The Evolution of Trees
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What is the main topic discussed in this episode?
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This is In Our Time from BBC Radio 4, and this is one of more than a thousand episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. Hello. We might take them for granted in parks, in our streets and in forests. Yet trees play a crucial role in regulating our climate and providing the air we breathe. We build with them, rely on them for food and shelter, use them for paper, clothing, medicine and energy. But trees aren't simply a backdrop to human life. They're the product of a deep evolutionary history that transformed our planet. When they first appeared, they reshaped our ecosystems, altered the atmosphere and created entirely new environments on land.
Well, with me to discuss the evolution of trees are Jenny McIlwain, the 1711 Chair of Botany at Trinity College Dublin and Director of Trinity Botanic Gardens... Christopher Berry, Senior Lecturer in Earth and Environmental Sciences at Cardiff University. And Bill Baker, Senior Researcher at the Royal Botanic Gardens in Kew. Welcome to In Our Time. And Jenny, let me start with you. I think like most of the listeners, I can say that I recognise a tree when I see one. But what is a tree?
Yeah, that's actually such a hard question because we have to define a tree without using the word tree. So there is actually, there's a lot of arguments and discussion among plant scientists about what a tree is. But what we can say is that it has to have a trunk, so a main stem, and it has to have a canopy. And the canopy, we think, usually has leaves today, but not necessarily. So it is a canopy of branches. that trunk has to not die back. So we have structures that look like trees, but they're actually flowers.
What do we mean by die back exactly?
So it means that it's a perennial structure, so that it will last for many, many years and not just die after one or two years. And then to be a tree, the trunk and the crown have to be self-supporting. So what that means is that this beautiful organism can't rely on leaning on a wall, or before walls leaning on another tree or leaning on other vegetation. And then other people use a definition of a particular height and a particular width. So there's loads of arguments about that. But I think a good definition is a tree has to be above three metres. And then if you took your hand and at chest height measured the width of the trunk, it has to be wider than your hand to Classify as a tree. And then the big controversy comes with wood.
Yes, tell us, what is wood exactly? And can you have trees without wood?
Yes, we absolutely can have trees without wood. Some botanists are really strict and they say you can't be a tree unless you have wood. But I think that's far too strict. And if you think of a tree, it can be woodless, it can have wood. Trees have worked out many ways of becoming a tree. So what is wood? Wood is a tissue made up of lots of cells and those cells are really complex and they are hollowed out and they really look like straws in their simplest form. So they have a hollow centre and then the cell wall of the straw is thickened with a polymer that is called lignin. And lignin enables that tree to self-support and not fall down and not rely on other structures to lean on. So if the tree has lignin, we call it wood.
Actually, the Greek for wood is called xylem. And that's what those wood cells are called, xylem.
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Chapters
5 chapters
1
What is the main topic discussed in this episode?
0:00–13:34
2
How do scientists define a 'tree' and can non-woody plants be trees?
13:34–22:53
3
What is wood (xylem) and why did lignin enable trees to stand upright?
22:53–37:12
4
When and what did the earliest trees (e.g., Calamophyton) look like in the fossil record?
37:12–49:30
5
Where are the earliest fossil forests found and what do they reveal about ancient ecosystems?
49:30–57:37