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This Arbor Day, give your favorite tree the gift they really want: this tree-themed episode of SciShow Tangents. Just put the headphones around its trunk or in a knothole! Follow us on Twitter @SciShowTangents, where we’ll tweet out topics for upcoming episodes and you can ask the science couch questions! While you're at it, check out the Tangents crew on Twitter: Stefan: @itsmestefanchin Ceri: @ceriley Sam: @slamschultz Hank: @hankgreenIf you want to learn more about any of our main topics, check out these links![Truth or Fail]Dragon’s Blood Tree Tree Fatalities [Fact Off]Vampire stump redwoods[Ask the Science Couch]Tree definition / Pygmy Forests trees[Butt One More Thing]Butt rot
[SciShow Tangents Intro theme music plays]


Hank: Hello and welcome to SciShow Tangents, the lightly competitive knowledge showcase starring some of the geniuses that make the YouTube series SciShow happen. This week, as always, I'm joined by Stefan Chin!


Stefan: Hello!


Hank: Stefan, what kind of drink should I have tonight after Tangents is done?


Stefan: Ooh, watermelon vodka. [Laughs]


Hank: A bold choice! Makes me think a little less of you.


Stefan: Oh no!


Hank: [Laughing] What's your tagline?


Stefan: If I was a rapper, I'd be Lil' Barbecue Sauce.


Hank: Sam Schultz is here with us as well.


Sam: Hello!


Hank: Sam, what's your tagline?


Sam: That's an egg, not a fish.


Hank: Ceri Riley's here with us as well. What's your tagline, Ceri?


Ceri: It's foggy in here!


Hank: Oh! And I'm Hank Green and my tagline is: four or five eyelashes. Every week here on SciShow Tangents, we get together to try to one-up, amaze, and delight each other with science facts. We’re playing for glory, but we’re also keeping score and awarding Sam Bucks from week to week! We do what we can to stay on topic, but we aren't great at that. If the rest of the team deems a tangent unworthy, we will force you to give up one of your Sam Bucks. So tangent with care! And now, as always, we will introduce this week’s topic with the traditional science poem... this week from Stefan.


Stefan: Trees cover the Earth like some kind of planetary hair. Most with rings that add some girth for each passing year. For the species Pennantia baylisiana we know of just one plant—it's rare! But taken all together, you could count over 3 trillion trees out there. There are trees whose tissues can survive a deep, deep freeze, and it turns out trees and forests make a great carbon sink. Trees provide useful shade with their dense canopies, and homes—and homes for flying squirrels and other little beasts. And of course, humans also benefit, for us there's much to loot. Every part of a tree can be useful, from the leaves to the roots. They give us nuts, syrup, rubber, and all kinds of fruit. And sometimes we just need that lumber... sorry, Groot.


Hank: Our topic for the day is trees! You know, planetary hair.


Sam: Yeaaahhh... That's a challenging idea.


Hank: God, I never thought until this moment that I have no idea what a tree is!


[Sam laughs]


Ceri: So you know a tree when you see one, right?


Hank: Yeah.


Ceri: Like, you can look outside and be, like, "That is a tree. That is not a tree." That ,as far as I know, is basically what scientists think about trees too. They were, like, that's a tree and it's a plant with a more-or-less permanent shoot system... so, like, the roots in the ground that is supported by a single woody trunk. So woody like the bark around a tree, as opposed to herbaceous, which is like a green stem that you'd see on a tulip or something like that. And then there's a lot of debate over the specifics?


Hank: Yeah, uh huh. 'Cause there are definitely trees that have more than one trunk.


Ceri: Yeah, some organizations of scientists get really specific about how thick the trunk has to be to count as a tree, or how tall the plant has to be to be counted as a tree. Like, one set of measurements is it needs to be three inches in diameter at a point four-and-a-half feet above the ground with a definitely formed crown of foliage and a mature height of at least 13 feet, which excludes lots of, like, smaller trees. So... who knows really.


Hank: That—that seems—that seems completely unnecessary. What? Like, that's not science. That's just—that's just like arbitrary classification. They're like, "We're tired of seeing papers from people studying small trees." The more important thing is that it's—it's planetary hair.


Stefan: As long as we can agree on that!


Hank: On the scale of the size of the planet, they're very small. So they're more like planetary stubble, like five o'clock.


