Tip:
Highlight text to annotate it
X
NARRATOR: They don't have
eyes or ears,
but they can find
their own food.
They lack a brain,
but some scientists
think they can communicate,
cooperate, and even wage war.
Perhaps plants lead anything but
solitary, sedentary lives...
MAN: They actively respond to
the nutrients,
and the predators,
and the herbivores
that are around them.
NARRATOR:
But not everyone believes
there's a "social side"
to plants.
If you talk to a lay person
about plant behaviors,
they'll just think
you're crazy.
If you talk to a scientist,
they'll think you're crazy
and wrong.
NARRATOR: From nurturing
their young
to eavesdropping on
their neighbors,
it seems plants are doing...
Whoa!
Yeah.
And saying quite a bit.
MAN: It's the plants' way
of calling for help.
NARRATOR:
We just need to listen...
NARRATOR:
For the past few months,
plant ecologist J.C. Cahill
has been criss-crossing
the continent,
researching a new book
that focuses on
one central question --
do plants behave like animals?
An idea that seems a little
far out to a lot of people.
If you talk to just a lay person
about plant behaviors,
they'll just think
you're crazy.
If you talk to a scientist
about plant behavior,
they'll think you're crazy
and wrong.
NARRATOR: And you can
understand the skepticism.
Out in the field,
observing plant behavior
is a little bit like
watching paint dry --
unless, of course,
you speed things up.
They may not swing from branches
or gallop across the savannah,
but plants do move,
and they do behave.
And one of the ways they behave
is through growth.
But does all of this growth
really constitute behavior?
Are the movements of plants
in any way comparable
to this?
Right now,
this fox is hunting for mice,
using every weapon
in his evolutionary arsenal
to find a meal.
And this plant is doing
pretty much the same thing.
When an unsuspecting insect
roams into a Venus Flytrap,
all it takes is a brush
with two of the plant's
trip hairs,
and the trap is sprung.
The bug is then
slowly digested,
providing the plant
with much needed nutrients --
unless, of course,
a lucky victim
manages to escape.
For years,
we just assumed that
the flytrap was the exception
that proved the rule --
"plants don't behave."
Turns out,
we were wrong.
What people don't know is that
all plants are doing this.
All plants are not necessarily
eating living organisms,
but they're having
elaborate behaviors
above ground
and below ground,
but they're slower than
the snapping of the flytrap,
or they are happening in
the soil so we can't see them.
But all plants are complex,
and all plants have
complex feeding behaviors.
NARRATOR: In fact,
every plant on Earth
is on a constant
hunt for food,
including the light they need
to photosynthesize.
And with the help
of time-lapse cameras,
we can now enter their world
and see how they do it --
climbing upward
and tracking the sun
as it wheels
across the sky.
But plants don't just
need light to thrive --
they also need nutrients,
food that lies
in a hidden world
that's just below
our feet.
As much as 80% of
a plant's total mass
lives below the ground,
in a secret world scientists
once called the "black box."
But with the aid of
new technology,
we're now exploring
that world
and discovering that,
when it comes to finding food,
plant roots are
a lot more animal-like
than we ever
imagined.
Not unlike
this grizzly family,
who are busy foraging for
berries and other edible plants.
CAHILL: So, when an animal
moves through the forest
and it's foraging for berries,
like a grizzly would,
it will find a berry patch
and it'll slow down and it'll
spend more time there,
maybe walking, without really
going in a forward distance.
The plants do something
roughly similar to that.
NARRATOR: Back in his lab
at the University of Alberta,
Cahill has been using
this high-tech camera
to explore the underground world
of foraging plant roots.
Some have
even nutrients
and some have
patches of nutrients.
Yeah, so we have...
NARRATOR: These grow boxes
have been seeded
with nutrient patches,
and Cahill
and student, Pamela Belter,
have taken thousands
of pictures,
documenting how long
it takes the roots
to reach the nutrients,
and how they behave
once they find them.
...the plant itself,
about two and...
NARRATOR: It takes long hours
to review those images,
but the surprises
are worth the wait.
Let's go ahead
a couple of days.
BOTH: Whoa!
So, this is huge growth
over three days.
This goes, what...
NARRATOR: The sudden root growth
confirms their suspicions.
CAHILL:
Almost three centimeters.
NARRATOR:
Over three days,
the growth rate of one root
suddenly accelerates,
as it homes in on
a nutrient patch.
