No part of the earth is more hostile to life than the frozen wastes around the poles.
850 miles north of the Arctic Circle, this is Ellesmere Island.
No animal can live permanently on these ice-fields,
and even plants face almost insuperable problems,
for the four things they must have are in cripplingly short supply.
Water: it's true there's a lot of frozen water all around me,
but water has to be liquid for plants to make any use of it.
Nutrients: there's virtually none in this frost-shattered rock.
Warmth and light: for six months of the year, it's dark,
and in the brief summer, as now, the sun doesn't rise high in the sky,
and devastating winds can carry away what little warmth it brings.
And yet, there are plants here.
Some live actually inside the rock.
This thin green line is made by algae, microscopic plants.
They're so small, they can live actually between the grains of this sandstone.
And there at least, they're out of this desiccating wind.
On the surface of the rocks are lichens.
They grow incredibly slowly, and may take 50 years to cover a square centimetre.
But they can survive
even if there are only two days a year when it's warm enough for them to grow.
In spite of these bleak conditions, there are, in fact, flowers to be found here,
but you have to look hard to find them.
Here's one. It's a kind of mustard,
but it's much smaller than its more southerly relatives.
But by being so small, it manages to keep out of the crippling wind.
In midsummer, for a few weeks, enough water melts from the glaciers for streams to flow.
And then, miniature gardens burst into bloom.
The searing wind compels them all to keep close to the ground.
None keeps closer than this.
It is in fact a tree, a willow.
These are its catkins.
But the trunk grows horizontally, not vertically, and it can stretch almost as far along the ground
as its more southerly relatives stand up above it.
Even so, it still produces enough leaves to sustain a few grazers, musk ox.
Nothing is wasted up here.
Not a moment of sunshine, not the tiniest shelter, not a scrap of food.
When a musk ox dies, its decaying body releases a rich flush of nourishment into the soil,
and tiny gardens appear in the shelter of its bones.
The arctic poppy, like all plants, needs warmth to grow.
But it is unusually efficient at collecting it.
As the midsummer sun skims round the horizon, all 360 degrees in 24 hours without setting,
the poppy turns its flowers to track it.
The slanting sun may not be strong,
but it is at least continuous during the few weeks of high summer.
The heat the poppy gathers by staring continuously at the sun
enables it to develop seeds in the centre of each flower
before summer comes to an end and the sun disappears below the horizon for months.
Conditions may be just as severe on the high peaks of the Alps, 2,000 miles to the south,
at least during the winter.
But here, spring brings a greater benefit than in the Arctic.
The sun rises higher in the sky and is warm enough to melt all but the highest snow-fields.
As it melts, it reveals the snowbell, already in flower.
The plant formed its flower buds last autumn,
before the increasing cold shut down its activities for the winter.
The buds stayed dormant until spring sunshine, filtering down through the snow,
triggered them into action,
and they opened even before the snowy blanket above them had melted.
In summer, the high meadows, newly freed from snow, fill with flowers.
Because for so much of the time it's so cold, the vegetation here decays only very slowly.
So a peaty soil forms.
But it's only a thin layer over solid rock and boulders,
and trees find it very hard to get root.
Not only that, but avalanches regularly sweep these slopes,
carrying away saplings before they can get firmly established.
So shallow-rooted plants have these parts of the mountains largely to themselves,
and in summer they bring a rich display of colour.
But for every thousand feet you climb, the temperature drops by about three degrees.
Plants living in the high mountains have to be able to survive extreme cold.
It's very important to keep out of the worst of the chilling winds,
and many plants here form small rounded humps.
And that brings them a number of advantages.
Growing into the shape of a cushion is an excellent way of conserving heat.
No plants do it more spectacularly than these, high in the mountains of Tasmania.
These are the largest cushion plants in the world, they grow to over 12 feet across.
Any one square yard contains over 100,000 shoots,
so I guess this one cushion around me contains several million.
This rounded shape does more than just reduce wind chill.
The air temperature around me here at about 3,500 feet high,
is only a degree or so above freezing.
But if I put this temperature probe on the surface of this cushion,
I can see that there it is several degrees warmer.