[Stefan laughs]


Ceri: If trees are planetary hair, what are the oceans?


Hank: Yeah, I'm—uh, I don't have any, like, pools of water on my body... ever, but like—


Sam: It's kinda like your guts, right?


Hank: [Thinking about it hard] Ohhh.


Ceri: So if you had, like, pools of stomach acid on your top of your skin?


Hank: Right. We only have one place an ocean can be on our body. It's just the belly button.


[Sam and Stefan laugh]


Sam: Well if you opened your mouth, you could pour some water in there too.


Hank: That's true. That's a great point, Sam. Inside of the nose, eye sockets, ear holes.


Sam: [Laughing] Yeah!


Stefan: Oh god.


Hank: Yeah, so many options available to us. Everyone dock all of us a point!


[Ceri, Sam, and Stefan laugh]


Hank: Um, no one deserves to benefit from this. Do you know the etymology of the word tree, Ceri?


Ceri: Yeah, it seems like the word tree or some variation thereof has existed for a while. In Proto-Indo-European, there is a root word deru- or drew-o-... or two root words, I guess. That means to be firm, solid, steadfast, with specialized meanings of wood or tree when they're referencing specifically a wood or tree.


Hank: And now it's time for Truth or Fail.


[Truth or Fail theme music plays]


Hank: One of our panelists has prepared three science facts for our education and enjoyment, but only one of those facts is real. The other three have to figure it out either by deduction or wild guess. And if we get it right, we get a Sam Buck. If we don't, then Sam gets the Sam Buck. Sam, what are your three facts?


Sam: Close your eyes and think of a tree.


Hank: [Hesitantly] Okay.


Sam: What do you see?


Ceri: One woody trunk! That's above 13 feet tall! [Laughing]


[Hank laughs]


Ceri: With branches and, uh, whatever I said. An umbrella of leaves?


Hank: Yeah, I see a bunch of leaves too...


Sam: So the odds are that it is tall and green and most importantly... peaceful. Is it peaceful?


Stefan: [Laughing] Yes.


Hank: Yeah!


Sam: Wouldn’t hurt a fly! But what if I told you that there were some trees out there that were capable of carnage. These are three trees that dabble in ultra violence, but only one of them is real.


Hank: [Laughing] Ultra violence! Okay.


Sam: So number one: the dragon's blood tree is an odd-looking, squat tree with a dome-shaped canopy of needles and a dark secret. Its roots contain a compound that erodes the roots of trees around it. Eventually, those trees fall and the dragon's blood tree uses the nutrients of the rotting tree to supplement its own diet. Its nickname? The vampire tree. Number two: the possumwood tree is a giant South American tree with a spiky trunk and a potentially deadly weapon in the form of pumpkin-like fruit that swells until it explodes, shooting shrapnel-like seeds at a speed of 160 miles an hour.


Hank: [In disbelief] What? What? What? WHAT? Don't get too close!


Sam: Its nickname? The dynamite tree.


[Ceri and Stefan laugh]


Sam: Or number three: the west coast tall coconut is a tall coconut-producing palm native to India. It looks just like any other palm tree, but it hides a deadly booby trap. It produces wedge-shaped fruits that are easily jostled loose from the tree. Uh, and the relatively pointy shape of the coconut combined with the height of the tree make the falling nuts extremely deadly, injuring and killing many people every year. Its nickname? The Gillade or executioner tree.


Hank: Oooooh, the executioner tree. So we have the dragon's blood tree that erodes the roots of its enemies and then consumes them... The possumwood tree, which also is called the dynamite tree 'cause it shoots shrapnel seeds from its pumpkin-like fruit... And the executioner tree, which drops heavy scythe-like coconuts down upon people, raining its reign of terror upon us for centuries or something.


[Ceri, Hank, Sam, and Stefan all laugh]


Hank: I know people get killed by coconuts. I've heard about that. But I didn't know—I—I haven't heard that there are special coconuts that are, like, axe-shaped. [Laughs]


Ceri: Yeah. And I can imagine that a coconut seed or coconut, I guess that is the seed, would evolve to be spikier for some reason? 'Cause that seems like it would help plant it.