Then, just as suddenly,
growth slows down,
while the root,
like the grizzlies,
eats its fill.
Roaming legs
or multiplying growth cells --
the mechanism may differ,
but the foraging behavior
is still the same.
The question is,
how do they do it?
How do plants find
the food they're looking for,
both above the ground
and below it,
when they have no eyes,
no ears,
let alone no brain?
Well, the feeding habits of
this strange,
snake-like vine
may hold the answer.
It's called the "dodder vine,"
the Count Dracula
of the plant world.
The vine has no roots
and can't produce
its own food,
so it lives entirely
off a host plant.
And it has just 72 hours
to find that host,
or it dies.
Its tiny teeth-like probes
pierce the stem
and grow into its victim,
draining it of its
life-giving sap.
And this botanical "vampire"
seems to prefer some plants
over others.
Tomatoes are among
its favorite victims.
So, how does it
find its host,
and how does it choose between
one plant or another?
J.C. Cahill
has come to Pennsylvania
to meet Consuelo De Moraes
and Mark Mescher,
the scientists
who solved that mystery.
MESCHER:
So, there's a patch here
of our species
that grows locally.
MORAES: We brought this plant
to the lab,
this parasitic plant,
cuscuta -- dodder.
We're looking at
how these plants interact,
but how do they
find a host.
And we thought for sure somebody
had already done that,
and we went to back to
the literature,
and there was
nothing on that.
So, what would happen to dodder
if it just was really poor
in its ability
to detect its host?
Well, these guys are
obligate parasites,
so they're completely dependent
on the host plants.
So, a seedling of dodder
has to find a host plant
within, you know,
a few days,
or they'll exhaust their
energy resources and die.
So, really, we expect really
intense pressure on these guys
to be good at foraging
and identifying their host.
NARRATOR: But while
the dodder vine
may be good at
finding a victim,
could it actually choose between
two different host plants?
De Moraes and Mescher made it
their mission to find out.
In a series of experiments,
they placed
wheat and tomato seedlings
in the same pot,
and planted
a newly sprouted dodder vine
between them.
Then, they set up
a time-lapse camera
to see if the seedling
was actually
making a choice.
For hours, it circles
the air like a snake,
as if sniffing out
its victims.
And nine times out of ten,
its preferred victim
is the juicy tomato,
a tender plant
that's easier to attach to.
MORAES: You really get the sense
of a behavioral response.
So, really, there is some fairly
strong selection here
for this plant to make
the right decision,
otherwise it will die.
NARRATOR: But how was the little
stem making its choice?
The team decided to
play a hunch.
They knew that all plants
produce green leaf volatiles,
chemical scents released by
their leaves as they breathe.
So, maybe
this predatory plant actually
was sniffing out its victim.
To test that theory,
the team devised
another experiment.
First, they captured
the scent of a tomato,
essentially condensing
the chemical odor
released by the plant.
[ Gas hissing ]
Once it's distilled,
they present the tomato perfume
to the vine,
along with
a real tomato
it can't possibly smell.
Time after time,
the dodder homes in on
the chemical language
that says,
"Yes, I'm a tomato."
There's no doubt with the dodder
there's choice.
There's choice
involving the, uh,
a suitable host
or non-suitable host.
This is a very familiar thing
in animal foraging behavior
that we're seeing in this
plant foraging behavior.
NARRATOR: But the dodder
isn't the only plant
that's exhibiting
animal-like behavior.
Once it's under
full-scale dodder attack,
the tomato releases
the chemical equivalent
of a scream.
In fact, many plants emit
a chemical SOS
when they're under attack,
and we've all caught
a whiff of it.
It's the smell of
freshly cut grass.
CAHILL: We all love
the smell of freshly cut grass,
we all love the smell of flowers
that we put into a vase,
we all love
the smells of plants.
But those smells
mean one thing to us
than they mean to the other
individuals in that environment,
and we are causing stress,
we are causing trauma
to these plants.
It's the plant's way
of calling out for help.
NARRATOR:
So, if it's a cry for help,
who or what are plants
calling out to?
Well, if this unassuming
desert plant is any indication,
they may be calling in
some pretty effective
reinforcements...
insects that eat
the insects that eat them.
The desert isn't
the most welcoming place
for people,
but it can be an ecological
nightmare for some plants.
Unlike us, plants can't
escape the heat
or walk for miles
to find water.
