 So last time, I introduced you to primary growth in plants. And we discussed eudaicot examples. But primary growth occurs across vascular plants in similar ways, this lengthening growth. And today we'll be talking about secondary growth, which only occurs in woody plants. And we'll be focusing on a eudaicot example. But of course, there are other woody plants as well, although monocots are not woody. And so the secondary growth doesn't apply to those. And then we'll go on into discussing the root system and hopefully get through all of primary growth and these topics up here today so we can move on to some physiological material next time. OK, so as I started to get to last time, secondary growth is thickening rather than elongating growth. And it occurs from lateral rather than apical meristems. And this is a pretty nice diagram here that shows how back from the growing tips of both the roots and the shoots, the root and the shoot, we have beyond the zone of elongation and these older tissues in woody plants, we start to get the development of these lateral meristems, which are found inside the tissues of the stem and roots, typically cylindrical. And these increase the width of stems and roots. And we're not going to talk about secondary growth in roots, it's very similar to that in shoots. So we just have time to really go through that in this course. So in most eudaicot stems, the vascular bundles, these things seem to be burned out now. Oh, there we go. They're arranged in a ring we've talked about before with the xylem to the inside and the phloem to the outside. And this is up here in the region where we still have elongating growth occurring. But if you get back beyond that region of elongating growth in a woody plant, a woody eudaicot, what you'll see is that between the xylem and the phloem, the new meristem form, so we had de-differentiation of tissue in between the xylem and phloem and in the ground tissue between the vascular bundles to form this, what's shown here is a red ring, a red cylinder of meristematic cells. And these meristematic cells then are producing xylem to the inside of the stem and phloem to the outside of the stem. And by doing that, they're increasing both the amount of vascular tissue for conduction as well as increasing the size of the plant and adding to its some structural support. So this is a figure from your book where you can see this cartoon of a vascular cambium here. This is just a stem cross section and a bleak view. So here you can see this cylinder of vascular cambium and you can see that it's only a single cell layer and thickness from inside to outside. It's just one cell layer. And if you look at this stem in cross section then here, we start out with one of these cells and it makes a division to the inside of the stem which gives rise to a xylem element like a trachea or vessel element for example. And then subsequent to that it make another division toward the outside to produce for example a sieve tube element and add to the phloem. And this goes back and forth. The same cell is dividing in one direction to give rise to xylem, dividing in the other direction toward the outside of the stem to give rise to phloem. So after a year of growth, this you can see here, this bluish green is new secondary xylem. And secondary xylem is what we call wood. I mean wood is by definition secondary xylem produced from the vascular cambium. And to the outside of the vascular cambium here, this sort of tannish region is secondary phloem which is marked over here. And that'll become part of the bark and I'll get to what bark is in a minute. Basically bark is everything to the outside of the vascular cambium though that has a number of different, it has a couple of different tissues involved. So you can see that this type of growth then is gonna result in the stem becoming wider. And it's interesting in the way that it becomes wider because it's producing cells to the inside becoming new xylem. And as it's doing that, it has to compensate and the vascular cambium has to get bigger in diameter to accommodate that. You can see how that as the stem gets wider with this additional secondary xylem here being produced. It's forcing out that vascular cambium which has to make divisions to accommodate that and get bigger in diameter at the same time. And also the tissues to the outside of the vascular cambium are being forced to expand as well but they're not meristematic. They're not capable of division any longer. So that pressure, that outward pressure is gonna cause those cells to be destroyed. They're gonna eventually split. The expansion pressures are gonna be destructive. And so that's what you're seeing here. You can maybe barely make out these cells sloughing off. So the epidermis is split and destroyed and this wouldn't be a great thing for the plant if it couldn't replace that tissue with some other dermal tissue. So what happens is that shortly after this vascular cambium starts producing cells here to the inside and outside and starts this expansion process of the stem, we get another lateral cambium forming, another lateral meristem right here called the cork cambium, that dark blue ring. And it starts producing cork cells and parenchyma toward the outside which replaces the epidermis. And these cork cells have their cell walls impregnated with suberin which is a waxy substance so it serves a similar purpose to the cuticle on the epidermis, that waxy covering. And so the cork cambium actually can't keep up with this expansion pressure either as it turns out. And so it will start reforming at deeper and deeper points into the stem as the stem continues to enlarge by growth of the xylem and phloem. So initially that cork cambium here forms in the cortex just inside the epidermis and then it'll reform at a deeper point in the cortex and finally it'll start forming in the phloem further to the inside. And so the