Hi. It's Mr. Andersen and in this podcast I'm going to be talking about plant control.
So if we were talking about animals we'd be talking about the brain, the nervous system,
muscles, sensory system. How we respond to our environment. But plants don't have any
of those things. And so how are they able to do things like this? A mealworm is placed
on a venus fly trap and it closes quickly? Digesting it eventually. So we're not quite
sure but basically we think there's some active transport going on that's pumping water down
here to the end, swelling up those cells and then closing in on it. There's a misconception
where a lot of kids think that somehow they are eating this. They don't do photosynthesis.
Venus fly traps live in an area where there's not a lot of nitrogen in the soil. It's kind
of a swampy area. And so they're still doing photosynthesis, but they're eating these bugs
just to get nitrogen that's not really available in the soil. And so if they don't have nervous
systems, if they don't have muscles. Basically what they're using is similar to our endocrine
system. They're basically sending out hormones. Those hormones are diffusing around the plant
and then they're going to have different consequences depending on what cells are picking it up.
And different actions depending on what hormone is sent. And so the five that we are going
to learn are going to be auxin, cytokinins, gibberellins, abscisic acid and then finally
ethylene. And so there are a lot of other hormones that are out there but basically
these are five ones that are found in a number of different plants. And so it's a good place
to start. So let's start with auxin. It's really one of the first ones that was ever
studied. And so if you put a plant next to the window and just let it sit there for a
long time what you'll find is that plant is going to start to orient itself towards the
sun. And you can see all of these leaves are pointing towards the sun. And they're also
going to move throughout the day. And so how do they do that? Well one of the first scientists
ever to do any work on auxin was Charles Darwin and his son. And basically what we found out
is that auxin is a chemical. And basically what auxin will do is auxin is going to move
to the shady side of a stem. And so if we've got auxin here, auxin is going to migrate
to the other side. Let's say that the sun was now on the other side. Then the auxin
has moved back to this side. But basically what the auxin does is it's going to loosen
up the cellulose in the cell wall. And as we loosen it up it's basically going to make
those cells a little bit bigger. And so I couldn't quite do this in the animation, but
if you think about it, if the auxin were to move to this side of my finger and make that
side of my finger longer, it's going to bend in that direction. Likewise, if it were to
move to this near side or my palm side of my hand it's going to bend it in the other
direction. And so basically depending on where the auxin goes, it's going to make the cells
grow faster there and it's going to make that bend in the opposite direction. Likewise if
auxin moves up to the top it's going to make it go farther and farther. It's going to grow
straight up. And so it's a great way for them to know which way the light is. Auxin is also
going to effect the roots. Basically what's going to happen is auxin is going to move
against gravity. And if you think about that, how's that going to play out in the roots?
Well let's say that we have a root, let's say we tip our plant on its side. Now we have
root that looks likes this and we have a little root cap on it like that, well gravity is
going to be pulling us down like this. And so auxin is going to move towards the top.
So auxin is going to move up here. Well what's that going to do? It's going to cause the
root to grow more on this side. And that's going to cause the root to bend down. So it's
going to go like that. And so it's basically, auxin's a great way for us to respond to our
environment. The next one is going to be something called cytokinins. Cytokinins, if you breakdown
the word it's really easy to figure out this one. It's basically cytokinesis. So we're
taking one cell and we're dividing it into two cells. And so when does this happen? Well
if you think about it, a plant is going to grow in two different directions. There's
going to be a plant that's going to grow up like this. And so we have this, it's going
to be moving up. But there's also going to be lateral growth. So it's going to be movement
in this direction. And so if we're moving in that direction we can't just expand ourselves.
We're actually going to have to make new cells. And cytokinin is responsible for that. It's
basically a hormone that causes our cells to divide. And that's going to cause us to
grow to the side. And so if you think about it, we can respond to light that way. And
so let's say that we have sun up here. So here's our sun again smiling away. And let's
say that there's nothing in the way. Well then a plant is going to secrete a lot of
auxin. So it can grow right up towards the sun. But let's say that we put something in
the way. Let's say that we put a barrier here so the light can't get through. Well now a
plant is going to increase the amount of cytokinin. So that's going to cause more lateral growth.
And so it can grow around like that. Once we get pure light again, then we're going
to use auxin to grow right up towards it. And so that's, I mean if you look at a forest,
basically everything in the forest is in competition with itself. Trying to get the most light.
This is a cool little study here. Basically what they did is they applied nothing to this
plant, a control. And then they applied a lot of cytokinins to this one. And you can
see even though they're the same age, this one is going up laterally and this one is
dividing. Next thing we have is the gibberellins. Gibberellins actually comes from an old story.
And basically we had rice. And rice was growing in rice fields or rice paddies. And a fungus
infected it. It's called gibberellic, I can't remember. It's the Japanese term. I'm not
sure of the species name. But basically what it did is it caused the rice to grow incredibly
fast. And then fall over. And then die. And so basically they called foolish seedling.
Because all of the rice would grow as quickly as it could. Couldn't support its own weight
and then it would fold over on itself. And basically what we found is it was causing
those plants to increase gibberellins which are natural plant hormones. And basically
what's that used to do? It's to control the yearly cycles of the plant. It tells them
when it's time to become dormant. When to flower. Humans have started using gibberellins
because we can cause seedless grapes to actually grow really, really quickly just by spreading
it on them. Next one is abscisic acid. Abscisic acid, I always remember that as like the scissors.
Like a garden scissors. Basically what abscisic acid is going to do is going to do the opposites
of what gibberellins are doing. And so in the fall when our amount of daylight starts
to drop off, at least here in Montana, then the trees are going to turn colors. And so
how does a plant know when to do that. Well they're looking at the quality of the light
and the amount of light that they get. But basically what that abscisic acid is saying,
it's time to cut off those leaves and basically causes them to fall. It causes them to suppress
this fruit formation. It causes them to stop transpiration. And so I think of abscisic
acid as basically a hormone that's causing everything to slow down. And enter into dormancy.
And then the last one I want to talk about is ethylene. Ethylene, unlike the other ones
is actually a gas that's going to go outside of the parts of the plant. And so this is
used in a positive feedback loop. And you probably notice that all of the fruits on
a tree will all ripen at the same exact point. And so basically what happens is if you get
fruit that starts to ripen. So these apples right here. Ripening fruit will start to give
off ethylene, this gas. That ethylene will spread to all the fruits around it causing
them to give off more ethylene and more ethylene. And pretty soon all of them have started to
go to become ripened at the same time. And this is super smart. Because you can attract
things like birds and humans so they can eat the fruit and transfer the seeds somewhere
else. And so again, positive feedback loop. And ethylene we find in different plants as
well. So you could take a really moldy banana. Put it right next to some apples and they're
going to go moldy quickly as well. And so basically those are hormones. It's how plants
control and respond to their environment. And I hope that's helpful.