Plants - Chapter 9

- Hello, I'm Lindsay Turnbull, and I teach biology at the University of Oxford. In this episode, I hope to convince you that plants are every bit as interesting as animals. Armed with stolen technology, these glorious green beings stormed onto the land 400 million years ago, long before any vertebrate set foot there. And they're the subject of Chapter Nine of my book, "Biology: The Whole Story".

(birds chirping) (frog croaking) (water rushing) (bird chirping)

Try to imagine for a moment, a world without plants. I hope you are imagining somewhere pretty barren and inhospitable, because even from space, our continents glow green, and nearly every habitat on Earth is defined by the plants that live there. What's any forest without its trees? The problem actually is though, that plants are so ubiquitous that we kind of don't even notice them, and we just take them for granted.

And lots of people when asked, think that plants maybe are a bit boring, and that's because they don't move very quickly, and they don't have faces, so we don't really know what they're thinking. But I hope to convince you in this video that that's just totally the wrong attitude. Plants are properly awesome, and I wanna tell you their story.

Well, the story of plants isn't so different to the story of vertebrates. Plants moved onto the land, and in order to do that, they had to make some significant changes to their bodies. And once they were on the land, many of them supersized to produce giants. But there is one very big difference between plants and any animal. Animals eat other cells and they digest those cells into their component molecules, and they use those molecules to build their own bodies. But plants don't do that.

They build themselves from scratch. Now, in order to understand how they do that, we need to look inside their cells. And we might start with something like this piece of onion skin. When you put that under a microscope, you can see a single layer of box-like cells. We can see the nuclei inside, so we know they're eukaryotic cells. And those box-like structures are the cell wall. So plants have evolved a new kind of cell wall made from cellulose, which animals find quite hard to digest.

Now the onion belongs under the ground. If we looked at a slice of a leaf, we might see something different in the cells. We are struck by these little green blobs, and those are a special kind of organelle called a chloroplast. Now, if we remember in the chapter about eukaryotic cells, we looked at mitochondria, and we said that they are in fact, domesticated bacteria. They're the powerhouse of the eukaryotic cell now, but once they were free-living bacteria.

And these chloroplasts are exactly the same. Once they were also free-living bacteria, a special kind of bacteria called a cyanobacteria. And they are famous because they invented what was probably the most disruptive piece of technology that ever evolved on Earth, photosynthesis.

Now, photosynthesis involves capturing energy from the sun and using that energy to smash up water molecules. Water molecules consist of hydrogen and oxygen atoms, and the reason you wanna smash them up is you wanna get the hydrogen out. If you combine hydrogen with carbon dioxide, then you can make glucose. And once you've got glucose, you can make a whole range of other organic molecules. The oxygen that's left over just trickles off into the atmosphere. That's just a waste product.

Now, this machinery to carry out this process is very sophisticated, and it only evolved once from the cyanobacteria. But that didn't stop other cells from stealing that technology. So the first algal cell captured a cyanobacteria and put it to work, and that obviously lived in the ocean. And today there are many kinds of algae living in the ocean and in fresh water. Many of those are single-celled, but some of them are multicellular, like these seaweeds.

There's a whole range of different kinds of seaweed. And at some point in the past, one of those algae came out onto land and founded the dynasty of the land plants.

Well, thanks to the Oxford Herbarium, I'm holding in my hand a rather old microscope slide. And stuck to this slide is a very thin slice of rock. Now, the type of rock is a chert, and the rock came from a village in Scotland called Rhynie. So this is a Rhynie chert, and it's about 407 million years old. Now, today Rhynie just looks like fairly normal farmland, but 407 million years ago, it looked like this.

Now, this is Yellowstone National Park in the USA, and you can see these hot springs and geysers, and sometimes those boil over and entomb the organisms growing around them. Now, let's put this slice of rock under the microscope. And what we see are some circular structures, and those are the stems of some of the very earliest land plants that were growing in a landscape like that.

