0:00:53 > 0:00:57The Great Barrier Reef, Australia, at night.
0:00:57 > 0:01:00I'm surrounded by corals.
0:01:02 > 0:01:06They do look extraordinarily like plants,
0:01:06 > 0:01:10branching into fans and twigs and bushes.
0:01:11 > 0:01:16At night, the similarity is particularly marked.
0:01:16 > 0:01:23All over their stony surface, tiny buds open into what look like flowers.
0:01:43 > 0:01:48But these structures don't behave in a flower-like way.
0:01:49 > 0:01:57They seize and eat any edible particle that drifts by. They are clearly animals.
0:01:57 > 0:02:01But even so, they look like plants. Why?
0:02:03 > 0:02:10It was only comparatively recently that we understood the answer in full detail,
0:02:10 > 0:02:14and it only becomes evident when the sun comes up,
0:02:14 > 0:02:19for then the corals change their behaviour in a radical way.
0:02:32 > 0:02:35Corals, like plants, must have light.
0:02:35 > 0:02:42They can't grow if the water is cloudy or the depths so great that the rays of the sun can't reach them.
0:02:42 > 0:02:46These resemblances are not just coincidences.
0:02:46 > 0:02:51If I go back underwater now, now that it's day and the sun is up,
0:02:51 > 0:02:58I shall see that many of these corals are feeding in a way that is not like animals at all.
0:03:08 > 0:03:13Now the plant-like form of the coral is even more obvious.
0:03:13 > 0:03:20The rosettes of groping arms have withdrawn into their stony sockets on the surface of the coral skeleton.
0:03:20 > 0:03:25But they're still within the reach of sunlight.
0:03:25 > 0:03:30And within their tiny bodies are microscopic green plants, algae,
0:03:30 > 0:03:34and they are feeding by making starches and sugars.
0:03:38 > 0:03:41But the corals are feeding too.
0:03:41 > 0:03:46They have partly digested the walls of these captive plants
0:03:46 > 0:03:53and 80% of the food the algae make leaks out of them and is consumed by the coral.
0:03:53 > 0:03:59Having dined on meat all night, the corals are now getting their vegetables.
0:03:59 > 0:04:07The corals provide their internal gardens with the best possible light by growing into these shapes,
0:04:07 > 0:04:14which is just what bushes do for their food factories, their leaves, when they grow in the same way.
0:04:14 > 0:04:19The coral algae do get some benefit from this arrangement.
0:04:19 > 0:04:26These glassy waters are very poor in nitrates and phosphates which algae need.
0:04:26 > 0:04:30Those substances are in the coral's waste.
0:04:30 > 0:04:34So the algae can absorb their fertiliser directly
0:04:34 > 0:04:39and live in waters that otherwise could not support them.
0:04:40 > 0:04:45Other animals on the reef also cultivate similar gardens.
0:04:46 > 0:04:52Giant clams keep their algae not inside their cells,
0:04:52 > 0:04:59but in special compartments just beneath the surface of the mantle that form long, brown lines.
0:04:59 > 0:05:06To give them the light they need, the clam has to open its shell wide, so exposing itself to danger,
0:05:06 > 0:05:13but the blue spots are sensitive to light and warn of unexpected shadows
0:05:13 > 0:05:17that might indicate an approaching threat.
0:05:17 > 0:05:22A few jellyfish maintain algal populations as well.
0:05:22 > 0:05:26These, in a lake on the Pacific island of Palau,
0:05:26 > 0:05:30pamper theirs in an extraordinary way.
0:05:32 > 0:05:40This lake is cut off from the sea by ramparts of coral limestone and there are very few fish here.
0:05:40 > 0:05:46So these jellyfish can't live, like most of their relations, by catching animal prey
0:05:46 > 0:05:51and their tentacles no longer carry stings for hunting.
0:05:51 > 0:05:56Instead, they have been converted into allotments for algae.
0:06:01 > 0:06:06The lake is surrounded by a tall forest growing on the limestone wall.
