaves begin to change colour.

In New England and the Appalachian Mountains, day after day,

whole hillsides of maples and aspens begin to flush red.

As the leaves dry out, they are sealed off.

A hard corky partition develops within the base of the leaf stalks.

Now the slightest breath of air will detach them.

The loss is great, but it's not total.

The falling leaves will soon decay.

That releases much of the nutriments used in constructing them.

And in spring, the trees through their rootlets just below the earth's surface

will be able to reclaim what they've lost.

So by the time winter grips the land the trees are reduced to skeletons.

Growth has virtually stopped.

The processes of life barely tick over.

This alternation of growing in summer and shutting down in winter leaves its mark in a tree's trunk

annual rings.

The white wood are large cells formed in summer,

and the dark wood, small dense cells laid down more slowly in autumn and winter.

So by counting the rings I can be absolutely certain that this beech tree lived for over 200 years.

That's longer than any animal lives.

The record for longevity, however, is much greater than THAT, and is held elsewhere.

Here, 10,000ft up in the White Mountains of eastern California,

grow the oldest living things on earth the bristle-cone pines.

This part is already dead.

But here, there is life and growth.

Those rings in the trunk tell us exactly how old these trees are.

Because the conditions are extreme and it gets very cold in winter,

some years there's little growth.

As a consequence, the rings are very much more close together.

This is a cross-section of one tree.

The outermost ring is the year in which it died 1958.

Count 100 rings inwards - 1858.

Another century 1758.

Around here is the ring it was developing when Columbus arrived on this continent in 1492.

It was in the full vigour of youth when the Pharaohs were ruling Egypt.

So we can be sure when the first human farmers were just beginning to plant seeds for themselves,

this ancient ravaged tree was just sprouting.

It's over 4,000 years old!

Pine leaves are very different from the leaves of oak and maple.

Instead of being broad and flat, and easily damaged by frost,

they are needle-shaped and tough.

Instead of having pores all over the flat surface as oak and maple do,

The pores are restricted to a groove running the length of the needle.

It's partly filled by a tough, waxy deposit.

Beneath that there are lines of small pores

Few compared with those on an oak leaf.

Even at the height of summer leaves like these can't manufacture food as swiftly as broad leaves do.

On the other hand, needle-producing trees don't discard them every year.

They keep them much longer with all the energy saving that implies.

The conifer's policy is "slow, but sure".

And it's produced not only the oldest plants, but OTHER record holders.

And this is the most massive living thing on earth

the giant sequoia.

They don't live as long as bristle-cone pines, but almost over 3,000 years.

They grow up to 300ft tall.

And every year they put on as much wood as there is in a 60ft tree of normal proportions,

so that the really big ones weigh over a thousand tons.

Although they may be loaded with snow for months in the winter, and baked dry in the summer,

the conifers have produced the largest and the longest-living of all organisms on earth.

Like all plants they have done it with the simplest of ingredients

water and minerals from the earth,

carbon dioxide from the atmosphere, and light.

Subtitles by Carolyn Donaldson BBC Scotland, 1994