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Hello, my name's Lindsay Turnbull and I'm an associate professor in the Department of Plant Sciences

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at the University of Oxford, and we're right in the middle of this very serious corona virus crisis right

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now. And my students are all stuck at home and we want to keep them in touch with biology

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and keep in touch with us. And so we're going to make a new series of videos and they're going to be called back garden

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biology.

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Hello and welcome to this episode of Back Garden Biology. When, in fact, I'm in the front garden

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next to me is a plant that I'm sure is familiar to all of you. And if you're a gardener at the moment, you

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may have some of these in your garden mail, roses. And of course, English gardens are really famous for their

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roses. And this one's doing very well. It was planted by the person who owned the house before me

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and raised it get better and better. As they get older, they get more vigorous. They get a good root system down. And now

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it produces lots of these wonderful flowers. You can see that it's not a wild rose

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because it just has so many petals in each flower. So roses belong to the family.

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It's named after them. The roses see, and they're characterised by having simple flowers with exactly

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five petals. And this one clearly has got a lot more maps. The efforts of the breeder

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and the breeder in this case is called David Austin. He's very famous for breeding English roses

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and the varieties called Gertrude Jekyll. And it's a very famous, very popular rose in England.

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It smells really wonderful. So that's one of the good things about David Austin's roses and some of the other breeds

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they've kept the sense of phrase says, which is so important, the smell of Strether say, well,

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he wants to smell up. Okay. Today's question is, why is the world

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green? Now the leaves off the rose are green and so are the leaves, just about every other

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plant in the world. And that's the first part to that question. Why are the leaves of plants green

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and not some other colour? The second part to that question, though, is different. It's an ecological

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question. Why do the world's plants still have all their leaves? Why

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have they not been stripped to the bone by the collective actions of the world's herbivores?

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Now, the productivity of plants is is normally called primary productivity.

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It's not secondary productivity which comes from herbivores eating plants. And in the oceans,

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there's also significant primary productivity, not by plants, but by all different kinds of

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algae. And in the oceans, a lot of that primary productivity does end up in the

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mouths of herbivores. But on land, we know that although we can often find plants with a small

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amount of damage, it's pretty rare to see. I've certainly never seen an entire tree stripped to the bone.

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So why is that? Why does most of the primary productivity on land stay in the plants?

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Well, ecologists don't have full answers to that question. I can tell you, but we have. But

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I've been doing some investigations in my home from garden, and I think I can start to shed a little bit

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of light on why that might be. So we'll have to come inside to try to address the first part

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of that. Why is the world green question? The first part is the physiological part. Why are leaves

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green? And we've got a prison set which is refracting sunlight

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onto this whiteboard. And I hope you can see it. And it makes this beautiful rainbow. And that's because the light

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from the sun is actually in the visible spectrum is actually a mixture of colours. So

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you see the classic rainbow, Roy G. Dave, the I'm sorry, red, you gave us my red,

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orange, yellow, green, blue, indigo and violet. And the red are the longer wavelengths, the

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blue or the shorter wavelengths. And as the light passes through the glass, it is slowed

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down. But the but some wavelengths, the blue wavelengths are slowed down more

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so they get more and more bang, more refracted. And by going through two surfaces

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of a prison, they separate away from the reds. I've got somebody holding it up at the window

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and managing to get it at just the right angle to throw it onto here. Now, we know now

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that visible light consists of these different colours. So what does that mean to do the leaves being green?

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Well, inside the leaves there is a molecule called chlorophyll, and it

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absorbs radiation from the sun. And it can only absorb certain

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wavelengths. So it absorbs the blue light very well and

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it absorbs the red light very well. And so what it doesn't absorb is

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the wavelength in the middle. So what you're left with there is the green

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light keeps moving. It's about there fairly quite a narrow band that looks green.

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And the leaves reflect that green light. So they absorb the blue light. They absorb

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the red light. I may reflect the green light. Now, what's going on there? Well, when they

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absorb light, either the blue or the red, they use the energy inside

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it to excite electrons and electrons or the small negative particles, the orbit

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atoms, and by firing electrons out of the chlorophyll molecule.

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They are able to split water. So water is two hydrogens

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bound to an oxygen very tightly. And for the plant to make carbohydrates from

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carbon dioxide, the plant needs a source of hydrogen. So carbon that carbohydrates

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contain carbon, oxygen and hydrogen. Can get the carbon and the oxygen from

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carbon dioxide from CO2, but it needs hydrogen as well. And the only place on earth you can get

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a lot of hydrogen from is water. But getting the hydrogen out of water is really

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very difficult. And that's what the photosystem of the chlorophyll does. So it absorbs light

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energy. Fires an electron out of the chlorophyll molecule. And the chlorophyll

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molecule is now so desperate to get an electron back that it will smash up a molecule

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of water to grab an electron out of it. And that frees the hydrogen, which the plant

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then captures and settles away to make carbohydrates. So it's sort

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of amazing rally. But the leaves have this incredible system going on with absorbing the sun's

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energy to split water. And they're going to use the hydrogen there to make carbohydrates.

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Why can't they use the green wavelengths? Nobody knows. We do not understand

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now why it is that chlorophyll absorbs very well and the red wavelength and the blue wavelengths,

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but not in the green. But it really makes me think if you went to an alien planet, how likely

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is it that the chlorophyll that had evolved there would also not be able to use

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the green light? That is really, really unlikely. So it's quite likely that photosynthesis has

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evolved on another planet. But I bet you any money that all the plants wouldn't be green.

