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

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

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Now it will rot and release a thousand seeds.

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

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

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

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They can see.

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Each, as you might expect, sprouts upwards.

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But now these infant plants behave very strangely.

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

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

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

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That enables it to travel six feet.

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

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These have made it to first base.

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They've reached a vertical surface

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a tree trunk.

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

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It starts growing upwards.

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As it does, it puts out its first leaves.

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

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

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It holds these small circular leaves flat against the bark.

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As it gains height it produces bigger ones.

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And now, 50ft above the forest floor,

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

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

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

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

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and the stem that bore them,

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have now shrivelled and died.

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

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And THESE are its adult leaves.

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

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

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These are ferns.

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A tropical Alocasia.

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The needle-shaped leaves of a larch.

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The broad, five-fingered hand of a chestnut.

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Sycamore.

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Leaves are the factories in which plants make their food.

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

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air, water, and a few minerals.

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The process is the unique talent of plants.

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No animals can do such a thing.

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

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This is the very basis of life.

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Air seeps into the leaves through pores on their surface.

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

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

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And produces carbohydrate sugars and starches.

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

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

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

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BIRDSONG

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In open country in a hedgerow, perhaps

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

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the plant easily gets all it needs. 

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In thick forest, it's not so easy.

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

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to catch the shifting shafts of sunlight.

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

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

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There ARE leaves, of course.

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

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

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

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It can wait 20 years for that chance. 

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

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

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Most will die as saplings.

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

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This begonia, for example.

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

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How?

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The secret is in the leaves.

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To start with, they have red undersides.

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

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is reflected back into the leaf.

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

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This species of begonia gathers light differently.

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These patches on their leaves are transparent,

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

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But plants need something else to make food for themselves.

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They need water and the nutrients dissolved in it.

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

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

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

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

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

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

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

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

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

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then some plants will become very big indeed.

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

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

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and spreading out a huge surface area of leaves.

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Through their pores it sucks in carbon dioxide.

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It also brings considerable problems.

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

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

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

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

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

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

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it takes that huge, noisy engine down there.

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

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and manages to do so in total silence.

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How?

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

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The central part of this is wood.

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

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

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

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These are the ones that carry the water up.

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

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

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and, ultimately, into the leaves themselves.

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

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The rest evaporates through the leaf pores as vapour.

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Of course, leaves can't absorb water directly.

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

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

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Plants in the tropical rainforests have particular difficulties.

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For here the rain drenches down in torrents.

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They have to be tough to withstand the pounding.

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They must have gutters to carry away the water.

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Many have pointed tips at the end,

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ensuring water doesn't linger on the leaf

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and doesn't obstruct air passing through the pores.

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Others use dense hairs to keep their pores free.

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

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

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They eat pretty well nothing else.

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

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

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

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

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

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

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And those animals that eat a lot of leaves

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

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

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

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The plants' most numerous attackers by far are insects.

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

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

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look at this.

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

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This is it  a tiny caterpillar.

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It's soft. It's defenceless.

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It's an excellent mouthful for many a bird.

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To survive, it must take steps to protect itself.

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

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

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It spins silk across the hinge.

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That, as it dries, contracts,

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helping the caterpillar pull it over to form a roof.

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

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

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The caterpillar can feed in safety,

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

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And at significant cost to the plant.

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

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Plants do what they can to defend themselves.

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Some develop long, ferocious, needle-sharp spines.

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These APPEAR sufficient to deter anything.

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But not so.

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

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

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

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The attacker is a giraffe.

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

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

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

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

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

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The branches up at the top, in the centre.

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

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

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

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

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Some involve very refined armaments.

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

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In summer, many might think it TOO abundant.

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Beneath its leaves, it produces sprays of tiny flowers.

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

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for the very good reason they have painful stings.

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

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Ow.

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

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And it's filled with poison.

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

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

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

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This one knows leaves are edible.