The cushion, in fact, acts as a solar panel, absorbing heat directly from the sun,
so that even on very cold days,
providing it's not covered in snow and is exposed to direct sunshine,
it can photosynthesise and grow.
The plants that form these spectacular cushions come from several families,
sedges and rushes, daisies and dandelions.
One cushion may contain several species,
tightly packed together and growing to exactly the same height.
For one kind to grow higher than those around it would be suicidal.
In the New Zealand Alps,
one of these cushion-forming species also protects itself by developing a blanket of hair.
Its colonies form conspicuous white humps on the mountainside.
New Zealand farmers, whose flocks sometimes stray up onto these high slopes,
call such cushions vegetable sheep.
This tall pillar growing on Mount Kenya also covers itself in a blanket.
It's a giant lobelia.
Its long leaves are fringed with dense hairs.
Its flowers are hidden away from the frost beneath this downy covering.
There is no point having bright petals if they can't be seen, and these are just simple tubes.
But the lobelia's pollinator, a sunbird, knows where they are and how to reach them.
During the day, it can get quite warm, as Mount Kenya stands almost exactly on the equator,
but up here at 14,000 feet, once the sun goes down it gets bitterly cold.
Then the lobelia will have real need of its hairy blanket.
There are other giants here, too - tree-groundsels,
relatives of the little yellow weed that grows in European gardens.
They have a different way of dealing with the cold nights.
Their dead leaves remain attached to the stem, so that they act like lagging,
and prevent the liquids in the pipes running up inside the trunk from freezing solid.
Conditions here can change with extraordinary speed.
One moment the equatorial sun is blazing down from a cloudless sky,
the next, a chilling wind begins to blow and the great mountain collects a cloud cover.
As well as the tree-groundsel,
there's another member of the family that grows close to the ground like a cabbage.
As night falls, it makes its own preparations for surviving the bitter cold.
The most precious and vulnerable part of the plant
is the bud in its centre from which all growth comes.
That must be protected at all costs,
and folding the thick leaves over it does the trick.
The temperature has now fallen by as much as thirty degrees.
Water in the muddy swamps is beginning to freeze.
As it does so, it expands and the ground begins to heave.
It's impossible for small plants to keep a root-hold under these conditions.
The Mount Kenya moss doesn't even try.
It grows into balls that are lifted up by the ice pinnacles and it rolls around during the night.
The sun returns, the temperature rockets upwards,
the threat of death by freezing has passed, at least for now.
The cabbage groundsels stretch out their leaves to catch the light
and start making food once more.
The ice in the swamps melts, and the streams flow again.
This is just as well, for now the plants, baking under the sun,
are losing a lot of water by evaporation from their leaves.
Excessive water loss, of course,
is the other great disaster that can kill the hardiest of plants.
If the sap-filled vessels in the trunks of the tree-groundsels had frozen,
their leaves would now be baked dry.
Here we are still in Africa, but about 14,000 feet lower down.
I'm on the southern edge of the Namib Desert.
Here, plants can't get water, not because it's frozen,
but because rain hardly ever falls, only about one or two inches in a year.
Most of the time, it's bone-dry and devastatingly hot.
Yet, almost unbelievably, there are trees standing out in the sands, totally unsheltered,
with no sign of moisture anywhere around them.
Water storage is the great trick here.
These green, succulent leaves are full of it,
and so are these bloated branches.
The local bushmen used to cut off these branches,
hollow out the spongy tissue and use them as containers for arrows,
which is why this tree is called the quiver tree.
Its branches are covered with a blindingly white powder which reflects the heat,
and its leaves have thick rinds with very few pores,
which minimises the amount of water they lose through evaporation.
The trunk, even of an old tree ravaged by the years,
remains smooth and impermeable.
But even the quiver tree can't seal itself off totally from its surroundings.
Living involves breathing, and some water vapour is inevitably lost in that process.
But this tree has a way of reducing that:
self-amputation.
It can cut off a leaf rosette and seal the stump.
This branch will never grow leaves again,
but with luck, the tree will just survive with a reduced number of leaves
and put out new shoots when conditions improve.
Most of the plants in this desert, however, are less conspicuous,
and there are rather more of them than you might suppose.
This little plant has fused its leaves together in pairs
to form a set of cones, which is why it's called conophytum.
