(VARIOUS ANIMAL NOISES)
High in the canopy of the South Amecan 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.
If the seedlings are to survive, they will have to gain a position like their parents.
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, as most seedlings do,
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 is enough to enable it to travel about 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, and 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 and bigger ones.
And now, fifty feet above the forest floor,
and many months since it first emerged as a slim green shoot from its seed,
this extraordinarily active plant has changed the shape of its leaves once again.
They've developed the slits and the holes that give it the name of cheese plant.
The small, round leaves 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 plants must open to catch the sun.
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 are powered by sunshine,
and they 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 the 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 tiny granules that contain a green substance, chlorophyll.
This is the key facilitator, that uses the energy of the sun
to bond carbon dioxide to hydrogen, derived 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 during the night, when the leaf factory has shut down.
Come the dawn, the sun reappears and the process starts up again.
In open country, in a hedgerow, perhaps,
there is so much light that, as the sun climbs higher,
a plant has little difficulty in getting 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 what it can from the shifting shafts of the sunlight.
Above, the trees position their leaves with such accuracy
that they form a close - fitting mosaic.
The canopy is so efficient at gathering light that very little filters down here.
There are leaves, of course - this is the young sapling of one of the canopy trees,
but it is growing hardly at all.
It's waiting for the chance that one of the adult trees will fall, releasing a flood of light.
Then it can grow, and it'll race upwards to try and claim the vacant space.
It can wait ten, twenty years for that chance,
but until it comes, there's simply not enough light for it to grow any further.
For most, of course, that chance will never come. Most will die as saplings.
But some plants that spend their whole lives here 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 surface of the leaf and going through it is not lost
but reflected back into the body of the leaf.
So when sunlight does, for a short time, fall on the leaf,
the plant is able to take maximum advantage of it.
Another species of begonia has a different light - gathering trick.
Small patches on their leaves are transparent, and act as tiny lenses,
gathering the feeble light and focusing it onto the grains of chlorophyll within.
But plants need something else as well as light in order to make food for themselves.
They need water and the nutrients that are 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 have to position themselves with just as much accuracy
as the leaves do when finding light.
Having found water, they put out thin rootlets, and from them, a fur of tiny hairs,
so multiplying 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 that it gets its fair share of moisture.
If the rainfall is reasonably good for much of the year,
if the water in the ground can dissolve enough nutrients from the soil,
then some plants will become very big indeed.
Growing seventy feet tall, like this sycamore, brings it great advantages.
It allows it to overtop its neighbours, so it can get all the sunshine it needs,
and it enables it to spread out a huge surface area of leaves,
and through their pores it can suck in carbon dioxide from the air.
But it also brings considerable problems.
As well as carbon dioxide, the leaves need water in order 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 seventy feet up in the air, here,
it takes that huge, big, noisy engine down there.
But this tree pumps up about 100 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 the water evaporates in the leaves above,
the long, thin 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.
Indeed, water lying on their surface can cause problems by clogging up the pores.
So some leaves have shapes which help to reduce that problem.
Plants growing in the rainforests of the tropics have particular difficulties,
for here the rain drenches down in torrents.
They have to be tough to withstand the pounding.
They also have to have gutters to carry away the water.
Many have pointed tips at the end,
ensuring the water doesn't linger on the leaf but drains rapidly and completely away
and doesn't interfere with the intake of air through the leaf 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 and supper for these proboscis monkeys in Borneo.
They eat pretty well nothing else.
Maybe a few flower petals now and then, perhaps a little fruit,
but otherwise entirely leaves.
But leaves have a great drawback as food - they are not, in fact, very nutritious.
So these monkeys have to spend hours and hours and hours every day,
stripping the trees of their leaves.
Leaf sap, loaded with starch and sugars, is certainly nutritious.
The problem comes from the walls of the cells that enclose that sap.
They are made of cellulose, and mammals' digestive juices can't deal with that.
Bacteria, however, can, and those animals, like these monkeys, that eat a lot of leaves,
have to sit around after feeding,
to give time for the bacterial colonies in their stomachs to deal with their difficult meals.
