Why leaves are one of nature’s most versatile creations

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Every Monday and Friday, you'll hear world leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Jason Goodger, commissioning editor at BBC ScienceFenkers. From the neatly curated English country garden to the wild jungles of the Amazon, the earth is host to a staggeringly diverse range of plant life. One of the key reasons for the great evolutionary success of plants is the wide raging versatility and utility of the leaves that grow on them. In this episode, we're joined by Professor Jonathan Silvertown, an evolutionary ecologist based at the University of Edinburgh to talk about his latest book, Nature's Magicians, how leaves conjure up our world. He tells us how plants act of one of the natural world's master chemists, explains how they contain internal clocks that allow them to track the time of day and even the time of year. And details how they detect and respond to light, sounds and even predatory attacks. (upbeat music) - So, welcome to the podcast. Thanks so much for joining us. - Thank you, Jason. - Pleasure to be here. - So, today we're talking about your latest book, Nature's Magicians, How Leaves conjure up our world. So, as the title suggests, it's about the many sort of fascinating properties that leaves have. So, let's start with the basics then. So, we all know that leaves of the green structures found on plants. But as a biologist, you know, how do you define them in the first 50 million years of plant evolution on land? There were no leaves. How do we know if we were to find them? Well, there were none of them. There were stems. When we were at a stem, it's a cylindrical thing. It's green. These are the early ones where they had tomato on them. These little pores through which water, vapor escapes and carbon dioxide enters. And they managed to get by doing photosynthesis without these laminar structures. There we are. There we are. This is a laminar structure that sticks out at an angle from a plant stem on a little stalk called a pithio. - So, you mentioned their photosynthesis. So, that's sort of one of leaves, key signature magic tricks. And we all learn in school that, you know, they're taking carbon dioxide, breathe out oxygen, it's powered by light. But that's probably the extent of our knowledge. So, let's start with this sort of, you, you call them sort of like solar panels. So, how do they respond to light? How do they collect energy from light? - Well, yes, so leaves are packed full of, or the cells in leaves are packed full of chloroplasts. And the photosynthetic machinery is inside the chloroplast. Chloroplasts themselves are fascinating. And they have an independent evolutionary history when you go back. So, we could, we could come back to that, if you like. So, what's happening is that the leaves, essentially, have evolved as, I think, are almost like display cases for chloroplasts. So, there's a whole lot of physics and biology and chemistry about leaves. A lot of it is to do with light capture. Not all of it. And that's the thing. The leaves are very multi-functional things. They also are sort of in charge of how water moves through the plant. And that's really important because it's water, the turquoise pressure that keeps cells expanded, that keeps stem standing up, right, keeps leaves. And so, they're multi-functional. - In the book, you say that leaves capture about 1% of the energy from sunlight. And that sounds like pretty inefficient. And you talk about, you know, we often think of the perfection of nature. So, you know, what can we say about that? - Well, on the perfection of nature, it doesn't exist. Is what I can say about that. Why, after all, we look at stuff and how beautifully adapted our eyes are for seeing and how other bits and pieces are for walking around et cetera, our brains are for doing podcasts. And we think this is wonderful. But evolution doesn't have any direction. So, it's not aiming for perfection. Basically, what the way evolution works or natural selection works, that's been more precise, is that it favors those organisms with characteristics that transmit themselves referentially to the next generation. And if being photosynthetic does that for you, then those organisms that are capable of photosynthesis will be favored over those others or not. And if they slightly better the photosynthesis, if that improved photosynthetic capacity actually means you transmit more offspring, you have more offspring, then it'll be favored. But if it's simply better at capturing light, but that doesn't actually turn into more seeds or more, more importantly, more descendants, more successful seeds, if you like, then natural selection doesn't care. So, if we're judging the effectiveness of the efficiency, the energy efficiency, if you like, of a light capture of a leaf, by the standards that we would use for a solar panel, we will quite rightly say, well, this is appalling. If I try and sell you a solar panel and you say, well, this sounds very interesting, and you mean the solar panels grow? And I say, yes, and so he says, well, where's the catch? And I say, well, I'm afraid it's only 1% efficient. You put the phone down. So, but it's good enough. And that's one of the many extraordinary things. There are so many, I mean, just to mention another one. Plants are extracting an extremely rare gas from the atmosphere. There when, you know, before industrialization