Nor can they run and hide when
they're attacked by insects.
But it's precisely because
they can't move
that plants have evolved
some pretty nifty methods
of self-defense.
We used to think
or used to view plants
sort of as just
sitting there,
whatever happens, happens,
they make their seeds,
and they go on.
But we're realizing
it's much more complex.
They're actively engaging
with the environment
in which they live.
They actively communicate.
They actively respond to
the nutrients,
and the predators,
and the herbivores
that are around them.
It's a really
dynamic system.
So, when you
take a look at a plant,
and if you were to
rip off a leaf,
and then think about this
from the plant's point of view,
what just happened was
something came around
and ate some of
its body.
And so this plant that was just
damaged by me ripping it off,
is likely to start changing
its defensive chemistry,
it can start communicating
with its neighbors or insects,
and all those
processes begin.
NARRATOR: And here
in the Utah desert,
there's a wild species
that's showing us
just how dynamic plants are
when it comes to self-defense.
It's called
nicotiana attenuata,
the wild tobacco plant.
For more than a decade,
Ian Baldwin has been studying
the wild tobacco
and the amazing ways
it responds to threats
in its environment.
BALDWIN: This plant's genome has
probably an order of magnitude
more genes involved in
environmental perception
than most animals do.
Most plants have to,
because they sit still
and they have to really tune
their physiology
and biochemistry
to what's going on,
and they need a very
sophisticated system
of perception
and response.
NARRATOR: And being able to
respond quickly
is essential for
wild tobacco,
because its seeds need wildfire
to kick-start their growth,
and they can wait for hundreds
of years for that to happen.
So, when they
finally do emerge,
they may face enemies
they've never seen before.
BALDWIN: It has no idea
what it's gonna face
when it germinates
out of that seed bank
and has to cope with
whatever's there.
There are all these
other organisms
that rain in on this habitat
that's just been cleared out
by a fire.
Just about every part
of the plant
is attacked in
a different way,
by a specialist that feeds on
that particular part.
It's a very
complex problem
they've got to solve.
NARRATOR: And it's not
just one problem.
This plant's enemies
are as plentiful
as desert sunlight.
But it turns out that
the wild tobacco
has a secret chemical weapon
to deploy.
As soon as an herbivore attacks,
it ramps up a toxin --
one that some of us
are all too familiar with.
It's evolved a toxin
that will poison
any organism
that has a muscle,
and that is this molecule
we call nicotine,
the one that human beings
have such a relationship with.
So, anything that moves
and wants to eat this plant
is going to be
poisoned by this thing.
NARRATOR: But while its nicotine
cocktail poisons some bugs,
it has absolutely no impact
on this one.
In fact,
the hornworm caterpillar
can mow down a tobacco plant
in a matter of days.
But this cunning
little plant
has a few more defensive tricks
up its leaves.
Once the caterpillar
starts feeding,
the plant's leaves release
an SOS --
chemical messages
that drift up into the air
where they're picked up by
the enemies of their enemies --
predators that just love
feasting on
caterpillars.
And if you find it
hard to believe
that plants can call in
insect mercenaries,
Baldwin has proof.
In one experiment,
he captured
the chemical signals
released by the leaves of
plants that were under attack.
Then, he glued
caterpillar eggs onto a leaf,
smeared them with
the chemical scent,
and waited to see if
anyone would show up.
Within a matter of hours,
this insect has responded to
the plant's call for help.
It's called
the big-eyed bug,
a pint-sized predator that
devours eggs and larvae alike.
In fact, it's even been known
to take a bite
out of a full sized
caterpillar.
But wait a minute --
how does the plant even know
who's attacking it,
let alone which predator
to call in?
Well, the answer lies in
yet another chemical message,
this one
delivered by
the caterpillar itself.
BALDWIN: When the caterpillar
chews on a plant,
it has to have saliva
in its mouth,
and in that saliva there are
these various compounds
that provide information
to the plant,
and the plant uses those
compounds to say,
"Ah, it is the hawk moth
and not a *** bug
that's feeding on me,"
and so it adjusts
and tunes its responses
to that particular
herbivore.
NARRATOR: And Baldwin has
discovered that this plant
has another secret weapon,
specifically designed
to rid itself
of caterpillars.
This is a trichome,
a sweet little treat
deposited by the plant
and irresistible
to caterpillars.
Beautiful, yes, but it's
as lethal as a land mine.