0:06:06 > 0:06:09The sun doesn't rise above the trees
0:06:09 > 0:06:12until several hours after dawn.
0:06:15 > 0:06:20But, at last, its rays strike the water at one end of the lake
0:06:20 > 0:06:26and there, several million jellyfish have assembled awaiting the sunlight.
0:06:29 > 0:06:32As the sun moves across the sky,
0:06:32 > 0:06:37so the vast fleet travels slowly towards the other side of the lake,
0:06:37 > 0:06:39keeping always in the sunshine.
0:06:41 > 0:06:48So reluctant are the jellyfish to leave the light that, on the edge of the shadow,
0:06:48 > 0:06:52they crowd together in a tightly-packed shoal.
0:07:06 > 0:07:10But without stings, the jellyfish are defenceless.
0:07:10 > 0:07:15Now, if they blunder into the arms of a sea anemone,
0:07:15 > 0:07:20they have no way of repelling the tentacles. They're eaten.
0:07:35 > 0:07:43The daytime voyage across the lake is not the only action the jellyfish take to nurture their algae.
0:07:43 > 0:07:47Come the evening, they swim down to the bottom.
0:07:47 > 0:07:51There the water is murky with decaying vegetable matter
0:07:51 > 0:07:56and there, in the night, the algae absorb the fertiliser they need.
0:08:01 > 0:08:06That animals should sometimes kidnap plants is not surprising.
0:08:06 > 0:08:13All animals, including ourselves, have always exploited plants in one way or another,
0:08:13 > 0:08:16directly or indirectly.
0:08:16 > 0:08:21It's more surprising that sometimes things are the other way round.
0:08:21 > 0:08:26Sometimes it's plants that keep animals for the plants' benefit.
0:08:27 > 0:08:33Here in the forests of Borneo, the rattan cane does just that.
0:08:34 > 0:08:39No plant benefits from being eaten, but most can't do much to stop it.
0:08:39 > 0:08:43Not so the rattan. Watch and listen.
0:08:45 > 0:08:51Out of a nest around the stem of the rattan, close to its tip,
0:08:51 > 0:08:54come angry ants.
0:08:54 > 0:08:57RHYTHMIC HISSING
0:09:01 > 0:09:05They're making this throbbing hiss
0:09:05 > 0:09:10by banging their heads synchronously against the rattan stem.
0:09:13 > 0:09:19These ants have oh! a particularly vicious bite,
0:09:19 > 0:09:21as I well know. Ow!
0:09:21 > 0:09:26And I certainly try to keep clear of them when I'm in the forest.
0:09:26 > 0:09:29I'm sure plant-eating animals do too.
0:09:30 > 0:09:33So when I, or they,
0:09:33 > 0:09:38hear this alarming noise, we try to steer clear of what's making it,
0:09:38 > 0:09:43and the rattan's tip, its most vulnerable part, remains undamaged.
0:09:51 > 0:09:57In Africa, there are a great number of very determined plant-eaters.
0:10:05 > 0:10:12Acacias protect themselves with spines, but they're by no means a total defence.
0:10:12 > 0:10:14Some animals are put off by them,
0:10:14 > 0:10:19but others, like the giraffe, seem able to ignore them.
0:10:31 > 0:10:36But a few acacias, like the rattan, have recruited ants as guards
0:10:36 > 0:10:39and provide them with special barracks
0:10:39 > 0:10:43the swollen bases of their thorns.
0:10:46 > 0:10:52One nibble from the giraffe is enough to bring out the defenders.
0:11:03 > 0:11:08They attack the animal's tongue and lips.
0:11:10 > 0:11:15Eventually, the irritation becomes too much.
0:11:15 > 0:11:21Even though there are a lot of good leaves left, the giraffe moves away.
0:11:23 > 0:11:28Several different acacias employ ants as defenders.
0:11:28 > 0:11:30As well as providing accommodation,
0:11:30 > 0:11:38the trees pay their security staff with a sugary nectar that wells up from little glands on their stems.
0:11:45 > 0:11:51This South American species rewards its ants even more extravagantly.