bark on the outside of the plant will start to be a mixture of phloem and cork paradigm we call it, the tissues produced by the cork cambium. So here are some of the terms here. So the paradigm, P-E-R-I-D-E-R-M is basically the tissues produced by the cork cambium and as that cork cambium has to form deeper into the cortex and secondary phloem through development there'll be phloem tissue also in the bark that's manifested on the outside of the plant. So you probably notice that bark changes in appearance as trees age and some of these characteristics, the way in which that cork cambium forms and produces cells and the presence of phloem and later time changes the appearance of bark. Okay so you can also see these radial lines of red cells here, these are what are called rays and vascular rays are produced by the vascular cambium as well so at the same time they're producing secondary xylem to the inside and phloem to the outside here they're also producing mostly living cells, parenchyma in these little sheets that run radially through the vascular tissue and these allow for communication between the xylem and the phloem and for storage in the stem. So you can see as we go down through the stem at these different levels how the outside of the stem here is changing and this is the formation of paradigm here replacing the epidermis as we go deeper and deeper. Are there questions about this at this point that anyone has? So basically xylem to the inside, phloem to the outside, paradigm replacing epidermis via another cambium here the cork cambium in addition to the vascular cambium. Okay so if you've looked at the outside of a stem you've probably noticed outside of a stem that's undergone woody growth say of a tree you've probably noticed these little raised areas that are on stems that have kind of a characteristic appearance in different species. These are what we call lenticels and this is a case where the epidermis has been replaced now by paradigm so mostly cork cells and the lenticels basically serve the function of stomata that we had in the epidermis but now the epidermis is gone. These little raised areas are loosely packed cells that can allow for gas exchange with the inside of the stem. So this is something that the plant produces basically as an alternative to stone mates and they have a characteristic appearance that you've probably noticed before. All right so here's a cross section of an actual photograph of a section of a woody stem of a eudaicot and what you see here, main thing I want to point out is that you have a lot of secondary xylem here just after a few years. This is all secondary xylem and you can see there's relatively little secondary phloem so there's not as much phloem produced from those vascular cambium, from the vascular cambium as there is secondary xylem. It's not a one for one production of cells to the inside and then another alternating cell production of a cell to the outside. There are more divisions toward the inside of the stem than toward the outside of the stem and also as I mentioned, the bark is constantly sloughing off as the stem is expanding. So this cork cambium which you can see right here is continually reforming at deeper and deeper points in the stem and at this point, after only three years, it's forming inside the secondary phloem and you can see this layer of paradigm here which consists of both cells of the produced by the cork cambium as well as some secondary phloem that's off here to the outside of this cork cambium. So the secondary phloem will be a smaller and smaller proportion of the overall stem diameter as the stem increases in size. So this xylem's not being put under pressure compared to the secondary phloem as the stem expands. All right, so that's a perspective from a real photograph of a section. So here's another cartoon in oblique section of a cross section of a stem that's grown for quite a while. It's undergone woody growth for some time and what starts to happen after a tree ages is that the plant starts to use the older wood as a depository for resins and other secondary compounds that are resistant to decay and this older wood then ceases to be used for conduction of water and minerals and you can actually see a difference in the color of this wood typically and its texture. It's tougher wood, it's like I say, it's impregnated with resins and other secondary compounds and it becomes really important for structural support but no longer for conduction. This is what we call heartwood because it's found near the heart of the plant but it's also different in its function now and this is the type of wood that's most valuable as lumber. So for timber, the heartwood is what is most sought after and the sapwood, which is still conducting wood less desirable and it's a relatively small part of an old stem, just these outer younger layers of secondary xylem that are close to the vascular cambium. Okay, so if you look at a cross section of a stem of say it's many years old, you can see that the, here's the sapwood, here's the heartwood and the vascular cambium is out here. It's actually pretty far, pretty close to the edge of the stem where we have secondary phloem and periderm out here and you've noticed these rings before, no doubt in trees of climates like this where we have seasonal growth of vegetation. You wouldn't notice these in the tropics where there's less seasonality but in this part of the world where there's seasonality you can see that the plant undergoes periods of growth. The early season of growth, the early wood has relatively large in diameter xylem elements like tracheids and vessel elements that are large in diameter and look fairly light in color like this, whereas the later wood in a growth season under less optimal conditions tends to be smaller in diameter the vessel elements and tracheids and it has this darker appearance. So this is the early wood and