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Plants! If oxygen and good smells aren't enough for you, here's a collection of episodes that might win you over.

Hosted by: Olivia Gordon

SciShow has a spinoff podcast! It's called SciShow Tangents. Check it out at http://www.scishowtangents.org
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Links to original videos and sources:

How Plants Tell Time
https://www.youtube.com/watch?v=6zlNwSHOWm8


Can Houseplants Improve Air Quality?
https://www.youtube.com/watch?v=AmeiXikh0v8

the Earth's Internet: How Fungi Help Plants Communicate

https://www.youtube.com/watch?v=_tjt8WT5mRs

The Terrifying Truth About Bananas
https://www.youtube.com/watch?v=ex0URF-hWj4

Darwin's Darlings: Meat-Eating Plants
https://www.youtube.com/watch?v=PxQj0xjObVE

Plants. Can't. Count. - ...except they kinda can…

https://www.youtube.com/watch?v=WHOpQhrJr-U

*intro*

There's a good chance you know the cats and dogs on your block better than you know the trees and flowers.  And hey, that makes sense, no judgement here.  After all, plants don't greet you on the sidewalk, get in fights with raccoons or sneak up on you when you're walking home in the dark.  They keep a low profile. 

But really, plants are amazing.  They give us food and oxygen, they decorate our homes, and they're also really fascinating organisms.  We've talked about plants a ton on this channel and thought it was about time to compile some of our favorite episodes about them.  Speaking of time; here's Hank to explain how plants can tell what time it is!  Because, you know, apparently that's something they can do.

 NewSection (0:53)


Hank: Plants don't have brains; that's 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 fly trap. 

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 woodsorrow 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 roll in this response, the cooler night air can help signal the plant's reaction and the warming sun in the morning does the oppisite.  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.  Initally phytochrome takes the form of Pr; so called because it absorbs red light which there is 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 bulbus structure 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.  Aditional leaf chemicals called leaf-closing and leaf-opening substances also play a part in nigh-time 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 flowers 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 and pulvini; if you wanted a lot of new terminology in one sentence.  So 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 plant's seeds devolop. 

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 light thanks to a higher level of auxin activity on the shady side.  That tilts the developing flower toward the sun.  At night in the absence of sunlight, sunflowers 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 which direction things are if you're in the middle of a sunflower feild.  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.  

 NewSection (5:56)


Olivia: So, plants know when the sun rises and sets.  That means, in my part of the world, they know that the sun is starting to set earlier and things are cooling down.  Around our office we've started keeping the windows and doors shut to keep out the winter chill.  But, if you're doing the same thing at home things might start to feel a bit stuffy. 

So, in place of that fresh air wafting in with the breeze could you just keep a few house plants to purify the air instead?  Here's what the science says.

It seems like every work place has that one person who swears that the potted spider plant that they have sitting on their desk has an almost magical ability to purify the air in the office.  Are they actually on to something?  There is evidence to show that plants can remove pollutants from indoor air.  But a single potted plant probably won't go the distace. 

If you've heard that bringing plants into your home or office will improve your indoor air quality; you probably have a study conducted back in 1989 by NASA to thank for that.  The goal was to test whether a variety of types of indoor plants could be used for air purification both on Earth and in space.  The plants were put into sealed containers and the air pumped full of chemicals including benzene and formaldehyde.  Those chemicals fall under the broader umbrella of volatile organic compounds or VOCs. 

Some VOCs are known to cause health issues like headaches or liver and kidney damage.  And they're noteworthy as indoor pollutants because household products and building materials can give them off.  The plants were kept under controlled conditions with plenty of light and water, and the air quality was measured over a 24 hour period. 

The study found the majority of plants included in the experiment removed much of the benzene and formaldehyde from the containers.  Plants are thought to remove pollutants from the surrounding air by absorbing the gasses through their leaves and roots with some possible help from the microorganisms in the soil. 

