For a long time, we have known that plants can move.
But recent studies have found that they can also smell, touch
and taste, respond to sound,
perceive shapes and hold memories.
We knew plants were sensitive to temperature, sunshine and humidity,
but new research proves plants also have other surprising senses,
offering us new insights.
When seen up close, and even very, very close,
nature takes on a whole other dimension.
Our eye perceives plants as a branch with leaves, a flower,
the silhouettes of trees.
But when closely examined, the intimate details of plant life
are both curious and stunning in their complexity.
These strange-looking antennae are in fact the epidermis of an ordinary
leaf, enlarged hundreds of times through digital 3D microscopes.
Scientific imagery enables us to see the world of plants in ever greater
detail, whilst revealing that plants still hold many mysteries for us.
Like humans, plants have a vascular system similar
to our veins and arteries.
An electric network like our nerves, hormones and other similarities.
But with no heart or brain - essential for us -
plants have developed other solutions for survival
throughout their evolution.
Solutions we could learn from.
Stefano Mancuso, a professor at the University of Florence
in Italy, founded the first international laboratory
of vegetal neurobiology over ten years ago.
The lab studies the interaction between plants
and their environment.
- Plants are able to sense the environment in an exquisite way.
They are much more sensitive than animals,
and they need to be more sensitive because
they cannot run away from the danger.
- Rooted to the ground, plants have evolved fine-tuned
and highly efficient sensory capabilities.
- Sensitivity is not something that is linked to a brain.
The brain in itself is a stupid organ.
It's just an amount of cells that we have here,
and every animals have.
There is nothing mysterious or supernatural
about the cells of our brains.
They are just a kind, a specific kind of cells called neurons.
But you don't need neurons to make this stuff.
You can have other kind of cells having the same function,
so it means transmitting signals from one cell to the other.
- Humans react instantly to their environment,
especially when confronted with danger.
Nerve impulses transmit through our nervous system at a speed
of 100 metres a second.
But do plants have a similar response?
To calculate the speed of electric signals in a plant,
Bruno Moulia and his team from the French National Research
Institute for Agriculture, Food and the Environment place electrodes
on the stem of a shrub.
The electric impulse, seen in this sped-up footage,
is only six to eight centimetres per minute.
The slower speed is logical given the plant's sedentary nature.
To create the organic matter required for growth,
a plant needs light provided by the sun,
carbon dioxide present in the atmosphere, and water.
Light is absorbed through chlorophyll,
the tiny round globes contained in plant cells.
Thanks to chlorophyll,
light, water and carbon dioxide are transformed into sugars,
which the plant feeds on.
This process, known as photosynthesis, releases oxygen
into the Earth's atmosphere and provides all the organic matter
and the energy necessary for life on Earth.
The procedure seems simple, but in everyday life,
plants are regularly confronted with vital problems.
Water is capricious and plants don't want too much or too little,
and it must fall at the right time.
Roots capture water in the soil through a network of hairs.
When water is scarce, the roots perceive the threat,
causing the stomata to close.
Stomata are mouth-like pores found on the epidermis of leaves...
..enabling gas exchange and the release of water vapour
produced by photosynthesis.
When roots signal a lack of water, the stomata pores shrink.
The plant captures less carbon dioxide and growth slows.
Daniel Chamovitz, a botanical genetic scientist, discovered a gene
in the hairs of plants that is similar to those responsible
for the vibrating hairs in the inner ear of humans.
- From a genetic point of view,
plants have the same genes that are needed for hearing
that animals have. This gene in animals is a gene which enables
the hairs of our inner ears to be formed, and these are the hairs
that vibrate in sound waves.
These are genes that are needed for us to hear.
Plants have essentially the same genes, which might lead us to think
that plants hear the same way.
But when we look more in depth at what these genes do,
they are necessary for forming of hairs at the tips of roots.
But these hairs apparently are not listening, but they're necessary
for the roots to absorb water.
So while the genes cause the same structures to be formed - hairs -
in humans, it's needed for hearing,
and in plants, it's needed for drinking.
- This unexpected proximity fascinates and drives Stefano Mancuso.
- We are quite sure that plants are able to use the information
coming from the sounds in the soil, for example, to detect the quality
of the soils, the amount of water in the soils, the presence
of obstacles in the soil.
So they are, in some way, using information coming from
sounds for... To have an idea of the space around them.
