Plants cover much of the land surface of our planet, but there is another extraordinary green world that is often hidden from us.

It's one where plants have overcome huge challenges in order to survive.

The world of freshwater（淡水）.

(资料图片仅供参考)

Water Worlds

At first sight, a lake like this would seem to have everything that life needs in order to thrive, clear, oxygen-rich water, plenty of dissolved nutrients and minerals and lots of sunlight. But in fact, life in freshwater presents plants with huge problems.

present... with: 使面临，赠（礼物）

To succeed, plants have had to abandon many of the adaptations that served them so well on land and evolve something quite new, and in doing that they have created some of the most beautiful andbizarreand important habitats on earth.

There are few places where it's more difficult to make a permanent home than a freshwater torrentlike this one. Violent currentsrip across the riverbed, scouring it clean, rippingland plants from their margins and drowning them. How could any plant survive in a place like this?

rip: to rush headlong; to tear or split apart or open

scour: to remove by rubbing hard and washing

Yet, even here, some do manage to, quite literally, hold on! They can grasp the bare rock with remarkable strength. This ability allows plants to thrive in these otherwise hostile environments.

This is the Cano Cristale River in Clombia（位于哥伦比亚的卡诺克里斯塔莱斯河）.

These plants are Red Macarenia（虹河苔）, sometimes called the 'Orchid of the Falls'. They cling tothe riverband, not with their roots, but with their stems - glued to the rock surface by one of the most powerful adhesives（粘合剂）in nature.

The rock itself will break before these anchors lose their grip. These feathery filamentsare their modified leaves and they do what roots normally do - gather the minerals and nutrients they need that are dissolved in the water.

filament: a single thread or a thin flexible threadlike object, process, or appendage

With such spectacularcolours, it's hardly surprising that the Cano Cristales is sometimes called 'the most beautiful river on earth'.

But being rooted to the spot is not always the best strategy for living in a water-world.

This is a water lettuce（大薸）, and it has some remarkable adaptations. Its roots hang free, so it's not anchored to the ground, and its leaves are thick and spongy and covered in fine hairs. So the plant itself is more or lessunsinkable.

This combination of characteristics enables the water lettuce to do something that almost no land plant can do. It can travel.

It is an ability that becomes invaluable when, during thewet season, flooded rivers become great highways as they do here in South America.

This is the largest inland water-world on Earth - the Pantanal（潘塔纳尔湿地）.

For a few months every year, it provides water plants with ideal conditions. But all too soon, it becomes a battleground.

Plants are racing to claim their space on the surface. The water lettuce rapidly expands its network of hanging roots so that it starts absorbing nutrients before other competitors arrive.

Water hyacinth（凤眼蓝，水葫芦） appears. Its leaves are carried on stalks, filled with air, that also make it virtually unsinkable.

The race for spaceintensifies.

A new competitor arrives - Ludwigia（重根丁香蓼）.

It spreads by developing a chain of tiny rafts, andjostles for space with the densely packed leaves of mosaic plants（菱叶丁香蓼）.

jostle: to make one's way by pushing and shoving

All are racing to claim as much sunlight as possible.

They flower quickly before the flood waters recede. And these surface dwellers also have competitors including one that has been waiting in the depths and is now stirring.

recede: to move back or away: WITHDRAW

It's a monster. It's well-armed. It clears space for itself by wielding one of its buds like a club. And now, it dominates the surface.

This is a leaf of the giant water lily（亚马孙王莲）. It expands by over twenty centimetres a day and eventually measures more than two metres across. Its immense leaves are supported by a network of air-filled strutsand protected by spines, two centimetres long. 

strut: a support for a structure such as an aircraft wing, roof, or a bridge.

The leaves float high in the water and their surfacesare dotted with tiny holes - drains - that help them ensure that rainwater doesn't accumulate and sink them. Nutrients from the fertile mud below are carried up by tubes in its stem to fuel the leaf's expansion. 

Over the next few months, the lily will produce some forty or so of these giganticleaves. And as each one reaches the surface and expands, more and more light is taken from those plants that are trying to grow beneath.

Competitors are pushed aside. Some are crushed or skewered. Eventually its immense leaves press their margins against one another, totally cutting off the light from the plants beneath them. 

skewer(n.):  a pin of wood or metal for fastening meat to keep it in form while roasting or to hold small pieces of meat or vegetables for broiling.

skwer(v.):  to fasten or pierce with or as if with a skewer

The battle is over and victory is total.

The frozen water world of Lake Akan, in northern Japan. Home to one of the strangest and mostprimitiveof plants.

It's an alga, like those that appear so mysteriously in our ponds, but this one is truly extraordinary.

Each spring, the melting ice releases soft, velvety balls of interwoventhreads called marimos（湖球藻）. This one is small, no bigger than a walnut（胡桃）, but there are lots of them here.

velvety: having the character of velvet as in being soft, smooth, thick, or richly hued.

