0:00:42 > 0:00:49No part of the earth is more hostile to life than the frozen wastes around the Poles.
0:00:49 > 0:00:54850 miles north of the Arctic Circle, this is Ellesmere Island.
0:00:54 > 0:00:58No animal can live permanently on these ice fields
0:00:58 > 0:01:01and even plants face big problems,
0:01:01 > 0:01:06for the four things they must have are in crippingly short supply.
0:01:06 > 0:01:08Water?
0:01:08 > 0:01:11Yes, there's a lot of frozen water here,
0:01:11 > 0:01:16but water has to be liquid for plants to make any use of it.
0:01:16 > 0:01:21Nutrients? There's virtually none in this frost-shattered rock.
0:01:21 > 0:01:25Warmth and light? For six months of the year it's dark,
0:01:25 > 0:01:30and in the brief summer, as now, the sun doesn't rise high,
0:01:30 > 0:01:37and devastating winds can carry away what little warmth it brings. Yet, there ARE plants here.
0:01:37 > 0:01:42Some live actually INSIDE the rock.
0:01:47 > 0:01:53This thin green line is made by algae — microscopic plants.
0:01:53 > 0:01:59They're so small, they can live BETWEEN the grains of this sandstone,
0:01:59 > 0:02:04and there, at least, they're out of this desiccating wind.
0:02:04 > 0:02:09On the surface of the rocks, there are lichens.
0:02:09 > 0:02:15They grow incredibly slowly and may take 50 years to cover a square cm,
0:02:15 > 0:02:22but they can survive even if there are only two days a year when it's warm enough for them to grow.
0:02:22 > 0:02:28In spite of these bleak conditions, there ARE flowers to be found here.
0:02:28 > 0:02:31But you have to look hard to find them.
0:02:43 > 0:02:46Here's one.
0:02:46 > 0:02:48It's a mustard —
0:02:48 > 0:02:52much smaller than its more southerly relatives,
0:02:52 > 0:02:56so it can keep out of the crippling wind.
0:03:01 > 0:03:04In midsummer, for a few weeks,
0:03:04 > 0:03:08enough water melts from the glaciers for streams to flow,
0:03:08 > 0:03:12and then, miniature gardens burst into bloom.
0:03:21 > 0:03:27The searing wind compels them all to keep close to the ground.
0:03:41 > 0:03:46None keeps closer than this. It is, in fact, a tree — a willow.
0:03:46 > 0:03:50These are its catkins.
0:03:50 > 0:03:52But the trunk grows horizontally,
0:03:52 > 0:04:00and it can stretch almost as far along the ground as its more southerly relatives stand above it.
0:04:05 > 0:04:11Even so, it still produces enough leaves to sustain a few grazers —
0:04:11 > 0:04:13musk ox.
0:04:24 > 0:04:26Nothing is wasted up here —
0:04:26 > 0:04:32not a moment of sunshine, not the tiniest shelter, not a scrap of food.
0:04:32 > 0:04:39When a musk ox dies, its decaying body releases a rich flush of nourishment into the soil,
0:04:39 > 0:04:43and tiny gardens appear, in the shelter of its bones.
0:04:45 > 0:04:48The Arctic poppy,
0:04:48 > 0:04:51like all plants, needs warmth to grow,
0:04:51 > 0:04:55but it is unusually efficient at collecting it.
0:04:55 > 0:04:59As the midsummer sun skims round the horizon —
0:04:59 > 0:05:03360 degrees in 24 hours without setting —
0:05:03 > 0:05:08the poppy turns its flowers to track it.
0:05:14 > 0:05:19The slanting sun may not be strong, but it is, at least, continuous
0:05:19 > 0:05:22during the few weeks of high summer.
0:05:34 > 0:05:39The heat the poppy gathers by staring continuously at the sun
0:05:39 > 0:05:45enables seeds to develop in each flower before summer comes to an end
0:05:45 > 0:05:49and the sun disappears below the horizon for months.
0:06:02 > 0:06:08Conditions may be just as severe on the high peaks of the Alps,
0:06:08 > 0:06:122,000 miles to the south, at least during the winter.
0:06:12 > 0:06:17But here, spring brings a greater benefit.
0:06:17 > 0:06:24The sun rises higher in the sky and is warm enough to melt all but the highest snowfields.
