The Science of Everything Podcast - Episode 97: Plant Structure and Function

Episode Date: June 27, 2018

An overview of the basic morphology and physiology of plants, including a discussion of the main types of plants, stems, roots, leaves, plant transport, meristems, plant nutrition, and plant sensory systems.

You're listening to The Science of Everything podcast episode 97, Plant Structure and Function. I'm your host, James Fodor. So in this episode, we are going to start in on a several episode long series on plants. So in this first part, we're going to look at the basic structure and anatomy of plants, including looking at stems, roots, leaves, merry stems, plant transport, nutrition, sensory systems, and in some later episodes, we'll look at plant reproduction, pollination, fruits, seeds, and different types of plant products, including vegetables and plant

fibres and other things like that. No real recommended pre-listening episodes for this one, although if you've got a little bit of background on cells, then that might help. But basically, nothing is central. Also, I wanted to apologise that it's been a few months since the last episode. I've had a few issues in my personal life come up lately that have taken a bit of my time, but never fear the podcast lives on and we move forward. So let's get a start on today's episode. So before I get into talking about the anatomy and morphology of plants, I just want to talk a little bit about the main types of plants that we find in the plant kingdom. Now, the basic definition of a plant is that it is a multicellular organism that gets most of its energy through

photosynthesis. So plants have chloroplasts, and it's also traditional to define plants as being land-based organisms that photosynthesize, although there's plenty of room for disagreement around the edges of this definition, so we won't worry too much about it. that but the basic idea is a very important event that occurred in the early history of the evolution of plants is that they adapted from early photosynthetic basically algae organisms that lived in the oceans and moved on to land but anyways so in this series of episodes we're going to focus on land plants and focus on multicellular organisms so first of all there are the green algae

there are about 8,000 different species of these they're not particularly interesting from our purposes they are photosynthetic organisms that can live in a diversity of places, including in the oceans. We're not going to worry too much about them other than it's generally thought that land plants evolved from them, as I just said, and they're sometimes grouped into the plant kingdom, but not always. Again, I just signpost them here.

We're not going to focus too much on those. The next main type of plants, though, that certainly count as plants, are non-vascular land plants. These are the biorephites. There are about 20,000 species of these. They include liverworts, hornworts, and mosses. So the most familiar of these are probably mosses for most people. So these are fairly small plants that look like moss or maybe certain types of grass.

Most grass aren't nonvascular, but some of the nonvascular plants look like grass. And importantly, there's very little distinction between the stem and the leaf in nonvascular plants, and they also have no xylem or phloem. Now, I'll talk a little bit about what those are later on, but xylem and phloem are transport tissues. So nonvascular plants are distinguished by the fact that, that they don't have a vascular system. They don't have a specialized tissue structure that transports nutrients and water throughout the plant. It's just transported directly through between the cells and the intertitial fluid. So that significantly limits the size that the plants can

obtain, obviously. Just like with animals, in order to have a larger, more complicated, more specialized body, you need to have specialized tissues moving around water and nutrients. So the non-vascular landplants were the first type of land plants to evolve. The next type to evolve were the seedless vascular plants. So seedless vascular plants have a vascular system, obviously, but they don't have seeds. So seedless vascular plants have about 12,000 different species and include club mosses, ferns, and horse tails. So we'll talk about seeds in a later episode when we talk about plant reproduction, but everyone should have, at least an intuitive idea about what a seed is.

So seedless vascular plants don't have seeds. so they're less specialized in that sense. And as I said, the most familiar example for a lot of people are ferns. That also means that ferns are not actually trees, because trees probably defined are seed plants. So ferns are not really trees, although they can sometimes look a bit like trees. So the most developed type of plants, evolutionarily speaking,

and the most specialized for life on land, are the seed vascular plants. So they have vascular systems and they have seeds. So most plants are, seed plants. They're also called spermatophytes and they're about 160,000 species. These include the conifers, cichads and flowering

plants. The seed plants are some divided into two main categories, gymnasperms and angiosperms. Angiosperms are the flowering plants and they comprise by far the largest number of plant species in terms of things that people are familiar with. So most

trees, bushes, flowers, grasses the large majority of plants that people are familiar with are angiosperms. which are, again, vascular seed plants. Gymnasperms are also seed plants, but they're distinct from the angiosperms. The basic characteristic that distinguishes gymnospers from angiosperms is whether the seed is fully enclosed in an ovary

or ovules within an ovary. So in gymnasperms, they are not in angiosperms, that the seeds are fully enclosed. So gymnasperm essentially means naked seed. And gymnospers comprise mostly conifers, including cedars, furs, pines, redwoods, spruces, junipers, cypresses and larches. They're typically the source of softwoods, which are usually used to make paper. They're typically found in cold environments that get a lot of snow, so, you know, northern Canada

and Siberia and so on, although, of course, they do grow elsewhere too. They have cones instead of fruits. So all of the flowering plants are angiosperms. They have fruits, and their seeds are enclosed within the ovary structure, which then often gives rise to the fruit, whereas gymnospirons have cones and their seeds are not fully enclosed. So that's the main difference between the two. Most plants are angiosperms. In fact, they're overwhelming majority of them. A few hundred species of the hundreds of a couple hundred thousand total seed plants are gymnasperms. So genusperms are important because they are, as I said,

the source of a lot of paper and trees like ferns, pines, redwoods and so on are quite prominent in terms of our understanding exposure to trees, but in terms of the overall diversity of plants, they're quite unusual. Okay, so what we're going to be talking about mostly in this episode are angiosperms. So I'm going to be focusing on flowering plants, and hopefully that should be understandable because the overwhelming majority of plant species that people are familiar with are angiosperms, and they also produce the overwhelming majority of food that the human species eats. Okay, so let's move on and talk a bit about plant anatomy and morphology.