Despite these drawbacks,
Iots 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.
All around me in this Borneo rainforest, millions of tiny mouths are 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.
Clearly it's a badly - damaged leaf,
but where is the creature that's doing the damage?
This is it, a tiny caterpillar.
It's soft, it's defenceless, it's clearly an excellent mouthful for many a bird.
So if it is to survive, it has to take steps to protect itself.
It starts by making a semi - circular cut into the leaf from the margin.
But when the cut is only half - complete, it starts from the other end.
It spins silk across the hinge. That, as it dries, contracts,
and helps the caterpillar pull over the segment to form a roof.
To make its tent a little 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.
Now the caterpillar can feed in safety,
shaving off the soft surface layers of the leaf out of the sight of any hungry bird,
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, of course, do what they can to defend themselves.
Some develop long, ferocious, needle - sharp spines.
These, you might think, would be sufficient to deter anything.
But not so.
This tongue is so mobile that it can pick out the soft leaves from between the spines.
This hide is so tough that even the sharpest spines don't puncture it easily.
And these rubbery lips seem able to survive the most prickly of mouthfuls.
The attacker, of course, is a giraffe,
and it can reach leaves fifteen feet above the ground.
It's the tallest of all living animals.
Such intensive grazing makes it very difficult for plants here
to grow much bigger than stunted bushes.
Thanks to their thorny defences, some acacias do succeed in growing to maturity.
Then they develop the umbrella shape so characteristic of the East African grasslands.
And now, at last, the acacia tree has some parts that even a giraffe can't reach:
the branches up to 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 as 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 indeed.
Some involve extremely refined armaments.
This is one of the commonest plants of the European countryside.
In summer, many might think its tall stems are only too abundant in the hedgerows.
Beneath its leaves, it produces sprays of tiny flowers.
We can all recognise these as nettles, and learned to do so when we were young,
for the good reason that they have painful stings.
But this sting is actually quite a complex weapon. Watch.
Ow!
It's a hollow hair made of silica, the mineral from which we make glass,
and it's filled with poison.
Its tip is so sharp that the slightest touch cuts human skin,
and so fragile it breaks at that touch and releases poison into the wound.
The result is a painful swelling.
It's not just young humans who learn to avoid nettles, so do young rabbits.
This one already knows that green leaves are good to eat.
It's yet to learn that some can defend themselves.
The nose has 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 great thickets.
But there are two kinds of nettles growing here.
The kind on the right is slightly different.
Its leaves look just like those of a stinging nettle, but its white, tubular flowers
are quite different from those small brown ones of the true nettle.
In fact, this is a relative of mint and thyme, this is the deadnettle,
and it has no sting of any kind.
But even an adult rabbit seems not to know the difference,
and it certainly doesn't risk a sting.
The deadnettle, without going to the expense and trouble
of producing poisoned hypodermic needles, has found protection in mimicry.
And this is another mimic.
A tortoise in the desert of Southern Africa is always on the lookout for a juicy mouthful,
but it walks right over as good a one as it might find all day
and feeds instead on a few shrivelled leaves.
The pebble plant mimics its surroundings so accurately
that it even varies its colour to match that of the gravel around it.
Few animals even notice it.
The passionflower uses mimicry to defend itself in perhaps the most extraordinary way of all.
It's much pestered by heliconius butterflies.
This is because its leaves are the favourite food of heliconius 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 little bright yellow globe.
There's another one.
The caterpillars are particularly voracious.
They will tackle leaves, stems, shoots and buds, pretty well every part of the passionflower.
Because her young need so much food,
a female heliconius won't lay on a passionflower if there are eggs already there.
And before she starts, she makes a careful survey.
This female has decided not to lay here.
Hardly surprising - the leaves are already covered with eggs.
Except that they are not eggs.
These yellow spots are imitations, fakes produced by the plant as a deterrent.