So just put a loudspeaker close to these roots, and you will see
that if the frequencies are in the range between
100 to 1,000 hertz, the roots will turn toward the source of frequency.
If the frequencies are higher than 5,000, the roots will grow
against the source of the sound.
- The plant Desmodium gyrans was once thought to dance to music,
but in fact, it moves its leaves towards the best light source.
Some may say that plants do better and grow faster
when music is played.
- What is important for plants is not the music, but the frequencies.
We found that there are a range of frequencies
in the low part of the spectrum, between 100 and 1,000 hertz.
There are many frequencies that we could say plants like.
They are completely insensitive
to the quality - if it's classical rock, blues and so on -
of the music, but they are able to detect specific frequencies
and to react to these frequencies accordingly.
- If there is one sound which humans find disturbing,
it's the whining of a mosquito in the dark.
Are plants aware of buzzing insects?
In Israel and in the global scientific community,
the work of Lilach Hadany is creating a buzz.
Her research shows that plants can identify the sounds
made by pollinators.
Flowers produce nectar to attract the pollinators that feed
on the sugar it contains.
When harvesting the nectar, pollinators fertilise the flower
with pollen brought from other flowers.
A researcher at the University of Tel Aviv,
she has recorded bees as they sip nectar.
She plays the recorded buzz through a loudspeaker held above
a flowerbed of primroses.
Lilach and one of her colleagues then harvest the nectar secreted
by the flowers and measure its sugar levels with an analyser.
- Much sweeter nectar this time.
- INSECT BUZZES
- It's estimated up to 30% of the energy of the plant
is wasted on nectar. So, to keep nectar high in all times
is a very expensive process
and also that attracts all sorts of robbers - bacteria, fungi, birds.
If you can have nectar low when pollinators are not around
and increasing sugar concentration only in times where pollinators
are around, that could save the plant a lot of energy.
- While the forms and shapes of flowers are very diverse,
many are concave - shaped like a bowl or a bell.
- If we look at this flower,
it has this, uh, bowl shape,
a bit like a small satellite dish.
So, we suggest that the flower is the external ear of the plant.
An ear that has evolved to respond specifically to the sounds
of the pollinators of this very plant.
Bees or moths.
This flower vibrates in response to the sound of bees.
But if we remove some of the petals...
..this remaining petal would not be able
to respond so much to the sounds.
- Lilach picks a few primroses to take back to her lab.
She wants to see if the petals can detect the sound of bee flight.
She observes them under a high-resolution optical microscope.
When she plays the recordings of buzzing bees and other
pollinators, a laser beam measures the petal vibrations
to establish a scale of the flower's reactions.
- With the laser vibrometer, we were able to detect tiny
vibrations in response to sound,
and we saw that indeed the flower vibrates in response
to bee sound, but not in response to much higher
sounds like bat sounds.
- The flower does not react to sounds made by other animals.
But a buzzing bee results in the flower
producing a sweeter nectar.
- If plants hear, if plants sense sound waves,
they do it in a completely different way.
It didn't say, "Oh, what should we listen to today?"
But it did develop the ability to respond to the sound
of a pollinator, and then to differentiate between a pollinator
and a non-pollinator.
And that makes sense, also, because if you're responding to every
wavelength, then you're just wasting energy in making
nectar for someone who will never drink it.
- We are still thinking of the possibility of plant
communication after hearing the bee sound. For example,
within the same bush of the evening primrose,
there is one flower exposed directly to the bee.
But is the signal transmitted to other flowers on the same plant?
This is something we intend to study.
- MUSIC: Flight Of The Bumblebee
Could the Flight Of The Bumblebee by Nikolai Rimsky-Korsakov
have been composed for the wellbeing of his plants?
If plants can sense specific sounds for their own benefit,
can they emit sounds?
RUSTLING
Over the years, scientists have wondered how the clicking sounds
in plant stems were produced.
Many believed that the clicks were air bubbles bursting
within the stem's vascular system.
At Tel Aviv University,
Lilach and her team have shown that the plant clicks
are very high frequency sounds,
like bursting popcorn.
To avoid interference from any background noise,
the team installed ultra-sensitive microphones ten centimetres
from the test plants.
- It actually worked.
Within a few hours, we discovered that plants indeed emit sounds -
brief ultrasonic clicks high above human hearing ability.
- CLICKING
- Moreover, these sounds contained information
about the stress of the plant.