was about 0.02%, carbon dioxide in the atmosphere, it's now over 0.04%, but that's still tiny. And that's what wood is made of. That's, you know, this rare gas is turned through photosynthesis and using the energy of sunlight into something as solid as the table that I'm sitting at now or the chair that you might be sitting on. This is extraordinary stuff. Yeah, so let's sort of stick with photosynthesis for a bit before we move on. Because as I said, like, a lot of people would say, oh, you know, name 10 things that plants do. One of them probably most people would say photosynthesis. But there were organisms that existed millions of years before plants that actually photosynthesized themselves. Yes, there were. And in fact, plants have inherited that. So you're talking about cyanobacteria. And the whole history of this, which we now understand through gene sequencing and reconstructing evolution retreats, otherwise known as phylogenies, of the genes involved in photosynthesis. And they go back an extraordinary length of time. So life emerged on Earth around four billion years ago. And it seems likely that photosynthesis appeared about three and a half billion years ago in some form of bacteria. It didn't emerge in cyanobacteria. This is really weird. So nowadays cyanobacteria, photosynthetic, and chloroplasts are actually in evolutionary history captured cyanobacteria-- things like cyanobacteria-- that were captured by early eukaryote cells. And they became photosynthetic as a result. So how do we know that? Well, chloroplasts have their own DNA. Well, the only things that have DNA are cells that can reproduce. So bacterial cells, eukaryote cells, human animal cells, and so on. So if chloroplasts, which live inside plastic, cells remember. If they have DNA and we know this pretty clearly now, they had an independent existence as free living bacteria. But it turns out that the genes that confer this ability on cyanobacteria are older than cyanobacteria or older than photosynthetic cyanobacteria. So they came from somewhere else where we don't really know. But the genes fall photosynthesis. Even older than chloroplasts, around three and a half billion years old. That is incredible. So let's have a look at some of the other of these magical properties of leaves that you talk about. You know, anyone who's walked around in the evening and things like that, you know, certain plants kind of, they change along with the cycle of the day. So you talk about, you know, plants like animals, having an internal clock. Yes, they do. The cells have clocks, what's fascinating is that they're coordinated because I mean, you know, there are billions of cells in, you know, a large plant, just as they're in your eye. And somehow they have to be coordinated. And this happens actually through cells in the flow. And the flow is this network of tubes that carries food from the leaves down to the rest of the plant. And it turns out, this is a fairly recent discovery that the coordination is actually carried out through cells in the flow on the borders of the flow. But yes, there are, there are clocks and they are very important. After all, if a plant, if an organism slashes a plant, an autotrophic organism, we call them, so in other words, it feeds itself. It doesn't eat other organisms. It creates its own food, hence the light and CO2 and water, which are like the feedstocks for a plant. Those things, particularly the light, of course, are available only some of the day. So everything about a leaf has to be kind of ready for that when sun up. And ready for what happens when the sun goes down. And of course, the sun is not only transmitting light, but also when that light hits the surface, it creates heat. And the heat is a real issue as well. And leaves have to cope not only with absorbing light in this part of the spectrum, only about half the spectrum is photosynthetically active. So that's where we talked about how leaves are about 1 percent efficient at capturing light. The first loss is basically, they aren't actually able to capture the full spectrum of radiation. It's about half of it. So it goes from 100 percent coming from the sun to about 53 percent in the right spectrum, the right ballpark, you might think, and then losses continue from there. Yeah, so let's move on to the idea of plants having senses. So most people will know about phototropism, for example. You know, we're talking about light. And you can see in the way that something grows, that it's growing towards the light. You think, well, you know, that makes perfect sense. But you also talk about other senses. And especially the one that you talk about is the response of plants to sound and vibrations, like they have a sense of hearing. Well, let me just add a caveat here. When we say that plants are sensitive to light, we don't mean they can see. Right. There's no image. There's no image capture going on in a plant. It is, of course, sensitive to light. And plants will bend towards the light very obviously. There are occasionally, by the way, plants that will actually bend away from light towards darkness. That's another story. So plants don't hear in the way that listeners to this podcast are hearing what I'm saying. But they do have a sense of touch that's mediated by these little hairs called trichomes. And certain frequencies vibrate those trichomes. And it turns out that there's a study, I mean, these things sound extraordinary, but it's all in the literature. There