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It has yet to learn that SOME can defend themselves.

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

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It's better to stick to grass!

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

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

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Its leaves look like those of a stinging nettle,

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

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

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And it has no sting of any kind.

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

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It certainly doesn't risk a sting.

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

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has found protection in mimicry.

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And this is another mimic.

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

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

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

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Few animals even notice it.

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

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It's pestered by heliconias butterflies

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because its leaves are the favourite food of heliconias caterpillars.

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So the female butterflies always lay their eggs on the plants

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

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The egg is a bright yellow globe.

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There's another one.

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The caterpillars are particularly voracious.

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

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

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Before she starts she makes a survey. 

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This female has decided NOT to lay here.

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

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

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

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Surely one of the subtlest of strategies based on mimicry.

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Bracken has adopted a rather more straightforward defence.

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

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I can see none.

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

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

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is liable to go blind or get cancer.

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

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

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

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

242
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Plants seem to have evolved every conceivable defence for their leaves,

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

244
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But this sensitive mimosa, common beside tropical roadsides,

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

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One touch makes it fold its leaflets.

247
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Another tap and it flops to the ground.

248
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How does that help? Well, watch how a hungry grasshopper gets on.

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

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But before it even takes a bite... 

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the meal vanishes.

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

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

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

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Watch.

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This is Venus's-flytrap.

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

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Here comes a meal.

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Touch the hair, and the trap is sprung.

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There's now no escape.

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

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

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

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

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These are the trumpet pitchers.

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

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they supplement it with the bodies of animals.

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Their traps are also formed from leaves,

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

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

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

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It advertises a delicious reward.

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The reward itself is under here.

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Sweet nectar.

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

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

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

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But if it strays off it...

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it falls into a pond of water and drowns.

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

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And where one ant goes others are likely to follow.

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The marsh pitcher attracts other animals too.

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

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Marsh pitchers have comparatively simple traps.

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The pitcher plants proper, producing more elaborate ones,

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live on the other side of the world. 

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

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

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

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that grows only on this great mountain, Kinabalu.

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And they're all around me.

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

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

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

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

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

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

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

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

300
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But then the tip of the midrib continues to grow.

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Once it touches the ground it begins to inflate.

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

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

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The shape and placing of the pitchers varies between species.

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But essentially they're all the same. 

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They attract their prey with nectar,

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

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

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

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manufacture food for a plant.

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But leaves are delicate structures. 

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This plant

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the giant arum of Borneo

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develops the biggest undivided leaf of all.

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

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

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

318
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Neither is likely to happen in a tropical rainforest, which is why immense leaves develop.

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

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In northern lands where the winters can be very severe,

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many trees have to take drastic measures to protect themselves.

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

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

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

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

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and the leaves begin to change colour.

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

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whole hillsides of maples and aspens begin to flush red.

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As the leaves dry out, they are sealed off.

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

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Now the slightest breath of air will detach them.

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

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The falling leaves will soon decay. 

334
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That releases much of the nutriments used in constructing them.

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00:42:21,960 --> 00:42:28,400
And in spring, the trees through their rootlets just below the earth's surface

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will be able to reclaim what they've lost.

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

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Growth has virtually stopped.

339
00:42:40,840 --> 00:42:44,760
The processes of life barely tick over.

340
00:42:55,920 --> 00:43:03,920
This alternation of growing in summer and shutting down in winter leaves its mark in a tree's trunk

341
00:43:03,920 --> 00:43:06,560
annual rings.

342
00:43:06,560 --> 00:43:10,560
The white wood are large cells formed in summer,

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

344
00:43:17,760 --> 00:43:25,240
So by counting the rings I can be absolutely certain that this beech tree lived for over 200 years.

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That's longer than any animal lives.

346
00:43:31,440 --> 00:43:38,640
The record for longevity, however, is much greater than THAT, and is held elsewhere.