So, that's it right?  Houseplants help clean the air.  Well, not quite.  Later studies doing similar experiments have yielded mixed results.  A 2009 study tested 28 varieties of indoor plants and found four that were the best at capturing every VOC the researchers threw at them.  So plants definitely can remove some pollutants in a laboratory test chamber.

But these idealized lab conditions don't exactly represent the air in your office.  Stuff like the amounts of light, water and air circulation are all going to be different.  on top of that, the lab plants were purifying the air in a small area; some of the chambers in the NASA study were less than a cubic meter in volume.  That means for the same amount of purification to happen in your house, you would need an indoor rain forest.  One critic of the NASA study suggested that to get the same effect in your home you would need 680 separate plants. 

There have been a handful of other studies investigating the effects on indoor air quality.  But at least one review published in 2014 has attempted to take stock of the existing research and it concluded that while lab based studies generally show some effect not many have been performed in living spaces and the ones that have been done haven't yielded a consensus.  So don't count on your poor desk fern to fix all of your indoor air problems.  

 NewSection (8:57)


Olivia: So a single spider plant on the window sill isn't going to fix that stuffy air.  But even if plants aren't doing that much for you, many of them are looking out for each other.  In the wild plants are constantly communicating.  And many of them have a lot to say.  Here's how plants talk to each other with the help of some friendly fungi.  

 NewSection (9:15)


Olivia: The internet connects more than half of the world's population through an invisible web of servers, computers, and devices.  It's changed our world in countless ways, by allowing otherwise separated people to interact and by providing access to vast amounts of information.  But humans aren't the only organism on the planet with an invisible interconnected network. 

While plants may seem like isolated, solitary individuals, they're capable of communicating with each other, sometimes over considerable distances.  All thanks to their special relationship to fungi.  Nearly all plant species we know of have a mutually beneficial relationship with soil fungi called mycorrhizae. 

Mycorrhizae can grow a network of small branching tubes called a mycelium that extends through the soil including inside or around plant roots. And these allow the fungi to absorb nutrients from the soil like Nitrogen and Phosphorous which plants struggle to extract.  So they basically barter; in exchange for those hard to get nutrients the plants trade the fungi Carbon in the form of sugars.  And ultimately, together both can thrive when they otherwise wouldn't. 

This symbiotic relationship between plants and fungi was discovered in the early 1900s.  But it wasn't until 1997 that we understood just how deep this underground network goes.  Ecologist Suzane Samard had a hunch that plants weren't just sharing nutrients with fungi, but also with each other. 

To test her hypothesis she and her colleagues infused trees in a forest with a traceable radioactive form of Carbon and later took samples from neighboring trees.  And it turned out that many nearby trees had the radioactive carbon too, proving that plants could send nutrients back and forth to one another.  Not only that, they seemingly distributed the nutrients where they were needed most. 

Plants need light energy to turn carbon dioxide and water into sugar and oxygen thanks to that magical process called photosynthisis.  So those in shade had less sugar to go around.  Samard found that these shaded energy-deficient trees ended up with more of the radioactive carbon than their sunbathing counterparts.  So it is basically the plant/fungi equivalent of feeding the hungry. 

Continued research into these underground networks, called common mycelium networks, has revealed that plants are not only able to gain access to more nutrients they can also engage in sophisticated communication by "talking" chemically through mycelia and it turns out they're saying quite a bit.  Generally any seedling that's plugged into the CMN has a higher likelihood of surviving and the plants that are "online" are generally healthier too. 

Researchers think this has to do with having access to an early warning system.  When a plant is attacked it releases chemicals telling nearby plants something bad is coming their way.  This communication happens through airborne compounds but also through a CMN.

And other plants heed this warning.  For example, when tomato plants are connected by a CMN and one plant is attacked by a pest, nearby plants will activate their defenses before the pest reaches them.  Scientists are only just starting to understand how important these plant networks are.  They've discovered that entire forests can be interconnected. 

