Hello, my name's Lindsay Turnbull and I'm an associate professor in the Department of Plant Sciences
at the University of Oxford, and we're right in the middle of this very serious corona virus crisis right
now. And my students are all stuck at home and we want to keep them in touch with biology
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
biology.
Hello and welcome to this episode of Back Garden Biology. When, in fact, I'm in the front garden
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
may have some of these in your garden mail, roses. And of course, English gardens are really famous for their
roses. And this one's doing very well. It was planted by the person who owned the house before me
and raised it get better and better. As they get older, they get more vigorous. They get a good root system down. And now
it produces lots of these wonderful flowers. You can see that it's not a wild rose
because it just has so many petals in each flower. So roses belong to the family.
It's named after them. The roses see, and they're characterised by having simple flowers with exactly
five petals. And this one clearly has got a lot more maps. The efforts of the breeder
and the breeder in this case is called David Austin. He's very famous for breeding English roses
and the varieties called Gertrude Jekyll. And it's a very famous, very popular rose in England.
It smells really wonderful. So that's one of the good things about David Austin's roses and some of the other breeds
they've kept the sense of phrase says, which is so important, the smell of Strether say, well,
he wants to smell up. Okay. Today's question is, why is the world
green? Now the leaves off the rose are green and so are the leaves, just about every other
plant in the world. And that's the first part to that question. Why are the leaves of plants green
and not some other colour? The second part to that question, though, is different. It's an ecological
question. Why do the world's plants still have all their leaves? Why
have they not been stripped to the bone by the collective actions of the world's herbivores?
Now, the productivity of plants is is normally called primary productivity.
It's not secondary productivity which comes from herbivores eating plants. And in the oceans,
there's also significant primary productivity, not by plants, but by all different kinds of
algae. And in the oceans, a lot of that primary productivity does end up in the
mouths of herbivores. But on land, we know that although we can often find plants with a small
amount of damage, it's pretty rare to see. I've certainly never seen an entire tree stripped to the bone.
So why is that? Why does most of the primary productivity on land stay in the plants?
Well, ecologists don't have full answers to that question. I can tell you, but we have. But
I've been doing some investigations in my home from garden, and I think I can start to shed a little bit
of light on why that might be. So we'll have to come inside to try to address the first part
of that. Why is the world green question? The first part is the physiological part. Why are leaves
green? And we've got a prison set which is refracting sunlight
onto this whiteboard. And I hope you can see it. And it makes this beautiful rainbow. And that's because the light
from the sun is actually in the visible spectrum is actually a mixture of colours. So
you see the classic rainbow, Roy G. Dave, the I'm sorry, red, you gave us my red,
orange, yellow, green, blue, indigo and violet. And the red are the longer wavelengths, the
blue or the shorter wavelengths. And as the light passes through the glass, it is slowed
down. But the but some wavelengths, the blue wavelengths are slowed down more
so they get more and more bang, more refracted. And by going through two surfaces
of a prison, they separate away from the reds. I've got somebody holding it up at the window
and managing to get it at just the right angle to throw it onto here. Now, we know now
that visible light consists of these different colours. So what does that mean to do the leaves being green?
Well, inside the leaves there is a molecule called chlorophyll, and it
absorbs radiation from the sun. And it can only absorb certain
wavelengths. So it absorbs the blue light very well and
it absorbs the red light very well. And so what it doesn't absorb is
the wavelength in the middle. So what you're left with there is the green
light keeps moving. It's about there fairly quite a narrow band that looks green.
And the leaves reflect that green light. So they absorb the blue light. They absorb
the red light. I may reflect the green light. Now, what's going on there? Well, when they
absorb light, either the blue or the red, they use the energy inside
it to excite electrons and electrons or the small negative particles, the orbit
atoms, and by firing electrons out of the chlorophyll molecule.
They are able to split water. So water is two hydrogens
bound to an oxygen very tightly. And for the plant to make carbohydrates from
carbon dioxide, the plant needs a source of hydrogen. So carbon that carbohydrates
contain carbon, oxygen and hydrogen. Can get the carbon and the oxygen from
carbon dioxide from CO2, but it needs hydrogen as well. And the only place on earth you can get
a lot of hydrogen from is water. But getting the hydrogen out of water is really
very difficult. And that's what the photosystem of the chlorophyll does. So it absorbs light
energy. Fires an electron out of the chlorophyll molecule. And the chlorophyll
molecule is now so desperate to get an electron back that it will smash up a molecule
of water to grab an electron out of it. And that frees the hydrogen, which the plant
then captures and settles away to make carbohydrates. So it's sort
of amazing rally. But the leaves have this incredible system going on with absorbing the sun's
energy to split water. And they're going to use the hydrogen there to make carbohydrates.
Why can't they use the green wavelengths? Nobody knows. We do not understand
now why it is that chlorophyll absorbs very well and the red wavelength and the blue wavelengths,
but not in the green. But it really makes me think if you went to an alien planet, how likely
is it that the chlorophyll that had evolved there would also not be able to use
the green light? That is really, really unlikely. So it's quite likely that photosynthesis has
evolved on another planet. But I bet you any money that all the plants wouldn't be green.