0:06:27 > 0:06:32As it melts, it reveals the snowbell, already in flower.
0:06:42 > 0:06:46The plant formed its flower buds last autumn,
0:06:46 > 0:06:51before the increasing cold shut down all its activities for the winter.
0:06:51 > 0:06:57The buds remained dormant until the spring sunshine, through the snow,
0:06:57 > 0:07:03triggered them into opening even before the snow had melted.
0:07:15 > 0:07:20In summer, the high meadows, newly freed from snow,
0:07:20 > 0:07:22fill with flowers.
0:07:26 > 0:07:33Because for so much of the time it's so cold, the vegetation here decays only very slowly,
0:07:33 > 0:07:36so a peaty soil forms.
0:07:36 > 0:07:40But it's only a thin layer over solid rock and boulders
0:07:40 > 0:07:45and trees find it very difficult to get root.
0:07:45 > 0:07:49Also, avalanches regularly sweep these slopes,
0:07:49 > 0:07:53carrying away saplings before they're established.
0:07:57 > 0:08:04So, shallow-rooted plants have these parts of the mountains largely to themselves
0:08:04 > 0:08:08and in summer, they bring a rich display of colour.
0:08:12 > 0:08:19But for every thousand feet you climb, the average temperature drops by about three degrees.
0:08:19 > 0:08:25Plants living in the high mountains must be able to survive extreme cold.
0:08:25 > 0:08:30It's very important to keep out of the worst of the chilling winds
0:08:30 > 0:08:34and many plants here form small rounded humps,
0:08:34 > 0:08:39and that brings them a number of advantages.
0:08:41 > 0:08:46Growing into the shape of a cushion is a good way of conserving heat
0:08:46 > 0:08:51and no plants do it better than these in the mountains of Tasmania.
0:08:51 > 0:08:55These are the largest cushion plants in the world.
0:08:55 > 0:08:58They grow to over 12 feet across.
0:08:58 > 0:09:06Any one square yard contains over 100,000 shoots, so this one cushion around me contains several million.
0:09:25 > 0:09:30This rounded shape does more than just reduce wind-chill.
0:09:30 > 0:09:35The air temperature around me here, at about 3,500 feet high,
0:09:35 > 0:09:38is only a degree above freezing.
0:09:38 > 0:09:43But if I put this temperature probe on the surface,
0:09:43 > 0:09:47I can see that there it is several degrees warmer.
0:09:47 > 0:09:52The cushion acts as a solar panel, absorbing heat from the sun
0:09:52 > 0:09:58so that, even on very cold days, provided it's not covered with snow,
0:09:58 > 0:10:01it can photosynthesise and grow.
0:10:02 > 0:10:09The plants that form these spectacular cushions come from several different families —
0:10:09 > 0:10:13sedges and rushes, daisies and dandelions.
0:10:13 > 0:10:20One cushion may contain several species, tightly packed together and growing to the same height.
0:10:20 > 0:10:26For one kind to grow higher than those around it would be suicidal.
0:10:27 > 0:10:33In the New Zealand Alps, one of these cushion-forming species
0:10:33 > 0:10:37also protects itself by developing a blanket of hair.
0:10:43 > 0:10:48Its colonies form conspicuous white humps on the mountainside.
0:10:48 > 0:10:53New Zealand farmers, whose flocks can stray up onto these slopes,
0:10:53 > 0:10:57call such cushions "vegetable sheep".
0:11:02 > 0:11:07This tall pillar, growing on Mount Kenya,
0:11:07 > 0:11:09also covers itself in a blanket.
0:11:09 > 0:11:12It's a giant lobelia.
0:11:13 > 0:11:18Its long leaves are fringed with dense hairs.
0:11:19 > 0:11:24Its flowers are hidden away from the frost beneath this downy covering.
0:11:24 > 0:11:28Bright petals are no use if they can't be seen,
0:11:28 > 0:11:31and these are just simple tubes.
0:11:33 > 0:11:39The lobelia's pollinator, a sunbird, knows where they are and how to reach them.
0:11:40 > 0:11:44During the day, it can get quite warm,
0:11:44 > 0:11:48for Mount Kenya stands almost exactly on the equator.
