The Private Life of Plants (1995–…): Season 1, Episode 4 - The Private Life of Plants - full transcript

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Howling storms and hurricanes,

devastating droughts and raging forest fires
may come only once or twice in our lives,

so we think of them as exceptional.

But for plants with longer lives
than ours,

such events
may be crucial for survival.

And those with briefer lives

may only be able to exist at all
in the aftermath of such disasters.

The greatest British hurricane
of recent times

struck on the night
of October 16 1987.

Ancient woodlands, centuries old,
were devastated.

For individual trees,
like this 250-year-old beech,



it was, of course, a catastrophe.

But for other plants
it was an opportunity

that they had
been waiting for, for decades.

The seeds from which
these young plants are springing,

may be from adults growing elsewhere
in the wood over a century ago.

Since then,
they have lain dormant in the soil.

But now, the light
has triggered them into life.

They won't have this clearing
to themselves for long

so they grow swiftly.

And the soil, enriched by leaves,
feeds them well.

In their second year, they flower.

They are foxgloves.

Their seeds won't sprout here.

There's no room for them.



They'll have to find a new clearing
and, in turn, may wait decades.

Willowglove competes with the
foxglove for this kind of territory,

so it too needs to reproduce
urgently.

Willowherb seeds, supported by no
more than fluff,

will be carried in millions,
far and wide, by the wind.

After a few years,
other claimants appear

and contest the ownership
of the clearing - young birches.

The willowherb may not be able
to stay here for much longer.

A few years later,
the willowherb has gone.

The birches, it seems, have won.

But the soil
is now losing its richness.

Most of these birches
started their lives ten years ago,

but there were lots
and competition was intense.

So most of them didn't do well at
all. This one is pretty well dead.

This is one of the winners,
overshadowing the rest.

But even this one will not be
the final holder of this territory.

That prize will go to a little
seedling like this - an oak.

Birches, on the timescale of a wood,
have short lives.

After 40 or 50 years, they begin
to succumb to fungus and disease.

Oaks grow more slowly
but more strongly.

Eventually, they overtake the ailing
birches and capture the sunshine.

The social struggle in the woodland
is over, at least for the time being.

The oaks rule.

But as monarchs, they have to support
a multitude of lesser mortals.

The wide, spreading expanses of
leaves, full of starches and sugars,

are rich meadows where vast numbers
of creatures can graze in the summer.

They protect themselves to an extent
by generating toxins in their leaves.

But that doesn't deter weevils.

Or bush crickets.

Sawfly larvae munch away
within the thickness of the leaves.

A caterpillar
of the yellow-tailed moth.

The tortrix moth caterpillar
converts a leaf into a tube,

eats it from within, then uses it as a
shelter while it changes into an adult.

The bagworm caterpillar tunnels inside a
leaf but shelters inside a portable tube

which hangs from the leaf
it's eating.

The meals the oak provides for
these insects then nourish others

when the insects become meals
for bigger diners.

A great tit with chicks will collect
at least 300 caterpillars a day.

The banquet only lasts a few months.

As summer gives way to autumn,

the oaks drop their leaves
and halt all activities.

And so do their lodgers within their
cocoons or hibernating in crevices.

As the year turns,
sunshine warms the soil

and plants, that were bulbs through
the winter,

race to catch the light
before the oaks' leaves regrow.

First are the snowdrops.

As spring proceeds and the sunlight
strengthens, bluebells take over.

The glorious carpet of uninterrupted
blue is a British specialty.

Twenty thousand years ago, glaciers
advanced over most of Britain,

driving many plants southwards.

When the ice melted,
the English Channel began to form

and became a barrier that prevented many
plants spreading back from Europe into Britain.

The bluebell was one that made it.

In North America,
things are rather different.

These lilies grow
in the Appalachian Mountains.

Spring here is even richer
than it is in Britain.

Instead of the two or three species

of flower per square yard
we're used to,

here, there are a dozen or even more.