They attract the attention of visiting whooper swans（大天鹅）. But there is one way for the Marimo to escape from the danger and it depends on a change in the weather. Fortunately in the spring, winds sweep across the lake, creating currents that carry some of the marimos beyond the reach of hungry swans.

It's a start of a remarkable journey.

They are gently carried back and forth by the currents, so that the Marimos become more and more spherical. And slowly, they travel into deeper water.

Here there are great numbers of them, certainly many millions! Some are the size of basket balls. They are safe from swans and the water is still shallow enough for some sunlight to reach them. It seems a perfect home. And so it is, almost.

The snag is that these waters also carry a fine sediment that can clogthe marimo's surface, cutting off the all-important（非常重要的，首要的） light. But the marimos are not entirely immobile.

snag: a concealed or unexpected difficulty or obstacle

They 'dance'!

The winds blowing overthe lake's surface create currents beneath that are sufficiently strong to move the marimos. Theyrubagainst each other, and in just a couple of hours of gentle movement, they're all clean once more.

As they spin, every part of their surface gets enough time in the sunlight to keep growing.

This is the heart of the Amazon. 

There are water worlds here that are so remote that even today few people have ever seen them. This barely exploredtributary（支流） is the Rio Claro（里奥克拉罗河）.

And here, when conditions are just right, it's possible to witness a rare and remarkable spectacle. 

The river is so crystal clearthat its bed is bathed in sunlight. A magical landscape of miniature mountains and valleys. 

It's carpeted by pipewort（谷精草属）, fanwort（水盾草属） and star-grasses（沼车前属）. 

spactacle: something exhibited to view as unusual, notable, or entertaining

As the sun climbs in the sky, bubbles of gas appear. Evidence of photosynthesis. 

Deep inside the plant's cells, tiny structures calledchloroplasts（叶绿体） move towards the light. They absorb carbon dioxide and use the sun's power to synthesisethe sugars that the plant needs to grow. And, as a by-productthey release oxygen. The gas that we - and all other animals - must have in order to breathe.

Now in late afternoon, bubbles of oxygen make the river water fizzlikechampagne.

The plants can become so buoyant with gas that they rise to the surface even carrying the bedrock with them. Only in this remote water world can this spectacular natural wonder be seen.

boyant: capable of floating

Eastern Venezuela（委内瑞拉）.

Hererectangular mountains known as tepui stand above the tropical forests. There are more than fifty such isolated mountains plateaus here, each home to a unique community of plants. 

Downpours are so torrential that no soil can accumulate on their broad rocky summits and some plants living up here have to find their nutrients from another source.

torrential: resembling a torrent in violence or rapidity of flow

These are bromeliads（凤梨科植物）. Their leaves are shaped like a funnel and collect rainwater which accumulates in the centre. This small pond is colonisedby all kinds of tiny animals. And it is their bodies, when they die, that provide some of the nutrients the bromeliads need. 

This makes a good partnership in which both parties can thrive. 

But it can be exploited by a plant predator.

This probing stem belongs to a plant called a bladderwort（洪堡狸藻）. It too is in need of nutrients. And a well-stocked bromeliad pool is just the place to find them.

This one is full ofaquatic animals.

The bladderwort begins to change into a hunter. It develops bladders and removes sufficient of the water within them to create a partial vacuum. Each bladder has a trap door beside it with trigger hairs. Now, all the bladderwort has to do is tobide its time.

It only takes one touch for a trap door to snap open and suck in its prey. It's all over in a milli-second. And after it has fed, a bladderwort has enough energy to produce another tendril, to search for another bromeliad pool.

Swamps and bogs（泥塘）are also poor in nutrients. So several plants that live in such places catch insects too, if they can.

The leaves of sundews（叉叶茅膏菜）are covered with long red hairs, each tipped with a droplet. Theseglistening globules are, in fact, glue!

globule: a tiny globe or ball especially of a liquid

Once the Sundews detect the taste of their victim's body, they flood it withdigestive enzymes. The little body disintegrates. And the sundew gets the nutrients it needs.

Another plant has an even more elaborate way of catching a meal. The Venus Fly-trap（捕蝇草） has leaves that are lined withinterlocking teeth. It attracts insects by producing a sweet perfume, just as a flower does. It too has a hair-trigger. And another insect is caught. But the technique is more complex than it might seem.

The Venus Flytrap has a problem. It needs to avoid false alarms. Snapping shut onsomething inedible, like a raindrop or little bit of twig, that would be a waste of both time and energy. So how does it avoid that? Well, it does it by counting. 

If I touch this one sensitive hair just there, no reaction. That could be a false alarm, but the plant remembers that for 20 seconds, and if I touch it a second time, within that time, then that's much more likely to be worth eating. And so, it closes. So far, so good.

But now, it needs to be absolutely certain that it's got something worth eating, so it continues counting. Only after it has totted up（合计）five separate touches to those hairs will it give the final squeeze and then begin to produce the liquid from the surface of the leaf which will dissolve the body of its unfortunate victim.