0:11:51 > 0:11:56It not only provides nectar for them, but packets of protein,
0:11:56 > 0:12:00little beads that grow on the tip of its leaflets.
0:12:05 > 0:12:08But these are not for the adults.
0:12:08 > 0:12:13They're special baby-food which the workers take back to their larvae.
0:12:21 > 0:12:27These infants are housed in the swollen bases of the thorns.
0:12:38 > 0:12:45The worker tucks the bead into a special pouch, just beneath the larva's jaws.
0:12:52 > 0:12:55Whenever the youngster wants a meal,
0:12:55 > 0:13:00it just bends its head down and takes a nibble.
0:13:11 > 0:13:15In return for these lavish provisions and amenities,
0:13:15 > 0:13:18the ants mount an energetic defence
0:13:18 > 0:13:22of the acacia, rushing to attack intruders.
0:13:25 > 0:13:32Any insect that lands on a tree, hoping to nibble a leaf or two, is soon dealt with.
0:13:45 > 0:13:50The ants even defend their tree against rival plants.
0:13:50 > 0:13:55Patrols go down the trunk and range for a long way over the earth.
0:13:57 > 0:14:04Seedlings that sprout within this area, so threatening to take some of the acacia's sustenance, are mauled.
0:14:04 > 0:14:08The ants aren't eating this plant.
0:14:08 > 0:14:11They're chewing it to death.
0:14:13 > 0:14:20The tendrils of any plant that reach over and try to climb onto the acacia
0:14:20 > 0:14:22get similar treatment.
0:14:22 > 0:14:27It's well worth the acacia's while to provide food and lodging
0:14:27 > 0:14:32for such a valiant and dedicated defence force.
0:14:37 > 0:14:41This plant is even more accommodating.
0:14:41 > 0:14:46It has inflated most of its stem into an ant mansion.
0:14:52 > 0:14:55It grows in New Guinea,
0:14:55 > 0:15:03clinging to the branches of other trees, and it's called, with good reason, an ant plant.
0:15:03 > 0:15:10Ants are continually running about on its surface on their way to, or returning from, a hunt for insects.
0:15:11 > 0:15:17The accommodation the plant provides for the ants is truly spacious
0:15:17 > 0:15:20and suited to their requirements.
0:15:20 > 0:15:23Immediately within its walls,
0:15:23 > 0:15:29a network of corridors ensures that the structure is air-conditioned,
0:15:29 > 0:15:33an essential for any well-appointed residence in the tropics.
0:15:44 > 0:15:47Farther inside,
0:15:47 > 0:15:49there are the nurseries
0:15:49 > 0:15:54smooth-walled chambers where the larvae are reared.
0:16:01 > 0:16:06And there are also special refuse tips.
0:16:06 > 0:16:10The workers dump the droppings of the colony.
0:16:16 > 0:16:18These chambers are not only middens,
0:16:18 > 0:16:21they are mortuaries
0:16:21 > 0:16:27the last resting place of members of the colony that die within the mansion.
0:16:33 > 0:16:37The chambers in which these bodies lie
0:16:37 > 0:16:40have walls covered with warts.
0:16:40 > 0:16:44These absorb nutrients from the rotting piles.
0:16:44 > 0:16:49This is how the plant collects its rent.
0:16:57 > 0:17:01Fungi may seem unlikely, even dangerous,
0:17:01 > 0:17:05organisms with which to form a partnership.
0:17:05 > 0:17:09After all, they do feed on plants.
0:17:09 > 0:17:12Fungi are neither animals nor plants.
0:17:12 > 0:17:16They're fundamentally different from either.
0:17:16 > 0:17:20They can dissolve all kinds of substances
0:17:20 > 0:17:23rock, metal, even plastic
0:17:23 > 0:17:27but most notably, they consume the bodies of plants,
0:17:27 > 0:17:30and these bracket fungi eat trees.
0:17:30 > 0:17:38We tend to notice them only when they provide spectacular structures like these their fruiting bodies.
0:17:39 > 0:17:46Spores fall from their underside in astronomical numbers millions a minute.