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So when the plants have splits the water molecules and shuttle the hydrogen away

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to attach that to CO2, to make carbohydrates glucose, then the oxygen

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in the water molecule is free and it's just put into the atmosphere. Of course, that's one of the amazing

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roles of the primary producers play. They generate all the oxygen that's in

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the atmosphere and that's their combined activities of all the plants on land and all the various kinds

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of algae in the oceans. Now, what about the second part of our question? Why is

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the world green? We were asking, why do all the plants have so many leaves? Why haven't they just been gobbled

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up by the world's herbivores? Well, it's partly because mature leaf like

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this one from this ivy here is not a very exciting meal. It's very tough.

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It's fibrous. It's packed full of a molecule called cellulose, which is what plants use to make

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their cell walls. And actually, animals don't have the enzymes to digest cellulose.

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There are specialist herbivores Caruth and various large mammals called ruminants,

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which can digest cellulose, but they can't do it themselves. They have to have these ridiculously enormous, elaborate

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stomachs that are full of bacteria that actually digest the cellulose for them.

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But there are other ways to eat plants that don't just involve chomping on the leaves. And on the 22nd

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of April. I went out into my front garden, which, as you say, is full of roses and saw a kind

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of gardeners nightmare, which was that the tips of the roses, the new shoots which are producing the flowers

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look like this, absolutely covered in aphids. Now, why the aphids all collected

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there? Well, it's because Humetewa leaves are brown sugar factories. They're making all

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the sugar, but they're exploiting a lots of it to the growing parts of the plant, which means

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that sugar to build new leaves and new flowers. So that's where the aphids aggregate

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and they just stick their stylist's into the pipes of the plant, the sugar carrying pipes,

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and the plant just pumps sugar into them. The fantastic trick for them,

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of course, there are lots of things that want to eat those aphids and they're coming after them. And what I notice is just

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a few days later, those aphids practically disappeared. Now, what

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could have accomplished that in such a short space of time? Now your attention might

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naturally turn to ladybirds, their very famous predators of aphids. That could be the seven spot Lady

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Bird. I do see those around in the front door. This was one on the rosemary. Very early in the year, they

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hibernate as adults over the winter. It could be one of these amazing parasitic wasps.

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Lots of those attack aphids like this little Picone. It was here. They lay their eggs inside

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the aphids. And you can see the aphids here kicking their butts around, try and deflect.

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And they turn the acorns in so-called mummies. So they lay the eggs inside them. The aphid

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becomes sort of paralysed and then the larvae just eat it from the inside out. And when they finished

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their development, they bust out in a kind of alien style manner. The problem with those kinds

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of predators, like ladybirds and parasitised wasps is early in the year they're really behind the curve.

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So you imagine that the leaves come out on the plants in the early spring. The aphids arrive first

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and they stop building their populations and then the ladybirds and the wasps have to arrive.

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And it takes them time to build up their populations because they have to eat to make its first

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and then turn those aphids into new wasps or new ladybirds. And so they tend to be behind

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the curve. So it takes them quite a bit longer. By later in the summer, you'll see aphid

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populations crashing on a much wider scale and lots of ladybirds looking around for something

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to eat. But they call. I've done that in my front garden in the space of a few days.

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So could it have been? It couldn't be. No, surely not. Not this

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mild mannered house. Sparrows dawn on a Sunday morning.

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And you can see this colony of sparrows flying in and out of the eaves of the house

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opposite. It's quite a large group of the lot there. Twenty to twenty 25, perhaps. And I have to

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admire the tolerance of that of the occupant of the house. The human occupants of the house. He's obviously happy

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to put up with them because sparrows. Well, I didn't really contribute to the dawn chorus. They just sort of engage

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in a bit of a dawn shouting match with each other. Anyway, this was hardly the smoking gun. Just because

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I can see their nesting opposite doesn't mean they're doing anything to my roses.

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I hung out the bedroom window. I staked out the front garden by lurking behind the dustbins.

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But most of the footage I obtained just looked like this. Now, this might come as a bit of a surprise to cafe

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owners up and down Britain who can barely keep the sparrows off the scones of their customers.

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But these sparrows are not that tame. And as a Sparrowhawk lurking around our neighbourhood, too,

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so they're not very keen to be caught out in the open. In desperation, I actually

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set up a trail cam in the bay window looking out over the front garden, and I managed to film Mason

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and the mess and the mess and the mess, but it wasn't really the footage

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I was after. Anyway, eventually, one morning very early, you can

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see the angle of the sun's rays here. I did manage to film small grief as far as flying

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into one of my rosebushes, messing about a bit and then flying away again. But then finally,

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one male sparrow took pity on me and I just gave up and filmed him through the window.

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And I managed to get this footage of him really gleaming away. And you can see just how

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effective those sparrows are at removing aphids in a very delicate way from

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every part of your roses. Let's end this episode

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where we began sitting next to Gertrude Gee, call that beautiful Rose. I can smell it from

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here is wonderful. Gertrude Jekyll was a very famous gardener, by the way, and that's why she had this race

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named after her. So what we learnt this week was the world is green,

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partly because of the nature of chlorophyll, that it is unable to use green

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light and it reflects that green light back and only absorbs the red and the blue light. That's

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one reason why the world is green. The other reason is that the world's herbivores are not really able

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to remove a large percentage of the leaves the plants produce, partly because they're really rather indigestible

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and partly because there are lots of predators who are on them. And we learnt that the generalist predator

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like the sparrow is actually really effective in biological control because its population

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is not closely tied to the prey, unlike the lady birds who have to track the prey

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through time. They can be very effective, but they're tending to boom and bust as the prey

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populations go up and down. So don't forget to feed those sparrows through the winter because they'll really

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pay you back. Come spring.