Another species of passionflower produces even more convincing bogus eggs,
on the stalks of the leaves,
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 young leaves like this
would show lots of 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 to get cancer.
When they're young, the leaves are packed with cyanide,
which deters most things, including insects.
But as the plant matures, it starts to synthesise even more complex poisons
that deter almost every living creature.
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 by their very nature have to be spread wide to catch the light, so are very visible.
But this plant, the sensitive mimosa, common beside tropical roadsides,
has perhaps 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 leaf - eating 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.
It shapes its traps from the ends of its leaves.
One or two hairs on their surface 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 even more tightly.
It now produces digestive acids from glands on the inner surface of the leaf
which first kill and then dissolve its victim's body.
Growing in the same Carolina swamp, there's another carnivorous plant.
These are the trumpet pitchers.
Like the Venus's flytrap,
they find so little nutriment in this impoverished, waterlogged soil,
that 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 they're not.
Though, in a sense, this bright yellow top serves the same purpose as a petal,
it's an advertisement of a delicious reward.
And the reward itself is under here.
A sweet nectar.
But if an insect comes to collect it and strays into the mouth of the trumpet, then 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 may be hoping to eat some of the insects before the pitcher does.
But if it loses its footing, the plant will eat it.
Marsh pitchers have comparatively simple traps.
The pitcher plants proper produce more elaborate ones.
And they live on the other side of the world.
The headquarters of the pitcher plants are in South - East Asia.
There are 76 different species of them, 30 of which grow only on the island of Borneo.
And 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... 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
and open out to form the blade of a leaf.
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 from species to species,
but essentially they're all the same.
They attract prey with nectar,
they have slippery sides, so many of their visitors tumble into them,
and the fluid within contains juices which actively dissolves the bodies.
So leaves, one way or another, either by catching insects
or, much more usually, by absorbing gases and harnessing the energy of sunlight,
manufacture food for a plant.
But leaves are actually quite 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 gets so cold the water freezes and bursts the cell walls,
the leaf will collapse.
Of course, neither is likely here in the tropical rainforest,
one reason why such immense leaves can develop here.
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're 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 leaves falling to the ground will soon decay.
That will release much of the nutriments used in constructing them,
and in spring, the trees, through their rootlets just below the surface of the earth,
will be able to reclaim much of what they have lost.
So by the time winter grips the land, the trees are reduced to skeletons.
Growth has virtually stopped, the processes of life are barely ticking over.
This alternation of growing in summer and shutting down in winter
Ieaves its mark in the tree's trunk - annual rings.
The white wood are large, open cells that were laid down in the 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 lived for over two hundred years before it fell.
And that's longer than any animal lives.
The record for longevity, however, is much greater than that, and is held elsewhere.
Here, 10,000 feet up in the White Mountains of Eastern California,
grow the oldest living things on earth, the bristlecone 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 conditions here are so extreme and it gets so very cold in winter,
some years there's very little growth at all.
As a consequence, the rings are very much more close together.
This is a cross - section of one of these trees.
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 its youth when the Pharaohs ruled Egypt.
So we can be sure that when the first human farmers were beginning to plant seeds,
this ancient, ravaged tree was just sprouting.
It's over 4,000 years old.
Pine leaves are obviously 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 very tough.
Instead of having pores all over the flat surface, as oak and maple do,
these pores are restricted to a groove which runs down the length of the needle.
It's partly filled by a tough, waxy deposit,
and beneath that there are lines of small pores.
Very few compared with those scattered all over an oak leaf.
Even at the height of summer,
Ieaves like these can't manufacture food as swiftly as broad leaves do.
But on the other hand, the needle - producing trees, the conifers,
don't discard them every year, but keep them very much longer,
with all the saving of energy 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 bristlecone pines but almost, over 3,000 years.
They grow up to 300 feet tall,
and every year they put on as much wood
as there is in a 60 - foot tree of normal proportions,
so that the really big ones weigh over 1,000 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.
And like all plants, they've done it with the simplest of ingredients,
with water and minerals from the earth,
carbon dioxide from the atmosphere and light.