Hank: The coconut seed dispersal mechanism is to float to a better place. So, like, that's why—I think that's why coconuts exist is so that they can float around and find another place to sit. And so I don't—I don't necessarily think that they like to go right where they land, 'cause there's no, like, branches of a coconut tree or a palm tree. So, like, right there is no good. 'Cause you're, like, right next to another tree already.


Ceri: Yeah, you're just fighting with your dad. I've read a little bit about dragon's blood trees, but I've not heard about this, but they are extremely weird so I would not put it past them. I think they're, like, reddish on the inside? Which is part of why—.


Hank: Hmm. Me too.


Ceri: [Laughs] Yes. Uh, which is part of why they get their name. And I think people have used them for a variety of medicinal treatments or dyes or something like that... That has nothing to do with how it destroys other trees, though.


Stefan: I sort of feel like trees are somewhat likely to end up near trees that are of the same species. And so it seems maybe weird that it would just, like, cannibalize everything around it.


Hank: It's true. I mean, in—in Montana, definitely we have a lot of the same tree over and over again. But in other places where there's sort of more production, you know, more—more energy in the environment, there tends to be more species that are sort of fighting it out. Where's the dragon's blood tree from, Sam?


Sam: The dragon's blood tree is from Socotra, which is an island off the coast of Yemen and Somalia.


Hank: That sounds like a place where there's plenty of sunlight and water. And then we have the possumwood tree, which sounds definitely not real. [Laughs]


Stefan: Really?! This is—this is the one! This is the one, I think. I remember hearing that trees can explode when it's cold, uh—


Sam: [Laughs] Yeah, for way different reasons.


Hank: Not on purpose!


Stefan: Oh!


Hank: Seed dispersal is a thing. So, like, shooting your seeds out like a snapdragon is definitely a thing that—that some—some plants do. And they do go fast. But fast enough and, like, with enough mass to injure a person seems unlikely. But, you know, I've never been there.


[Ceri and Sam laugh]


Stefan: Well now I don't know. [Sighs in resignation] If the two sciencey people think exploding fruit is not likely...


Hank: And the best thing is that if—if we're all wrong, then Sam gets the points. And he needs them!


Sam: I need them pretty bad.


[Ceri, Hank, and Stefan laugh]


Sam: It won't really hurt you guys to get it wrong. [Laughs]


Hank: Ooooh.... gah. [Thinking hard] I'm going to go with the, uh, executioner tree.


Stefan: Let's go bomb—bomb fruit.


Hank: Bomb fruits. Okay.


Ceri: [Self-assuredly] I'm also going to guess bomb fruit.


[Hank and Stefan gasp dramatically]


Sam: All right, the correct answer was: the bomb fruit!


Hank: Oh no! Woah!


Stefan: [Gasps] Whaaaat? So these can kill you if they explode in your face.


Sam: Yeah, they can hurt you, I don't know. I couldn't find a lot of firsthand accounts of people actually getting exploded by one. I mean, they shoot 'em at 160 miles an hour.


Hank: Yeah, but if they're, like, little...


Sam: Yeah, but imagine if you're, like, a chipmunk or something. You're going to get creamed. So the possumwood AKA sandbox, AKA dynamite tree: they're really spiky lookin' and they have exploding fruit! It can shoot 160ish miles per hour and averages a distance of 98 feet—each seed does after it shoots.


Hank: [Amazed] Woooow.


Sam: That sounds like a pretty good way to do it. You don't need—need birds anymore!


Stefan: Yeah.


Hank: Yeah. And I'm looking at—I'm looking at the seeds and they look like I wouldn't want to get hit by one!


Sam: No. Uh, and then on top of all that its sap is toxic and it's used by fishermen in the area to poison fish. So it's a very mean and useful tree.


Hank: Is there any truth to the other, the other facts?


Sam: Well, the dragon blood tree is a cute little dome-shaped tree that lives in Socotra. It's an island full of endemic species, kind of like the Galapagos islands, and this tree is one of them. It's considered vulnerable by the IUCN, the International Union for the Conservation of Nature, and they're trying really hard to protect it. And killer roots... they maybe exist? I don't know. But that was kind of an inversion of the fact that trees that are dying or stressed out can send nutrients to the trees around it. Via the mycorrhizal network.


Hank: Yeah, the Wood Wide Web.


Sam: Yeah, the Wood Wide Web. I'll just say that instead. So they're like opposite vampires. They're friendly vampires.