The white surface of each cone is the skin of last year's leaf.
The plant is now waiting for the rains to arrive.
Let's see what happens if I make them arrive just a little early.
One of the greatest of all water reservoirs
is held by the saguaro cactus that grows in Arizona and New Mexico.
One of these giants can hold several tons of liquid.
They don't risk losing precious water through the leaves, for they have none.
Instead, the task of making food has been taken over by the stem,
which has become green with chlorophyll and keeps its pores well protected in grooves.
The 50-foot-high columns are crowned with tufts of flowers.
The pleats in the trunks enable the plants to expand rapidly
and suck up rain falling in a sudden storm before it evaporates in the heat and disappears.
Such a store is very precious. Lots of desert animals would raid it if they could.
They can't because cacti, like desert succulents everywhere, defend themselves with spines.
The other way of protecting yourself against robbers is to hide underground.
You might think that these are pebbles. You would be wrong.
This is a window plant.
These little studs are the flat tops of the pillar-like leaves.
And these tops are transparent. They allow the light to pass through them,
where it's transmitted by a series of lined-up crystals
down to the bottom of the leaf, where there's green pigment.
So, although this little plant is several inches under the ground,
it can catch the sunlight and turn it into food.
And in the driest times of all, when sandstorms blow across the Namib,
it may be covered up completely.
Many plants take refuge underground at the hottest time of year,
and survive as bulbs and tubers,
swollen with their stores of food and water gathered during the good times.
Underground is undoubtedly the coolest place to be, but it's not necessarily the safest.
Mole rats.
They burrow ceaselessly, searching at random for their food.
Some bulbs they eat immediately,
but others they take away down their tunnels and stack in special larders.
Being carried away and put in store is not necessarily a disaster for the plants.
Mole rats seldom eat all their reserves,
and as they extend their burrows, more distant larders may get forgotten.
Then the bulbs will sprout, and benefit from a new location.
This plant is totally dead.
It didn't store its food underground in bulbs.
Instead, it adopted a very different and drastic strategy.
It condensed its entire life into a few short weeks.
And its last act was to release into the sand a few hundred seeds.
They're easy enough to find.
And there are some.
They can wait here in this hot sand, apparently lifeless,
for one year, two years, even twenty years.
But when the rains do come, their moment arrives.
One day the land is so dry the withered plants crunch to pieces underfoot.
Two or three weeks later and it's ablaze.
Arid lands around the world, not only here in South Africa,
but in Australia and Arizona,
all respond to rain by rapidly producing dazzling displays of colour.
The sudden flush of flowers and leaves attracts lots of plant-eaters.
For them, too, the pressures of desert living are momentarily relaxed.
It may seem a paradox that some of the harshest environments
should produce such unrivalled glories,
but the desert soil will not remain moist for long,
and in that short time, plants must not only grow leaves but produce seeds,
so the need for pollination is urgent.
Those with the most brilliant flowers have the best chance of attracting an insect.
This is competitive advertising at its most intense.
So, a few days of rain once every year or so
are enough to enable plants to survive in some of the driest places on earth.
And this is one of the wettest places on earth.
Here it rains almost every day, and sometimes for days on end.
I'm in South America, on the top of an immense sandstone plateau
9,000 feet high, five miles across, surrounded by huge vertical cliffs.
This is Mount Roraima.
Plants cut off up here from the hot rainforest below
adapt to their surroundings in their own individual way,
so there are species here that occur nowhere else in the world.
The near-continuous rains produce torrents that cascade over the edge of the plateau
and form some of the highest waterfalls on earth.
Much of this extraordinary landscape is naked rock.
Only here and there do clumps of plants get a root-hold,
and even when they succeed, life is very difficult for them.
Their main problem is lack of nutrients.
These rushing streams wash away everything in their path,
and are flowing over bare rock.
Only in a few places does a little gravely sediment accumulate.
So many of the plants that grow up here have to have ways of augmenting their food.
And some of them do it by eating animals.
This is about the simplest way in which a plant can catch and eat an insect.
This is the marsh pitcher,
and this particular species lives only on Mount Roraima.
There are four others, but they, too, live on other mountains just near here,
and nowhere else in the world.
