Hello. This is 6 Minute English from
BBC Learning English. I'm Neil.
And I'm Sam.
Over the past 18 months, we've heard
a lot about the human immune
system - the cells in our bodies
that fight diseases like coronavirus.
We know that in humans the blood
stream carries immune cells
around our body.
But what about trees and plants?
They don't have blood, so how
do they protect themselves?
That's a good question, Sam, and
the answer involves memory. Us,
humans, store memories in our brain,
but our body also remembers things,
including stressful situations
from the past, which it stores
in our genes. The information
gets passed on to our
children genetically.
But surely trees don't have
memories, Neil! I mean, do
you think a tree can remember
being young or what it
was doing last year?
Well, not exactly, but trees
grow rings - a layer of wood
for each year of growth.
That could be a kind of memory.
In this programme, we'll be
asking whether trees can
remember - and if so, does
it make them stronger and
better able to fight disease?
But before that I have a
question for you, Sam. As I
said, trees grow a new ring
every year and by counting
them we can estimate their
age. One of Earth's longest
living trees is The Great
Bristlecone Pine, found on
the west coast of America.
But how long can these
trees live? Is it:
a) over 1,000 years?
b) over 3,000 years? or
c) over 5,000 years?
Wow, it'd be a job to count
the rings on those trees! I'll
say b) over 3,000 years.
OK, Sam, we'll reveal
the correct answer later.
Unlike us, trees don't have
blood and bones to protect them
from outside attacks, so how
exactly does a tree's
immune system work?
That's what BBC World Service
programme, CrowdScience, asked
bioscientist, Jurriaan Ton.
Here's what he said:
Plants in particular need to
have a very efficient immune
system for two important reasons.
Firstly, they sit at the bottom
of the food chain so there
are a lot of opportunistic
organisms out there, including
insect herbivores and microbial
pathogens who want to tap
into that biochemical energy
that is stored in plants.
The other reason is plants
are rooted to the ground -
they cannot escape from the
stressful conditions in
their environment.
It's hard for trees to protect
themselves. Unlike animals, they
can't run away, and they're
at the bottom of the food
chain - the plants and animals
linked in a chain of eating
weaker things and then being
eaten by stronger ones.
Rabbits eat grass and,
in turn, are eaten by foxes.
Right. If you are at the
bottom of the food chain,
everything wants to eat you,
including opportunistic animals.
If something is opportunistic,
it takes advantage of a
situation to gain some
benefit for itself. Tree
leaves are opportunities for
hungry insects and
caterpillars to eat.
So, trees need immunity
because they're under attack,
either from disease or from
living things wanting to
eat them. But what
about memory, Sam?
If trees can remember stress -
types of insects that eat it,
for example - they might be
better prepared in future.
For me, stress is a work
deadline or moving house,
but for trees it's more basic,
something like not
getting enough water.
Dr Estrella Luna-Diez believes
trees record stress in their
rings. A small ring, showing
that the tree didn't grow
much that year, indicates
some outside stress. She
explained more to BBC World
Service programme, CrowdScience:
Our hypothesis would be that,
depending on the level of
that stress - if it was a
really long-lasting drought
of a few years, then maybe
the tree can remember it for
a long time because it needs
to adapt to that hostile
environment. Now, maybe the
hypothesis would be the other
way around, maybe if it was
a very dry July for instance,
maybe the tree is not even
that bothered and then it
forgets within one year
because that memory of
stress is gonna be holding
it back on its growth,
for instance.
Dr Luna-Diez has a hypothesis -
an idea that explains how or
why something happens which
has yet to be tested to
see if it's correct.
Her hypothesis is that trees
remember stressful outside
events, something like a
drought - a long period of
time with little or no rain.
For a tree which has lived
for hundreds of years it
might be useful to remember
that 1947 was a
very dry summer.
On the other hand, maybe
that stressful year is
best forgotten. Maybe the
tree is not bothered - not
worried or concerned
because it's not
important to it.
So, trees do have memories -
but they don't let it
get them stressed!
Maybe that's the secret to
a long life! But what's
the answer to your
question, Neil?
Ah yes, I asked you how long
Earth's oldest trees, Great
Bristlecone Pines, can live.
I said b) over 3,000 years.
Was I right?
You were wrong, I'm
afraid, Sam. They live
even longer - over 5,000 years,
in fact all the way
back to the Bronze Age.
What memories those trees must
have - if only they could speak!
Right, let's recap the vocabulary
we've learned, starting with
immune system - the body's way
of fighting infection
and disease.
A food chain describes the
ways plants and animals get
eaten and eat each other.
Opportunistic people
take advantage of
a situation to get some
benefit for themselves.
A hypothesis is an idea to
explain how or why something
happens that hasn't been
tested to see if it's correct.
A drought is a long period
of time with little or no rain.
And finally, if you're not
bothered about something,
you're not worried because
it's not important to you.
Our six minutes are over.
Bye for now!
Bye!