Hank: It's really—it's especially sad that the dragon's blood tree is vulnerable because of how, when you cut it down, it literally bleeds. You'd think that people would be, like, let's stop.


Sam: Too sad about it, yeah.


Hank: That seems too—that seems too mean.


Sam: And then the west coast tall coconut tree is just a normal, really tall coconut tree. And I looked up the stats of death by coconut, and it's more of an urban legend than anything.


Hank: Oh no.


Sam: Hawaii's first recorded death by coconut happened in 1973, and I think that looked like the only one recorded. But more coconut deaths than you'd think were situations where a trained monkey, who was trained to gather coconuts, accidentally threw a coconut too hard onto the head of the person who had trained it. There were three of those on Wikipedia.


Hank: Okay. So, like, it wasn't—it wasn't that, like, they were just walking by. It was like, we knew a coconut was going to fall. There was a person there waiting for the coconut. It's like when your kid throws you a ball without, like, making eye contact first and it's just like whaps you in the cheek.


Sam: Yep.


Hank: All right, next step, we will take a short break. Then we will be back with the Fact Off.


[Two transitional snippets of the SciShow Tangents Intro theme music plays]


Hank: Welcome back, everybody! Sam Buck totals: Ceri has zero, Sam has zero, I have negative one, and Stefan is in the lead with a positive one. Uh, 'cause we were all—we were all punished for our earlier conversations. Now get ready for the Fact Off!


[Fact Off theme music plays]


Hank: Two panelists have brought science facts to present to the others in an attempt to blow their minds. Each of the presentees have a Sam Buck to award to the fact that they liked the most. And we will decide who goes first—is it going to be Ceri, or is it me—with a trivia question. Who's got that question?


Stefan: A tree named Hyperion was discovered in 2006 and is considered the tallest tree on earth. How tall is it?


Ceri: What unit do we have to give our answers in?


Stefan: Uh, guess in meters to two decimal places.


[Ceri laughs]


Hank: [Surprised at the specificity] Oh! All right.


Ceri: 137.86 meters.


[Sam laughs]


Hank: I will—I'm gonna—I'm gonna say 100.00 meters.


Stefan: Hank wins, but barely! The answer was almost directly in between at 115.85 meters.


Hank: Ohhh, wooow.


Ceri: I almost got the decimals right, though. So that... is thrilling.


Hank: Yeah, it's true! You should get extra points for that, but you don't. So I guess it's time for me to go first. Do you guys want to hear about a tree?


Sam: [Low enthusiasm] Yeah!


Stefan: Ohhhkay.


Hank: A couple of years ago, two professors from Auckland University of Technology were hiking around New Zealand when they saw something that I wouldn't think would be surprising. It was the stump of a tree that—to me—that would look like a dead stump. But to them it did not! Even though it did not have other foliage, it was not able to do photosynthesis—they were not normal people, they were ecologists. And so they looked closer and they could tell that the stump was alive. Now, living tree stumps have been observed before. So it's a dead tree, no branches, no leaves, but it is alive. And it is not clear how they survive. So these Auckland ecologists decided to investigate how this works by attaching sensors that measured the movement of water in sap through the stump, but also through its two nearest living tree neighbors. And they found that during the day, the big living trees were moving lots of water and sap around, as you would expect. And the stump wasn't moving stuff around. But during the nighttime that switched: the stump would take in water and sap while the—the living trees had less activity. Now researchers know about these underground networks that connect trees through symbiotic fungi, which we already talked about because it's awesome. I like to call these networks, the Wood Wide Web, and you could also call it the mycorrhizal fungi networks or something. But they share nutrients, they share carbon, they also share information between trees. So trees will know things that are gather—like, information gathered by other trees. But this hydraulic connection that the researchers found is weird because it's not clear, like, what the living trees are getting out of this. So it's not like—they're not doing it when the—when the trees are photosynthesizing, but like the—the trees appear to be keeping the stump alive. Now this might be that this stump just grafted onto the other tree roots, and then when it died, it just hung there like a lazy tree vampire. Or maybe the stump serves some kind of connection as part of this, like, Super organism that protects all of them from, you know, possible threats or droughts or something. I think that it's a lazy vampire, but this other theory is much more beautiful.