Like animals, plants have organs, although it's, perhaps less common to think about it in that way, but they do. The organs of plants can be divided to two main types, vegetative and reproductive. So the reproductive organs are quite variable and depend on the type of plant. So for example, in gymnasperms, the organ bearing of reproductive structures is called a cone because the seeds are not enclosed, whereas in flowering plants, in angiosperms, the reproductive organs include the flower, the seed and the fruit. So those are quite variable. We'll talk about those in a future episode. The focus of what I want to about here are the vegetative organs of the plant. So these are the roots, the stems, and the leaves.

Many fewer systems than animals typically have, but nevertheless important that they are structurally distinct. So remember that in nonvascular plants, there's little distinction between the different vegetative organs, and the distinctions and specializations become more pronounced in vascular plants than they are in non-vascular plants. So the plant stems are the structural axes on which leaves grow, basically. That's a simple way of thinking about it. So another thing, another way to look at it is that the two main structural axes of a plant are both be referred to as shoots. Shoots that are above the ground are called stems, whereas shoots that are below the ground are called roots. So that's not always strictly speaking true because there are exceptions of roots that can grow above the ground and some

type of stems that can go below the ground, but for our purposes, that's a useful simplification. So basically when a plant seed grows, it grows in two directions, it grows downwards into the soil and upwards out into the air. The growths upwards out into the air, the ultimate purpose of those is to gather sunlight that's used to produce sugars that allows the plant to grow and increase in size and continue to survive. The shoots that grow below the ground do not house leaves, their purpose is mainly to gather water and other key nutrients that the plant needs, that it can't make for itself. And these are called the roots. So this is the main distinction between stems and root. It's whether they're above or below ground and their purpose.

Stems grow in a modular fashion, which means they don't grow all at once, but they grow bit by bit in distinct segments. So the basic structure of plant stems consists of nodes separated by internodes. So a node is basically like a small little region on the plant stem that can give rise to new growth. And specifically, nodes hold leaves.

Now, not every node necessarily has to have a leaf on it. The leaf may not have grown yet, or perhaps it fell off, or it was eaten or something. But the basic idea is that the purpose of the nodes is to grow a leaf, one node to one leaf usually. Internodes are the lengths of stem between the nodes that separate out the leaves. Obviously, you can't crowd in the leaves too much, because otherwise they won't have enough access to the light, and the purpose of leaves, of course, is to gather sunlight. So they need to be separated enough, and that's the purpose of the internodes. So the internodes are sort of like the smooth sections of the stems, and the nodes are sort of the rough bumps in between the smooth bits where the leaves grow.

So that's what we mean by plant stems being modular, because there's this regular internode node node, interode node structure to them. Now, another important aspect of stem structure is what's called the axillary bud. So axillary buds are embryonic tissues or regions of embryonic tissue that it's located what's called in the axillary. of a leaf. So basically just above where the leaf grows out from the node. And each axillary bud has the potential to form new shoots, which can be specialized as either vegetative shoots, so more stems and branches or reproductive shoots, flowers. So basically, as the stem is growing, it can either produce leaves, and those form off at those butt off at the nodes, or it can sort of divide and branch off into new stems, and that happens at the axillary buds, right?

So, axillary buds give new stems and also flowers, whereas nodes give rise to leaves. So they're actually growing from, they grow from distinctive anatomical parts of the stem. One key difference between plant organs and animal organs is that plant organs, like stems, in particular, and also roots, typically grow by elongating. That is, they grow at their ends, and the cells divide at the end in what's called the merry stems, which we'll get to talking about it in a little bit, and then gradually increase in length and thicken and broaden.

Now, that's very different to how, say, human arms grow, or legs or most of the other organs, which essentially fully form in their basic shape and then just get bigger and more complex. Your arm doesn't grow from the shoulder out to the elbow and then to the hand. That's not how it works. So plants are quite different in that respect.

Okay, so that was the first main type of vegetative plant organ, the stems. And remember, the basic function of the stems is to house all of the leaves and also to transport substances between the leaves and the roots. So let's move on to the next organ, next main plant organ, which are the roots. So again, in botany roots are the parts of the plant that are normally found underground. You can have above-ground roots, but generally they're underground. its main function is to anchor the plant, so to stop it from falling over or being washed away, that's obviously important, and also to absorb water and other minerals and nutrients found in the soil.

Roots also connect up the most outlying regions of the roots, where a lot of the absorption takes place to the central stem region, which then obviously connects it to the leaves. So it's a little bit like an arterial erode system, and at the very extremer of the stems, you've got the leaves, at the very extreme of the roots, you've got the root hairs, which we'll get to in a moment. And it's those places where the action's taking place. At the root hairs, you've got most of the absorption of nutrients from the soil.

At the leaves, you've got most of the production of the photosynthesis, and you have to connect up the nutrients and water coming from each to the other. So essentially, they gradually sort of come together in thicker and thicker stems or roots, as they're all sort of directed towards the central trunk of the tree. or plant, which then sort of forms a highway connecting all of the leaves to roots. So that's obviously a simplification, but it's perhaps a helpful analogy. The primary root of a plant is called the radical, and it's the first organ to appear when the seed germinates.