0:11:48 > 0:11:54But at 14,000 feet, once the sun goes down, it gets bitterly cold
0:11:54 > 0:11:59and then the lobelia will have real need of its hairy blanket.
0:12:00 > 0:12:05There are other giants here too — tree groundsels,
0:12:05 > 0:12:10relatives of the little yellow weed in European gardens.
0:12:10 > 0:12:14They have a different way of dealing with the cold nights.
0:12:15 > 0:12:21Their dead leaves remain on the stem, so that they act like lagging
0:12:21 > 0:12:26and prevent the liquids in the pipes inside the trunk from freezing solid.
0:12:29 > 0:12:34Conditions here can change with extraordinary speed.
0:12:34 > 0:12:39One moment the equatorial sun is blazing down from a cloudless sky,
0:12:39 > 0:12:46the next, a chilling wind begins to blow and the great mountain collects a cloud cover.
0:13:06 > 0:13:08As well as the tree groundsel,
0:13:08 > 0:13:14there's another groundsel that grows close to the ground like a cabbage.
0:13:14 > 0:13:20As night falls, it makes its own preparations for surviving the bitter cold.
0:13:24 > 0:13:32The most precious and vulnerable part of the plant is the bud in its centre from which all growth comes.
0:13:32 > 0:13:35That must be protected at all costs,
0:13:35 > 0:13:40and folding the thick leaves over it does the trick.
0:13:41 > 0:13:46The temperature has now fallen by as much as 30 degrees.
0:13:46 > 0:13:51Water in the muddy swamps is beginning to freeze. As it does so,
0:13:51 > 0:13:55it expands and the ground begins to heave.
0:14:00 > 0:14:06It's impossible for small plants to remain rooted under these conditions.
0:14:06 > 0:14:10The Mount Kenya moss doesn't even try.
0:14:10 > 0:14:17It grows into balls that are lifted up by the ice pinnacles and it rolls around during the night.
0:14:24 > 0:14:27The sun returns, the temperature rises,
0:14:27 > 0:14:31the threat of death by freezing has passed,
0:14:31 > 0:14:36and the cabbage groundsels stretch out their leaves to catch the light
0:14:36 > 0:14:39and start making food once more.
0:14:59 > 0:15:02The ice in the swamps melts
0:15:02 > 0:15:04and the streams flow again.
0:15:11 > 0:15:18This is just as well, for now the plants, baking under the sun, are beginning to lose a lot of water
0:15:18 > 0:15:21by evaporation from their leaves.
0:15:21 > 0:15:27And severe water loss is the other disaster that can kill hardy plants.
0:15:27 > 0:15:32If the sap-filled vessels in the tree groundsels' trunks had frozen,
0:15:32 > 0:15:35their leaves would now be baked dry.
0:15:41 > 0:15:44Here we are...still in Africa,
0:15:44 > 0:15:48but about 14,000 feet lower down.
0:15:48 > 0:15:53I'm on the southern edge of the Namib Desert.
0:15:53 > 0:15:59Here, plants can't get water, not because it's frozen, but because rain hardly ever falls —
0:15:59 > 0:16:03only about one or two inches a year.
0:16:03 > 0:16:06Most of the time, it's bone dry...
0:16:06 > 0:16:09and devastatingly hot.
0:16:12 > 0:16:18Yet, almost unbelievably, there are trees standing out in the sands,
0:16:18 > 0:16:23totally unsheltered, with no signs of moisture anywhere around them.
0:16:24 > 0:16:27Water storage is the trick here.
0:16:27 > 0:16:31These green succulent leaves are full of it,
0:16:31 > 0:16:33and so are these bloated branches.
0:16:33 > 0:16:41The local bushmen used to hollow out these branches and use them as containers for their arrows,
0:16:41 > 0:16:45which is why this tree is called the quiver tree.
0:16:45 > 0:16:52Its branches are covered with a blindingly white powder which reflects the heat,
0:16:52 > 0:16:56and its leaves have thick rinds with few pores
0:16:56 > 0:17:00which minimises water loss through evaporation.
0:17:04 > 0:17:07The trunk, even of an old tree,
0:17:07 > 0:17:11remains smooth and impermeable.
0:17:11 > 0:17:16But even the quiver tree can't seal itself off completely.
