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Plants Have Proprioception and That’s Weird
Plants Have Proprioception and That’s Weird
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Subtitles (157)
0:00
Plants need light to grow.
0:02
But when a seed is first planted, it’s underground in total darkness.
0:05
Under those circumstances, you or I would not find our way to the light.
0:11
So how do seeds do it?
0:13
As it turns out, they have some unexpected ways
0:16
of navigating the world and literally growing up.
0:19
Here’s how they perform that amazing feat.
0:24
[♪INTRO]
0:26
Imagine you’re a seed, and you just germinated.
0:28
Congratulations! Have a cookie!
0:30
The first thing you need to know is
0:32
which way is up and which way is down.
0:34
Typically light – which you desperately need to photosynthesize
0:37
and start making your own food – will be up, where the sun is.
0:39
And water and nutrients will be down in the earth.
0:42
That’s where you generally want your roots to grow.
0:44
So you need to orient yourself. But you’re a seed.
0:47
You have no brain, no mammalian nervous system,
0:50
nothing that looks like eyes or anything that
0:53
I would use to figure out this puzzle.
0:55
But you can detect the gravitational force of the planet!
0:59
I know, it sounds bizarre. But here’s how we know it’s true.
1:03
In 1806, a British botanist sprouted seeds –
1:08
this is a crazy idea – on a disk rotating in the dark.
1:12
And because it was dark the plants were not moving toward the light.
1:16
Instead, the centrifugal force generated from spinning made roots grow
1:21
away from the center of the disk, and shoots grow toward the center.
1:25
This is a cool experiment. I love it!
1:28
It demonstrates that plants can sense the pull of gravity.
1:31
But we need to dig deeper to understand how they sense gravity.
1:34
We need to dig deeper.
1:36
It turns out that plants use sensory cells
1:38
called statocytes in the shoots and roots.
1:40
See, And statocytes are chock-full of statoliths,
1:44
which work kind of like snow in a snow globe.
1:47
They settle on one side of a cell, thanks to gravity.
1:50
The seedling senses where its statoliths accumulate
1:54
and that’s what tells it where gravity is pushing.
1:56
But turns out statoliths aren’t the only things
1:59
accumulating on one side of a young plant’s tissue.
2:02
A growth hormone called auxin also concentrates
2:04
on one side or another of the growing bits,
2:08
spurred on by what’s going on in the statocytes.
2:10
And that’s what helps roots grow downwards
2:13
toward gravity and shoots push up against it.
2:16
The concentration of auxin can make one of the
2:19
seedling’s sides grow more than the other,
2:21
bending a root or shoot in the right direction,
2:24
like a paper fan opening up.
2:26
Really good work for a thing that has no mammalian nervous system!
2:31
Just...
2:32
Impressive plants.
2:33
So the first force that tells a seedling where to grow is gravity.
2:37
But gravity isn’t the only force acting on it.
2:40
Mathematical models – and yes, there are people who do plant math –
2:44
suggest that if seedlings only used statocytes to direct their growth,
2:48
they would overshoot the perfect upward orientation
2:51
and need to rebound to correct their angle.
2:54
And the thing that helps them wiggle their
2:56
way to a straight stem is self knowledge.
2:59
You may have heard of a human sense called proprioception
3:02
that helps people know where their bodies exist in space.
3:06
It’s what helps people touch their nose with their finger
3:09
when their eyes are closed, or balance on a bike.
3:11
Plants have proprioception too.
3:13
And this means they can auto-correct if they get bent out of shape. . .
3:17
Say, if they have too much auxin on one side.
3:19
The sense of self helps them know when they’ve
3:22
gone too far so they can straighten out.
3:25
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3:28
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3:33
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4:18
At this point in a seedling’s growth,
4:20
it has already overcome several obstacles.
4:23
It has used gravity to guide it upward,
4:25
and proprioception to ensure it’s growing straight.
4:28
Now it’s time to use light to guide its way
4:31
to its best photosynthesizing self.
4:33
And I know what you’re thinking. The sun is up.
4:35
So growing straight up should lead the plant
4:38
to light without needing to sense it another way.
4:41
But sometimes light isn’t straight up.
4:43
Maybe the seed sprouted under a park bench,
4:45
or it’s the subject of some random scientist’s experiment.
4:48
Scientists like Charles Darwin helped establish
4:51
the idea that plants respond to light back in the 1800s.
4:55
He covered growing grass shoots
4:57
in aluminum foil to block out the light.
4:59
Or as he probably called it, aluminium foil.
5:02
And he found that only the uncovered shoots grew toward the light.
5:06
That’s because, while seedlings don’t have “eyes,”
5:08
they do have their own version of photoreceptors,
5:11
like the rods and cones in human eyes.
5:13
For light, that helpful hormone, auxin, does its thing again.
5:17
It allows the plant to bend toward that sweet, sweet sunshine.
5:21
But even with a sense of gravity, self,
5:23
and light, a plant won’t last long without water.
5:27
While scientists are still nailing down how plants search for that,
5:31
they know that once they detect it, they extend their roots right to it.
5:36
Researchers figured this out by putting pea plant roots
5:39
in a maze that basically gave them a fork in the road.
5:42
One side of the fork had water under it and the other didn’t.
5:45
And eight out of ten plants grew in that direction.
5:48
…Even when the water was enclosed
5:51
in an underground drainage pipe.
5:53
So pea plants can sense something about water running
5:56
through pipes even when they can’t feel the moisture.
5:59
However they do it, they seem to use that
6:01
as a cue for where they want their roots to grow.
6:04
Which means we can add water to the list
6:06
of things that tell a plant which direction to grow.
6:09
But so far, I’ve only really talked about
6:11
each of those factors on their own.
6:13
And out there in the world, a plant will be shaped
6:15
by all of these forces at the same time.
6:18
They grow their shoots away from gravity while also
6:21
growing them away from themselves and toward the light.
6:24
And often, these effects are additive in directing growth.
6:28
Away from gravity means a plant grows straight up.
6:31
And if it’s in an open field, then away from itself
6:34
would mean up toward the light also grow straight up.
6:37
But sometimes these forces conflict with each other.
6:40
A plant can be growing toward the light,
6:43
but find itself arching toward its own stem.
6:45
When two forces don’t totally agree,
6:48
one of them has to take priority.
6:50
We’re talking about plants making decisions!
6:53
How much a plant bends in response to one force
6:56
over the other depends on factors like how much those
6:59
forces disagree with each other and how powerful the force is.
7:03
If light is faint, then gravity will dictate growth much more than light.
7:07
And if gravity dictates that a plant should grow straight up,
7:10
but it only gets sunlight from its right side because there’s a big tree
7:14
blocking the sun from the left, then it will grow at an intermediate
7:17
45 degree angle between the pulls of gravity and light.
7:21
Together, these forces are what shape a plant.
7:24
They’re all constantly interacting in the growth process.
7:27
And in the end, there is still so much we don’t
7:30
understand about how a seed goes from this to this.
7:34
But we know that they incorporate
7:36
a lot of data to reach their final form.
7:39
That little seed may not be able to pick up and walk away,
7:42
but it sure knows where it’s going.
7:44
[♪OUTRO]