So you remember I said when a seed germinates, it grows essentially up and down. The downwards part is the radical. It's the first thing to appear and grow downwards into the soil. The deepest observed living root, at least as far as I was able to uncover, was found 60 metres below the ground surface, which is pretty insane if you think about it. Roots can often grow as deep as the tree is high. So if you look at a tall tree,

it's not unlikely that its roots go down about as far as the tree is tall, which is not too surprising if you think about it because the tree does have to be stably anchored, and so that's going to require a lot of below-ground structure if it goes high above the ground. Most of the plant roots, however, are fairly close to the surface because that's where most of the nutrients and the water is. Now, as I mentioned, most of the actual absorption of the nutrients,

it occurs at the interface between the roots and the soil, and the surface area is the limiting factor here. So surface area of the roots is increased, and therefore absorption is facilitated by the existence of what are called root hairs. So these are little outgrowths of the epidermis, the sort of edge cells of the root, and they just increase the surface area to help absorption of water and other nutrients with the soil.

and because they're longer a thing, they can penetrate, sorry, they can penetrate in the spaces between soil particles, which, again, helps them with absorption. If soil is too tightly packed or it's really dense like clay, there's very little space between the particles, and it's very hard, then it can actually be difficult or impossible for roots to grow properly. Apart from the hair cells and the sort of cortex, basically the edge tissue surrounding the root,

there's another important functional aspect of roots, which is called the root cap. That's on the very end, like the tip of the root. The root cap is important because it's a sort of a tough section which protects the tip of the root and helps it to push through the source. So physically it gives it sort of an ability to push through and part the soil particles. It also contains statists which restrict the root to grow so that it grows against gravity. And we'll talk about more of those when we get to plant sensory systems,

but the statists there are located in the root cap. Now, I just mentioned the epical merri stem, which is sort of just behind the root cap. That's the location at the tip of the root that contains the merri stem cells which divide and proliferate so as to facilitate root allegation. That's directly analogous to the merri stems that are found in the epical merri stem of the stems of the plant, which grow upwards. Some types of plants called dicots have a taproot system, which means one large vertical root, with many smaller lateral. roots coming out of it. Other types of plants called monocots have more of a mat-like fibrous structure that spreads out below the surface. So the architecture of roots can be different depending on the plant. Okay, so that's the second main type of vegetative plant organ, the roots. Now I'll talk about

the third main type of vegetative plant organ, which are the leaves. So a leaf is an organ of a vascular plant that is the principal appendage of the stem. So we know that leaves grow out of the stems. And leaves are typically broad, flat, and thin. And the reason for that is because the purpose of leaves is to gather as much sunlight as possible. So shaping them into a broad and flat shape like that helps them maximize their surface area to the sun. Leaves are asymmetric. So the top of a leaf, the side that faces the sun, is different to the bottom of a leaf, which should be familiar if you've really ever looked at leaves before. The basic structure of a leaf is, so there's an epidermis, the upper and lower epidermis,

which contains waxy cells that help to protect the leaf. This includes protection from drying out, desiccation, and also protection from predators. These cells are mostly transparent, as you would expect, because they don't photosynthesize much themselves, but the light still needs to pass through them so that it can reach the photosynthetic cells below. Now, below the epidermis are two layers of paranthymar cells. So these types of cells are found throughout plants. They're sort of structural cells, for the most part. They help us store energy and just sort of keep the plant together.

But in this case, in the case of leaves, they're the main location of the chloroplasts. So in particular, the paliside cells contain the largest number of chloroplasts per cell. So they're the primary side of photosynthesis in the leaves. There are chloroplasts found elsewhere as well, of course. but they're largely found in these palisade cells in the epidermis. So the critical point there is that, to remember, that the main photosynthetic siding leaves is not at the very surface of the leaf. It's below that, and the light passes through the outer surface,

the outer protective surface, and into the parochymic cells underneath, which, where the photosynthesis actually occurs. There's also quite a bit of empty space within leaves to allow for the transfer of gases. So we'll talk about that a bit more when we get to aspects of plant transport, in nutrition. Leaves have determinant growth, so they're more analogous to animal organs, as I mentioned before, so animal organs tend to grow a certain amount and reach a final size and then stop growing, whereas many other types of plants, including stems and roots keep growing, essentially, as long as the organisms are alive, and they elongate so that they don't,

there's no final form that they'll attain, they'll just keep growing. So leaves are the exception to that. Leaves have a specific pattern of growth, and then they'll grow to their full and stop, and they'll retain that shaper. So plants won't grow bigger and bigger and bigger and bigger leaves as they age. They'll just grow more and more leaves on their larger stems and with larger roots. The most prominent structural elements of a leaf are the petiole, which is the leaf stalk and the laminar, which is the blade, the made flat part of the leaf. Okay, so that concludes the discussion of the three major vegetative organs of plants, stems, roots, and leaves. Now I'm going to talk a little bit more about Mary Starrie.

which are the cells that give rise to plant growth. So I've mentioned these before. There are primary merriestems found in the apex, so the end of stems and also roots, and these cells divide to keep elongating the roots and the stems as the plant grows. There are many other types of merri stems and sort of different subtypes of them found within plants. I'm not going to go through the full explanation

of how all the tissue types are related, because it's a little bit complicated, complicated and hard to keeping your head without a diagram. However, the most important thing to understand is the distinction between the primary marrow stems and lateral merri stems. So primary merri stems, as I said, are found at the apex, the endpoints of stems and roots. And the cells there divide so as to elongate the stems and roots and help grow the plant. When I say that the cells divide, the way that works is that these are effectively stem cells. So they are undifferentiated cells that can give rise to a range of different tissue types.

And so when these cells divide, at least one way they can divide, is by dividing into two different types of cells. Essentially, an initial stem cell will divide into, you know, cell mitosis. One of the daughter cells will be the same type of cell as the parent's cell, so it'll be a stem cell. But the other type will be a somewhat more differentiated type of cell, depending on what it's differentiating into. And then that cell can further divide and further,

differentiate. So the point is that as Mary stem cells divide, they maintain an existing population of merri stems, otherwise the plant wouldn't be able to keep growing, but then they also give rise to new tissues, which further specialize as needed, depending on the plant's growth. So that's how this sort of cell differentiation works. But so we've got the apical merri stems, the primary merri stems, at the ends, at the tips of the roots and the stems, essentially, but there is also another type of merri stem called lateral merri stems, and these are found basically around the sides of stems, at least in certain types of plants. They're not found in all plants.