0:17:46 > 0:17:50Fungal spores exist pretty well everywhere.
0:17:50 > 0:17:54They may enter a tree through a wound in the bark.
0:17:54 > 0:18:02They then develop into threads that slowly move inwards and start to digest the wood.
0:18:02 > 0:18:06The tree now, as we would see it, has a rotten core.
0:18:10 > 0:18:15Eventually, after tens or even hundreds of years,
0:18:15 > 0:18:22a tree may have its interior completely eaten away by fungal threads, as has happened here.
0:18:22 > 0:18:27It's not as disastrous as it sounds. The fungus only consumes dead tissue.
0:18:27 > 0:18:31It leaves the living tissue untouched,
0:18:31 > 0:18:36and it survives as an outer cylinder from which all new growth comes
0:18:36 > 0:18:39and that's all that the tree needs.
0:18:45 > 0:18:51So although this 800-year-old oak in Windsor Great Park is completely hollow,
0:18:51 > 0:18:54it's still thriving.
0:18:54 > 0:18:59Every year it puts out a fresh crown of green leaves
0:18:59 > 0:19:03and I guess it's got many more years of life in it yet.
0:19:06 > 0:19:11The change of form brings a positive advantage to the old tree.
0:19:11 > 0:19:17A hollow cylinder is better able to absorb great shocks than a solid pillar.
0:19:17 > 0:19:25Trees standing out in the open, as they do in parks, can get severely buffeted by stormy winds,
0:19:25 > 0:19:30and it's not unusual after a gale to see young oaks uprooted,
0:19:30 > 0:19:37whereas older ones, with the age and the girth to become hollow, are still standing.
0:19:39 > 0:19:44The surgery performed by the fungus brings other advantages too.
0:19:44 > 0:19:49It enables the oak to reclaim some of its lifetime's savings.
0:19:49 > 0:19:54Roots develop on the inside of the hollow trunk.
0:19:54 > 0:20:01They grow down and collect nutriment that the fungus has released from the wood as it digested it.
0:20:01 > 0:20:04That is not the only goodness here.
0:20:05 > 0:20:09Animals have come to live in the hollow tree.
0:20:09 > 0:20:14Owls may be roosting in its upper parts,
0:20:14 > 0:20:16bats hanging from its walls.
0:20:19 > 0:20:22Its lodgers,
0:20:22 > 0:20:25having fed out in the woodland,
0:20:25 > 0:20:28drop their dung within the hollow.
0:20:28 > 0:20:35So the tree receives food from places that otherwise would be far beyond its reach.
0:20:35 > 0:20:39So thanks to its fungal partner,
0:20:39 > 0:20:44an oak often has an old age that is both robust and well-fed.
0:20:49 > 0:20:54But fungi bring food to many plants throughout their lives,
0:20:54 > 0:21:01and that is particularly so in forests such as this one on the northwest coast of America.
0:21:01 > 0:21:09Even the tallest of these giant spruces, totally healthy and in the prime of its life,
0:21:09 > 0:21:13is dependent for its health and strength on a fungus.
0:21:13 > 0:21:16Its partner is down here.
0:21:20 > 0:21:25This is a rootlet through which the tree absorbs its nourishment,
0:21:25 > 0:21:30but wrapped round it are a mass of tiny white threads.
0:21:30 > 0:21:37They belong to the fungus and are part of a dense mesh increasing the surface area
0:21:37 > 0:21:41through which the tree can absorb water and nutrients.
0:21:41 > 0:21:47The partnership starts at the very beginning of a tree's life,
0:21:47 > 0:21:52when a fungus entwines itself around the seedling's infant roots.
0:21:52 > 0:21:59Seedlings which germinate in the soil without fungi are likely to starve to death.
0:21:59 > 0:22:04If there's a fungus to convey food, the seedling will get a good start.
0:22:05 > 0:22:08And that connection is never broken.