0:17:16 > 0:17:22Living involves breathing and some water vapour is inevitably lost in that process.
0:17:22 > 0:17:26But this tree has a way of reducing that.
0:17:26 > 0:17:32Self-amputation. It can cut off a leaf rosette and seal the stump.
0:17:32 > 0:17:36This branch will never grow leaves again.
0:17:36 > 0:17:41The tree will just survive with fewer leaves
0:17:41 > 0:17:46and put out new shoots when conditions improve.
0:17:46 > 0:17:51Most of the plants in this desert, however, are less conspicuous,
0:17:51 > 0:17:56and there are rather more of them than you might suppose.
0:17:56 > 0:18:01This little plant has fused its leaves together in pairs
0:18:01 > 0:18:05to form cones, which is why it's called Conophytum.
0:18:05 > 0:18:10The white surface of each cone is the skin of last year's leaf,
0:18:10 > 0:18:15and the plant is now waiting for the rains to arrive.
0:18:15 > 0:18:19Let's see what happens if I make them arrive earlier.
0:19:11 > 0:19:16One of the greatest of all water reservoirs is the saguaro cactus
0:19:16 > 0:19:19that grows in Arizona and New Mexico.
0:19:19 > 0:19:24One of these giants can hold several tons of liquid.
0:19:24 > 0:19:29They don't risk losing any water through the leaves — they have none.
0:19:29 > 0:19:34Instead, the task of making food has been taken over by the stem
0:19:34 > 0:19:41which has become green with chlorophyll and keeps its pores well-protected in grooves.
0:19:48 > 0:19:52The 50-foot columns are crowned with flowers.
0:19:52 > 0:19:57The pleats in the trunks enable the plants to expand rapidly
0:19:57 > 0:20:04and suck up rain falling in a sudden storm before it evaporates in the heat and disappears.
0:20:07 > 0:20:14Such a store of liquid is very precious. Lots of desert animals would raid it if they could.
0:20:14 > 0:20:21They can't because cacti, like other desert succulents, defend themselves with spines.
0:20:25 > 0:20:29The other way of protecting yourself against robbers
0:20:29 > 0:20:31is to hide underground.
0:20:38 > 0:20:42You might THINK that these are pebbles.
0:20:42 > 0:20:44You would be wrong.
0:20:47 > 0:20:51This...is a window plant.
0:20:51 > 0:20:56These little studs are the flat tops of the pillar-like leaves.
0:21:00 > 0:21:06And these tops are transparent. They allow the light to pass through
0:21:06 > 0:21:13where it's transmitted by a row of crystals to the bottom of the leaf where there's green pigment.
0:21:13 > 0:21:18So although this little plant is several inches under the ground,
0:21:18 > 0:21:23it can catch the sunlight and turn it into food.
0:21:25 > 0:21:31And in the driest times of all, when sandstorms blow across the Namib,
0:21:31 > 0:21:33it may be covered up completely.
0:21:33 > 0:21:39Many plants take refuge underground during the hottest part of the year
0:21:39 > 0:21:44and survive as bulbs and tubers, swollen with food and water stores,
0:21:44 > 0:21:47gathered during the good times.
0:21:49 > 0:21:54Underground is undoubtedly the coolest place to be,
0:21:54 > 0:21:57but it's not necessarily the safest.
0:22:03 > 0:22:05Mole rats.
0:22:08 > 0:22:11They burrow ceaselessly,
0:22:11 > 0:22:14searching at random for their food.
0:22:16 > 0:22:20Some of the bulbs they eat immediately,
0:22:20 > 0:22:25but others they take away and stack in special larders.
0:22:52 > 0:22:59Being carried away and put in store is not necessarily a disaster for the plants.
0:22:59 > 0:23:03The mole rats seldom eat all their reserves
0:23:03 > 0:23:06and some larders get forgotten.
0:23:06 > 0:23:12Then the bulbs sprout and benefit from doing so in a new location.
0:23:15 > 0:23:18This plant is totally dead.
0:23:18 > 0:23:25It didn't store its food underground in bulbs. It adopted a very different and very drastic strategy.
0:23:25 > 0:23:30It condensed its entire life into a few short weeks.
0:23:30 > 0:23:35And its last act was to release into the sand a few hundred seeds.