are certain plants that appear to be able to respond to the sound of a caterpillar munching. And now they must be picking this up through these little trichomes, which seem to be tuned to that sort of wavelength. And then they respond by upping, up regulating, increasing the concentration of chemicals in the leaf that deter caterpillars. And in the last 10 to 20 years, we've learned an extraordinary amount about how sophisticated plants are. And most of this stuff is happening in leaves, which is why we're talking about it today. But people think about plants doing this, they're, yeah, but it's in the leaves mostly. I guess their sense of hearing such as it is, given the caveat I've just mentioned, is really related to these touch sensors. Not that different, so how it works in animals as well. Ultimately, it's about something vibrating. And the cell detecting those physical movements, physical sound is pressure waves, so it kind of makes sense. So the touch sense sensitivity is fascinating. People have done experiments where every day they've run their fingers across a little pot of Arabodopsis, this plant, plant biologists use like the drosophila, the fruit fly, or the rat, or the mouth of the animal biologists, the plant biologists use this thing called Arabodopsis, Thailcress, a short-lived plant, small plant related to mustard. And apparently if you kind of, touch it daily and then compare it with a control, they grow much more poorly. The ones that you've kind of touched during, you know, over a period of a couple of weeks can be much smaller than the ones that haven't been, well, the controls that haven't been treated like that. And this is the trichomes again. And you think, well, what's going on here? And it turns out the trichomes actually and touch is part of the way that plants find their way around each other. So the thing we have to remember about plants is they're always surrounded by other plants. So they're competing, there are times when they, in a sense, cooperate. And the trichomes are a way of sensing that there's another plant there and there's a fascinating experiment which we've done. So the lovely thing about using Arabodopsis, like using drosophila, there were different, you know, different mutations that enabled you to find out, you know, okay, so what's the importance of this gene? We'll knock that gene out in drosophila and then see what difference it makes. You can do the same thing in Arabodopsis and when you knock out the sense of touch in Arabodopsis seedlings and grow them surrounded by controls, by ordinary plants, they get overtopped. They can't find their way through the canopy. And the wild type, if you like, the control plants that have the sense of touch actually do better. And so here's a perfect example of natural selection would work because the disabled plants, the plants that have no sense of touch will, of course, and leave no offspring. And that's the end of that. We can only do this experiment because you can engineer mutations into sale crest. I doubt that there are any wild sale crest plants out there and there are many billions of these things all over the place. They'll be growing in the cracks of the pavement as you walk to work or whatever. It's most unlikely that any of them carry that mutation because it will be very rapidly removed by natural selection. That's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block or finally break down that long article you've had open for weeks. When you need to build up your team to handle the growing chaos at work, use Indeed Sponsored Jobs. It gives your job post the boost it needs to be seen and helps reach people with the right skills, certifications, and more. Listeners of this show will get a $75 sponsor job credit at Indeed.com/podcast. That's Indeed.com/podcast, terms and conditions apply. Need a hiring hero? Zero Sugar and Vitamins. Propel hydrates better than water to help you get the most out of your workout and get back to your best self. What propels you? So how about another thing? When we're thinking of plants, if we leave, probably quite often we'll think of the herbs that we use in cooking and they're beautiful fragrance and smell. So what's going on there? Why do they produce such a vast degree of these different aromatic compounds? What do we know about what the kind of evolutionary purpose of those is? Well, this is the thing about plants. They are master chemists and I like to say that plants a light, a witty screen. school kid who's hopeless on the playing field but excels in the chemistry lab as a sort of compensation for that, it's almost me actually. I was a witty school kid not much interested in football but so what the plants do when they can't run away, they protect themselves in a different way and they use chemistry and it's reckon that there are probably about a million different compounds made by plants and I don't think anybody's been able to accurately count them all and I'm sure the numbers are going up all the time as we discover more and more but there are certain basic classes which are known to alkaloids for example so that would include nicotine, caffeine, things like this and they have biological activity so these these million different chemicals have biological activity not meaning they interfere with biochemical reactions sometimes positively, sometimes negatively but when it comes down to what they're doing for the plant it's virtually always negative. So tan in that stuff that