When this little guy
chows down on a trichome,
it gets a very bad case of
body odor.
BALDWIN: Twenty minutes after
eating a trichome,
they're smelling.
So, what we've learned from
these particular smells
is that they inform predators,
particularly
ground foraging predators.
The plant is offering
this nice, little sugary
first meal for the caterpillars,
but it's an evil lollipop,
because the caterpillar
gets tagged for predation.
It's the razor blade in
the apple at Halloween time.
Plants, after all,
can't run away,
so they have to do this.
They have to be able to solve
their environmental problems
by changing the organism
that they are.
NARRATOR: And being able to
change who they are
is critical to
the tobacco plants' survival,
because it turns out that
the mother of these
voracious caterpillars
is also the plant's
best friend --
its main pollinator,
the hawk moth.
Tonight, Baldwin is in the field
trying to lure the moth in
through the irresistible
draw of light.
Wild tobacco flowers
bloom at dusk,
the perfect draw for a nocturnal
pollinator like the hawk moth.
As the moth sips nectar,
it gathers pollen,
spreading it from
one plant to the next.
But while the moth happily does
the plant's *** bidding,
it has its own
reproductive agenda.
A single moth
can lay as many as 200 eggs --
eggs that grow into
plant-munching
caterpillars.
So, sometimes,
despite its best defenses,
the wild tobacco can still get
infested with caterpillars.
Even then,
the plant has
another card to play.
It simply
switches
pollinators.
Baldwin's colleague,
Danny Kessler,
was the first to observe
this astonishing
behavior.
He was out photographing
tobacco plants
in the early hours
before dawn.
Most were infested
with caterpillars.
As he worked, he began to
notice something unusual.
Instead of
blooming at night,
some of the flowers
were opening at dawn --
and the daytime flowers
didn't look
or smell
the way they should.
KESSLER:
They're different completely
from the night opening flowers
in terms of
nectar volume,
sugar concentration.
And what we found out later,
even they didn't emit
floral volatile as well.
When we walked around
and we saw that
almost any plant had
caterpillars on them,
it was really a huge outbreak,
and we felt,
"Hmm, what's going on here?"
It's kind of...
It was weird, right.
NARRATOR:
And the weirdness continued.
Not only had the bloom's nectar
and perfume changed,
the shape of the flower itself
had completely transformed.
Essentially,
by changing its flowers
and bloom time,
the plant
had stopped talking
to its nocturnal pollinator,
the hawk moth,
and struck up a conversation
with a daytime pollinator,
the hummingbird.
BALDWIN: The eggs
that hummingbirds lay
don't hatch into caterpillars,
they hatch into
little baby hummingbirds,
which don't eat plants.
So, by switching its pollinator,
it avoids a whole group
of herbivores
that it would normally get.
NARRATOR:
No one knows for sure
why the plant doesn't
permanently switch pollinators,
but the switch can happen
in less than eight days.
The ability to change
the shape and smell
and the quality of nectar
in flowers,
almost immediately,
is incredible.
It's incredibly complex,
and we have no idea
how common this is
across species.
It's a very novel
and new finding.
NARRATOR: And the surprises
don't end there.
When one plant is attacked
and starts to signal,
other plants
can eavesdrop
on its chemical messages,
and may respond by ramping up
their own defenses.
And Baldwin
has also discovered
that when you block the plant's
ability to hear itself talk,
it seems to go
a little crazy.
If you basically
plug the plant's ears
so it can't hear that volatile
that it's producing,
it begins to
scream louder.
For one thing, they don't know
when they're pollinated.
They will produce
floral scents continuously.
They'll yell and yell and yell
for pollinators,
even though they were
pollinated a long time ago.
One could interpret that
as evidence of self-awareness.
If they're not able to
perceive themselves,
everything goes wrong.
NARRATOR: So, if plants are
basing their behavior
on signals they receive
from their environment,
is it possible that
they're also interacting
with each other?
In other words, could plants
have a secret social life?
For a pride of lions,
social life isn't all about
childcare
and cooperative hunting.
Adult lions can be
fierce competitors,
fighting over everything from
mates and meals to territory.
[ Lions snarling ]
So, what about plants?
Do they fight over things
like food and terrain?
Well, if you speed things up,
you can actually see
how plants compete --
pushing and shoving
as they struggle
to capture sunlight.