The Flytrap now has enogh energy to produce flowers and attract pollinating insects.

Wind and insects between them pollinate virtually all land plants, but neither method can be used by plants that live entirely underwater. So some lead double lives!

A chalk stream（白垩溪流） in southern England, and swaying in the current is a plant for which these rivers are famous.

This is water crowfoot（笔刷毛莨）, a kind of aquatic buttercup（毛莨属植物）. For most of the year it is underwater, and if I take this underwater camera:

You can see its floppy stems grow horizontally. That reduces the risk of being swept away by the current. But each spring, when it's time to flower, it produces something crucially different. A stem that is stiff enough to resist the current and lift its flowers into the air above. And now of course, they can get help from insects.

flop: to swing or move loosely:FLAP

floppy: tending to flop, especially being both soft and flexible

So every year, in part at least, Water Crowfoot becomes a land plant. And provides us with one of the loveliest natural spectacles of the early English summer.

Water crowfoot is not the only water plant to lift its flowers above the surface. Plants do so, all around the world. From the swamps of the Pantanal

to lakes of Thailand,

they all burst into spectacular bloom.

Once they've been pollinated, they produce seeds. And now their flowers have done their job,  some return to a life underwater. Now, they must ensure that some of their seeds will find suitable places in which to germinate.

germinate: to begin to grow:SPROUT

Bullfushes（香蒲） every year produce these long brown velvety objects. Look what happens when I break one open. It contains almost a quarter of a million seeds. 

Each seed is attached to a delicate parachute. Even the slightest breeze will lift it and may carry it for very long distances indeed. So even though suitablestretchesof freshwater are few and far between, there's a good chance that at least one will end up in a place where it can grow. 

Much bigger seeds of course can't travel by air. A river can provide transport, but it's a one-way journey - down-stream that often ends up in the sea. And that's not ideal!

So how can any riverside plant avoid this and travel up-stream?

Here, along the Bonito River in Brazil, a variety of trees manage to do exactly that. They embed their seeds in the middle of soft sweet fruit.

Monkeys, such as these capuchins（黑帽悬猴）, make a meal of them just as soon as they're ripe.

But monkeys are very wasteful feeders and what's not eaten, ends up in the river and is washed away.

But not all.

In thefruiting season, hundreds of Pirapatunga fish（希式石脂鱼） gather beneath these trees. But the Pirapatunga want more than the monkey's left-overs.

The bright coloured fruits are clearly visible even to fish in the water below. And some manage to claim them even before a monkey does.

This isn't a skill mastered by just one particularly successful acrobatic（杂技的）fish. Many of the Pirapatunga can do this. 

Nor is this a disaster for the tree. Far from it.

These Pirapatunga are migratory（洄游鱼类）, heading many miles up-river to spawn. The trees, by enticingthe fish to eat their fruits have a perfect means of transport for their seeds. With luck, these seeds will be deposited many miles upstream.

The ability to colonisenew habitats has allowed one group of flowering plants to venture out of the fresh water and into a world that may look the same to us, but for a plant is crucially different.

The much greater, saltier world - the sea. 

This is a fruit from one of the most important plants on the earth today, sea grass（海草）.

This particular one is floating off the coast of Formentera（福门特拉岛） in the Mediterranean.

A hundred thousand years ago, a seagrass seed like this sank to the sea floor just here.

And eventually it produced a great meadow - a meadow that is still flourishing today.

It did so by cloningitself.  Now over ten miles across, it's not only one of the largest living organisms on earth, it's also one of the oldest. And it supports a rich community of many kinds of animals.

It's become a kind of marinesavannah.

savannah: a large, flat area of land covered with grass, usually with few trees, that is found in hot countries, especialy in Africa.

Over  a thousand species now live here. Some like these elegantly camouflaged pipefish（海龙）, live nowhere but amongst the sea grass.

Sea grassfringes many of the world's coasts. Turtles depend upon it too. And so do dugong（儒艮）, animals that are sometimes called, very appropriately, sea-cows.

Today, sea grass plays a critical role in maintaining the health of our planet. It creates stores of carbon around its roots at an enormous rate, thirty five times faster, in fact, than plants that live on the floor of a tropical rainforest.

Here, off Formentera（福门特拉岛）, it's possible to see, beneath the living sea grass, layer upon layer of trapped carbon that the plants have accumulated over the past two thousand years.

Sea grass, however, is easily destroyed by human disturbance. A third of the world's underwater meadows have already been lost and many more are in decline.

Biologists are now striving to not only protect the remaining meadows, but to restore them one plant at a time. Sea grass could be a valuable ally in our fight against climate change.

Today, water worlds everywhere are under threat. Many of their inhabitants are disappearing without us even being aware of their existence.

The plants that grow in water are probably the least noticed of all. They're certainly the least studied, 

but the more you know about the problems of living in that way, the greater the wonder of their success.

Surely, they deserve more of our attention and, most importantly, our care.