But, like with our internet connectivity throughout an ecosystem isn't evenly distributed.  Older, larger, trees are more connected, kind of like some servers in the human internet.  These highly connected trees are called hub, or mother trees.  They have big root networks that host a greater diversity of microbial fungi and that allows them to interact with a lot of other plants.  They do play favorites though. 

Scientists have shown that they can send care packages of nutrients to their kin to help them survive.  Which is how they got the mommy monacre.  And they can also help forests transition during times of change.  When they're injured or dying, they release a surge of carbon into the network which nurtures the next generation of trees, even if they're a different species.  Of course, no internet is complete without hackers. 

Some plants can claim territory and influence community dynamics by sending toxins into the CMN.  Black walnuts will use these networks to release toxins into the soil for example.  Those that are immune to the toxins thrive, while others struggle or die off.  And harmful worms, parasitic plants, and fungi can find their way to the plants they target by following the chemical trails emitted by the microrize underground. 

It's amazing to think that this chemical information superhighway was right beneath our noses for eons and yet we had no clue.  But now that we can finally plug in, it might just help us connect to the planets flora in much more constructive ways. 

Knowledge of this inter-connectivity is helping improve our relationship to plants including things like forest conservation and aggriculture.  For example, preserving the highly connected mother trees from deforestation ensures mycorrhizal fungal diversity and helps forest regrowth happen more quickly.

And farming in soil with a CMN means plants can warn each other of invading pests; which might reduce the need for pesticides.  Like with the human internet, the internet of the earth increases security awareness and knowledge for those connected to it; including us.  

 NewSection (14:19)


So, plants are just talking to each other, all the time.  And honestly, that kind of worries me, because some plants are keeping really big secrets.  Here's Hank with a terrifying secret about bananas.  

 NewSection (14:33)


Hank: First, the good, bananas are healthy, packed with nutrition and energy.  They fit in your hand and give nice little cues when they're ripe and are easy to peal and eat.  Shocking statistic; the banana is Walmart's number one selling item, not the potato chip, not Coca Cola, not Fifty Shades of Grey, bananas!  They appear to be so perfect for human consumption that Kirk Cameron used them to prove the existence of god. 

Of course, this banana was not created by God, or really, even nature.  Bananas, at least the ones you see at the store, were created by people.  Don't get me wrong, there are wild banana plants, lots of them, they're native to south and southeast Asia; and there are dozens of species and thousands of varieties. 

They're just not the ones we eat.  Some of those species, as you might suspect, have seeds; because that's what fruits are, they're fleshy bodies containing seeds.  So you might wonder, why have you never eaten a banana seed?  Well, you have, kinda.  In cultivated bananas the seeds have pretty much stopped existing.  If you look closely you can see tiny black specks.  Those are all that's left, and they're not fertile seeds; if you plant them nothing grows.

Today's bananas are sterile mutants.  I'm not trying to be mean, that's just the truth.  Unless you were alive in the 1960s, hat's off to all those older Scishow viewers out there, every banana you have ever eaten was pretty much genetically identical.  This is a Cavendish, the virtually seedless variety we all eat today.  But it wasn't always always our banana of choice.

Until the 1960's everyone was eating the same banana, it was just a different banana; the Gros Michel, a bigger sweeter fruit with thicker skin.  You might notice that banana flavored things don't really taste like bananas; well, they do, they taste like the Gros Michel. 

The genetic monotony of the Gros Michel crop was it's undoing; a fungicide resistant pathogen called panama disease began infecting Gros Michel crop; by the time growers understood how vulnerable their crop was the Gros Michel variety was all but extinct. 

The entire banana industry had to be retooled for the Cavendish.  Since they're seedless the only way to reproduce them is to transplant part of the plant's stem.  And for the last 50 years we've been good with the Cavendish since it's more resistant to the panama disease.  However, somewhat terrifyingly a strain of panama disease that affects the Cavendish strain that we all eat has been identified.  A global monoculture of genetically identical individuals is a beautiful sight to a pathogen. 