So when the plants have splits the water molecules and shuttle the hydrogen away
to attach that to CO2, to make carbohydrates glucose, then the oxygen
in the water molecule is free and it's just put into the atmosphere. Of course, that's one of the amazing
roles of the primary producers play. They generate all the oxygen that's in
the atmosphere and that's their combined activities of all the plants on land and all the various kinds
of algae in the oceans. Now, what about the second part of our question? Why is
the world green? We were asking, why do all the plants have so many leaves? Why haven't they just been gobbled
up by the world's herbivores? Well, it's partly because mature leaf like
this one from this ivy here is not a very exciting meal. It's very tough.
It's fibrous. It's packed full of a molecule called cellulose, which is what plants use to make
their cell walls. And actually, animals don't have the enzymes to digest cellulose.
There are specialist herbivores Caruth and various large mammals called ruminants,
which can digest cellulose, but they can't do it themselves. They have to have these ridiculously enormous, elaborate
stomachs that are full of bacteria that actually digest the cellulose for them.
But there are other ways to eat plants that don't just involve chomping on the leaves. And on the 22nd
of April. I went out into my front garden, which, as you say, is full of roses and saw a kind
of gardeners nightmare, which was that the tips of the roses, the new shoots which are producing the flowers
look like this, absolutely covered in aphids. Now, why the aphids all collected
there? Well, it's because Humetewa leaves are brown sugar factories. They're making all
the sugar, but they're exploiting a lots of it to the growing parts of the plant, which means
that sugar to build new leaves and new flowers. So that's where the aphids aggregate
and they just stick their stylist's into the pipes of the plant, the sugar carrying pipes,
and the plant just pumps sugar into them. The fantastic trick for them,
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
a few days later, those aphids practically disappeared. Now, what
could have accomplished that in such a short space of time? Now your attention might
naturally turn to ladybirds, their very famous predators of aphids. That could be the seven spot Lady
Bird. I do see those around in the front door. This was one on the rosemary. Very early in the year, they
hibernate as adults over the winter. It could be one of these amazing parasitic wasps.
Lots of those attack aphids like this little Picone. It was here. They lay their eggs inside
the aphids. And you can see the aphids here kicking their butts around, try and deflect.
And they turn the acorns in so-called mummies. So they lay the eggs inside them. The aphid
becomes sort of paralysed and then the larvae just eat it from the inside out. And when they finished
their development, they bust out in a kind of alien style manner. The problem with those kinds
of predators, like ladybirds and parasitised wasps is early in the year they're really behind the curve.
So you imagine that the leaves come out on the plants in the early spring. The aphids arrive first
and they stop building their populations and then the ladybirds and the wasps have to arrive.
And it takes them time to build up their populations because they have to eat to make its first
and then turn those aphids into new wasps or new ladybirds. And so they tend to be behind
the curve. So it takes them quite a bit longer. By later in the summer, you'll see aphid
populations crashing on a much wider scale and lots of ladybirds looking around for something
to eat. But they call. I've done that in my front garden in the space of a few days.
So could it have been? It couldn't be. No, surely not. Not this
mild mannered house. Sparrows dawn on a Sunday morning.
And you can see this colony of sparrows flying in and out of the eaves of the house
opposite. It's quite a large group of the lot there. Twenty to twenty 25, perhaps. And I have to
admire the tolerance of that of the occupant of the house. The human occupants of the house. He's obviously happy
to put up with them because sparrows. Well, I didn't really contribute to the dawn chorus. They just sort of engage
in a bit of a dawn shouting match with each other. Anyway, this was hardly the smoking gun. Just because
I can see their nesting opposite doesn't mean they're doing anything to my roses.
I hung out the bedroom window. I staked out the front garden by lurking behind the dustbins.
But most of the footage I obtained just looked like this. Now, this might come as a bit of a surprise to cafe
owners up and down Britain who can barely keep the sparrows off the scones of their customers.
But these sparrows are not that tame. And as a Sparrowhawk lurking around our neighbourhood, too,
so they're not very keen to be caught out in the open. In desperation, I actually
set up a trail cam in the bay window looking out over the front garden, and I managed to film Mason
and the mess and the mess and the mess, but it wasn't really the footage
I was after. Anyway, eventually, one morning very early, you can
see the angle of the sun's rays here. I did manage to film small grief as far as flying
into one of my rosebushes, messing about a bit and then flying away again. But then finally,
one male sparrow took pity on me and I just gave up and filmed him through the window.
And I managed to get this footage of him really gleaming away. And you can see just how
effective those sparrows are at removing aphids in a very delicate way from
every part of your roses. Let's end this episode
where we began sitting next to Gertrude Gee, call that beautiful Rose. I can smell it from
here is wonderful. Gertrude Jekyll was a very famous gardener, by the way, and that's why she had this race
named after her. So what we learnt this week was the world is green,
partly because of the nature of chlorophyll, that it is unable to use green
light and it reflects that green light back and only absorbs the red and the blue light. That's
one reason why the world is green. The other reason is that the world's herbivores are not really able
to remove a large percentage of the leaves the plants produce, partly because they're really rather indigestible
and partly because there are lots of predators who are on them. And we learnt that the generalist predator
like the sparrow is actually really effective in biological control because its population
is not closely tied to the prey, unlike the lady birds who have to track the prey
through time. They can be very effective, but they're tending to boom and bust as the prey
populations go up and down. So don't forget to feed those sparrows through the winter because they'll really
pay you back. Come spring.