Glaciers covered these mountains too

but no sea ever cut them off from
the rest of the American continent.

So nothing prevented spring plants
returning from the warmer south.

And here, instead of the uniform carpet of bluebells
there is one with a rich pattern of many colors.

Trillium lilies.

Dutchman's-breeches.

Wild geranium.

A member of the lily family,
the bellwort.

Blue phlox.

But the glories of spring
do not last long.

In April,
English oaks rebuild their canopy.

The bluebells' time is over.

In their few weeks of activity,

they manufactured enough food
not only to set seed,

but below ground, to grow new bulbs.

So next spring, if there's room,
they'll extend their splendid carpet.

A young beech reinforces the canopy
by adding another, lower layer.

Shade returns to the woods.

In tropical forests, it's never winter and
the trees are in leaf throughout the year.

If a plant on the ground beneath such a permanent
canopy needs sunshine, it will have to climb.

These youngsters search for some kind of ladder
by lashing around with their whip-like tendrils.

Once one gets a grip, it puts a coil
in the tendril, so shortening it,

and pulls itself closer to
the branch up which it might climb.

Other plants ascend by twining
their main stem around their support.

As a climber gets nearer the canopy,
and the light, it expands its leaves.

There are no more determined competitors
in this upward scramble than the rattans,

native to the forests of South-East
Asia and tropical Australia.

Mature rattans have
the longest stems of any plant.

One was measured at 560ft!

The mature plant doesn't have leaves
down here on the forest floor.

It only does that up in the canopy.

This luxuriant growth, basking in
the full sunshine 200ft above ground,

is the crown of the rattan.

It makes the plant's character
quite plain - it's a kind of palm.

The tendrils it climbs with are so
thin, they are easily overlooked,

but snag one of these, and it will
rip your clothes and your flesh!

The tendrils are rigid enough
to reach up

and hook onto
the branches of established trees.

They then hold the stouter, heavier,
main stem in position

while it grows up from the
fearsomely protected bud at its tip.

There is, of course, an easier way
to ascend - floating up, as a seed.

Other plants among these squatters

arrived here as seeds stuck to
a monkey's fur or in a bird's gut.

Orchids have seeds
that are as fine as dust,

so small, they can be lifted
by even the faintest breath of air.

So, even the highest branches
of the tallest trees

may carry spectacular displays
of breathtaking blooms.

But living up here has its problems.

Water must be collected and stored.

Above the ground, it's difficult
to get mineral nutriment.

This orchid deals with those difficulties by
wrapping its green roots round the branch it sits on.

They intercept rain, trickling down the
bark, carrying a trace of nourishing dust.

Some higher-altitude forests
have almost permanent mists.

And the canopies of all rainforests get
intermittent supplies of water from regular storms.

Many plants store water as pools in their centers
and have little difficulty keeping them topped up.

Orchids may keep water
in swollen, bulb-like stems.

The ponds in bromeliads
are more than reservoirs.

Tiny animals, such as mosquito
larvae, take up residence in them,

and their excrement and dead bodies form a nutritious
sludge. So the bigger plants save more food.

I'm 200ft above the ground,

suspended in the branches
of a koompassia tree,

the tallest of the trees
in the South-East Asian forest.

And here, beside me,
is a magnificent basket fern.

As it grew and put out leaves,
so it collected moisture.

And as the leaves got bigger,

they in turn collected other leaves
falling from above.

They decayed to form
a rich leaf mold.

And as they grew, so more leaves
and with them, more seeds,

so that now
there's a fig tree growing here.

So here you have a complete garden,
200ft up in the forest,

with no part of it
touching the ground in any way.

But some of these squatters
can become murderers.

A young fig, like this, may arrive
here as a seed, brought by a bird.

At first, it grows quite slowly.

As it gains in strength, its roots crawl
downwards over its landlord's branches.

Some dangle free but keep on growing.

Eventually, they reach the ground.

