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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 bloodstream 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.

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