0:23:35 > 0:23:37They're easy enough to find.
0:23:48 > 0:23:50And there are some.
0:23:50 > 0:23:56They can wait here, in this hot sand, apparently lifeless, for years...
0:23:56 > 0:23:58even 20 years.
0:23:58 > 0:24:02But when the rains DO come, their moment arrives.
0:24:14 > 0:24:21One day, the land is so dry that the withered plants crunch to pieces underfoot.
0:24:21 > 0:24:26Two or three weeks later... and it's ablaze.
0:24:36 > 0:24:41Arid lands around the world, not only here in South Africa,
0:24:41 > 0:24:46but in Australia and Arizona, all respond to rain
0:24:46 > 0:24:50by rapidly producing dazzling displays of colour.
0:25:22 > 0:25:28The sudden flush of flowers and leaves attracts lots of plant-eaters.
0:25:28 > 0:25:34For them, too, the pressures of desert-living are momentarily relaxed.
0:25:41 > 0:25:49It may seem a paradox that some of the harshest environments should produce such unrivalled glories.
0:25:49 > 0:25:54But the desert soil will not remain moist for long after rain
0:25:54 > 0:25:59and in that short time, plants must grow leaves AND produce seeds.
0:25:59 > 0:26:03So the need for pollination is urgent.
0:26:03 > 0:26:08The most brilliant flowers have the best chance of attracting an insect.
0:26:08 > 0:26:12This is competitive advertising at its most intense.
0:26:16 > 0:26:21So, a few days of rain once every year or so
0:26:21 > 0:26:26are enough to enable plants to survive in the driest areas on earth.
0:26:28 > 0:26:30THUNDER
0:26:48 > 0:26:54And this is one of the wettest places on earth.
0:26:54 > 0:26:59Here, it rains almost every day and sometimes for days on end.
0:26:59 > 0:27:04I'm in South America, on the top of an immense sandstone plateau,
0:27:04 > 0:27:089,000 feet high, five miles across,
0:27:08 > 0:27:11surrounded by huge vertical cliffs.
0:27:11 > 0:27:14This is Mount Roraima.
0:27:19 > 0:27:23Plants cut off up here from the hot rainforest below
0:27:23 > 0:27:28adapt to their surroundings in their own individual way.
0:27:28 > 0:27:33So there are species here that occur nowhere else in the world.
0:27:33 > 0:27:39The rains produce torrents that cascade over the edge of the plateau
0:27:39 > 0:27:43and form some of the highest waterfalls on earth.
0:27:46 > 0:27:50Much of this extraordinary landscape is naked rock.
0:27:50 > 0:27:55Only here and there do clumps of plants manage to get a root-hold,
0:27:55 > 0:28:00and even when they succeed, life is difficult.
0:28:00 > 0:28:03Lack of nutrients is the big problem.
0:28:03 > 0:28:09Streams wash away everything in their path and flow over bare rock.
0:28:09 > 0:28:14Only in a few places does a little gravelly sediment accumulate.
0:28:14 > 0:28:19So many of the plants here have to have ways of augmenting their food.
0:28:19 > 0:28:23And some of them do it by eating animals.
0:28:25 > 0:28:28This is about the simplest way
0:28:28 > 0:28:32in which a plant can catch and eat an insect.
0:28:32 > 0:28:39This is the marsh pitcher and this particular species lives only on Mount Roraima.
0:28:39 > 0:28:45There are four others, which only live on other mountains near here.
0:28:45 > 0:28:47And this is how they do it.
0:28:49 > 0:28:52The leaf, in the shape of a tube,
0:28:52 > 0:28:55is covered by downward-pointing hairs —
0:28:55 > 0:29:00easy to slide down, very difficult to climb up. One slip,
0:29:00 > 0:29:05and it's drowning and dissolution for the insect.
0:29:05 > 0:29:09And then digestion by the plant.
0:29:12 > 0:29:16The pond in a bromeliad is usually safe for aquatic insects,
0:29:16 > 0:29:22but a bladderwort is hunting inside Roraima's bromeliads.
0:29:31 > 0:29:34Not content with prey in THIS pond,
0:29:34 > 0:29:39the bladderwort is looking for new hunting grounds elsewhere.
0:29:49 > 0:29:53It explores with long, sensitive tendrils.