Stefan: No, it's like a tree hospital. But wait—are stumps...? At what point is the tree dead, if you cut it down?


Hank: Right then, the stump would still be alive. But if the stump doesn't have any way of photosynthesizing, the stump would—would theoretically, you would think, die pretty quickly.


Stefan: Unless it's in tree hospital.


Hank: Yeah, because it's going to have a bunch of, like, nutrients stored in the roots that it, like—it could try to, like, make another go of it, which they often do. But if that—but if that fails, then it is just a stump.


Sam: Okay, here's a dumb question. If you're a tree and you have leaves and you have roots, are both of them giving you nutrients in a different way and you need both those different kinds of nutrients?


Hank: Here's the rough, like, understanding of what's going on. Uh, a lot of transpiration. So there's photosynthesis happening in leaves and they're also—there's also, like, water evaporating out of the leaves. That evaporation is what draws nutrients through the whole tree. So the water and stuff that it gets from the—the roots has to be then drawn up through the whole tree and photosynthesis can't happen without that water.


Sam: Okay.


Hank: So yes, they're both necessary. You can't have tree without some way of getting nutrients and water, and you can't have tree without some way of photosynthesizing that—that stuff.


Sam: Right. Unless you're a lazy vampire tree.


Hank: Yeah. 'Cause it's like—it's still alive! But it—it's just, like, taking nutrients from other trees.


Sam: In my cursory reading of it for my Fact Off fact, the people that they were interviewing were talking about looking at what the fungus is getting out of it instead of what the trees are getting out of it. And, like, it seems like the trees are being nice to each other, but really the fungus have just, like, too much to gain to let any of the trees die.


Hank: I don't know exactly how the fun—fungus are benefiting, except that I think that they get some stuff from the tree roots.


Sam: Yeah, and I guess they can't go above ground and look at the cut-down tree and be like, "Oh, this one's cut down. We can't help this one anymore."


Ceri: [Laughing] So it's just in disguise. They don't know that it's a vampire tree, uh, they're just like, "Well, there are these roots here!"


Hank: And they have not actually dug up the tree roots to see how they're grafted together. So—so they're only looking at, like, what's happening based on, like, flow of water and sap.


Stefan: Do you know what information, like, other than, like, just nutrients is being shared? Like you said the trees are learning things and then sharing the knowledge like a tree library...


Hank: Yeah, this is a thing that I have heard people who know what they're talking about tell—say. And so they... I—but I do not know what—what it is. If it's just like, "This tree found some water." Or if it's like a FernGully situation where the trees are talking to each other about, like, the—the timber harvesters.


Sam: That's what I saw is that they can pass alert pheromones to each other.


Ceri: That makes sense. Because other plants do that too, like when grass is cut...The—the smell of freshly cut grass, I think, is an alarm pheromone. That's like, "Ah, we're being cut!" Not that grass can, like, get up and run or anything, but...


Sam: [Laughing] Yeah!


Ceri: ...that is some sort of plant communication as well.


Sam: Just preparing each other for the inevitable, yeah.


Hank: Prepare! You too will die.


[Ceri and Sam laugh]


Hank: Okay, so we've got my vampire trees.


Ceri: Okay! So my fact is actually sort of similar, but with a different ending. Redwood trees in the genus Sequoia are known for being super old, super giant evergreen trees... uh, some that are even eight or nine meters in diameter. They grow in places like along the coast of California in the U.S. in vast forests and can sprout in several different ways, like from seeds or growing from the stumps or roots of a parent tree. And tucked into these forests of giant trees are ghost redwoods, which only exist in the numbers of, like, tens or a few hundred around the world. And ghost redwoods only grow from a few inches to a couple meters tall because they're albino and, for plants, that's basically a death sentence because their pigment chlorophyll is what makes them green and allows them to photosynthesize. So these ghost redwoods can't photosynthesize, they have waxy needles, weak wood, slow growth, and are basically parasites since they have to grow from a parental trunk and get all their nutrients from the still-photosynthesizing parent tree. It doesn't seem very evolutionarily favorable to support an energy-sucking small ghost tree. And sometimes these ghost trees do starve to death, but they always grow back afterward. Does—as far as scientists have known—


Hank: [Interrupting] Wait, wait, wait. That's not how it—That's not what happens when I starve to death. They starve to death... but then—that's not death, Ceri! If they grow back!