0:22:08 > 0:22:14An adult tree is able to collect nutriment-laden moisture from fungal threads,
0:22:14 > 0:22:18suck it along its roots, up its trunk,
0:22:18 > 0:22:26into its leaves and combine it with that other essential raw material, carbon dioxide gas, to make food.
0:22:33 > 0:22:41So trees, including giants like this one, can't grow without the help of tiny organisms within the soil
0:22:41 > 0:22:45organisms that we don't even notice until they fruit,
0:22:45 > 0:22:50and that may not happen more than two or three days in twenty years.
0:23:07 > 0:23:14This is how the fly agaric uses its share of the profits from the partnership.
0:23:26 > 0:23:33About a quarter of the sugars and starches produced by the tree in its leaves
0:23:33 > 0:23:37travel back down the trunk and into the ground
0:23:37 > 0:23:41to feed its multitude of fungal partners.
0:23:59 > 0:24:04Fungi fruit so briefly and often so rarely,
0:24:04 > 0:24:09it's difficult to appreciate how widespread they are, and how varied.
0:24:09 > 0:24:15There are over a thousand different species in the coniferous forests.
0:24:15 > 0:24:23Although trees do have preferences, any one individual may have links with up to 200 different partners.
0:24:24 > 0:24:27And it is not only limited to trees.
0:24:27 > 0:24:30Many small plants are also dependent,
0:24:30 > 0:24:36and none more so than those most glamorous of plants orchids.
0:24:43 > 0:24:50It seems paradoxical that such opulent and flamboyant blooms should be totally dependent upon the help
0:24:50 > 0:24:55of drab, thread-like organisms wrapped around their roots.
0:25:06 > 0:25:11Most plants provision their seeds with stores of food
0:25:11 > 0:25:15to fuel germination and the first stages of growth.
0:25:15 > 0:25:20But not these orchids. This is an orchid seed capsule,
0:25:20 > 0:25:24and here...is orchid seed
0:25:24 > 0:25:28so fine it's blowing away in the air.
0:25:28 > 0:25:33Minute seeds like this have always been difficult to get to germinate.
0:25:33 > 0:25:41And, infuriatingly, the seed from some of the most dazzling and rare of orchids wouldn't germinate at all.
0:25:41 > 0:25:44Then scientists tackled the problem.
0:25:44 > 0:25:49They found that many orchids have their own special fungal partner.
0:25:49 > 0:25:56They found methods of isolating that fungus and then culturing it with the orchid seed.
0:25:56 > 0:26:03Under the right conditions, the two strike up their partnership immediately.
0:26:10 > 0:26:18The fungus extracts nutriment from the culture medium in a way that the orchid can't do for itself
0:26:18 > 0:26:21and supplies it to the young plant.
0:26:44 > 0:26:46Within a month,
0:26:46 > 0:26:51the fungus invades the seed and conveys nutriment to it
0:26:51 > 0:26:56and the seedling is on its way to becoming a vigorous plant.
0:27:08 > 0:27:15You could argue that it is the orchid which is the dominant member of this partnership.
0:27:15 > 0:27:20It is, after all, the one we can see with our naked eye.
0:27:20 > 0:27:25There are plant-fungus relationships where the balance is the other way.
0:27:25 > 0:27:30The fungus determines the shape into which that partnership grows.
0:27:30 > 0:27:35One of those shapes is flat and plate-like,
0:27:35 > 0:27:42but in order to see the two partners, you have to look at it through very high magnification.
0:27:42 > 0:27:44This is a section
0:27:44 > 0:27:49through one of those plate-like partnerships.
0:27:49 > 0:27:51The top is formed by the fungus.
0:27:51 > 0:27:56These threads are part of the fungus and this sphere here is the plant.
0:27:56 > 0:28:04To see just how intimate their relationship is, you have to look at them in greater magnification.
0:28:04 > 0:28:07This is magnified 10,000 times.
0:28:07 > 0:28:12Here are the fungal threads and this is the plant, the algae,
0:28:12 > 0:28:16from which they're getting their sustenance.
0:28:25 > 0:28:32Together, the different organisms form one of the most widely distributed of living structures
0:28:32 > 0:28:34lichens.