makes your tea orange come from leaves of course because tea leaves are what you make your tea from and those are present in leaves of quite high concentrations because they they interfere with proteins, proteins include enzymes, enzymes are essential for any organism to digest its food so the caterpillars are coming along and chewing on the leaf and the first thing that happens is the tannins in the leaves are interfering with the with the digestion of the of the leaf material and of course there's a good thing to try and poison your your predators in this way the herbivores in this way so that in a nutshell it's it's basically about defense not just against things with jaws but also against fungi and bacteria the thing about all these natural enemies of plants is they they all have some tell if you like to use some thing about them that can alert the plant that it's being attacked because I mean the basic biochemistry of anything that's alive is the same I mean you know we have something like 60% of the genes of our banana for example so and the basic processes are are the same so how do you how do you know that you'll be attacked by something that's not you right and bacteria many often have a flagellum a little tail so that's one way in which plants can tell they're being attacked if suddenly the proteins in a flagellum which are characteristic are detected by a plant it responds accordingly by increasing the concentration of antibacterial compounds in the leaf so this creates a selection pressure against for bacteria that can hide so there's a very common bacterium called pseudomonas that sometimes is pathogenic sometimes not on leaves and there are strains that in you use a flagellum to invade they need it because they need to move into the plant they need to get into the plant often through the stomata little holes in the leaf but once they're inside they multiply and their offspring have no tails they actually jettison their tails well they don't produce them I think rather than jettison them because if they hang onto them the plant can then you know can react against them so there's so there's this constant it's been described as an arms race between natural enemies and plants in fact this is true of animals and us as well evolution of covid virus for example yeah so it's sort of sticking with that idea so you know there's a huge variety the different shapes and textures of leaves and so you know we've all have picked up stinging nettle listening or touched a sharp plant and got you know got pricked by it so that's the same sort of idea isn't it's they're all defense mechanisms to keep various things away from them yes they are I mean when you can't run away you know other other other mechanisms of defensive old for the kinds we've just been talking about and you mentioned nettles so one of the nice things about the nettle story is of course it has these stinging hairs on which it really aimed at I guess mammals things with with with sensitive mouths not caterpillars caterpillars of many speeches butterfly love nettles nettles are very rich in the nitrogen phosphorus nettle leaves have this this fairly characteristic shape and anybody who knows some of their wildflowers will have heard of a dead nettle and the dead nettle which is unrelated to these stinging nettle has leaves of quite similar shape to stinging nettles and they often grow together but the dead nettle as its common name kind of suggests doesn't sting so I'm not aware of anybody who's actually tested out whether this is mimicry but it's certainly a good hypothesis that dead nettles are imitating again not deliberately this is the natural selection of work if it's true they're imitating something that animals be wary of eating in order to benefit from the same defense essentially without having to produce stinging cells or anything else yeah so let's flip this around then to some examples of plants that you know insectivorous plants the most famous of which is it's the Venus fly trap which ensure you know everyone's aware of but you know it's got these jaw-like crescent so you know how how does that what happens how does that work you know how does it capture a fly we understand this in quite some detail now so basically looking at the jaws by the way they're often red colored which on the inside they're leaves by the way these jaws are leaves on the inside they're often red which is attractive to flies some flies like red anyway but they think it's meat I guess and there are these little trichomes these hairs inside three on each of the leaves so you imagine the pair of leaves and each of them the left half and the right half have these three triggers their hairs and if one of them is triggered on the leaf nothing happens if two of them are triggered then within 30 seconds of each other it doesn't have to be simultaneous then that the jaw snap shut and basically the it's a fascinating mechanism that depends upon a combination of things that the structure of the two leaves which are sort of bowed outward slightly like if you imagine a pair of symbols you know musical symbols and this is maintained by hydrostatic pressure by water inside the leaves kind of in the cells it leaves maintaining this shape and when the right stimulus arrives these these two of these trichomes are touched they generate an electrical impulse that changes the water pressure in the cells of the leaf and they then they then collapse inwards essentially then closing the leaf