Now, supplied with nutrients from
the soil, the fig grows really fast.

The rootlets wrapped around the main
trunk thicken and fuse into a lattice.

The host tree's fate is now sealed,

for it is in the clutches
of a strangler fig.

As years pass, the fig's roots thicken,
embracing the trunk ever more completely.

Trees grow by increasing their girth.
For this tree that is now impossible.

Growth is difficult anyway as the fig has a huge crown
in the canopy cutting off sunshine from its host,

and its roots are stealing
most of the soil's nutrients.

Eventually the host tree is killed
and its trunk rots away.

But the fig does not fall.

Its roots now form a hollow cylinder that is
quite capable of standing upright by itself.

This strangler
is about 300 years old.

It's maybe misleading to refer to it
as one tree.

Probably, 300 years ago, there were
several young figs up in the canopy.

Now, centuries later,
their roots are down to the ground.

They got rid of their victim's body,

and they cling to one another
in this amazing interlace of pillars

in order to maintain their dominance
of this part of the forest.

Nor are these monsters always
satisfied with just one victim.

This 500-year-old, having strangled its
first victim and lost its support,

toppled into a second; killed that;
and then a third.

And now its roots are ready
to embrace a fourth.

Dead tree trunks are not wasted.

And neither are dead leaves
when they fall.

Both are food for fungi.

Some leaves are captured
even before they reach the ground.

The fungus constructs a net, stretched
between the twigs of the undergrowth.

Once they've caught their leaf,
the threads put out white filaments.

These produce powerful acid which
dissolves the leaves' cellulose.

Why doesn't it dissolve the fungus?

Because fungi are not plants. Their
bodies do not contain cellulose.

They are made instead from a material
much more akin to animal horn and hooves.

Fungi are neither plant nor animal.

They belong to a category of life
that is all their own.

Nourished by the liquid tissues of leaves,
this fungus puts out more threads.

But fungi do require moisture.

They can only live out in the open
like this in the moist rainforest.

In cooler, drier woodlands they have great
difficulty in living out in the open.

Instead, they hide in the ground or
the tissues of the bodies they eat.

A fungus has no stem, no root,
no leaves.

Most of the time, it's only a
tangled tissue of branching threads.

These produce digestive acids,
absorb the resulting soup,

and then use it to construct more threads and
widen their search for more dead plant tissue.

But cellulose is very low
in nitrogen.

To get that,
some fungi trap living animals.

The microscopic threads
develop tiny lassoes.

These give off a chemical that attracts
microscopic worms - nematodes.

One of them nuzzles into the ring.

And the fungus
suddenly draws its lasso tight!

The worms are killed
and the fungus has its nitrogen.

All this takes place out of sight,
below ground or inside a dead plant.

Only when a fungus is ready to reproduce
does it make itself more visible.

From such apparitions as these
come spores -

the fungal equivalent of seeds.

They are so small
that they drift away like smoke.

But the appearance
of these constructions is brief.

As soon as their spores are shed, sometimes
after only a few days, they collapse.

Now they are merely food for maggots.

So plant corpses
don't retain nutriment for ever.

Some of it is consumed by fungi,

and the remainder, now in soluble form, seeps
back into the soil to sustain the next generation.

But it's not always easy for
that new generation to get a start.

On the north-west coast of America
and British Columbia

fir, spruce, and hemlock are so dense
that little light filters down to below.

Except when one of them dies.

This giant tree fell
about ten years ago.

The gap it left in the canopy above
is still open,

but there's little light
on the forest floor.

These ferns and mosses
are so very thick.

But up here, on the fallen trunk,
things are very different.

The next generation.

Bare bark doesn't hold moisture.

Thick moss could bury a seedling.

But in moss like this,
in such a position,

a young plant can get all
the moisture and light it needs.

Its thread-like roots grow downwards
over the surface of the trunk.