Instead of increasing the length of roots or stems, like primary merri stems do, they increase the width or the girth of the plant's stem or root as well. So these are most prominent in trees, what we call trees. So trees have to have, in order to count as a tree, a plant has to have significant secondary growth from the lateral merri stems. and this produces wood and produces a thickened trunk, which gets wider and wider as the tree gets older. So that is the result of a different type of tissue than the elongation of the stems and roots. So this then leads me into a discussion of the different layers of cells in plant stems, particularly tree.

So I'm going to focus on the layers of tissue in a tree trunk. Obviously, this basic structure applies to other stems as well, but it becomes, most fully differentiated and reaches its sort of peak form in the structure of a mature tree trunk. So, you know, if you imagine looking through the cross-section of a tree trunk, and certainly you would have seen that before, you look at it and you see circular patterns radiating out from the centre with mostly wood in the middle and bark around the edges, basically. That's the extent of what a layperson sees. So how can we understand what we see here?

The first thing to understand is that there are three main tissues that tissue types that apical meri stems, remember that's the meri stems on the end of the stems, the cells that are dividing and elongating the stem. Three main types of tissues that these divide into. Protoderm, pro-Cambrian, and ground merri stem. The protoderm is the easiest to understand because it gives rise to a tissue called the epidermis.

Basically these are at the very edge of the plant. the very edge of the stem, and they just provide some protection for the cells underneath. Below that is what's called the periderm. So these are layers of cork. When you hear of cork, that's essentially a commercial product that's produced from a particular species of plant. But more generally, other plants have cork as well. It's a type of tissue that's found just at the outer edge of the bark,

not the very outermost layer, but just beneath that. So these form the peasant. periderm. Again, it's largely sort of protection. Below the periderm is a sort of a three-layered structure. You've got the phloem, then something called the vascular cambian, and then the xylem. So let's try and unpack that. If you recall before, I talked about transport in plants. Vascular plants with specialized transport structures have specialized tissues called xylem and

phloem. I'm going to talk about more about the difference between them later on. But the important thing to note is that they're both produced by a special type of tissue called the vascular cambium. And that derives from the pro-cambium, as you may have expected. So basically, if you think about it as a sort of a layered cake, towards the outside is the phloem. Just interior to that is the vascular cambium that gives rise to the phloem, and then interior to that is the xylem. and that also comes from the vascular cambium. So the vascular cambium sort of provides the place where the xylem and the phloam grow from, or grow out of, divide from. When we talk about wood, we're actually just talking about secondary xylem tissue.

Secondary xylem tissue is produced by the lateral merri stems. Remember those are the ones that increase the girth of the plant. primary xylem, as well as primary flowam, there's primary and secondary of each, primary zylam is produced by the apical meri stem. But as the tree grows and the tree trunk thickens and thickens, the primary zylam and the primary phloem both become quite insignificant compared to the secondary zylam and secondary phloom. Basically, simplifying things a bit, you can think about it as if there's only a certain amount of primary zylam and primary phloom that are produced by the apical mary stem, because it's, it's, it's, it's, it's, it's, it's

So if I'm thinking about a given section of the tree trunk, the apicalomeric stem starts out near there, but then grows further and further away as the tree gets taller and taller. And so there isn't new primary xylem, primary phloom coming into existence. So as the tree trunk gets fatter and fatter, the primary salem and flambe become not very important. So I'm just emphasizing that because you might wonder why we're just talking about the secondary isleum and secondary phloom. Well, basically because the primary ones are insignificant once the tree trunk grows a significant amount. So,

we've got the vascular cambium. It's giving rise to the secondary xylem and the secondary floren. When we look at the tree trunk, most of the material that we see is just the secondary zylam. So all of those circular rings that you see when you look at the cross-section

of a tree trunk, the overwhelming majority of that, if it's a fairly mature tree, is secondary zylam. So it's almost all one type of tissue. What happened to all the other types of tissues, you might ask? Well, at the very center of the tree trunk, you might see this sort of circle or pattern of some sort.

It's a bit hard to describe, but looks a bit different depending on the tree. But this is called the Pith, and this is the, remember the paranchimer cells, these were the sort of structural cells that house the chloroplasts in the leaf. Well, there's parochymar cells in the center of plant stems as well. If you cut a cross-section of a fairly new stem, you'll see that there's quite a large, pith with a lot of spongy paranchimus cells there. But as the stem grows, it elongates, and particularly as the secondary merist stems give rise to greater and greater girth, the pith sort of decreases in relative size and importance until it's largely obscured by the secondary isleum.

So in the middle, you've got the pith, which comes from the ground mary stem, but by the time we're talking about a mature tree trunk, it's fairly insignificant and small. So you can only just barely see it at the center there. Then the overwhelming majority of all of those rings surrounding the center and radiating outwards, that's the secondary xylem, and that's wood. Just outside of that is the vascular cambium. Remember, that's what gives rise to the secondary zylam and also the secondary floam, which is what sits just outside of the vascular cambium.

And then on top of that is the laser periderm that I mentioned. That's the cork. And then on top of that, again, is the epiderm. So we've got a very out of out of layer of epidermis and periderm, the cork cells, interior to that, you've got your phloem, interior to that, you've got your vascular cambion

that gives rise to the phloem, interior to that. Again, you've got your secondary xylem, and then that provides most of the structure of the tree trunk until you get to the very center, which is the pith. Now, what we talk about is bark. That's basically the layers of periderm

and the secondary phloam. So the outer regions, there are different types of bark, there's the dead tissue, which is on the very outer most edge of the tree, and then interior to that, you've got your secondary flow, and that's the living butt. That's the living tissue. That's softer, but it's still part of the bark.

