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- Hello, I'm Lindsay Turnbull,

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and I teach biology at
the University of Oxford.

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In this episode, I hope to convince you

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that plants are every bit
as interesting as animals.

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Armed with stolen technology,
these glorious green beings

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stormed onto the land
400 million years ago,

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long before any vertebrate set foot there.

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And they're the subject of
Chapter Nine of my book,

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"Biology: The Whole Story".

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(birds chirping)
(frog croaking)

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(water rushing)
(bird chirping)

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Try to imagine for a moment,
a world without plants.

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I hope you are imagining
somewhere pretty barren

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and inhospitable, because even from space,

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our continents glow green, and
nearly every habitat on Earth

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is defined by the plants that live there.

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What's any forest without its trees?

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The problem actually is though,

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that plants are so ubiquitous

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that we kind of don't even notice them,

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and we just take them for granted.

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And lots of people when asked,

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think that plants maybe are a bit boring,

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and that's because they
don't move very quickly,

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and they don't have faces,

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so we don't really know
what they're thinking.

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But I hope to convince you in this video

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that that's just totally
the wrong attitude.

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Plants are properly awesome,

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and I wanna tell you their story.

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Well, the story of
plants isn't so different

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to the story of vertebrates.

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Plants moved onto the land,
and in order to do that,

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they had to make some significant
changes to their bodies.

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And once they were on the land,

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many of them supersized to produce giants.

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But there is one very big
difference between plants

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and any animal.

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Animals eat other cells
and they digest those cells

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into their component molecules,
and they use those molecules

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to build their own bodies.

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But plants don't do that.

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They build themselves from scratch.

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Now, in order to understand
how they do that,

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we need to look inside their cells.

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And we might start with something

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like this piece of onion skin.

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When you put that under a microscope,

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you can see a single
layer of box-like cells.

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We can see the nuclei inside,

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so we know they're eukaryotic cells.

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And those box-like
structures are the cell wall.

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So plants have evolved
a new kind of cell wall

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made from cellulose,

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which animals find quite hard to digest.

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Now the onion belongs under the ground.

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If we looked at a slice of a leaf,

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we might see something
different in the cells.

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We are struck by these little green blobs,

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and those are a special kind of organelle

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called a chloroplast.

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Now, if we remember in the
chapter about eukaryotic cells,

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we looked at mitochondria,
and we said that they are

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in fact, domesticated bacteria.

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They're the powerhouse of
the eukaryotic cell now,

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but once they were free-living bacteria.

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And these chloroplasts
are exactly the same.

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Once they were also free-living
bacteria, a special kind

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of bacteria called a cyanobacteria.

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And they are famous because they invented

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what was probably the most
disruptive piece of technology

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that ever evolved on
Earth, photosynthesis.

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Now, photosynthesis involves
capturing energy from the sun

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and using that energy to
smash up water molecules.

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Water molecules consist of
hydrogen and oxygen atoms,

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and the reason you wanna smash them up

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is you wanna get the hydrogen out.

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If you combine hydrogen
with carbon dioxide,

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then you can make glucose.

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And once you've got glucose,

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you can make a whole range
of other organic molecules.

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The oxygen that's left over just trickles

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off into the atmosphere.

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That's just a waste product.

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Now, this machinery to
carry out this process

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is very sophisticated,
and it only evolved once

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from the cyanobacteria.

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But that didn't stop other cells

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from stealing that technology.

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So the first algal cell
captured a cyanobacteria

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and put it to work, and that
obviously lived in the ocean.

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And today there are many kinds
of algae living in the ocean

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and in fresh water.

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Many of those are single-celled,

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but some of them are
multicellular, like these seaweeds.

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There's a whole range of
different kinds of seaweed.

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And at some point in the past,

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one of those algae came out onto land

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and founded the dynasty
of the land plants.

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Well, thanks to the Oxford Herbarium,

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I'm holding in my hand a
rather old microscope slide.

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And stuck to this slide is
a very thin slice of rock.

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Now, the type of rock is a chert,

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and the rock came from a village
in Scotland called Rhynie.

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So this is a Rhynie chert,

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and it's about 407 million years old.

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Now, today Rhynie just looks
like fairly normal farmland,

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but 407 million years
ago, it looked like this.

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Now, this is Yellowstone
National Park in the USA,

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and you can see these
hot springs and geysers,

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and sometimes those boil over

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and entomb the organisms
growing around them.

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Now, let's put this slice of
rock under the microscope.

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And what we see are some
circular structures,

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and those are the stems of some

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of the very earliest land plants

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that were growing in
a landscape like that.

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They only grew to about
20 centimetres high,

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so they were quite small,

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but they possessed some
of the key adaptations

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that plants need to grow on land.