But here in Montana,
there's a new plant in town,
a beautiful,
nasty weed
that doesn't just compete
with its neighbors --
underground, it's waging
territorial war.
It's a foreign invader
from Eastern Europe
called "spotted knapweed,"
and it's killing off
the native grasses
the local cattle
love to eat.
Bad news for rancher,
Dave Mannix.
Dave's family has been raising
cattle on this land
for more than
a hundred years.
And like all ranchers,
he's fought the weather
and taken on predators
to keep
his herd alive.
Never in his wildest dreams
did he imagine
that a single plant
could take him down.
Succession with our children
is a big thing,
and the economic viability of
our industry is a big thing,
and then right there with that
is knapweed.
That scares us worse than
predators scare us,
that's for sure.
And if we lose that battle
to knapweed, you know,
then we've lost the base
for our whole enterprise.
You know, sometimes there's
a dominant species out there,
that's a plant that has
an unfair advantage for a time,
and right now is its time.
We focus a lot of our attention
and monetary resources
trying to take care of
our range, battling knapweed.
NARRATOR: That battle has
involved everything
from chemical sprays
to biological weapons,
like these sheep.
A hired hand
moves this herd
across hundreds of acres
of ranch land,
because sheep are happy
to mow down knapweed,
unlike cattle
and other herd animals.
MAN: Wildlife, like elk, deer,
consume it, but it doesn't
seem to be preferred.
The reality is, it's taken over
a lot of territory,
and it's both
a destructive plant
and it's also an absolutely
fascinating organism.
NARRATOR: Fascinating because
even the insects
that eat knapweed
don't seem to slow it down.
Imported from
the plant's home range
in Europe,
this weevil's offspring
burrow into knapweed's tap root,
where they feed
until fully grown.
But they seem to have
very little impact
on the health of
the plant.
CALLAWAY: There's two of
these insects in here,
blasting away at the root,
and look at the plant,
it's healthy as hell.
NARRATOR:
But a lack of natural enemies
can't fully explain
the phenomenal success
of this land hungry weed.
As J.C. Cahill
is about to discover,
spotted knapweed has invaded
some 4.5 million acres
of Montana rangeland.
And no one knows more about
its aggressive behavior
than plant ecologist,
Ray Callaway.
CALLAWAY: So, J.C.,
this is a pretty good example
of a really dense
knapweed monoculture.
It's not a very big patch.
But you notice
it follows the road.
Knapweed loves disturbance.
Usually when you get
something this dense,
disturbance is a part of it,
but there's nothing
native in here.
So, this is maybe
a little more typical
of a spotted
knapweed invasion.
You know there's
a lot of knapweed,
maybe it's more of it
than anything else,
but there's still
a few natives left in here.
I mean, this kind of scene
covers a lot of Montana.
CAHILL: It's been neat,
and sort of sad,
to see this,
at the same time.
This isn't just a plant
that academics use
to answer neat
academic questions.
This is a plant that is causing
people hardship on the land.
But I would say again,
this is a lot more typical.
So, there's a huge drop
in biodiversity
that follows this invasion,
and so these plants are just
knocking off the natives.
CALLAWAY: Yeah.
NARRATOR: So, if knapweed is
the plant equivalent
of the Terminator,
how does it wage war?
Well, there may be
a lot of factors
giving this invader
a leg up,
but the root of the problem may,
quite literally,
be in the roots.
To uptake essential
soil nutrients,
knapweed roots deploy
a variety of chemicals.
And when they're released,
the chemicals have a nasty
added benefit.
They seem to kill off
a lot of native grasses,
allowing knapweed
to capture and hold
huge swaths of territory --
a behavior that's
very animal-like.
CALLAWAY: Interestingly enough,
this could be
one neat behavioral sort of
analogy with plants,
holding a territory
through the release of chemicals
that harm your neighbors,
and then making the resources
that are there available to you.
So, J.C., this experiment...
NARRATOR: And experiments
like this one
show just how lethal
those chemicals can be.
...competitive ability
of this spotted knapweed.
Callaway planted
native grass alone
and others in pots
with knapweed.
In the latter group,
half the samples
were treated with
activated carbon,
a substance
that would neutralize
at least some of the toxins
released by the knapweed roots.
The results
are astonishing.
So, these grasses that
I'm showing you here
were all planted
at the same time.
This one's growing by itself,
no big surprise,
but it's huge,
it doesn't have
any competitors.