0:28:37 > 0:28:43The partners operate so closely together that each pairing is given a single name
0:28:43 > 0:28:47and there are over 13,000 of them.
0:29:02 > 0:29:07They not only form these hard skins and curling crusts.
0:29:07 > 0:29:11Some lichens grow into little branched bushes.
0:29:24 > 0:29:28And very successful organisms they are too.
0:29:28 > 0:29:32They come into their own in the harshest of conditions.
0:29:32 > 0:29:39No grass can grow on these arid slopes here on the edge of the Namib Desert in southern Africa.
0:29:39 > 0:29:46This extraordinary orange colour is produced entirely by a carpet of lichen.
0:29:50 > 0:29:55It can get so hot here that it's painful to put your hand on rock.
0:29:55 > 0:30:00And there's no relief with a shower of rain, for it hardly ever falls.
0:30:00 > 0:30:04Yet 29 species of lichen flourish here.
0:30:04 > 0:30:09The red one is particularly successful.
0:30:20 > 0:30:23One of the functions of the fungus
0:30:23 > 0:30:27is to absorb moisture and deliver it to the algae.
0:30:27 > 0:30:33If there's no moisture, the organism shrivels and becomes brittle.
0:30:33 > 0:30:37And that's what's happened to this here.
0:30:37 > 0:30:43For this lichen, salvation is going to come from a surprising source.
0:30:45 > 0:30:48The sea lies only a mile or so away.
0:30:48 > 0:30:52A cold current sweeps up the coast from the south.
0:30:52 > 0:30:55The hot air rising from the desert
0:30:55 > 0:31:00pulls in cold air from the sea and the mixture produces fog.
0:31:07 > 0:31:12The moisture condenses as droplets on the lichen's branches.
0:31:12 > 0:31:16It's swiftly absorbed by the fungal skin
0:31:16 > 0:31:19and conveyed to the alga within
0:31:19 > 0:31:25and suddenly and miraculously, the desiccated branches turn green.
0:31:58 > 0:32:04But even in the best circumstances, lichen grow only very slowly
0:32:04 > 0:32:07often only a millimetre or so a year.
0:32:07 > 0:32:14One place shows vividly and accurately just how slowly that is a churchyard.
0:32:14 > 0:32:18The lichens, with their ability to live on bare rock,
0:32:18 > 0:32:21flourish on the tombstones.
0:32:25 > 0:32:33The dates of the inscriptions can tell us exactly when the bare stone surface was exposed to the elements
0:32:33 > 0:32:35and was available for colonisation.
0:32:35 > 0:32:38Some of these blotches,
0:32:38 > 0:32:42only an inch or so across, may be centuries old.
0:32:53 > 0:33:01Lichens also grow in undisturbed ancient forests such as those on the Pacific coast of North America.
0:33:02 > 0:33:07Trees here may live five or six hundred years,
0:33:07 > 0:33:11but well before they reach such an advanced age,
0:33:11 > 0:33:16they have usually been colonised by different kinds of lichens
0:33:16 > 0:33:21that hang in great tufts and blankets from their branches.
0:33:34 > 0:33:41So plants form intimate partnerships with members of the other great kingdoms of life
0:33:41 > 0:33:48in tropical forests, with members of the animal kingdom particularly ants and other insects.
0:33:48 > 0:33:53In the forests of North America, partnerships with fungi are common,
0:33:53 > 0:33:58ranging from those that produced these lichens,
0:33:58 > 0:34:02dangling from the boughs of this great spruce tree,
0:34:02 > 0:34:10down to the tangle of tiny threads meshed around the roots of the tree 250 feet below me.
0:34:10 > 0:34:17There are also partnerships within the plant kingdom between plant and plant.
0:34:17 > 0:34:21Some are just simple these mosses and ferns
0:34:21 > 0:34:26which use the spruce tree simply as a perch,
0:34:26 > 0:34:30but some partnerships are much more intimate.
0:34:30 > 0:34:33This is a mistletoe.