So if you peel away the bark, it's basically the vascular cambium, and interior to that, it's just sort of wood all the way down until you get to the pith in the center. So the point to take away from this, I know it's quite a complicated structure to describe without being able to see your diagram, and as usual, I'll put some up on the Facebook page. But the basic point is that most of the structure

of mature tree trunks, so we're talking about fairly old plant growth here. Most of it is secondary xylem, which has derived from the vascular cambium, and then there's just a layer of bark, secondary floren plus and peridermis, and epidermis, above that. These different types of tissues

derive from different types of primary merostem cells, the protodermium, the corcambium and the ground merri stem, plus the lateral merri stems. Remember, that's the vascular cambium, the gisrises. secondary xylem and the secondary flowam. Okay, so enough on that, that's potentially a bit confusing,

but I just wanted to give you a sense of the structure that's inherent to trees and the different layers of cells that are found in a trunk. One other thing that I did want to mention before moving on is, I mentioned the circular rings that you see in the secondary zilum, in the wood, in tree trunks. The reason for those, these is called the annual growth rings. The reason for these is because the rate at which trees grow, especially the division of the masculine cambium into further flow and asylum cells, the rate at which this occurs varies over the course of the year. So basically, plants are growing pretty much continually. They typically slow down a lot

in winter, though, and grow more during the summer. So the differing rates of growth give rise to visible structures that you can see in terms of annual rings. So this is used, as you probably have heard of, to date trees as well, because you can actually count them and correlate them across different trees and so on to be able to see how old the tree is. Okay, let's move on and talk about plant transport. So this mostly concerns the vascular tissue, the xylem and the phloem. Because you may not have seen these words written down before, xylem is spelled kind of like xylophone, it's x-y-l-em,

and phelome is spelled p-h-h-l-e-m, just in case you want to look any of those words up. So xylem and floan. These are the two components of the vascular tissue. their purposes to transport fluids and nutrients internally throughout the plant. The basic architecture, as I mentioned before, is that sugars are produced in the leaves, as a result of photosynthesis using light as a source of energy, whereas water, as well as some other minerals and necessary nutrients,

are absorbed in the roots. Now, those have to exchange those substances with each other, because obviously all of the cells in the plant need both sugar and water in order to survive. plus the other minerals. So water and the other minerals are transported mostly up to the leaves from the roots, whereas sugars are transported mostly down from the leaves, down the stem to the roots. Obviously, that's the simplification of the process, but that's the basic idea of what's happening in plant transport. Xylem transports primarily water, transporting it from the roots to the stems and the leaves. Whereas the primary purpose of floam is to transport soluble organic compounds, so basically sugars,

from the photosynthetic parts of the plant, that is the leaves, down to the roots. So, key thing there, xylem transports mostly water, flowem transports mostly sugar. Recall that xylem is the main structural component of tree trunks, because it forms the annual growth rings, it form the majority of the tree trunk there, and that gives rise to wood. So wood is basically plant tissue, the purpose of which is to transport water. That kind of makes sense, if you think about it, because trees are very big, they require a lot of nutrients,

they require a lot of water brought up to their leaves, and they require a lot of sugar brought down to their roots. So most of what the tree trunks actually doing is facilitating that transport. It's like the big highway of the plant, ensuring everything gets to where it needs to go. Now, vascular tissue is highly specialized for its transport function. So cells in vascular tissue are long and slender, generally with essentially holes in the middle.

They're basically hollow cylinders that allow transport of water and organic soluble compounds, so like sugar throughout the plant. And they connect up to each other. It's sort of like, if you imagine bamboo, it's a little bit like that. It's not literally like that, obviously, because they're much smaller, and each cell, one cell doesn't extend right from the base of the tree up to the leaves. They connect up to each other in little segments. But bamboo stems will give you the basic idea of what sort of looks like at a microscope.

level. In roots, vascular tissue form bundles, which is located mostly in the center. So, xylem of phloin are located essentially in the center of roots, but they spread outwards, forming either a ring near the edge, not at the very edge, but near the edge of the stems, or in other plants, they're sort of scattered throughout the middle of the stems. Obviously, this is less relevant in the case of highly differentiated, mature secondary growth, like tree trunks that I was talking about, where it's pretty much all of the stem is dominated by the secondary zylam, but in less mature stems, it's actually most of the internal structure of the stem is the pith, the parochymar cells that I mentioned, the basic structural cells.

Then you've got the vascular bundles, the xylem and the phloem, interspersed throughout that. So this is important to understand because the basic structure of plant stems, if you look at diagrams and think about it sort of in terms of the morphology of the different cells is actually different to how you would sort of naively think about what plants look like if you have a tree trunk cross-section in your mind when you think about it. Maybe that's just me. I tend to think in terms of trees. So tree trunks are actually sort of atypical in that sense. So here's something you might not have thought of before, but the water that plants absorb through the roots is mostly lost via transpiration, essentially evaporation, in the leaves.

I'll explain in a little bit why that happens, but that's the main reason why plants need a lot of water. It's because they're constantly losing large amounts of water while their photosynthesizing. So they've got to replace that somehow, otherwise they dry out. But how does that water get from the ground to the leaves of the tree? Sometimes it may have to travel many meters, even dozens of meters, in order to reach the full distance.

