Plants need light to grow.

But when a seed is first planted,  it’s underground in total darkness. Under those circumstances, you or I  would not find our way to the light.

So how do seeds do it? As it turns out, they have some unexpected ways of navigating the world and literally growing up. Here’s how they perform that amazing feat. [♪INTRO] Imagine you’re a seed, and you just germinated.

Congratulations! Have a cookie! The first thing you need to know is which way is up and which way is down.

Typically light – which you  desperately need to photosynthesize and start making your own food  – will be up, where the sun is. And water and nutrients will be down in the earth. That’s where you generally  want your roots to grow.

So you need to orient yourself. But you’re a seed. You have no brain, no mammalian nervous system, nothing that looks like eyes or anything that I would use to figure out this puzzle.

But you can detect the  gravitational force of the planet! I know, it sounds bizarre. But  here’s how we know it’s true.

In 1806, a British botanist sprouted seeds – this is a crazy idea – on a  disk rotating in the dark. And because it was dark the plants  were not moving toward the light. Instead, the centrifugal force  generated from spinning made roots grow away from the center of the disk,  and shoots grow toward the center.

This is a cool experiment. I love it! It demonstrates that plants  can sense the pull of gravity.

But we need to dig deeper to  understand how they sense gravity. We need to dig deeper. It turns out that plants use sensory cells called statocytes in the shoots and roots.

See, And statocytes are chock-full of statoliths, which work kind of like snow in a snow globe. They settle on one side of  a cell, thanks to gravity. The seedling senses where  its statoliths accumulate and that’s what tells it where gravity is pushing.

But turns out statoliths aren’t the only things accumulating on one side  of a young plant’s tissue. A growth hormone called auxin also concentrates on one side or another of the growing bits, spurred on by what’s going on in the statocytes. And that’s what helps roots grow downwards toward gravity and shoots push up against it.

The concentration of auxin can make one of the seedling’s sides grow more than the other, bending a root or shoot in the right direction, like a paper fan opening up. Really good work for a thing that  has no mammalian nervous system! Just...

Impressive plants. So the first force that tells a  seedling where to grow is gravity. But gravity isn’t the only force acting on it.

Mathematical models – and yes,  there are people who do plant math – suggest that if seedlings only used  statocytes to direct their growth, they would overshoot the  perfect upward orientation and need to rebound to correct their angle. And the thing that helps them wiggle their way to a straight stem is self knowledge. You may have heard of a human  sense called proprioception that helps people know where  their bodies exist in space.

It’s what helps people touch  their nose with their finger when their eyes are closed, or balance on a bike. Plants have proprioception too. And this means they can auto-correct  if they get bent out of shape. . .

Say, if they have too much auxin on one side. The sense of self helps them know when they’ve gone too far so they can straighten out. But before we get to the sense that  we started the video with light, I need to enlighten our viewers  with a quick advertisement.

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It has used gravity to guide it upward, and proprioception to ensure  it’s growing straight. Now it’s time to use light to guide its way to its best photosynthesizing self. And I know what you’re thinking.

The sun is up. So growing straight up should lead the plant to light without needing to sense it another way. But sometimes light isn’t straight up.

Maybe the seed sprouted under a park bench, or it’s the subject of some  random scientist’s experiment. Scientists like Charles Darwin helped establish the idea that plants respond  to light back in the 1800s. He covered growing grass shoots in aluminum foil to block out the light.

Or as he probably called it, aluminium foil. And he found that only the uncovered  shoots grew toward the light. That’s because, while seedlings don’t have “eyes,” they do have their own version of photoreceptors, like the rods and cones in human eyes.

For light, that helpful hormone,  auxin, does its thing again. It allows the plant to bend  toward that sweet, sweet sunshine. But even with a sense of gravity, self, and light, a plant won’t last long without water.

While scientists are still nailing  down how plants search for that, they know that once they detect it,  they extend their roots right to it. Researchers figured this out  by putting pea plant roots in a maze that basically  gave them a fork in the road. One side of the fork had water  under it and the other didn’t.

And eight out of ten plants  grew in that direction. …Even when the water was enclosed in an underground drainage pipe. So pea plants can sense  something about water running through pipes even when they  can’t feel the moisture. However they do it, they seem to use that as a cue for where they want their roots to grow.

Which means we can add water to the list of things that tell a plant  which direction to grow. But so far, I’ve only really talked about each of those factors on their own. And out there in the world, a plant will be shaped by all of these forces at the same time.

They grow their shoots away  from gravity while also growing them away from  themselves and toward the light. And often, these effects are  additive in directing growth. Away from gravity means a plant grows straight up.

And if it’s in an open  field, then away from itself would mean up toward the  light also grow straight up. But sometimes these forces  conflict with each other. A plant can be growing toward the light, but find itself arching toward its own stem.

When two forces don’t totally agree, one of them has to take priority. We’re talking about plants making decisions! How much a plant bends in response to one force over the other depends on  factors like how much those forces disagree with each other  and how powerful the force is.

If light is faint, then gravity will  dictate growth much more than light. And if gravity dictates that a  plant should grow straight up, but it only gets sunlight from its  right side because there’s a big tree blocking the sun from the left,  then it will grow at an intermediate 45 degree angle between the  pulls of gravity and light. Together, these forces are what shape a plant.

They’re all constantly  interacting in the growth process. And in the end, there is still so much we don’t understand about how a seed  goes from this to this. But we know that they incorporate a lot of data to reach their final form.

That little seed may not be  able to pick up and walk away, but it sure knows where it’s going. [♪OUTRO]