- The frequency of sounds emitted by tomato plants and vines
in distress was between 20 and 100 kilohertz.
This is beyond human hearing, which is limited to 16 kilohertz.
The experiment continued in the acoustic chambers
of the university's soundproof bunker.
Scientists stress the plants by cutting them,
pinching them, or depriving them of water.
CLICKING
- You see, this is a strong sound, this drying plant is actually
emitting a lot of sounds and is in significant stress.
In our experiments, they peaked about the fifth day without water,
and then, the sounds start declining until a very dry plant does not
emit much sound.
We cannot say yet if plants speak, in the sense of emit sounds
for communication purposes
that are identified by someone else in a constructive way.
- CLICKING
Plant sounds increase in number and intensity
as stress levels increase,
but long before they are visually dehydrated.
Lilach and her team are pursuing this research.
If plants emit sound signals that have a meaning, especially
at the start of a drought, this information could help farmers.
In humans, some body odours attract mosquitoes,
while other body odours do not.
Are there any similarities in the green kingdom?
Each plant species has its own olfactory signature,
which comes from the complex structure of the fragrant
compounds it releases.
Odiferous capsules and bulbs are very common on the leaves
of tomatoes or cannabis.
These capsules release a strong scent which is easily identifiable.
The strong smell of lavender is produced by large pockets
that burst upon contact.
Professor Consuelo De Moraes,
from the Swiss Federal Institute of Technology in Zurich,
has spent many years studying plant odours
and how plants detect odours.
Odours are measured under these glass domes,
where tomato plants are infested by a strange vampire plant.
- Cuscuta is a parasitic plant, so it needs a host to survive.
So, it's a true parasite.
It doesn't have roots.
It doesn't do photosynthesis.
So, in order to survive, it has to attach to a host.
It has to find that host.
And the question is - how do they find the host plant?
And what we discovered was that they actually smell
the host plant.
- The cuscuta seed does not have long to find a host once it sprouts.
One might think it latches on to the first plant within reach,
but that's not the case.
It makes a choice.
This high-speed footage shows the cuscuta hunting for a host.
- I think they're smelling the tomato plants as,
"This is something that I can survive on."
They really can sniff.
- How do the cuscuta find the tomato plants?
- Cuscuta, like, germinates, it comes out of the ground.
And then they, like, you know, start growing
towards the tomato plant.
And our hypothesis is that the perception of the smell
is at the tip of the vine.
We now know that plants can perceive smells, but we don't know yet
how they do that.
What are the mechanisms of this nose perception?
- The differing odours allow the cuscuta to select a plant
which smells strong enough to support it.
- One thing that we were amazed to see is that the cuscuta
have this ability also to perceive, you know,
a healthy plant versus a sick plant.
They have - I mean, in my view - an incredible sense.
- This plant is unhealthy,
so the cuscuta rejects it.
Once a host has been selected, the cuscuta wraps around it
and sucks its sap, leaving just enough to keep the host alive.
When the cuscuta have flowered and produced their seeds,
they exhaust their host and die with it.
If plants can smell each other,
some can also smell pest insects.
- One of our recent discoveries is that actually plants can also
perceive insect pheromones.
So, we showed that eurosta flies produce pheromones that attract
the females, and the plants,
these plants that they have co-evolved with,
these goldenrod plants can perceive these pheromones
by smelling the specific compound that the flies are producing.
This pheromone indicates an upcoming threat for the plants.
- When it detects the eurosta parasite fly pheromones,
the plant defends itself.
For instance, it flowers at a different time.
Discovering that plants can perceive the smell of some insects opens
a wide field of research,
including applications for ecological farming.
If water is vital for plants, light is just as important.
- What is light for a plant?
But it's its energy to make food through photosynthesis.
A plant has to know where the sunlight is, so that it could do
photosynthesis, in order to make its sugars,
in order to have energy.
And so, therefore, it has to be exquisitely sensitive
to where the sun is.
- Plants do not sense light with an organ like the eye,
but with each and every cell.
Translating light signals into action -
germination, growth, flowering.
- Compared to plants, humans are visually impaired.
While we can see the colours of a rainbow,
that's the whole spectrum we see from blue all the way
through to red,
plants see, respond to all of the visual light that we see,
but they also perceive, sense and respond to light waves
that we're blind to, such as UV light or far-red light.
- Hydroponic farms light their plantations
with ultraviolet and infrared light, which favour photosynthesis.