0:29:59 > 0:30:02It has found another bromeliad.
0:30:06 > 0:30:09And descends into this new territory.
0:30:17 > 0:30:20Now it prepares to hunt.
0:30:32 > 0:30:38Its traps — the bladders from which it gets its name — are tiny capsules.
0:30:38 > 0:30:44Glands inside them extract water, so creating a partial vacuum.
0:30:54 > 0:30:58Each bladder has a little door fringed with bristles.
0:30:58 > 0:31:03A mosquito larva has only to touch one of these triggers
0:31:03 > 0:31:08and the door will implode and sweep the prey inside.
0:31:08 > 0:31:11The glands pump out water,
0:31:11 > 0:31:15so the bladderwort starts its meal
0:31:15 > 0:31:20and resets its trap which is ready for another customer in two hours.
0:31:28 > 0:31:33Roraima also has sundews. Like sundews elsewhere,
0:31:33 > 0:31:38they catch insects in a way that is a family speciality.
0:31:42 > 0:31:48The drops on the leaf hairs are not sweet, but still attract insects.
0:31:48 > 0:31:53They are, however, extremely sticky —
0:31:53 > 0:31:56as any inquisitive insect discovers.
0:32:00 > 0:32:06The hairs move swiftly. One can turn 180 degrees in less than a minute.
0:32:06 > 0:32:11So even though an insect may have been caught by only one or two hairs,
0:32:11 > 0:32:16others nearby quickly fold over it and soon it is held fast.
0:32:43 > 0:32:50The sundew species on Roraima, like the bladderwort and carnivorous pitcher,
0:32:50 > 0:32:52occur only on these plateaus.
0:32:54 > 0:33:01Indeed, about a third of the species on the mountain have evolved here and are found nowhere else.
0:33:02 > 0:33:10The water sluicing over these rocks has caused problems for Roraima's plants by washing away nutrients.
0:33:17 > 0:33:23After leaving the mountain, it joins the biggest river of all, the Amazon,
0:33:23 > 0:33:26and then, lying in swamps and lakes,
0:33:26 > 0:33:29it will create different problems.
0:33:29 > 0:33:33But again, there are plants that have solved them.
0:33:39 > 0:33:46Access to light is the great problem here. Those plants that can command the surface can rule the lake,
0:33:46 > 0:33:52and none does so on a greater scale and more aggressively than this —
0:33:52 > 0:33:55the giant Amazon water lily.
0:34:04 > 0:34:08Its gigantic leaves are armoured with spines
0:34:08 > 0:34:13that protect them against any fish that might try to eat them.
0:34:28 > 0:34:33Their huge form is kept outstretched and floating on the surface
0:34:33 > 0:34:38by a lattice of buoyant, air-filled struts.
0:34:43 > 0:34:48The crinkles in the surface swiftly flatten out
0:34:48 > 0:34:51as the leaf expands to its full size.
0:34:58 > 0:35:04The edges are turned up so that the leaf can shoulder aside any rivals.
0:35:06 > 0:35:10Fully grown, a single leaf is six feet across.
0:35:10 > 0:35:17Virtually no other plants can live in the black, shaded water beneath these leaves.
0:35:17 > 0:35:23They cover the surface so completely and their girders are so strong,
0:35:23 > 0:35:25that birds, like the lily-trotter,
0:35:25 > 0:35:31can spend their entire lives walking around on them, collecting insects.
0:35:31 > 0:35:37The giant lily's flowers are on an equally monumental scale.
0:35:37 > 0:35:39They're about a foot across.
0:35:51 > 0:35:54The life of any one bloom is short.
0:35:54 > 0:35:59It opens in the evening and gives off a strong perfume.
0:35:59 > 0:36:02During the night, it closes
0:36:02 > 0:36:07and it stays closed for the whole of the next day, slowly flushing pink.
0:36:07 > 0:36:11On its second evening, it opens again.
0:36:16 > 0:36:19Then it closes for the last time.
0:36:21 > 0:36:25Why does it behave in this extraordinary way?
0:36:25 > 0:36:31It's a way of avoiding any chance of being fertilised by its own pollen.
0:36:31 > 0:36:36The perfume it produces on its first evening attracts beetles.