Well, the answer is that it's a combination of factors. Basically, the water is pushed upwards by the pressure of the water crossing the barrier being absorbed through the hair cells into the roots. So it's sort of pushed by the higher pressure of the water down in the roots and also at the same time pulled by the lower pressure of the water up in the leaves. And the reason for that is because the water is evaporated. There's continual transpiration of water from the leaf stoma, the little holes in the leaves, I'll talk about it in a moment. So the water pressure there is lower than it is down at the roots, and so there's this sort of combination of a pulling and a pushing mechanism,

which helps the water to get up from the roots to the leaves. Another important mechanism that helps in the xylent transport of water is cohesion tension. Basically, this is an intermolecular attractive force between the water molecules and the cell walls of the edges of the xylem cells, the little pipes essentially, the water is climbing up. This tension helps to pull the water up from the roots to the leaves. So by itself it wouldn't be enough in order to get the water up there, but basically it helps to pull the water up by giving an extra pulling force more than you would get, if you just had, say, one very wide pipe. The idea is that you keep the pipes that the water's traveling up fairly

narrow so that there's a larger surface area for the water to sort of stick on the sides. I mean, it's sort of similar in a vague way to, you know, you see little beads of water stuck on the sides of the tap or something like that. That's because water coheres to certain surfaces and it will just sort of stick there. If you have lots of very thin surfaces, sorry, if you have lots of very thin pipes with large surface area, then that can help to pull the water up. Just like a it's easy to drink water through, you drink water through a straw. You have to suck on the straw. That reduces the pressure and so the water is sort of pushed outwards by the higher pressure at the base, but also the cohesion of the water molecules with the sides helps that process as well.

Anyway, so the basic point here is that the transport of water from the roots to the leaves is quite a complicated process and it's still under active studies to exactly the contribution of all these mechanisms. But the basic idea is that lots of the lots of thin pipes in the the xylem cells help provide cohesion forces, but the main underlying mechanism is higher water pressure at the roots compared to the lower water pressure at the leaves as a result of evaporation, and that leads to the pushing of water upwards. This also then leads to the idea that plants can control the rate of transpiration, thus the rate of water pumping, by opening or closing these tiny openings on the lower side of leaves called stoma. So these are on the lower side of the leaves, the side facing

away from the sun. Remember that leaves are asymmetrical, the top is different to the bottom. Now the purpose of these holes, the stoma, is to allow the leaves to receive carbon dioxide into air pockets inside the leaf. This is essential for photosynthesis because carbon dioxide is the ultimate source of carbon fixation, which then forms the sugar molecules. That's the whole point of photosynthesis is to form these sugar molecules. So the carbon has to come from somewhere. The energy comes from sunlight, but the actual material that forms the tree, the carbon, comes from the air, carbon dioxide. So that means that while leaves of photosynthesizing,

there must be ready access to the outside air, a ready source of carbon dioxide. And so stoma allow that airflow to continue, to continue cycling as photosynthesis occurs. However, the problem with that is that air contains water vapor. And furthermore, the air pockets exist inside the leaf, as I mentioned previously, air pockets exist that help the cells to absorb the carbon dioxide that they need and also pass out the oxygen. But because cells exist in aqueous environment, essentially they're wet, there's water inside them and also surrounding them in the interstitial fluid.

So the actual leaf cells are saturated with water. That's just the way cells are. However, air can absorb water vapor essentially. This is the humidity level. And so when air comes into contact with these water-saturated plant cells, it takes up that moisture, which of course removes moisture from the plant. So the basic point here is that in order to gain carbon dioxide from the atmosphere, plants have to lose moisture. They have to lose water. It's a trade-off.

The more carbon dioxide the plant wants to take up, the wider it can open its stoma, and so it gets more air circulation, but then the more air is circulating, the more contact this air is going to have with the water-saturated cells inside the leaves, and therefore the more water it's going to lose. There is therefore a trade-off between water loss and the efficiency of photosynthesis. You can have more efficient photosynthesis by pulling in and cycling more carbon dioxide at the expense of losing more water,

by putting more water into contact with the air, and therefore carrying out more water. Droughts cause loss of water and hence cause cells to lose their terger, which basically means they kind of sag, if you like, and that causes leaves to wilt, because cell turdure is what gives sort of strength and rigidity to the cells and the leaves that they're made up of.

So that's why plants wilt when there's a drought or when they haven't been watered. This will also cause the stoma to close, and so reduces the rate of water loss, but also at the cost of reducing the rate of plant growth. stoma typically are open during the day and close at night. Now, if you think about that, that makes perfect sense because at nighttime, there's no photosynthesis, and therefore there's no point in keeping the stoma open because the point of them is to bring in carbon dioxide for photosynthesis,

but if there's no photosynthesis, you don't need extra carbon dioxide, so you'd just be losing water for no good reason, so they typically close at night to conserve water. So far I've been talking about xylem transport, which is, remember, mostly transports water. but what about Floam? Flowam sap moves along sieve tubes, which are sort of similar to those found in Zylam, but is a little bit different structurally. And they mainly, their main direction of transport

is from the sugar sources to sugar sinks. So sugar sources are obviously the leaves, where photosynthesis occurs. Sugar sinks is basically everywhere else in the plant. So everywhere that needs energy, that needs sugar, and where growth is occurring, but that doesn't photosynthesize directly. So this is the stems and the roots of,

the plant. Sometimes sugar is also moved around to storage organs that can be below the ground or in the stem. So these are often found in vegetables, for example, that have storage organs that store lots of sugars underneath the ground. So those ultimately, all of that has come from the leaves and been as a result of photosynthesis and then being stored in the organ below the ground. The way Flom works is essentially that the sucrose, the sugars that's produced in the leaves, is loaded into the sieve cells, so loaded into the tubes that carry the flowam, and then there's just basically bulk flow of the flam sap. Osmosis will tend to lead to a gradient whereby the fluid is pushed towards the sugar sources, and the sugar tends to move away from that, and will gradually