It's a happy plant.
It's a nice, happy plant,
nothing affecting it.
This one is growing
with the knapweed, obviously,
and there's no activated carbon
in the soil,
so this plant is, you know,
what, 50 times smaller.
CALLAWAY: Knapweed is absolutely
waging war with its neighbors,
but it's doing it
not just in the standard
light and nutrient
sort of way,
it's also doing it with these
chemical interactions
in the soil,
and so it's adding
this whole new dimension.
This chemical warfare is novel,
and it has these abilities
to kill the native plants,
which is very rare
among plant species.
NARRATOR: But there's one native
plant that can fight back.
These colorful little flowers
belong to the wild lupin.
And lupins can launch a chemical
counterattack of their own.
Like their knapweed neighbors,
lupin roots release a chemical,
called oxalic acid,
to get food from the soil.
And amazingly, that chemical
also acts as a defensive shield
against the knapweed toxins.
But these feisty little plants
don't just defend themselves --
they also seem to protect
the plants around them.
CALLAWAY: My lab
has shown in the field
that not only does lupin seem to
do well against knapweed,
if we experimentally
plant grasses
near lupin
in a knapweed patch,
they do much better
next to lupin.
It's a really exciting thing
to consider
in terms of potential ways
that plants may interact,
that really has not
been explored.
It's a totally behavioral
sort of analogy.
NARRATOR: So, what does
this complex behavior
teach us about
the social lives of plants?
Well, when you look around
at the incredible diversity
of the plant world,
you start to realize
that knapweed's style of
"killer competition"
isn't the dominant form of
social interaction among plants.
If it were,
we'd be looking at
far fewer plant species.
CAHILL: So, what we see in
the natural world isn't just
struggle for resources.
It's a balance of positive
and negative interactions
that sometime is beneficial,
sometimes is detrimental,
but it all depends on
the environment.
There is no "plants compete,"
and that's the end of story.
That idea is dead.
NARRATOR: So, what other forms
of social interaction
are at play
in the plant world?
If plants aren't
the lone wolf competitors
we once imagined
them to be,
are they as sociable
as the animals that eat them?
For herd animals,
like these elephants,
recognizing family members is
an important part
of social life.
In fact,
for many animals,
kin recognition
is an essential skill.
WOMAN: Animals use kin
recognition for two things --
to recognize their relatives
and avoid mating with them,
and to benefit their relatives
in social interaction.
So, animals may give off
warning calls
if there's a predator near,
only if their relatives
are near,
but not if strangers
are around,
and animals would generally
avoid mating with relatives.
It's well known that plants have
a mechanism to at least avoid
mating with themselves, so it's
a kind of kin recognition.
But people hadn't known
if plants could benefit
their relatives
in social interaction,
so, and that's what
we've been working on.
NARRATOR: When she began
her research,
Dudley knew that plants could
sense the presence
of other plants
above the ground.
Using photo or light receptors
in their leaves,
they can sense when they're
being shaded by a neighbor
and respond by growing taller
or in a different direction,
to compete for light.
And plant roots
have a similar ability.
DUDLEY: People have shown that
plant roots
would respond differently
to their own roots
than to a neighbor's root.
So, the question is,
can do anything?
Do they know who they're
interacting with?
So, I said, well,
this is a good place
to look for
kin recognition,
'cause wouldn't it
be cool?
NARRATOR: So, Dudley
and student, Amanda File,
headed for the shores of
Lake Ontario
on a hunt for a very special
kind of plant,
to see if its roots
behaved differently
when growing
next to its kin.
It's called sea rocket,
and its reproductive behavior
made it ideal for
a kinship study.
Here's a good-sized plant.
NARRATOR: Sea rocket
produces two different
kinds of seed pods,
one of which
clings to its mother,
resulting in seedlings that
often grow up
side by side
in a family group.
It's in flower, and it's
got a few big fruits.
This is probably
the biggest one.
And 'cause they're
annual plants,
the mother plant dies,
and can get buried by,
you know,
by the wind and the waves,
and so you have a whole bunch of
seedlings potentially coming up
from that buried,
dead mother plant,
so they would all be siblings,
sharing the same mother.
NARRATOR:
And family members were
exactly what Dudley
was after.
In a series of experiments,
she and File
planted some
sea rocket siblings together,
and others with
unrelated plants.
After a few weeks, they cleaned
the roots, weighed them,
then checked to see if there
was a difference
between those growing with
siblings versus strangers.