0:36:40 > 0:36:44They bring pollen from other lilies,
0:36:44 > 0:36:48so this flower is about to be fertilised.
0:36:52 > 0:36:55But then, the lily closes its petals.
0:36:56 > 0:37:02The beetles will be held captive inside for 24 hours.
0:37:08 > 0:37:13The following evening, the beautiful prison opens its gates
0:37:13 > 0:37:15and the inmates are free to go.
0:37:22 > 0:37:27The flower has given the beetles its own pollen during their long stay.
0:37:29 > 0:37:34Now red and odourless, the flower is no longer attractive to beetles,
0:37:34 > 0:37:38so they'll search for white flowers on another plant,
0:37:38 > 0:37:43carrying the pollen and bringing about cross-fertilisation.
0:37:48 > 0:37:54Its mission completed, the flower withdraws back to its watery world.
0:38:08 > 0:38:14As swiftly-flowing streams enter the still water of a lake,
0:38:14 > 0:38:19so they slow down and shed their load of sediment.
0:38:19 > 0:38:23So, day after day, the lake fills up.
0:38:23 > 0:38:26As the water gets shallower,
0:38:26 > 0:38:31so it becomes possible for different, bigger plants to grow in it.
0:38:32 > 0:38:39The trees in the forefront of this invasion, here in the southern United States,
0:38:39 > 0:38:41are likely to be swamp cypresses.
0:38:41 > 0:38:48The bases of their trunks are broad and cone-shaped, so they can squat firmly on the lake floor.
0:38:53 > 0:38:58But they also make an ever-widening platform for themselves
0:38:58 > 0:39:02to get a head start on their competitors.
0:39:02 > 0:39:09Mud will be deposited wherever the current that is carrying it slows down.
0:39:09 > 0:39:16Cypresses encourage that to happen around them by growing their roots into flanges and spires.
0:39:16 > 0:39:20But the problems of a freshwater swamp are tiny
0:39:20 > 0:39:26compared with those of the coastal, salty swamps where mangroves live.
0:39:29 > 0:39:35EERIE ANIMAL NOISES Here, I am close to the sea
0:39:35 > 0:39:38and the ground is even more unstable.
0:39:38 > 0:39:43So the mangroves that grow here have to take more extreme measures
0:39:43 > 0:39:49in order to stand upright, and they develop this tangle of prop roots.
0:39:49 > 0:39:55Twice in every 24 hours, their land is invaded by the sea.
0:40:11 > 0:40:15Estuary mud is particularly fine and sticky.
0:40:15 > 0:40:19Aerating it is impossible and when the tide is out,
0:40:19 > 0:40:24the mangroves breathe through pores on their prop roots.
0:40:24 > 0:40:28But when the tide is IN, they can't do that.
0:40:28 > 0:40:32In effect, they hold their breath for several hours.
0:40:36 > 0:40:42Eventually, the tide begins to turn, and as the water ebbs away,
0:40:42 > 0:40:46the mangroves slowly begin to breathe again.
0:40:46 > 0:40:50Submersion is longest at the edge of the sea.
0:40:50 > 0:40:55It's the first part to be covered and the last to be exposed.
0:40:55 > 0:41:03Here the mangroves sprout fields of snorkels, each with pores through which the roots can take in air.
0:41:07 > 0:41:12It's especially tricky for young plants to get started here.
0:41:12 > 0:41:15The adult trees deal with that
0:41:15 > 0:41:20by keeping hold of their young until the very last moment.
0:41:40 > 0:41:46This long spike, green though it is, is, in fact, a root.
0:41:46 > 0:41:51The seed has germinated while it's still attached to the tree.
0:41:51 > 0:41:59And now, the young plant is about to stake its claim for territory in a quite literal way.
0:41:59 > 0:42:03A shoot that falls when the tide is out may stick in the mud.
0:42:09 > 0:42:14If the water is too deep, the shoot won't reach the bottom.
0:42:14 > 0:42:17But all is by no means lost.
0:42:17 > 0:42:20The young plant simply floats away.
0:42:21 > 0:42:26Like this, it may be carried into a different estuary.
0:42:28 > 0:42:31There, when the tide goes out,
0:42:31 > 0:42:37it may snag its tip in the mud. So it ends up far from its parents
0:42:37 > 0:42:42and colonises newly-formed mud flats on the very margins of the sea.