And this is how they do it.
The leaf, joined at its margins to form a tube, is covered by downward-pointing hairs.
Easy to slide down, very difficult to climb up.
One slip and it's drowning and dissolution for the insect...
..and then digestion by the plant.
The pond in a bromeliad is normally a safe haven for aquatic insects,
but a bladderwort is hunting inside Roraima's bromeliads.
Not content with prey in this pond,
the bladderwort is looking for new hunting grounds elsewhere.
It explores with long, sensitive tendrils.
It has found another bromeliad...
..and descends into this new territory.
Now, it prepares to hunt.
Its traps, the bladders from which it gets its name, are tiny capsules.
Glands inside them extract water, so creating a partial vacuum.
Each bladder has a little door fringed with bristles.
A mosquito larva has only to touch one of these triggers,
and the door will implode and sweep the prey inside.
The glands pump out water,
so, at one and the same time, the bladderwort starts its meal and re-sets its trap,
which is ready for another customer within two hours.
Roraima also has sundews.
Like sundews elsewhere, they catch insects in a way that is a family speciality.
The sparkling drops on the leaf hairs are not sweet,
but insects nonetheless are attracted to them.
They are, however, extremely sticky, as any inquisitive insect immediately discovers.
The hairs move swiftly. One can turn through 180 degrees in less than a minute.
So even though an insect may have been caught by only one or two hairs,
others nearby quickly fold over it, and soon it is held fast.
The sundews on Roraima, like the bladderwort and the carnivorous pitcher,
occur only on these plateaus.
Indeed, about a third of the species on the mountain evolved here
and are found nowhere else.
The water that sluices over these rocks has caused problems for Roraima's plants
by preventing the accumulation of nutrients.
After it leaves the mountain, it joins the biggest of all river systems, the Amazon,
and then, lying in swamps and lakes, it will create a different set of difficulties.
But again, there are plants that have solved them.
Access to light is the great problem here.
Those plants that can command the surface can rule the lake.
And none does so on a greater scale and more aggressively than this,
the giant Amazon water-lily.
Its gigantic leaves are armoured with spines
that protect them against any fish that might try to make a meal of them.
Their huge expanse is kept outstretched and floating on the surface
by a lattice of buoyant, air-filled struts.
The crinkles in the surface swiftly flatten out as the leaf expands to its full size.
The edges are turned up so that the leaf can shoulder aside any competition.
Fully grown, a single leaf is six feet across.
Virtually no other plants can live in the black, shaded water beneath these leaves.
They cover the surface so completely, and the support of their air-filled girders is so effective,
that birds, most famously the lily-trotter,
can spend their entire lives walking around on them, collecting insects.
The giant lily's flowers are on an equally monumental scale.
They're about a foot across.
The life of any one bloom is short.
It opens in the evening and gives off a strong perfume.
During the night it closes,
and it stays closed for the whole of the next day, slowly flushing pink.
On its second evening, it opens again.
Then it closes for the last time.
Why does it behave in this extraordinary way?
It's a neat way of avoiding any chance of being fertilised by its own pollen.
The perfume it produces on its first evening attracts beetles.
They bring with them pollen from other lilies, so this flower is about to be fertilised.
But then the lily closes its petals.
The beetles will be held captive inside for 24 hours.
The following evening, the beautiful prison opens its gates and the inmates are free to go.
The flower has showered the beetles with its own pollen during their long stay.
Now red and odourless, the flower is no longer attractive to beetles,
so they'll go in search of white flowers on another plant,
carrying the pollen and bringing about cross-fertilisation.
Its mission completed, the flower withdraws back to its watery world.
As swiftly-flowing streams enter the still water of a lake,
so they slow down and shed their load of sediment.
So, day after day, the lake fills up.
As the waters get shallower, so it becomes possible for different, bigger plants to grow.
The trees in the forefront of this invasion here in the southern United States
are likely to be swamp cypresses.
The bases of their trunks are broad and cone-shaped,
so that they are able to squat firmly on the lake floor.
But they also have a way of creating an ever-widening platform for themselves,
and getting a head start on their competitors.
Mud will be deposited wherever the current that is carrying it in suspension slows down.