that's our rather approximate definition of exploration but it's fascinating stuff and what's extraordinary is it's the only plant we know of that does don't works like that it's just there's one species in this genus dyneo that does this thing there are other plants that use similar tricks but nothing quite that well some of them are small and live in water so we don't see them very much bladder work is one that actually has these little bladders that have a little trap door that operates a bit like the leaves of a Venus fly trap yeah so we've talked about the ability of plants thanks to their leaves to be able to sort of you know in a way sense things that are going on around them and we've talked there about you know the mechanism by which the Venus fly trap can catch and consume a fly so in the in the book sort of by way of a sort of final question you talk about this which I'll which I gather you're not very much a fan of is this idea of planting your biology you know do do plants display any signs of intelligence well it's an understatement to say I'm not a fan I spend a chapter in the book explaining why not and the reason why one even bothers to talk about this in or wait why I've even bothered to talk about it is because others are obsessed with the idea and have published some very successful books based on this idea so if you're into this kind of thing you will know exactly what which books I'm talking about and there's not one there's about half a dozen more dozen of them the first problem is what is intelligence and psychologists you know are somewhat have some difficulty over this at the moment when you know artificial intelligence is in every news bulletin this evident confusion there too about you know whether what amounts to a sophisticated matching exercise with you know the entire contents of the of the internet is intelligent or is this just you know smart searching general intelligence is clearly a lot more than that. But what can a plant do? So this is the issue. Can a plant do anything that you could describe as intelligent as what your dog could do, right? I mean dogs can learn things. They do learn things. That's one reason why they make good pets is because you can you can teach them not to poop all over the carpet and stuff like that. But is there evidence that plants can learn none whatsoever. None whatsoever. So how could you, I mean this is a short version, you know, the long version you can read. So how could an organism that can't learn be intelligent because it can't remember anything, it's experience and so on. Now somebody on the other side of the argument would say, oh, but there are there are plants that can learn. For example, they can they know whether they've been through winter. So this is absolutely so. Or they know that the total number of sunlight hours in the in the day has changed. So that must require some kind of memory. Well, it's the sort of memory that your toaster has, right? Because, you know, your toaster is programmed to deliver toasted bread of a particular color, which can be changed to your liking. So you can you can say, I want it, you know, black almost burning or I can have it, you know, scarcely toasted at all. And you can set that on a dial. And of course, your your toaster responds accordingly. But why does it do so? Because it's been designed to give you these options. And that's the kind of intelligence that some that plants have. Essentially, the what in the experiments I described earlier, where, for example, if they have intact trichomes, they can find their way through a canopy of leaves to at least keep up with the with the Joneses with the plant Joneses, you know, and not not be overshadowed is that intelligence? Well, no, it's adaptation. And when you push certain people who are of the plant intelligence persuasion on this, they will say things like, but all life is intelligent because it's adapted to which I reply, well, we understand adaptation. Why call it intelligence? You know, I mean, adaptation is a well understood process. We tie it to genetics, to genes, to chemistry, you know, to population genetics. This is this is not new. And it is exciting. And it is extremely productive science. It's, you know, it works. Calling plants intelligent doesn't tell us anything. So if you want to define intelligence so loosely that it includes plants, what is it telling you about plants or anything else that you wouldn't otherwise know and my answer to that is zero. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Fakus. That was Professor Jonathan Silvertown. To discover more about the topics we've just discussed, check out his book, Nature's Magicians, How Leaves Conjure Up Our World. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our guests and hosts in person, then why not check out our YouTube channel at Science Fakus. If you've enjoyed this podcast, why not take your curiosity a step further with BBC Science Fakus magazine. Receive an extra £5 off when you subscribe using the code SF5off. For more details, see the show description. The current issue of BBC Science Fakus magazine is out now. Pick up a copy wherever you buy your favourite magazines or downloads on your app store of choice. You can also find us on Apple News or online at sciencefakus.com. Tammy regularly haul stuff to her kids at college. That's why she shops at BJ's wholesale club saving up to 25% off grocery store prices. 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