0:34:33 > 0:34:38It exists in partnership with a tree, for it has no roots of its own.
0:34:38 > 0:34:41It's a very one-sided relationship.
0:34:41 > 0:34:45The mistletoe has leaves, so it can manufacture food,
0:34:45 > 0:34:51but it draws all the liquid it needs from the tree to which it's fastened.
0:34:51 > 0:34:55The tree gets nothing from the arrangement.
0:34:55 > 0:34:58The mistletoe, in short, is a parasite.
0:34:58 > 0:35:02The mistletoe family has over 1,000 species.
0:35:02 > 0:35:07Here in Australia alone, there are 75.
0:35:07 > 0:35:11Somewhere there is always one in fruit.
0:35:11 > 0:35:16And that makes it possible for one bird to eat almost nothing else.
0:35:16 > 0:35:21The mistletoe bird knows exactly how to extract the fruit.
0:35:36 > 0:35:41The bird digests the fleshy coating of the seed with extraordinary speed.
0:35:41 > 0:35:46It takes less than half an hour to travel from beak to bottom.
0:35:46 > 0:35:51The seed when it emerges is still phenomenally sticky
0:35:51 > 0:35:55and has to be wiped off, which suits the mistletoe.
0:36:05 > 0:36:08The seed, when it comes out,
0:36:08 > 0:36:14remains attached to the bird's behind by a long sticky thread.
0:36:14 > 0:36:18The bird has a technique for breaking it.
0:36:29 > 0:36:34Every time it needs to detach a seed, it has to perform this little dance.
0:36:54 > 0:36:59It's this stickiness that is the key to the mistletoe's success
0:36:59 > 0:37:02 in getting from one tree to another.
0:37:02 > 0:37:07Once parked on a living branch, the seed quickly plugs itself in.
0:37:15 > 0:37:23With a connection to its host's liquid supply, it can build leaves and start making food for itself.
0:37:34 > 0:37:36This is another mistletoe.
0:37:36 > 0:37:41It grows only in Western Australia and it flowers in December,
0:37:41 > 0:37:46which is why it's known locally as the Christmas tree.
0:37:46 > 0:37:53I know it's a mistletoe because of the character of its flowers and its green, fleshy leaves.
0:37:53 > 0:37:58But from other points of view, it's very unlike other mistletoes.
0:37:58 > 0:38:03It's a free-standing tree that does not seem to be parasitising anything.
0:38:03 > 0:38:10But it gives us a very good idea as to how parasitism might have started in this family.
0:38:10 > 0:38:13Have a look at its roots.
0:38:21 > 0:38:26This is the root that belongs to the Christmas tree,
0:38:26 > 0:38:31and this root belongs to another completely different bush nearby.
0:38:31 > 0:38:36And the Christmas Tree has encircled this other root with a white ring.
0:38:36 > 0:38:41It's plugged itself in to the root system of another plant,
0:38:41 > 0:38:45and it gets all its water and minerals in that way.
0:38:45 > 0:38:52And it's not at all fussy about what kind of plant it parasitises grasses, sedges,
0:38:52 > 0:38:57small bushes, big trees, gumtrees, cycads it will go for the lot.
0:38:58 > 0:39:03At least the mistletoes have leaves for making some food for themselves.
0:39:03 > 0:39:07A few parasitic plants don't even have that.
0:39:07 > 0:39:12These are the germinating seeds of dodder.
0:39:12 > 0:39:17They have to find their host within a few days or they will die.
0:39:27 > 0:39:30A favourite target is the nettle.
0:39:30 > 0:39:37Well-armed with stings it may be, but they are no defence against dodder.
0:39:40 > 0:39:47The seedlings can detect whether a nettle stem is feeble or well-nourished
0:39:47 > 0:39:51and they pick their victim with care.
0:39:52 > 0:39:57This is a strong, healthy one good to feed on.
0:39:57 > 0:39:59In goes a nozzle.
0:40:07 > 0:40:13The dodder sucks the nettle's sap, which then fuels its growth
0:40:13 > 0:40:16and its hunt for another victim.