sort of disperse outwards through the plant. I think I've discussed osmosis in one of the previous chemistry episodes, so I won't go through the detail here, but yeah, but the idea The idea here is that the sugar is produced at particular sites and water comes from the roots, and so there's going to be a natural tendency because of essentially concentration, moving down concentration gradients for the sugar to move away from its high point of concentration and dispersed throughout plant, whereas water will move in the opposite direction to regions where it is in the lowest concentration. And this process is mediated by the flow of the sieve tubes. Okay, so that concludes the discussion of plant transport. I now want to talk

a little bit about plant nutrition. There's obviously a lot more I could say about this, but I just want to cover a few basic points here. So as I said, plants need water, which they need in order to offset the water that's lost during transpiration as part of photosynthesis. So this is critical, otherwise the plant will wilt. They also need water as a source of oxygen and hydrogen for forming other organic molecules. And they need carbon dioxide as a source of carbon. So carbon is the basic, well, basic structural element, I guess carbon along with hydrogen, the basic structural element of any organic substances, including plants, and it is the main ingredient of the sugars and other compounds that make up cell walls

and many of other structural elements of cells. So carbon is obtained from the atmosphere. So basically the large majority of the dry weight, of plants is literally taken out of the air. That's the process called carbon fixation. The carbon is taken out of the air and essentially fixed in organic molecules, which then form the structure of the plant. However, in addition to those sort of key elements of water and carbon, there are a number of other elements and substances that plants also need to grow. So one sort of large class here are what we broadly call fertilizers. In particular, nitrogen. So nitrogen,

is an essential critical component of proteins and nucleic acids, which obviously plants need. However, nitrogen cannot be extracted by plants directly from the air, which you might naively have thought would be a logical source of nitrogen, because about 80% of the air is nitrogen, so why can't plants just use that? After all, they take carbon dioxide from the air, or they take the carbon from carbon dioxide out of the air. Trouble is that plants lack the enzyme to do this directly. It's actually hard to split out nitrogen from those N2 molecules that are in the air and get it into a biologically usable form. And plants can't do that themselves.

So in order for them to gain nitrogen, they have to absorb it in a biologically accessible ionic form from the soil. This is ultimately produced by nitrogen fixing bacteria that live in the soil. Artificial fertilizers can also contain large proportions of nitrogen, and this is why fertilizers are often applied to plants. Basically, it's a source of nitrogen, because this is typically, in most environments, the main limiting factor to plant growth.

its lack of nitrogen. Artificial fertilizers have essentially the same chemicals as natural fertilizers, again mostly nitrogen as well as a wide variety of other minerals and other substances plants need. The main difference between natural and artificial fertilizers though is that artificial fertilizers tend to release their chemical substances much more quickly. And that in itself isn't a problem except when they're applied in excess quantities like the wrong time and so on, they're liable to leach out of the soil and pollute nearby rivers and lakes. Leaning to a process called eutrophication where there's an excess growth of algae and other plants that you don't actually want, which can lead to a wide variety of problems for ecosystems

there. Basically, that's excessive fertilizer, excessive nitrogen that's been added on, leached off because it wasn't needed by the plants where it was applied at that particular time, and instead it's promoting an undesirable form of plant growth somewhere else. Essentially what eutrification is that there are other aspects of that as well, which maybe we'll discuss in a few. future episode. Fertilizers are a very important source of nitrogen, but do need to be added carefully. Now, there's another aspect to this nitrogen story, which is that in addition to absorbing natural fertilizers from the soil, like from manure or decaying plant matter, or artificial

fertilizers applied by man, another source of nitrogen for plants are swellings on the roots of legumes. So not all plants have these, but some plants, particularly legumes, have these things called root nodules. And these are locations in the roots that nitrogen-fixing bacteria from the genus Rhizobium live. So they live in these vesicles, that the bacteria themselves actually live in these vesicles formed especially by the plant cells to facilitate a mutualistic relationship. So the bacteria literally lives as an independent organism inside the plant. It's sort of like they've got their own hotels set up for them there. And the reason though the plant does this, the reason that they have these special nodules on the roots is because these bacteria have

that enzyme that's necessary to fix nitrogen from the air and make it into a form usable by the plant. So the plant benefits from this relationship by getting a better access, more ready access to nitrogen. In return, the bacteria benefits by gaining access to a ready supply of sugars produced by the plant. So it's a win-win, that's what a mutualistic relationship is. This is also why legumes are particularly useful in crop rotation, which is where in the same region of soil at either different times in the year or in successive years, you grow different types of crops, because many crops, for example, will deplete the nitrogen in the soil, but then if you grow legumes there one year, because of the root nodules, they can actually return nitrogen to the soil and thereby providing a source of nitrogen for future years for other times. types of plants to grow in their future years. So that's one of reasons why legumes are especially useful. So one little final tidbit before moving on from plant nutrition is I just wanted to mention carnivorous plants. So this at first might seem a bit strange because we defined plants as being

photosynthetic organisms, but some plants in certain environments find it advantageous to derive some part of their energy requirements by consuming small animals, generally insects or other types of small arthropods. So the Venus flytrap is the most famous. the most famous example of a carnivorous plant, but there are many other types as well. And the basic idea is that the plant will grow some structure, which is specially designed to trap or to lure in and then trap a small animal, generally an insect or a little spider or something like that. And then they'll be slowly digested by enzymes that the plant produces and absorbs its nutrients. These are almost always found in nutrient-poor habitat, such as

bogs where the soil nutrients are very limiting, but there's readily available sunlight and water. So people have done modelling of this, so that they're only very limited environments in which it's worthwhile for plants to grow these elaborate structures in order to get the nutrients they need. And basically, it tends to happen in bogs. So carnivorous plants aren't very common. I think there are a few hundred species of the hundreds of thousands of plants, but, yeah, they're an interesting little addition to the plant kingdom. Now, I want to move on and talk about plant sensory systems.