The fungus only has to figure out how to infect and destroy a single individual and suddenly there's no diversity to stop it or even slow it down.  That's led to a lot of scientists worrying about, or even predicting the outright demise of the Cavendish, this wonderful most popular of fruits might completely cease existence.  The good news is we now have a much better understanding of genetics, epidemics, fungi and pathology. 

Scientists and growers have already taken steps to protect the Cavendish.  Some growers are creating genetically different bananas that might replace the Cavendish crop if it fails.  While scientists are attempting to genetically engineer Cavendish plants with immunity to panama disease.  Plus we learned a lot from the grow Michelle debacle, infected fields are quickly being destroyed and new crops are grown from pathogen free, lab grown plant stock. 

So thanks to the people who work tirelessly to grow and harvest bananas and bring them to us so that we can offer them inexpensively to our employees.  And thanks to the growers and scientists working tirelessly to make sure they don't go the way of the Gros Michel.    

 NewSection (17:52)


Olivia: So, bananas as we know them are mutants.  And they're basically one fungus away from extinction.  That is terrifying.  And speaking of terrifying plants, how about plants that eat meat?
Because that's a thing.  Here's Michael with the scoop.  

 NewSection (18:08)


Michael: Darwin's fascination with Drosera, a kind of plant known as a sundew stems from it's ability to capture and digest insects.  He categorized it, and other plants like it, as insectivorous plants.  We know them today as carnivorous plants because, well, they're not that picky. 

Several species of the plants have been known to trap and digest frogs and even small mamals.  You've probably heard of the Venus flytrap; but did you know that it's just one of more than 600 known carnivorous plant species, with more discovered every year?   Scientists generally look for two things when defining a carnivorous plant; it has to be able to absorb nutrients from a dead animal and it must have some adaptation that it uses to attract, capture, kill, and digest it's prey.  But where do these adaptations come from? 
And why would a plant need to eat meat when it gets it's energy from the sun for free?

Well, while most plants get their Nitrogen and nutrients from soil, through their roots; carnivorous plants are typically found in swampy environments, like bogs, where water is constantly washing those nutrients away.  So they get their Nitrogen from animal tissue, absorbed through glands in their specially modified leaves.  How exactly they do this varies widely among hundreds of species and at least nine plant families. 

There are pitcher plants, for example, which lure their prey with sweet nectar and leaves that resemble a long tube.  Insects fall from the slippery rim of the pitcher into what's known as a pitfall trap.  This is filled with a mix of rain water, digestive enzymes, and the leftovers of previous prey.  Not exactly a fun way to die. 

Then there are bladderworts, which with over 200 species make up the largest group of carnivorous plants.  They use bladder shaped leaves lined with trigger hairs and topped with a sort of trap door.  When an insect touches one of the hairs, the door opens and sucks in the victim.  Within fifteen minutes, the prey is digested, quite efficent. 

Species of the Sundews described by Darwin act much like a spiderweb; luring and catching insects with sticky drops disguised as nectar.  But those drops contain a thick mucus like substance that traps the prey on the leave's sticky tentacles.  And the Venus flytrap is well known for a good reason.  It's eponymous trap activates when an insect walks across the leaf and applies pressure to it's trigger hairs. But it dosn't initially close all the way. 

Scientists believe that this is the plant's way of letting smaller bugs escape so it dosn't waste time digesting a low nutrient meal; instead, it closes a second time soon after using enzymes similar to those in our stomachs' to slowly digest it's prey. 

Unlike other carnivorous plants, the venus flytrap can take up to ten days to finish it's meal.  For more than 150 years carnivorous plants and their astounding diversity have fascinated and perplexed botonists.  And until the late 1980s many scientists thought they all shared a common ancestor.  But studies in the last twenty five years have shown that carnivory, as it's called evolved independently at least six times within five orders of plants. 