They only grew to about 20 centimetres high, so they were quite small, but they possessed some of the key adaptations that plants need to grow on land. And let's have a look at those. So the first one is that dark blob in the middle of the circle, and that is a pipe that pipes up water through the stem of the plant. The second feature that they had is that when plants came onto land, they were in danger of being sucked dry.

So they evolved a thin waterproof layer, a waxy cuticle that stopped water loss. The problem with that is, if water can't get out, then carbon dioxide can't get in, and plants need that for photosynthesis. So they peppered that waterproof layer with tiny holes called stomata, and we can see those stomata in the Rhynie chert plants. Finally, if we look down to the bottom of the plant, we see some long thin cells that push down into the soil to take up water.

So these three features between them mean that plants take up water in the soil, they pipe it up through their bodies and out through holes in the leaf. And this means that plants can suck up huge amounts of water. In fact, a large tree in full leaf can suck up a tonne of water in a single day. And that's why forests are uniquely good at recycling water. And clouds often form over forests during the course of a day, and rain then falls down in the afternoon.

And that's why forests are very, very important for regulating the local climate.

Now, as well as having an impact on the climate, plants can also have a profound impact on the composition of the Earth's atmosphere. Photosynthesis removes one molecule of carbon dioxide from the atmosphere and replaces it with one molecule of oxygen. And this process creates glucose for the plant, which it uses to build its body.

Now, when the plant dies or it's eaten, that molecule of glucose is respired, and that returns a molecule of carbon dioxide to the atmosphere and uses up a molecule of oxygen. So as long as photosynthesis and respiration are in balance across the planet, then the composition of the atmosphere won't change. The levels of oxygen and the levels of carbon dioxide will remain roughly the same.

But more than 350 million years ago, as the first forest spread across the globe, that balance just wasn't there. Those forests were filled with very strange trees, and when they died, they weren't eaten, and they didn't rot away. Instead, they lay buried under tens of millions of years of rock. And that meant that because of all that buried plant material, the levels of oxygen in the atmosphere shot up, and the levels of carbon dioxide plummeted.

And that would've had all kinds of effects on the organisms that were living at the time. Now, what's happened to those trees? Well, those fossilised trees have become fossil fuels. So the massive coal deposits that lie around the world are all made up of trees that were buried hundreds of millions of years ago. And of course, we are digging up that coal, and we are burning it, and doing that returns carbon dioxide to the atmosphere that hasn't been there for more than 300 million years.

And that's why the concentration of carbon dioxide in the atmosphere today is rapidly rising again. And because carbon dioxide is a greenhouse gas, that is having profound impacts on global temperatures, and that warming of the planet causes all kinds of catastrophic events to become much more common.

Now, modern plants don't form coal. The vast majority of plants on the Earth today belong to a group called the Flowering Plants. And they came along relatively late in the Earth's history towards the end of the period when the dinosaurs dominated. And of course, they brought with them welcome colour, because the flowers of flowering plants are all incredibly beautiful. And the flowering plants have diversified into all kinds of forms and shapes.

So we have enormous trees, cacti in deserts, beautiful orchids, tiny duckweeds that float on the surface of ponds. And we even have some plants that have turned the tables on animals, like this Venus Flytrap that supplements its diet by catching insects. But of course, most plants have a much more benign relationship with insects, a mutually beneficial one, in which the insects visit the flowers and collect pollen and transfer it around in return for sugary rewards.

That's the nectar that the plant produces to tempt insects to come and visit. Now, that relationship between plants and insects is just one example of an ecological interaction, and the science of ecology is what we're gonna look at in the next episode.

Well, I really hope you enjoyed that video, and that I succeeded in convincing you that plants are really worthy of your attention. If you wanna get your own copy of the book, don't forget there's a link below, or share this video among friends and colleagues. There's a lot more information in Chapter Nine about plants, particularly about how plants are expert traders.

They monopolised the sugars commodity market, and they then use that monopoly to strike hard bargains with lots of other organisms. Otherwise, see you next time for the next episode all about ecology. (birds chirping) (water rushing) (birds chirping) (wildlife bustling)