So plants are not simply passive entities that just sort of sit there and grow. They actually sense the environment around them in a wide range of ways. Plants are not motile so they don't move, although they can move in certain ways, but they don't move around. However, they still have many important ways of detecting their environments. So I'll just talk briefly about a few of these. One prominent one is phototropism. So the basic idea here is that plants respond to sunlight, and this is not too small. surprising. So most plant stems exhibit what's called positive phototropism. That means they grow

towards the light. This is mediated in part by a chemical called oxen. So the basic idea is it seems to move away from sources of light and thereby stimulates growth on the opposite side of the stem to where the sunlight is coming from. So this might be a bit in counterintuitive, but if you think about it, if you have a stem that's pointing upwards and the sunlight is shining more from the right hand side, then what you want to happen is the left side to grow more quickly than the right, because if that happens, the growing left side will sort of stretch outwards and cause the stem to tilt towards the right, because the right side is not growing as fast, and it sort of can't keep up. So the whole thing will tilt towards the right. So oxen, by moving away from light,

stimulates growth on the opposite side of the stems to where the sunlight comes from, and thereby causes bending or tilting towards the light. And so the net effect is a rotation of the growth in the direction of the sunlight. Gravitotropism is response to a plant growth with respect to the force of gravity. So this I mentioned previously, it's... So gravitropism is found both in stems and in roots. So plant stems tend to grow against the force of gravity, whereas roots grow in the same direction as gravity.

So that's how a plant knows whether to grow down or up, depending on whether it's a root or a stem. This is easy to see if you tilt a plant seed sideways and watch what happens. the roots and the stem will slowly reorient it, so they won't just keep growing horizontally. The roots will bend around and grow downwards as they sort of should, and the stem will bend around and grow upwards. And that's called gravitropism. One of the ways plants can sense the direction of gravity is through statoliths, which I mentioned before. These live in the root caps on the roots.

These are dense organelles that store starch. It's thought that they work by being denser than the cytoplasm, and so they'll tend to sediment to deposit in a with gravity, and so there will be a way for the cell to determine which way is downwards, and this motion can then be detected and signalled by actin fibers within the cell. So basically it's like rocks pushing on a network of fibers in a lake, so that the rocks fall downwards, and they'll push the fibers around, and so that the outside of the cell can sense which direction is downwards. That's a loose analogy, but that's sort of what's happening.

Another type of plant sensory mechanism is called phygmatropism. This refers to differential plant movement as a result of touch. Usually when one side of the plant is touched, it will grow more slowly than the other side. This is thought to occur as a result of activating mechanoreceptors, which lead to the opening or closing of ion channels, which can thereby generate signals causing plants one side to grow more slowly at the other side. the main application or importance of this is that it tends to result in stems that will coil around

and cling to the surface that it's touching. Again, it's the same principle, the same basic principle as phototropism. If one side of the plant is touching a surface, then if the touching side grows slower, the non-touching side will sort of curve and grow around the surface, thereby causing the plant to cling to the surface and sort of coiling the stem around it. Now another very interesting phenomenon of plants is called rapid plant movement.

So obviously plants can move. They move in the wind and they grow and they grow towards the sun and according with gravity. But that takes a long time. And apart from that, most people think of plants as sort of static. And as I said, they're not motile, they don't move around, but plants can sometimes move rapidly or sometimes the plants can. and this is in response to external stimuli and motion that occurs in less than one second. So we're talking a sort of animal speed motion here. A famous example is this is the Mamoza plant where the leaflets fold up when you touch them

as a result of electrical signals that cause some of the leaf cells to lose their terger, which causes essentially the leaves to wilt, although it looks like they're sort of folding up. So plants don't have muscles, so the way that the leaves move here is by the cells losing turdure, losing the, basically some of the water flows out, leaving them to sort of wilt, and the leaf

to sort of collapse in on itself. They'll fall, the leaves will fall back down if left undisturbed for a few minutes. I don't know if it's exactly known why they do this, presumably it's some sort of defense mechanism, but anyway, it's quite remarkable to see, and recommend looking at a video if you haven't seen this before. One final phenomenon of plants that I wanted to talk about is called photoperiodism. This refers to the ability of plants to detect seasonal changes, either in the length of the day or the length of the night, and those can actually be sensed independently of each other. And some plants require day-length of a certain amount

in order to, before they'll flower, or conversely short enough nights before they'll flower. So this is one mechanism that plants use to detect the time of the year. They can actually detect the length of the sunlight or daylight. And you can actually interfere with this artificially by like turning a, shining a light on the plant, even for brief periods during the night, and that disrupts its process

so that it thinks the night was shorter than it was, or conversely covering them during the day, so they think the day was shorter than it was. Sometimes the plants absolutely require nights or days of a certain length. These are called obligate photoperotic plants, whereas others don't absolutely require a certain length, but they're just more likely. It's a probabilistic thing.

These are called facultative photoperotic plants. Okay, so that's all I wanted to discuss in this episode. Next time I'll talk more about plant reproduction and plant products, including fruits, and vegetables. So hopefully you enjoyed this episode and stay tuned for next time. Remember that you can show your support for the podcast by going on Facebook and typing in the Science of Everything podcast, giving a page a like. You can also send me an email with questions or suggestions or other feedback. My address is FODs12 at gmail.com. That's FODS12 at gmail.com. Thank you very much for

listening and I will talk to you next time.