0:40:32 > 0:40:37The dodder is a relative of the bindweed, convolvulus,
0:40:37 > 0:40:41and it climbs in the same sort of way.
0:41:03 > 0:41:06Wherever the feeding seems good,
0:41:06 > 0:41:11the parasite inserts a tube and draws off the nettle's sap.
0:41:22 > 0:41:24Once it's fully established,
0:41:24 > 0:41:29drinking from the nettle through hundreds of connections,
0:41:29 > 0:41:36the dodder is siphoning off enough nourishment from its victim to enable it to flower.
0:42:11 > 0:42:18Eventually, the whole bed of nettles is overwhelmed by writhing dodder stems.
0:42:47 > 0:42:50The dodder is completely parasitic,
0:42:50 > 0:42:54getting all it needs from another plant.
0:42:54 > 0:42:59But the relationship between parasite and host can be even closer.
0:42:59 > 0:43:06Here in the forests of Borneo is an enormous parasite whose relationship with its host is so intimate
0:43:06 > 0:43:11that the parasite is invisible for most of the year.
0:43:37 > 0:43:41This is the first that anyone or anything sees of it.
0:43:41 > 0:43:47The bud is coming from this root, but the root doesn't belong to this.
0:43:47 > 0:43:51The root is part of this great vine.
0:43:58 > 0:44:05Inside the massive trunk of this vine, there's a multitude of hair-like filaments.
0:44:05 > 0:44:10They don't belong to the vine but to a parasite called Rafflesia.
0:44:10 > 0:44:15Rafflesia has no stem, no leaves, and never will have.
0:44:15 > 0:44:19It feeds entirely on the sap produced by the vine.
0:44:19 > 0:44:24The only time Rafflesia emerges into the outside world is to flower.
0:44:24 > 0:44:31That bud was weeks old. If I follow the root of the vine, maybe I'll find more.
0:44:41 > 0:44:43Two more, but still small.
0:44:48 > 0:44:51A bigger one.
0:44:56 > 0:45:01And this one looks as though it might well open tonight.
0:45:37 > 0:45:44By the time dawn comes and the first rays of the sun filter down into the forest,
0:45:44 > 0:45:47the flower is almost fully open.
0:45:56 > 0:46:01Rafflesia produces the largest single flower on earth
0:46:01 > 0:46:04a big one can be three feet across.
0:46:11 > 0:46:18The surface of the warty petals look a little like that of a putrefying corpse.
0:46:18 > 0:46:22There is a faint smell of rotten fish
0:46:22 > 0:46:30and the huge flower quickly attracts those that find much of their food in carrion blowflies.
0:46:35 > 0:46:42In the bottom of the cup, a great disc covered in spikes stands on a pedestal.
0:46:42 > 0:46:46The flies go in to investigate and crawl all over it.
0:46:55 > 0:47:00Hanging from the underside of the disc are droplets of liquid pollen.
0:47:03 > 0:47:06As the flies explore,
0:47:06 > 0:47:11they touch the droplets and get saddled with a dab of pollen.
0:47:16 > 0:47:21This will only benefit Rafflesia if the fly is able to find
0:47:21 > 0:47:28another of these very rare flowers fully open in the forest to which it can deliver its load.
0:47:28 > 0:47:33Rafflesia produces the biggest single flower in the world.
0:47:33 > 0:47:37But why, when all it needs to attract are flies?
0:47:37 > 0:47:45Plants, like other living organisms, can only afford to spend a limited amount of food on reproduction.
0:47:45 > 0:47:48But Rafflesia does not earn its food.
0:47:48 > 0:47:51It takes it straight from the vine.
0:47:51 > 0:47:59Provided the vine is not fatally injured, there seems to be no limit to the amount Rafflesia may extract.
0:47:59 > 0:48:07Maybe an unearned income in the plant world, as elsewhere, can lead to extravagance
0:48:07 > 0:48:09on a truly monumental scale.
0:48:48 > 0:48:53Subtitles by Sarah Aitken BBC Scotland 1995