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Plants don’t have brains or muscles, and yet some of them can perform such feats as eating insects or following the sun. Scientists haven’t completely figured out how this happens, but they do have some pretty strong leads.

Hosted by: Hank Green

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Sources:
http://plantsinmotion.bio.indiana.edu/plantmotion/movements/leafmovements/oxalis/oxalis.html
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https://www.mobot.org/jwcross/duckweed/phytochrome.htm
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Images & Video:
https://commons.wikimedia.org/wiki/File:Desmanthus_pern_83565_b.jpg
https://commons.wikimedia.org/wiki/File:Desmanthus_pernambucanus_nyctinastic_(nighttime)_leaf_movement_0175.JPG
https://www.flickr.com/photos/schill/3787936438/
https://commons.wikimedia.org/wiki/File:Bauhi_purpu_151006-0058_tdp.JPG
[INTRO ♪].

Plants don’t have brains. This is probably not news to anyone.

Plants also don’t have muscles, or anything resembling a nervous system, and yet… they can move. In some plants, this is actually pretty dramatic—think Venus flytraps. But there are tons of plants that move more slowly, and they do it in time with the coming of day and night.

So how do they move, and how do they know when to do it, all without a brain or any of that other stuff? Many plants, such as members of the legume and wood sorrel families, tuck their leaves in at night. We don’t totally understand how this happens, and we have almost no idea why.

But scientists have identified some of the players involved. The process of how plants tuck themselves in at night is called nyctinasty. Nastic movements are a plant’s movement in response to a stimulus that doesn’t occur in a particular direction.

The leaves don’t follow the moon or anything—they just droop. Temperature change plays a role in this response: the cooler night air can help signal the plant’s reaction, and the warming sun in the morning does the opposite. But it gets quite a bit more sophisticated than that, involving not just temperature changes, but several different types of chemical reactions.

One player in this process is a molecule called phytochrome, which absorbs light. Phytochrome participates in a reversible chemical reaction, meaning it doesn’t just react to form a product and then stop. Instead, it can switch back and forth between two different forms, depending on the conditions.

These two forms are called Pr and Pfr. Initially, phytochrome takes the form Pr, so called because it absorbs red light, which there’s more of during the day when the sun is out. As Pr absorbs red light, however, it is converted into Pfr, which absorbs far red light instead—basically the less intense wavelengths as the sun sets.

Absorption of far red light causes Pfr to convert back to Pr. Some of it will change back over time in the absence of any light, as well. Which means the phytochrome automatically cycles back and forth between forms depending on whether it’s day or night.

These changing forms of phytochrome are important in structures called pulvini. A pulvinus is a region of bulbous tissue at the base of a leaf that acts as a flexible joint—it's like a “plant elbow”. When enough Pfr is present in the pulvinus, the plant pumps water to a specific section of the joint.

The change in water pressure within the cells, called turgor pressure, basically flexes the joint like a muscle, which bundles the leaves up for the night. When the chemical reaction reverses, the turgor pressure shifts back. Additional leaf chemicals called leaf-closing and leaf-opening substances also play a part in nighttime, well, leaf opening and closing.

There’s a lot of variety in these chemicals, but the general idea of oscillating chemical reactions is similar. In the same way, many flowers open in the morning and close at night, for reasons that are even more poorly understood. It might be to conserve a flower's scent, to protect their nectar, to keep pollen dry, or some other reason—but the mechanism might be similar.

Petals, after all, are just a type of leaf. This isn’t the only kind of day and night plant movement, either: many species actively follow the sun during the day, in a process called heliotropism. Unlike nastic movements, tropisms are plant movements that are oriented in a specific direction.

Heliotropism can help leaves get the most possible sunlight. Often, heliotropism in leaves is also controlled by turgor pressure in pulvini, if you wanted a lot of new terminology all in one sentence. Though in this case the leaves can move continuously to track the sun throughout the day, rather than just opening and closing.

And some flowers follow the sun too. It seems to have a few benefits, like providing a nice warm place for pollinators and helping the plants’ seeds develop. But many heliotropic flowers have no pulvini.

Young sunflowers instead turn to face the sun by growing their stem on one side at a time. It’s not totally clear what chemicals the sunflowers use to sense sunlight. But the changes in stem growth appear to be governed by a hormone called auxin, which in this case tells certain parts of the plant to grow in response to light.

The stem of the sunflower grows faster on the side that gets less sunlight, thanks to a higher level of auxin activity on the shady side. That tilts the developing flower head toward the sun. At night, in the absence of sunlight, sunflower stems reorient themselves to face east again, and in the morning, the light-directed growth process resumes.

But sunflower stalks don’t keep growing forever. Solar tracking only happens in young sunflowers. Once they’re fully mature, the flowers face east, and never move again.

So now you know how to tell what direction things are if you’re in the middle of a sunflower field. And you also know that plants don’t need brains or nervous systems or muscles to respond to their environments, as long as they’ve got chemistry on their side. And they’re more sophisticated about it than we think, able to keep track of time and act appropriately.

Which is… pretty smart. Thanks for watching this episode of SciShow, which was supported by our community of patrons. If you like what we do here, and you’re interested in being a part of it, check out patreon.com/scishow. [OUTRO ♪].