Ceri: Okay! They starve to almost-death that they prune back to nothing.


[Hank and Sam laugh]


Ceri: And then they go "Whoomp! Here's a new ghost tree!" Is it a new—I don't know. It happens infrequently enough that it could just be the same mutation growing from the same spot...


Hank: Right, right.


Ceri: ... or it, like, recedes to a little nub and then regrows. But they could provide a different kind of protection. In 2013, a PhD student from Colorado State University tested the green needles of a redwood and the white needles of an albino redwood for their chemical composition and found that the ghost redwoods had more than twice the concentrations of toxic heavy metals as the green ones. So they acted as a sort of, like, safe for these toxins and sequestered them away as a protective measure. His idea is that they could be a sign of adaptability to either, like, natural damaging factors or even human pollutants that have been introduced into the environment because these large trees just grow a little ghost and then shove all the bad stuff in it.


Sam: Are they purposefully giving the little trees the—the heavy metals? Is that what's happening?


Ceri: That's, like, the best guess so far. We're not a hundred percent positive why they exist. Um, but that would give a favorable reason for them. So, like, if the concentrations of heavy metal become too high in the parent tree, they would siphon it off along with the nutrients.


Hank: But then because that part of the tree is definitely going to die because it can't sustain itself, that would give it an opportunity to, like, get this stuff out of my body forever.


Ceri: It's like pooping for trees.


Hank: It's like pooping into a cyst...


Sam: [Grossed out] Ohh...


Hank: ...that is genetically different from you.


Sam: Okay.


Hank: So you can either go for Ceri's fact that there are albino trees that stick off of big redwood trees and they contain heavy metals and are maybe a way of sequestering those toxins away from the tree, or me—I had a stump that was dead, but it turned out it was alive and it's just a lazy vampire or possibly it's in tree hospital or possibly it is part of the super organism of the forest.


Sam: Wait is—is this anything? TREE-age? [Pronounced like "triage"].


[Ceri, Hank, and Stefan laugh].


Hank: Sam, Stefan, are you ready to—to vote your votes?


Sam: Yeah, this is a hard one.


Stefan: I think so.


Hank: Three, two, one...


Stefan: [Hesitantly] ...Ceri?


Sam: Hank.


Hank: So we split it?


Stefan: Yeah!


Sam: Yeah, you split it.


Hank: That's nice. I'm not negative! It's time to Ask the Science Couch. We've got a listener question for our couch/blanket fort/chair of finely honed scientific minds.


[Ask the Science Couch theme music plays]


Hank: This is from @felofHe, "What is the world's smallest full-grown tree species?" Well, it's exactly 13 feet tall is what I learned.


[Ceri, Hank, Sam, and Stefan all laugh]


Stefan: Some kind of bonsai?


Sam: Oh yeah.


Hank: Oh yeah! Bonsai trees are definitely trees because of how that's right there in the word.


Stefan: Yeah.


Ceri: Mhmm [laughs]. And you look at it and say, "That's a tree." This question is actually deceptively very tricky because unlike saying, like, "The blue whale is the biggest species!" Smallest, full grown tree species... makes me want to ask a lot of questions. Like, naturally full grown? I—I feel like that's what the question is getting at. And bonsai aren't naturally that big, like, they're not selectively bred to be small. They're just a big tree species that is pruned and shaped and, like, carefully manipulated into art, which is a miniature tree. So technically yes, if you count that as, like, a full grown tree in that it looks like a mature tree and could probably propagate more through cuttings or through something else. But that has a lot of human influence. Without human influence and just natural influence, there are things called pygmy forests and there are some in California and then there are some in the Philippines. They're all over the world and it's where the soil is poor enough—so usually this means really low water retention and really acidic—the—it's just like a horrible place to live. And so trees grow really slowly and are essentially stunted in their growth. And so you have a bunch of these pygmy variations of the same trees that are only five or six feet tall and people can walk through and—and be above the tree line because the soil is so poor that they're struggling to grow. And those are naturally—those are grown to the best of their capacity.


Sam: But if they were somewhere else they would be big, right?