347
00:43:53,480 --> 00:43:59,480
Here, 10,000ft up in the White Mountains of eastern California,

348
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grow the oldest living things on earth  the bristle-cone pines.

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This part is already dead.

350
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But here, there is life and growth.

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

352
00:44:20,600 --> 00:44:26,120
Because the conditions are extreme and it gets very cold in winter,

353
00:44:26,120 --> 00:44:28,880
some years there's little growth.

354
00:44:28,880 --> 00:44:33,680
As a consequence, the rings are very much more close together.

355
00:44:33,680 --> 00:44:37,080
This is a cross-section of one tree.

356
00:44:37,080 --> 00:44:41,720
The outermost ring is the year in which it died  1958.

357
00:44:41,720 --> 00:44:45,760
Count 100 rings inwards - 1858.

358
00:44:45,760 --> 00:44:49,040
Another century  1758.

359
00:44:49,040 --> 00:44:56,440
Around here is the ring it was developing when Columbus arrived on this continent in 1492.

360
00:44:56,440 --> 00:45:01,600
It was in the full vigour of youth when the Pharaohs were ruling Egypt.

361
00:45:01,600 --> 00:45:10,360
So we can be sure when the first human farmers were just beginning to plant seeds for themselves,

362
00:45:10,360 --> 00:45:14,240
this ancient ravaged tree was just sprouting.

363
00:45:14,240 --> 00:45:17,280
It's over 4,000 years old!

364
00:45:18,440 --> 00:45:23,480
Pine leaves are very different from the leaves of oak and maple.

365
00:45:23,480 --> 00:45:29,120
Instead of being broad and flat, and easily damaged by frost,

366
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they are needle-shaped and tough.

367
00:45:32,360 --> 00:45:37,360
Instead of having pores all over the flat surface as oak and maple do,

368
00:45:37,360 --> 00:45:43,200
The pores are restricted to a groove running the length of the needle.

369
00:45:43,200 --> 00:45:47,040
It's partly filled by a tough, waxy deposit.

370
00:45:49,040 --> 00:45:53,040
Beneath that there are lines of small pores

371
00:45:53,040 --> 00:45:56,520
Few compared with those on an oak leaf.

372
00:46:02,320 --> 00:46:09,680
Even at the height of summer leaves like these can't manufacture food as swiftly as broad leaves do.

373
00:46:09,680 --> 00:46:15,480
On the other hand, needle-producing trees don't discard them every year.

374
00:46:15,480 --> 00:46:20,520
They keep them much longer  with all the energy saving that implies.

375
00:46:20,520 --> 00:46:24,440
The conifer's policy is "slow, but sure".

376
00:46:24,440 --> 00:46:30,880
And it's produced not only the oldest plants, but OTHER record holders.

377
00:46:32,960 --> 00:46:38,200
And this is the most massive living thing on earth

378
00:46:38,200 --> 00:46:40,560
the giant sequoia.

379
00:47:02,240 --> 00:47:09,160
They don't live as long as bristle-cone pines, but almost  over 3,000 years.

380
00:47:09,160 --> 00:47:11,800
They grow up to 300ft tall.

381
00:47:11,800 --> 00:47:19,800
And every year they put on as much wood as there is in a 60ft tree of normal proportions,

382
00:47:19,800 --> 00:47:24,760
so that the really big ones weigh over a thousand tons.

383
00:47:51,120 --> 00:47:58,200
Although they may be loaded with snow for months in the winter, and baked dry in the summer,

384
00:47:58,200 --> 00:48:05,200
the conifers have produced the largest and the longest-living of all organisms on earth.

385
00:48:05,200 --> 00:48:10,240
Like all plants they have done it with the simplest of ingredients

386
00:48:10,240 --> 00:48:12,760
water and minerals from the earth, 

387
00:48:12,760 --> 00:48:17,040
carbon dioxide from the atmosphere, and light.

388
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Subtitles by Carolyn Donaldson BBC Scotland, 1994