Even before they reach the ground,
they find plenty of nourishment,

from soil accumulating
around the clumps of moss,

and from bark,
already being broken down by fungi.

Eventually, the roots make contact
with the earth

and, one by one, a whole group of
vigorous saplings grow on the trunk.

So, eventually, a line of trees stands in
a line of almost regimental straightness.

Each is propped up on arching roots
that never were beneath ground

but which still hold between them the
rotting remains of the huge trunk

that, 150 years ago,
nursed them into life.

Southern Australia in the rolling
mountains outside Melbourne.

It's drier than North-West America, but
there's still enough rain for tall forests

and with them,
a rich, dense undergrowth.

These ferns are even taller than the ones
in British Columbia - these are tree ferns.

But they're still only understory.

Above them, rises one of the tallest
of all trees.

These are called, locally,
mountain ash.

But they're no relation
of the European ash.

They're eucalypts
and they stand over 300ft tall.

They carry their tiny seeds
in small capsules.

They shed them, a few at a time,
throughout the year.

But in the deep shade,
the seeds have no chance.

Even landing on a fallen trunk they would not be high
enough above the ground to be clear of the tree ferns.

The fact is that, in a mature forest, the
mountain ash has a severe regeneration problem.

The solution
could not be more dramatic.

It's another event that for human
beings is a major catastrophe -

a forest fire!

Oil in the bark and leaves of eucalyptus
makes them extremely inflammable.

As the flames spread, the seed
capsules are singed, and they empty.

The fire grows
and becomes a firestorm.

In the wake of such a huge burn,
little is left alive.

But, safe in the soil,
below the worst of the heat,

many of the eucalyptus seeds
have survived.

In the sunshine, they sprout
with extraordinary speed.

Few of their competitors
can match them.

Nourished by the rich ash, they grow around the
charred logs, as thick and uniform as a crop of wheat.

Within a year or so, they are so tall that no
other competitors are able to invade their land.

This stump was burning
exactly ten years ago.

And these saplings sprouted from seeds that
germinated immediately after that fire.

The saplings are so close together,
their roots interlock,

forming a dense mat in which
few other trees can get a roothold.

This land still belongs
to the mountain ash.

As the trees begin to spread their canopies,
the less vigorous saplings are thinned out.

Now there is space on the ground
for the tree ferns to return.

Only Californian giant redwoods
are taller than the mountain ash.

And that
may only be because of the loggers.

Back in the 19th century, one ash
was felled here that was 435ft.

That's the tallest tree ever known.

The greatest threat to the survival
of this forest

is that it should grow
for hundreds of years without fire.

Should that happen, then these huge
trees will eventually die from old age.

They'll fall and lie buried among the
tree ferns, without having reseeded.

The paradox is that this magnificent forest can
only survive if it is first almost destroyed.

Another fire,
this time in the west of Australia.

The soil is so poor, the climate
so dry, tall trees can't grow.

Instead there is a rich variety
of bushy plants.

The grass tree has long leaves
that burn quickly but fiercely.

It is neither a grass nor a tree
but a strange relation of the lilies.

What looks like a woody trunk is a fibrous
stem that has a special protection.

It sheds its leaves each year.

The bases stay attached to the stem

making a gum that glues the whole lot
together into an effective fireguard.

Even if it doesn't save every one,
most will survive.

Every species here has to be able to
live through at least a brief fire,

otherwise it would quickly
lose its place in the community.

It's eight months since the fire and
the bush has recovered dramatically.

The fire stimulated the plants
in at least four different ways.

Its sheer heat baked these fruits of the Banksias,
opening these windows and allowing the seeds to fall.

The fire also produced great volumes
of ethylene gas

and that triggered the grass tree after
it had regrown its burnt leaves,

to put up this huge green spike which
will, ultimately, carry its flowers.

The smoke of the fire
was the stimulus

which caused seeds that had lain
dormant in this sand

for the last 20 years or so,
since the last fire,

it triggered them to produce
the annual plants now springing up.