Ceri: Yeah! But if they're somewhere else, they'd be big. Now, if you think of a tree living where it can, and then growing to its maximum height, some people think that the dwarf willow, Salix herbacea, is the smallest tree in the world because it can grow one to six centimeters in height. And so that's, like, very, very short—definitely below the 13 feet definition. But other—some people look at it.


Hank: Wait. [Laughs] That's—that's—how is it a tree?


Ceri: That's the thing! People—some people are like, "It's a woody—like has—sometimes has a singular stem." And so they're like, "That's basically a trunk!" But then other people are just like, "That is a—a bush. It's a shrub. It is a woody shrub. And, uh, there's no way I'm accepting that as a tree." This dwarf willow is part of a genus of plants called Salix, which as a group have both shrubs—like sprawling, branching, woody shrubs, and willow trees—like the characteristic by a pond, trunk goes up, long strands come down. And so there are some people who are like, "Eh, this is more like a tree than a shrub." And then there are other people like, "No, it obviously fits in the shrub category."


Hank: So it's either a tree or a shrub because it is a willow and people are like, "But it only has one trunk, so I guess it's a tree!"


Sam: Shrubs are really mud—muddying the water of this whole situation. What the heck is a shrub?


Hank: [Disappointedly] Yeah, shrubs.


Ceri: Yeah, plants are confusing. I feel like a shrub is when there are multiple branches. So instead of a single trunk, like, close to the ground or out of the ground, there are potentially lots of little branches.


Hank: I just wanted to give a shout out to the word shrub, which is great. It's just really good.


Sam: It's a good word!


Ceri: But yeah, so this is a really long and sprawling answer to say: I don't really know. There are other trees that are between, like, four to six feet tall. So, uh, Japanese maples can grow naturally about that big before they just level off. And there are a couple other tree species like that—that, fully grown, they just sort of level off at six feet. Don't know why. They're just sort of happy there.


Hank: Yeah, that's what I did!


[Ceri and Sam laugh].


Hank: If you want to ask your question to the Science Couch, you can follow us on Twitter, @SciShowTangents, where we tweet out topics for upcoming episodes every week. Thank you to @PaulPlaysGames2, @Pretty_empic, and everybody else who tweeted us your questions this episode. Final scores: I'm tied with Sam with zero, and Ceri and Stefan tied for the lead with one Sam Buck. Which means that Ceri is still in the lead! And now, Sam, you're in third and I am in last.


Sam: Yeah, I kind of needed a big win today.


[SciShow Tangents Outro theme music plays]


Hank: If you liked this show and you want to help us out, it's very easy to do that. You can leave us a review wherever you listen, that's very helpful and lets us know what you like about the show. You can also tweet out your favorite moment because that just makes us feel good. And finally, if you want to show your love for SciShow Tangents, just...


Ceri, Hank, Sam, and Stefan: Tell people about us!


Hank: Thank you for joining us. I've been Hank Green...


Ceri: I've been Ceri Riley...


Stefan: I've been Stefan Chin...


Sam: And I've been Sam Schultz.


Hank: SciShow Tangents is a co-production of Complexly & the wonderful team at WNYC Studios. It’s created by all of us and produced by Caitlin Hofmeister and Sam Schultz, who is also our editor. Our editorial assistant is Deboki Chakravarti. Our sound design is by Josef “Tuna” Metesh. Our beautiful logo is by Hiroka Matsushima. And we couldn't make any of this without our patrons on Patreon. Thank you, and remember: "the mind is not a vessel to be filled, but a fire to be lighted."


[SciShow Tangents Outro theme music plays]


Ceri: But! One more thing.


[Butt One More Thing theme music plays]


Ceri: Trees can suffer from an infection called butt rot, which is pretty much what it sounds like. Apparently, the butt of the tree is the base of the trunk... so I guess roots are legs. And it's when certain fungi invade the stump through injuries and make the butt really spongy, dying tissue, which disrupts the stability of the tree and can even kill it.


Sam: Are our butts named after tree butts? What came first? Where does our butt come from?


Ceri: I think human butts came from—


Sam: Where did that word come from?


Hank: Oh, god. Now I've typed, uh, "butt etymology" into—into Google and that—it will remember that forever. Oh, it's from—it's from [laughs in surprise]


Sam: Maybe we should save it for our butt episode.


Stefan: Ooh, yeah.


Hank: Yeah, we should save it for our butt episode.