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!