DUDLEY:
I was extremely shocked
because we got exactly
what we predicted,
and this doesn't happen often
in science.
We're predicting a response,
we found exactly that --
with siblings there's lower root
allocation than with strangers.
NARRATOR:
In other words,
strangers grew more roots
to compete for food,
while siblings politely
restrained their root growth.
So, was that evidence of
a family sharing resources?
Was it proof of
altruistic behavior?
DUDLEY:
Altruism is defined as
doing a benefit to others
at some cost to yourselves.
And it's one of the things
get involved by kin selection,
but it's not the only one.
Two plants that do not compete
avoid paying that
cost of competition,
and because they're
benefiting a relative,
their genes are passed on
to a greater extent.
NARRATOR:
But whether it's a case of
enlightened self-interest or
a selfless sharing
of resources,
no one knew how these plants
were identifying
their kin.
Dudley knew that all plants
exude chemicals
from their roots,
so maybe sea rocket roots
were identifying one another
via chemical signals.
To test that theory,
Dudley and her collaborators
turned to Arabidopsis,
a fast growing plant that
exhibited the same kin response
as sea rocket.
When they placed two siblings
in water
and blocked all
chemical signals,
the seedlings suddenly started
ramping up their root growth.
Without the ability to
chemically communicate,
siblings had become
strangers.
Is it communication?
Are they deliberately
sending a signal?
That we don't know.
That's something that
we can't answer yet.
NARRATOR: Since then, Dudley's
lab has gone on to document
sibling responses
in at least
three other
species of plants.
Now, she and File
are moving on
to study some of the longest
lived plants on Earth,
to answer an even more
surprising question --
do mother trees
nurture their young?
It's a question
that's been asked before.
On the other side of
the continent,
there's a species of tree that's
defying all of our expectations
about the social lives
of plants.
In the feature film "Avatar,"
an ancient mother tree
creates a magical network
that sustains
every living organism
in the forest.
Here, in the rain forests
of British Columbia,
scientist Suzanne Simard
has been studying
the real mother trees
and the vast underground
networks they create
to nurture
their own kind.
SIMARD: When I saw
the movie "Avatar,"
I was sitting in the audience
with my 3D glasses on,
and there is
the mother tree,
and the network growing
out of the mother tree,
and I just went -- [ Gasps ] --
"They read my papers!"
NARRATOR: Those papers focused
on the magnificent Douglas-fir,
a tree that can live up to
a thousand years
and grow to
a staggering height
of more than 300 feet.
But like all trees,
only part of its total mass
can be seen
above the ground.
SIMARD: I feel when I'm walking
through the forest
that I'm gliding past
an iceberg,
'cause what I am seeing is just
the tip of what's going on.
What we see above ground
is just such a small fraction
of the body of
the ecosystem.
In a lot of ecosystems,
over two-thirds of what you see
is actually below ground.
We're seeing one-third
of the body of the forest.
NARRATOR: That mysterious
underground world
has preoccupied Simard
for more than a decade.
But while evolutionary theory
told a story
about competition,
the trees themselves
were telling her
a different story.
SIMARD: We think of them
as these individuals
that are just
competing against each other,
you know, "I'm gonna get this,
and I'm gonna get it,
and you're not gonna get it,"
and that's really led
a lot of the thinking.
And we've long ignored,
you know,
a lot of the other interactions
other than competition.
But at the same time,
there's a community effect
that we haven't understood,
and we're just starting to
really look at this
and understand that
this is a system,
it's not just
a bunch of individuals
competing against
each other.
They're working together
to make
this system work.
NARRATOR: So, what does
that community look like,
and how do trees
work together?
To answer
these questions,
Simard focused not only on
the below ground world
of the Douglas-fir,
but on their relationship
with some of the strangest
looking organisms in the forest.
SIMARD: And these trees are all
connected below ground
by their roots
and also by fungi.
And you can often see
the fruiting bodies of the fungi
right next to old trees
like this.
NARRATOR: Neither plant
nor animal,
the fruiting bodies
of fungi like this
bloom in the spring
and fall.
Each of their gills
is filled with tiny spores,
the source of
the next generation.
And just like
the fruiting bodies
of trees,
mushrooms represent only a tiny
fraction of a vast organism
that lives below the ground,
in networks, made up of roots
and organic material,
like this bright yellow
fungal tissue.