This ensures maximum continuous plant productivity.
At France's Institute for Agriculture,
Food and the Environment, Elzbieta Frak is studying
how plants communicate through light signals.
Exposure to coloured filters reveals that plants absorb blue,
use red for photosynthesis, reflect green,
and communicate in infrared.
To demonstrate the infrared communication, Elzbieta tracks
a control plant as it moves along a rail.
A light source and an optical sensor are placed behind the plant.
Measured in nanometres,
the optical sensor detects the movement of the control plant
by the variations in the infrared light signal being reflected
as it moves along the rail...
..similar to the way an infrared camera starts
recording when an animal passes in front of it.
Elzbieta has installed masts equipped with high-precision
optical sensors.
Each sensor is placed in the middle of a ring of evenly spaced
identical young plants, and pivots through 360 degrees.
The sensor measures the signals exchanged by the plants
in the near-infrared,
the specific wavelengths that neighbouring plants reflect.
The signals reveal that each plant locates its neighbours
and adapts its growth so that it does not waste energy.
Elzbieta Frak is at the forefront of research revolutionising our
knowledge of plants.
In the near future, farmers could use mathematical models
to predict the interaction between plants of the same
and different species,
or with natural elements such as wind.
We are often sheltered from wind in our daily lives,
but wind is part of the natural environment of plants.
In Clermont-Ferrand, Bruno Moulia and his team are studying plant
reactions to all kinds of wind...
..from a caressing breeze
to the violent shocks caused by storms.
The experiment is repeated under natural conditions.
Several trees are restrained with straps and cables.
Monitors are installed that measure the degree a tree bends
according to wind strength.
Bruno attaches the same type of monitor to other trees,
which are not restrained and can move freely.
Distinguishing unusual, dangerous winds
from ordinary, inoffensive winds
supposes that trees have an active memory over long periods
of time, perhaps even throughout their entire lifespan.
The sharp angle at the base of the tree is damaged
from an exceptional gust of wind.
The tree was bent but survived.
The cells of the wood fibres that make up the trunk of the tree
stretch and retract permanently, like hydraulic cylinders,
depending on the strength of the wind.
This elasticity allows it to bend without breaking,
and, once the gust of wind has passed,
to recover a vertical position.
The forest of Krzywy in Poland is known as the Crooked Forest,
marked by a wind that almost destroyed it.
Events sculpt the silhouettes of trees telling us
about their lives.
If knocked over, why do trees straighten up again?
This irrepressible tendency has fascinated scientists.
In the 1900s, the botanist Wilhelm Pfeiffer was one of the first
to create moving pictures of this phenomena.
Do plants only straighten up to capture
sunlight more efficiently?
Bruno Moulia has designed an experimental lighting chamber
with over 90 fluorescent light bulbs shining from all directions.
Under uniform lighting conditions,
the plant still straightens up vertically in a single day.
The conclusion? It is not light perception that guides the plant
upwards, but another sense.
Could plants be sensitive to gravity?
To answer this, we need to examine how the plant cells react
to a change of direction.
The French team pursued its investigation.
They wanted to know if the plant could locate itself in space.
So, in the absence of gravity, could it still find its stem,
branches and leaves in relation to each other?
Bruno Moulia's team cancelled the effect of gravity by rotating
young plants in a device called a gravitron.
The plants were photographed over several days.
The experiment showed that plants can perceive
their location in space.
Like humans and animals,
they also have a sixth sense called proprioception.
To perceive the position of your own body in the space
around you is vital for organisms that grow
in an ever-changing environment.
But unlike humans, plants simultaneously inhabit
two environments. One above ground and another below,
with their roots also facing ever-changing conditions.
Roots endlessly explore underground,
each one searching for vital water and nutriments.
The neurobiologists Frantisek Baluska and Stefano Mancuso
are interested in the tips of the roots called the apex.
- All the root apices, they are coordinated in their activities
and growth, and they resemble some swarms,
for example insect swarms or birds.
Somehow there is some communication.
We still don't understand how it is going on,
but they are coordinating and behave as one swarm.
- Each plant root is like one unit,
like a swallow, an ant or a termite within its community.
Information collected by each apex is transmitted to every cell
in the root and body of the plant,
creating a collective intelligence
that determines future tasks and coordinates action.
- There is no central organiser,
and that's why these networks are not
vulnerable to some stress situations.