And it was the sudden superabundance of
nutrients produced by the fire in its ash

which let perennials, like this
cat's-paw, cover the ground with color.

Within a few years, the whole
community is fully restored.

The fire was an opportunity for
the social struggle to begin again.

Each plant had to fight once more
for living space,

profiting or not
from its recovery speed,

or its suitability to any change
in the terrain created by the fire.

But, overall, the character
of the bush remains unchanged.

The East African grasslands
are not so stable.

Here, fire or its absence
can trigger great change.

This fire is advancing quite fast.

It's so dry that as soon as the fire gets onto
a bunch of leaves they're gone in seconds.

As long as there is wind behind it
then it will travel.

But, in fact,
the line of the fire is very thin.

And, if I want to,
it's quite easy to cross it.

The land certainly looks ravaged and destroyed.
But, in fact, little damage had been done.

The leaves of the grasses have gone.

But the roots... are undamaged.

The heat is least intense
close to the ground.

And this part of the root, close to
the surface, is totally undamaged.

It's from here
the new growth will come.

And there are many around to relish
the succulent young leaves.

Grasses can survive such cropping

because their leaves break
at the base when pulled,

leaving the horizontal stems
and their buds undamaged.

Plants with vertical stems
grow from the top.

If they are cropped,
they're likely to be killed.

And if they're pulled,
they come up roots and all.

So the great herds of Africa, in effect,
weed out the grass's competitors.

It has vast acres to itself, which
suits the grazers AND the grass.

But something can upset this happy
arrangement - another of those catastrophes.

This time, it's drought.

The sun turns the soil to dust.

The animals, having eaten the last
of the dry stalks, move away.

The land is left bare.

Scavengers come to clean up the corpses
of those that starved to death.

But they too will soon leave.

When the rains at last return,
the grass springs again.

And so do other plants
whose seeds ended up here somehow.

With no animals to graze them
acacia seedlings grow into bushes.

And they grow very fast.

At a couple of feet high,
they'll survive a grass fire.

And their thorns will protect them
from most grazers.

By the time they're ten years old, their
branches cut off the light from the ground,

their roots
take most of the moisture,

and the grass
has virtually disappeared.

A patch of acacia scrub
has established itself.

All the trees are of the same age
and height.

They all took the same opportunity
to sprout.

But this situation
isn't permanent either.

A hungry elephant that fancies a mouthful
of acacia has no problem, or hesitation,

in pushing over the whole tree
in order to get it.

Strangely, it's not just acacias
they knock over.

They also push down connifera trees.

They don't even like the leaves
and seldom eat them.

So is that just wanton destruction?

Or can they possibly know what the
effect of removing the trees will be?

Whatever, a group of elephant,
once they have taken up residence,

can turn what looked like promising acacia
woodland back to grass in only four or five years.

The grazers return.

And the elephants once more have supplies
of their favorite food which is grass!

Elephant are, without any doubt,

prime factors in removing
thorn scrub from the plain

and allowing grass to spread.

You could say
they are farming the grass,

knocking over trees they don't like and
browsing thorn scrub to destruction.

But you could also
put it the other way.

You could say the grass exploits
the elephants.

By providing them with food, year after year, the
grass remains the dominant plant on the plains.

And if you look at things that way, then that is a
trick which other grasses have played across the world.

Wheat once grew only around
the Mediterranean.

Ten thousand years ago a small group
of humans began to eat its seeds.

It was so much to their liking that
they sowed it wherever they settled

and carried it with them
as they overran the Earth.

So, a few species of grass, by recruiting the aid
of animals, in particular, us, the human animal,

have succeeded in interrupting
the ecological cycles

that have operated for millions
of years in much of the Earth.

They've claimed for their own exclusive
use, not only wide, open plains

but fertile well-watered lands that once
supported rich communities of animals and plants.

These grasses have solved
the social struggle.

They have got rid
of their competitors.