0:42:50 > 0:42:55Rocky coasts present plants with yet other problems.
0:42:55 > 0:43:02The rocks are firm enough. The perils are the pounding waves and the surging currents.
0:43:02 > 0:43:09No flowering plant has evolved a solution to the difficulties of living here. But algae have.
0:43:09 > 0:43:13They have the simplest structure of all plants.
0:43:13 > 0:43:20They've never developed rigid stems, but here, the water provides support.
0:43:20 > 0:43:23Their holdfasts grip the rock so firmly,
0:43:23 > 0:43:29that in a strong current, the rock's more likely to break than the plant.
0:43:36 > 0:43:39They have long, cable-like stems
0:43:39 > 0:43:43that are rubbery and flexible but immensely strong.
0:43:43 > 0:43:49The great blades in which they make their food are kept near sunlight
0:43:49 > 0:43:52by huge, gas-filled floats.
0:43:57 > 0:44:00Such algae can reach immense lengths.
0:44:00 > 0:44:04They can grow in waters almost 100 feet deep,
0:44:04 > 0:44:11but because they stream out in the current, their total length can be several times that.
0:44:13 > 0:44:18One species has fronds that measure over 300 feet,
0:44:18 > 0:44:22about as long as the tallest of land-living trees.
0:44:22 > 0:44:30These thickets can, with justice, be regarded as the marine equivalents of terrestrial forests.
0:44:30 > 0:44:34Farther out to sea, the water becomes so deep
0:44:34 > 0:44:41that even these giant algae can't maintain a hold on the sea-floor and still reach the light.
0:44:42 > 0:44:45The open water of the deep ocean
0:44:45 > 0:44:49is the domain of the simplest plants of all —
0:44:49 > 0:44:52the microscopic single-celled algae.
0:44:56 > 0:45:01These, perhaps the least considered by humanity of all plants,
0:45:01 > 0:45:05have the four essentials of life in abundance.
0:45:05 > 0:45:09The water around them never drops much below freezing,
0:45:09 > 0:45:13they are always within reach of sunlight,
0:45:13 > 0:45:18they're provided with nutrients as currents bring plenty of rich ooze,
0:45:18 > 0:45:22and they have colonised not only salt water, but fresh.
0:45:38 > 0:45:43These simple plants are the basis of all life in water,
0:45:43 > 0:45:47just as higher plants are the basis of all life on land.
0:45:47 > 0:45:55Two thirds of the earth's surface is covered by water — most of it is out of reach of flowering plants.
0:45:55 > 0:46:02So floating algae, in the seas and lakes, play a greater part in enriching our atmosphere with oxygen
0:46:02 > 0:46:06than all the land-based plants put together.
0:46:11 > 0:46:14So we end as we began —
0:46:14 > 0:46:18with the simplest of plants — algae.
0:46:18 > 0:46:22Between them, plants, whether simple or complex,
0:46:22 > 0:46:29like these growing in the rainforest on the coast of tropical Australia, have colonised the whole planet.
0:46:29 > 0:46:36They live, not only in favourable environments, but on frozen rocks of the Polar lands
0:46:36 > 0:46:40and in the searingly hot sands of the deserts.
0:46:40 > 0:46:44They've developed ways of surviving fire and hurricanes.
0:46:44 > 0:46:49They can withstand animal attacks and can even eat animals themselves.
0:46:49 > 0:46:52But one thing plants CAN'T withstand
0:46:52 > 0:46:56and that's the determined onslaught of human beings.
0:46:56 > 0:47:04Ever since we arrived on this planet, we have cut them down, dug them up, burnt them and poisoned them.
0:47:04 > 0:47:07Today, we're doing so more than ever.
0:47:07 > 0:47:14Even this small, precious patch of rainforest in northern Queensland is under threat.
0:47:18 > 0:47:21We destroy plants at our peril.
0:47:21 > 0:47:25Neither we nor any other animal can survive without them.
0:47:25 > 0:47:32The time has now come for us to cherish our green inheritance, not to pillage it.
0:47:32 > 0:47:36For without it, we will surely perish.
0:48:12 > 0:48:16Subtitles by Gillian Frazer BBC Scotland 1995