Carnivorous plants are a pretty wonderful example of convergent evolution in which unrelated organisms develop similar traits in response to their enviroment.  In this case, nutrient poor swamps and bogs all around the world.  

 NewSection (20:49)


Olivia: Ok, fine, so some plants eat meat.  And by some, I mean hundreds.  But, who is counting?  Actually, maybe plants, as Hank explains.    

 NewSection (21:01)


Hank: Now, you might've heard that plants can count, and yes, I know that this sounds like a silly question to ask if it is true.  But even the New York Times has called venus fly traps, a plant that can count.  The thing is, they can.  But it's not counting the way you know it.  Which is to say, they don't have some kind of mental representation of numbers in their brains; because as far as we know they don't have brains.  They're not sentient, we're not talking, like, Groot here.  Can Groot count? 

There's evidence for some sort of counting mechanism in plants though.  And a lot more work needs to be done to properly understand it.  The whole idea of counting plants went viral after a 2016 paper in Current Biology.  The title of the study itself literally has the words Venus Flytraps count in it, so like, you can't really blame the headline writers here.  And yes we're talking about those infamous flytraps that snap unsuspecting flies with their leafy mouthes. 

You see, the study found that an insect needs to bump small trigger hairs on the plant's leaves at least five times before the plant would begin secreting enzymes to digest it. And this built on knowledge already known within the botany community that the plants close those leaf mouths if an insect bumps trigger hairs twice within 15 to 20 seconds. If the plants can discern one or five different triggerings they're counting, right?  Presumably, this means they think about and process numbers somehow; which would mean that they have to think. 

Well, as awesome as the idea of sentient plants might seem, they're probably not thinking, or really understanding numbers, not in the way that we do anyway.  You see, your brain actually has specific neurons that fire when you look at certain quantities.  One set will fire when there are three lemon drops on a plate, another when there are four.  Intregueingly these are different than the neurons that fire when you simply look at numbers written out; but both seem to give you some kind of mental representation of the amount. 

There just dosen't seem to be much concrete evidence for these kinds of mental representations of numbers and plants.  Though, at least one study says you can never be sure.  What plants do have, are sophisticated biological mechanisms that enable them to react to changes in their enviroment.  And while these responses may look like plant intelligence at first glance, they fall short of what you and I might associate with conscious thinking. 

There are some similarities though, like both you and plants can transmit information quickly thanks to electrical signals called action potentials.  The process isn't exactly the same for plants and humans since the chemicals involved are a little different.  But they're similar and they are how a fly trap counts.  It generates and transmits action potentials every time a trigger hair is touched. 

The number of action potentials not only dictates if a trap should close, but also how much digestive juice it should secrete.  Presumably, more triggering implies a larger insect that will require more enzymatic activity to break down.  Basically, having a measure of the size of their meal lets them meter their investment in it so they don't waste energy.

But the electrical communication networks in plants are much simpler than humans and other animals.  They're not considered true nervous systems.  Those signals never arrive at some central hub or brain where they're processed and translated into action.  So even if we know that venus fly traps can count and why; how it counts is still up for investigation.  And it's not just the venus fly trap; many other plants use action potentials and other types of electrical signals.  And even without brains plants can do things that involve some pretty impressive math. 

A 2013 study used mathematical models to show that a small mustard relative can do something akin to division to partition it's food so it lasts through the night.  Other plants have the equivalent of an internal thermostat that lets them regulate their temperature.  So yeah, so, plants can't really count numbers or think about them the way that we do.  But they're still pretty remarkable.  And they still "know" how many times their little hairs get shoved around by a fly.  

 NewSection (25:15)


Olivia: It's ok, if nothing else they're not counting how many days you forgot to water them.  Thanks for watching this leafy compelation.  And thanks to all of our supporters on Patreon who helped us make all these episodes.  And if you want to help us keep making content like this, head over to patreon.com/scishow to learn more.

*outro*