this YouTube series and I also run the website brilliantbotany.com over on tumblr, which

you should check out. You can also find me over on twitter.

Inspired by a comment over on my video about gravitropism, today I'm going to be talking

about phototropism and shade avoidance in plants. Now superficially, that might sound

a little bit boring, but how plants sense and respond to light is actually really cool.

So, phototropism. Phototropism is a word we use to refer to a change in growth or movement

in response to light. An awesome example of this is solar tracking or heliotropism in

sunflowers. What this means is that sunflowers literally move so that they're fully facing

the sun as much as possible. That way, they can get as much sun, and photosynthesize as

efficiently as they can.

This shows us how important sunlight is for plants. They use the energy from the sun to

create nutrients for themselves and alongside water and other nutrients absorbed through

the plant's roots, sunlight is the most important resource for a plant. For that reason, plants

track and follow and change in response to where the sun is.

Like gravitropism, there are two main parts of how this works. The plants have to sense

the light, and then they have to change in response to light. There are a bunch of sensors

that plants will use to sense where the sunlight is, and they'll use hormones to change how

their growth is so they can get closer to it.

Another example of plants changing their growth in response to sunlight is shade avoidance.

Now for a plant, a bad thing that can happen is they can get shaded by something that blocks

their sunlight. What's really cool, is that plants can tell the difference between a shadow

cast by another plant or a shadow cast by an inanimate object.

So how does that work? Well, when plants photosynthesize, they use a lot of red light but they don't

use far-red light. Now those are just different wavelengths that you'll find in sunlight.

So when light hits or passes through a plant, the plant will pull out some of that red light

and it will change the ratio of red light to far-red light. Now plants have these photoreceptors

called phytochromes. And these phytochromes can detect the differences in ratio between

red light and far-red light.

So a shadow that's cast by a plant will have a different red to far-red light ratio than

a shadow cast by something like a rock. These phytochromes in the plants can sense that

difference so the plant can react differently depending on what's shading them. This might

be them growing taller, or curving, anything like that, but anything they can do to hopefully

get more sunlight and outcompete that other plant.

Now those are just a couple of examples of how plants sense and react to sunlight. If

you want to see some really awesome videos of plants responding to light or even gravity,

check out plants in motion, I'll put the link to that in the description. They have really

great time lapse footage of plants moving and responding to different stimuli.

Now that was just a quick video for today, thank you so much for watching. Don't forget

to like and subscribe, and I will see you next time.

On my Amazing Plant Roots video YouTube user awesomeazure asked what I mean when I say

some plants can regenerate from their roots. Basically, what that means is if you cut down

a tree, for example a coastal redwood, as long as the root system remains underground,

new trunks can grow back from that root system and grow new trees. Which is pretty cool.