CAHILL: If we could see
in that soil,
we'd realize there's layer after
layer of just networks,
that -- and these are
big organisms --
just like these trees
are really, really big,
so too are the mushrooms,
they're just flat,
and they happen to be
predominantly in the soil.
SIMARD: When you're walking on
the forest floor,
you're walking on this massive
amount of fungus material.
And, so, anything that grows or
germinates in the forest floor
can't help
but be colonized.
NARRATOR: In fact, the massive
underground roots of fir trees
are colonized by
below-ground fungi,
because the fungi
can't produce their own food.
Instead, their vast
underground networks
tap into the roots of trees
and other plants.
The plants provide the fungi
with carbon-based sugar,
and the fungal network
returns the favor,
providing the trees
with nutrients.
Many plant species are dependent
on fungi for their survival,
and the Douglas-fir
is no exception.
In turn,
the underground fungi
are equally dependent
on the trees' carbon.
SIMARD: It absolutely
needs that carbon.
It cannot live without
the carbon from the tree.
There's a signaling that goes on
between the root and the fungus,
and they say, "Hey, we can
help each other out."
And the reason the trees
associate and do this
for the fungi is that
the fungi are so small, right,
they can crawl into little soil
spaces that roots can't grow.
It costs a lot less in carbon
to support the fungi
than to build
its own root system.
And so, it's a great
trading system.
It shuttles a lot of carbon
to the fungi,
the fungi then shuttle it
between themselves,
from fungus to fungus,
but also fungi can also
connect the trees together.
NARRATOR:
Simard has demonstrated
that vast underground
fungal networks
link the Douglas-firs together
into a kind of
resource sharing community.
SIMARD: We found that the big,
old Douglas-fir
are hubs for
this massive network
that is connecting pretty much
all the trees in the forest.
And the bigger the tree
and the older the tree,
the more roots
that its growing,
and the more mycorrhizal
connections that it has,
and what we found is that
those connections
are providing really
a rearing ground
for the new trees
that are coming up.
NARRATOR:
And experiments like this one
demonstrate how
that network operates.
First, Simard and graduate
student, Marcus Bingham,
bag the branches
of an older "mother tree,"
then expose it to radioactive
carbon-14,
a gas the tree naturally absorbs
to produce its food.
SIMARD: Okay, so, let's just
inject this C-14
or radioactive gas,
CO-2 gas,
and within a few hours
it's gonna be
coming down the trunk,
in its sugars
and into the root system,
and then in a day or two we
should see the radioactivity.
NARRATOR: Days later,
the team returns
to track where the radioactive
carbon has gone,
using a Geiger counter.
The results are nothing
short of amazing.
Let's pull out the Geiger
counter, see what we've got.
[ Geiger counter beeping ]
Oh, yeah.
NARRATOR: Not only has
the underground network
shuttled the mother tree's
carbon-based food
to surrounding trees,
experiments like this one reveal
that the biggest beneficiaries
are the youngest,
most vulnerable trees.
It's right in
the foliage.
So, that parent tree is really
giving lots of carbon
to its
offspring here.
That seedling over here, let's
see if that's picked up any.
Sure, okay. Oh, yeah,
this is all hot down here.
It's great, the whole
network's lit up, yeah.
That provides this great
source of nourishment
for those little
seedlings.
In an old forest,
you can imagine that
there's not a lot of light,
it's quite shaded,
they don't have the ability to
fix their own carbon very well,
they're really being nurtured
and grown up as a community,
as a family, almost.
And it's those relationships
that really build the forest.
It's really
a beautiful, self-organizing,
complex system.
What we're finding
is amazing.
And I know that there are
nay-sayers out there,
but those of us who are seeing
this are just going,
"Wow, it's awesome."
NARRATOR: So, if mother trees
can nurture their own kind,
if plants can recognize
family members
and communicate with
their friends and foes,
how are they doing it
if they have no brain --
no way to organize or integrate
the information they receive?
It's an interesting question,
both philosophically
but also biologically --
how do you
integrate information
if you don't have
a nervous system?
Plants may have
a parallel system.
There may -- there has to be
something that's doing
this integration,
we just don't know
what to look for.
NARRATOR: Maybe we're not quite
as smart as we thought we were,
and perhaps plants
are a lot more intelligent
than we ever imagined.
To learn more about what you've
seen on this "Nature" program,
visit pbs.org.