The cypresses encourage that to happen around them
by growing their roots into a maze of flanges and spires.
But the problems of a freshwater swamp are tiny
compared with those of the salty swamps of the coast, where the mangroves live.
Here I'm closer to the sea and the ground is even more unstable.
So the mangroves that grow here have to take more extreme measures to stand upright,
and they develop this great tangle of prop-roots.
Twice in every 24 hours, their land is invaded by the sea.
Estuary mud is particularly fine and sticky. Aerating it is virtually impossible,
and when the tide is out, the mangroves breathe through large pores on their arching prop-roots.
But now, when the tide is in, they can't do that.
They have no alternative but, in effect, to hold their breath for several hours.
Eventually, the tide begins to turn,
and as the water ebbs away, the mangroves slowly begin to breathe again.
Submersion is longest at the very edge of the sea.
It's the first part of the swamp to be covered, and the last to be exposed.
Here, the mangroves sprout fields of snorkels,
each carrying pores through which the roots can take in air.
It's especially tricky for young plants to get started here.
The adult trees deal with that hazard
by keeping hold of their young until the very last moment.
This long spike, green though it is, is in fact a root.
The seed has germinated while it's still attached to the tree.
And now the young plant is about to stake its claim for territory in a quite literal way.
A shoot that falls when the tide is out has a chance of sticking in the mud.
If the tide is in and the water deeper, then the shoot won't reach the bottom.
But all is by no means lost. On the contrary. The young plant floats away.
Like this it may be carried into a different estuary.
There, when the tide goes out, it may, with luck, snag its tip in the mud.
So it may end up far away from its parents
and colonise newly-formed mudflats on the very margins of the sea.
Rocky coasts present plants with yet other problems.
The rocks are firm enough - the perils come from the pounding waves and surging currents.
No flowering plant has evolved a solution to the difficulties of living here,
but algae have.
They have the simplest structure of all plants, they've never developed rigid stems,
but in the sea, the water itself provides support.
Their holdfasts grip the rock so firmly that if the current does rip them up,
it's likely to be the rock that breaks, not the plant.
They have long, cable-like stems that are rubbery and flexible but immensely strong.
The great blades in which they manufacture their food
are kept near the sunlight by huge, gas-filled floats.
Such algae can reach immense lengths.
They can grow in waters almost 100 feet deep,
but because they stream out in the current, their total length can be several times that.
One species has fronds that measure over 300 feet,
about as long as the tallest of land-living trees.
These thickets can with justice be regarded as the marine equivalent of the terrestrial forests.
Farther out to sea, the water becomes so deep
that even these giant algae can't maintain a hold on the sea floor and still reach the light.
The open water of the deep ocean is the domain of the simplest plants of all,
the microscopic single-celled algae.
These, perhaps the least considered by humanity of all plants,
have the four essentials of life in abundance.
The water around them never drops more than a degree or so below freezing,
they're always within reach of sunlight, and they're constantly provided with nutrients
as currents bring up rich ooze on an immense scale.
And they've colonised not only salt water, but fresh.
These simple plants are the basis of all life in water,
just as higher plants are the basis of all life on land.
Two-thirds of the surface of this planet is covered by water
and most of it is beyond the reach of flowering plants.
So floating algae in the seas and the lakes
play a greater part in enriching our atmosphere with oxygen
than all the land-based plants put together.
So we end as we began, with the simplest of plants, algae.
Plants, whether very simple
or highly complex, like these growing in the rainforest along the coast of tropical Australia,
have colonised the whole planet.
They live not only in such favourable environments as these
but on frozen rocks and gravels of the polar lands
and in the searingly hot sands of the deserts.
They've developed ways of surviving fire and hurricanes.
They can withstand the attacks of animals
and even, on occasion, find ways of eating animals themselves.
But one thing plants can't withstand, and that's the determined onslaught of humans.
Ever since we arrived on this planet as a species,
we've cut them down, dug them up, burnt them and poisoned them.
Today we're doing so on a greater scale than ever.
Even this small, precious patch of rainforest in northern Queensland is under threat.
We destroy plants at our peril.
Neither we nor any other animal can survive without them.
The time has now come for us to cherish our green inheritance, not to pillage it.
For without it, we will surely perish.