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And let's have a look at those.

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So the first one is that dark blob

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in the middle of the circle,

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and that is a pipe that pipes up water

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through the stem of the plant.

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The second feature that
they had is that when plants

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came onto land, they were in
danger of being sucked dry.

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So they evolved a thin waterproof layer,

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a waxy cuticle that stopped water loss.

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The problem with that is,
if water can't get out,

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then carbon dioxide can't get in,

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and plants need that for photosynthesis.

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So they peppered that waterproof layer

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with tiny holes called stomata,

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and we can see those stomata
in the Rhynie chert plants.

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Finally, if we look down
to the bottom of the plant,

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we see some long thin cells that push down

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into the soil to take up water.

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So these three features between them

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mean that plants take
up water in the soil,

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they pipe it up through their bodies

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and out through holes in the leaf.

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And this means that plants

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can suck up huge amounts of water.

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In fact, a large tree in full leaf

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can suck up a tonne of
water in a single day.

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And that's why forests are
uniquely good at recycling water.

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And clouds often form over forests

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during the course of a day,

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and rain then falls down in the afternoon.

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And that's why forests
are very, very important

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for regulating the local climate.

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Now, as well as having
an impact on the climate,

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plants can also have a profound impact

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on the composition of
the Earth's atmosphere.

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Photosynthesis removes one
molecule of carbon dioxide

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from the atmosphere and replaces it

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with one molecule of oxygen.

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And this process creates
glucose for the plant,

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which it uses to build its body.

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Now, when the plant dies or it's eaten,

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that molecule of glucose is respired,

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and that returns a
molecule of carbon dioxide

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to the atmosphere and uses
up a molecule of oxygen.

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So as long as photosynthesis
and respiration are in balance

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across the planet,

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then the composition of the
atmosphere won't change.

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The levels of oxygen and
the levels of carbon dioxide

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will remain roughly the same.

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But more than 350 million years ago,

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as the first forest
spread across the globe,

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that balance just wasn't there.

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Those forests were filled
with very strange trees,

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and when they died, they weren't eaten,

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and they didn't rot away.

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Instead, they lay buried

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under tens of millions of years of rock.

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And that meant that because of all that

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buried plant material,

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the levels of oxygen in
the atmosphere shot up,

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and the levels of carbon
dioxide plummeted.

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And that would've had all kinds
of effects on the organisms

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that were living at the time.

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Now, what's happened to those trees?

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Well, those fossilised trees
have become fossil fuels.

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So the massive coal deposits that lie

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around the world are all made up of trees

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that were buried hundreds
of millions of years ago.

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And of course, we are
digging up that coal,

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and we are burning it,

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and doing that returns carbon
dioxide to the atmosphere

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that hasn't been there for
more than 300 million years.

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And that's why the
concentration of carbon dioxide

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in the atmosphere today
is rapidly rising again.

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And because carbon dioxide
is a greenhouse gas,

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that is having profound
impacts on global temperatures,

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and that warming of the planet

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causes all kinds of catastrophic events

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to become much more common.

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Now, modern plants don't form coal.

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The vast majority of
plants on the Earth today

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belong to a group called
the Flowering Plants.

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And they came along relatively
late in the Earth's history

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towards the end of the period
when the dinosaurs dominated.

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And of course, they brought
with them welcome colour,

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because the flowers of flowering plants

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are all incredibly beautiful.

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And the flowering plants have diversified

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into all kinds of forms and shapes.

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So we have enormous
trees, cacti in deserts,

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beautiful orchids, tiny duckweeds

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that float on the surface of ponds.

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And we even have some plants

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that have turned the tables on animals,

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like this Venus Flytrap
that supplements its diet

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by catching insects.

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But of course, most plants have

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a much more benign
relationship with insects,

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a mutually beneficial one,

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in which the insects visit
the flowers and collect pollen

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and transfer it around in
return for sugary rewards.

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That's the nectar that the plant produces

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to tempt insects to come and visit.

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Now, that relationship
between plants and insects

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is just one example of an
ecological interaction,

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and the science of ecology
is what we're gonna look at

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in the next episode.

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Well, I really hope
you enjoyed that video,

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and that I succeeded in convincing you

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that plants are really
worthy of your attention.

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If you wanna get your
own copy of the book,

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don't forget there's a link below,

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or share this video among
friends and colleagues.

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There's a lot more information
in Chapter Nine about plants,

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particularly about how
plants are expert traders.

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They monopolised the
sugars commodity market,

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and they then use that monopoly
to strike hard bargains

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with lots of other organisms.

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Otherwise, see you next
time for the next episode

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all about ecology.

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(birds chirping)
(water rushing)

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(birds chirping)
(wildlife bustling)