These networks can self-organise very effectively
and they can survive any bad stress situation.
The network resembles very closely the internet network,
so one could call it a root-wide web.
- The networks are all connected, and depending on the information
collected by the multiple apex,
the root filaments change direction,
accelerate, slow down or branch out.
However, if a root apex is cut off, the root does not change direction.
The upper root with no apex grows straight,
while the lower root, with an intact apex,
moves around, constantly seeking information.
Strong electrical activity can be detected in the apex,
and it is the part of the plant that uses the most oxygen.
- This is the same kind of signal we can record in the animal brain. - Yes.
Really, these networks are also used to exchange information,
not just to provide water or nutrition.
We have still no idea how complex the communication there is.
- In every plant habitat,
roots are weaving infinite networks.
These root networks are further enhanced by uniting with another
complex underground network - filaments of fungi.
Roots and filaments grow in symbiosis to form the mycorrhiza.
In a forest, this network links the trees,
increasing the reach and capacity
of both root and filament networks 10,000-fold.
The mycorrhiza is a network of intense interaction.
Exchanging information, rare mineral elements
and sugars produced by the trees.
In this forest near Jerusalem,
pine, oaks, cypress, carob and pistachio trees
all grow together.
The Weizmann Institute is at the forefront of research
into mycorrhiza networks.
- On the root of this big pine tree behind us,
we see the mycorrhiza growing all over the root,
transporting materials from the root
of this tree to the next tree, and on and on.
- With a specialist underground camera, researchers map
the mycorrhiza networks.
- Wow. Look at these roots.
And it seems like they are full of mycorrhiza.
- Wow. It's amazing.
Maybe it's connected to the younger pine.
- The underground maps charted by the Weizmann Institute team show
that the trees share carbon, and therefore sugar,
with neighbours of the same species, but also with other species of tree.
- We showed that up to 40% of the biomass in the fine root
of this pine tree can actually come from carbon
exported from an oak tree nearby
through this mycorrhizal connection. We know that both
control the association.
For example, a tree can stop giving carbon to a fungi that is not
providing water or nutrients.
And the fungi themselves, they are very flexible and agile.
They can leave a tree and reconnect to another side of a tree
where they get the carbon from.
- The scientists measure the level of carbon in the tree roots
and in the mycorrhizal fungal networks.
They also measure the level of carbon in leaves with a device
that captures their photosynthesis data.
Back in the laboratory, a new generation of sequencing
techniques quantifies the carbon in the samples.
- Using the carbon measurement,
we discovered that the carbon can move from leaves to roots
in a tree within three days,
and it takes another six days to move
from the roots of one tree
over five metres to the roots of another tree
through the mycorrhizal hyphae.
- Are the mycorrhiza capable of feeding the trees
in extreme climatic conditions?
To find out, pine trees have been covered by opaque bags
to prevent them from photosynthesising.
- Wow!
Look at this poor plant.
It died because it was not connected.
It was separated from the middle plant here.
But this one survived the shading
because it was connected and got
carbon from the middle pine to this pine.
- The Israelis are concerned about the possible impacts of climate
change on their forests.
They have set up an experiment that restricts the amount
of rainwater in the soil.
It will take five years to understand how the drier
conditions impact the mycorrhiza network.
- In agriculture, you can breed specific mycorrhiza and apply
it to your fields in big scale,
but in forests, it is actually impossible.
We have to learn what the nature does.
We have to think about the diversity.
The more tree species that we plant,
the more mycorrhizal species
that will be there below ground,
and will make more intricate, more complex networks
that will make the forest more resilient and more sustainable.
- Science is still far from having made an exhaustive inventory
of the senses of plants and their resources.
Future discoveries will no doubt bring many new surprises.
The more we learn about plants, the more they amaze us.
The Swiss Federal Ethics Committee on Non-Human Biotechnology considers
that plants, like animals, have dignity and a moral value.
- If tomorrow, the humans would disappear from the planet,
in a few centuries, plants would take control of everything,
and every single trace of the human civilisation
would be hidden by plants.
But if tomorrow, the plants would start to disappear
from the planet, in a few weeks, probably, at least
all the terrestrial life would disappear,
transforming the planet in a desert.
- The latest discoveries teach us that the frontiers between
the human world, the animal world and the plant world
are not as distinct as we once thought.
Revolutionary thinking is bringing an immense change to how we see
the living world.