this is the late wood of a given year here, this is an annual ring. So the annual ring representing one growth season. So we can count the age of the plant by counting out these annual rings from the center toward the periphery as the plant has gotten older in this direction. So that's the basis for annual rings. Okay, so in monocots as I mentioned, secondary growth is rare. You do get it in a few groups like even out in our Mojave Desert the Joshua trees out there have a secondary growth but it's different than the secondary growth we've been talking about. It's been secondarily evolved from a non-woody ancestor and it doesn't really compare to what I've been showing you. So we're not gonna get into it. It's a fairly specific and relatively minor component of diversity that has that type of growth but most monocots have as I mentioned earlier their vascular bundles scattered in the stem. This is a cross-section of a palm and you can see these vascular bundles here scattered around in this ground tissue, ground tissue here. But you notice that palms get to tree-like size and they can get to be pretty large in diameter. So you might wonder how do they get to be that thick? And actually that thickening growth is happening during primary growth. So if you look at a palm as its first emerging a young palm that's very short, you'll notice that its stem diameter is as large as it is in a comparable, in a member of the same species that's grown to a great size. So if you look at a, and also if you look at this is, so basically the primary growth, the production of tissue from the apical meristem yield stems that are of this width and we don't have time to really get into how that happens, but monocots that are large in diameter often do not undergo any secondary growth at all and they start out as having very thick stems. Oops, oops, that's not what I meant to do. So the bamboo is another example. That's actually a grass. Grasses don't undergo secondary growth either. Of course grasses are monocots. And if you look at a cutting board like this made of bamboo under magnification you can see the vascular bundle scattered around in ground tissue. So it's the same kind of situation. And so look at these stems of these monocots. Here's a bamboo, here's a palm. Their diameter is roughly the same as we look from top to bottom whereas in a eudaicot like this hardwood the tree has sort of a narrowly triangular shape such that the older parts of the stem are wider than the younger parts of the stem closer to the apical meristam. So these, as we go down toward the base here these have had a longer and longer time that they've been undergoing secondary growth since the tip of the stem left that particular region. So here's a little quiz for you. If you've read, done the reading you probably know the answers to this already. So based on what I just told you about secondary growth what do you think about this question? If a sign is hammered into a tree a couple meters from its base that is up around Yay High and if the tree is 10 meters tall and subsequently elongates one meter each year afterwards how high would the sign be after 10 years? Yeah, two meters, same height, right? It's not gonna change. Two meters up from the ground after a tree is 10 meters tall that part of the stem is long ceased to elongate and it never will get any higher it just gets wider, becomes thicker. And this is a little redundant then. If you nail this, if you nail in the sign all the way into the secondary xylem past the vascular cambium, again two meters but here's another question for you that maybe isn't so obvious. So if you nail in the sign as far as only the secondary phloem not into the wood but just into the bark which is everything to the outside of the vascular cambium that is the secondary phloem and if there was any cortex left that would also be part of that and the paradigm. So it could be actually sloughed off. So eventually if you didn't get it all the way into the wood at some point the sign could well be sloughed off to the outside of the plant after sufficient time. If you nailed it all the way into the wood though the plant would ultimately claim it and grow around it like this which you may have seen before in some old trees. So there's some other consequences of secondary growth you might be familiar with and here's one of them. You've probably heard of girdling trees and this is a mean thing to do to a plant to a woody plant but if you remove all of the bark, remember the bark's everything to the outside of the vascular cambium that lateral meristem that produces the secondary xylem, secondary phloem such that what you have here is just exposed wood. If you remove it all the way around what do you think is gonna happen? I mean I've already told you here well you've already seen it here. It kills the tree right because you've destroyed the phloem in that whole region. Now that's not gonna be particularly destructive to the parts of the plant above the girdling. We're assuming this girdling has happened below the point of any branching and any production of leaves so down in the lower part of the stem. So the plant is producing photosynthate up there. It's producing food for itself up in the upper tissues and the leaves and it can nourish itself and it can also receive water from the roots through the sapwood just to the inside of the vascular cambium which is still intact. The problem is the roots are gonna starve because the photosynthate being produced for them leaves can't be moved down to the roots and the roots absolutely depend on the shoot for nourishment and so the roots will starve and ultimately they won't function at all and the plant will die. So the girdle is actually this region here where the bark has been removed and it can be a really thin region. You can girdle a plant without removing that much. Even just a tiny, tiny strip all the way around will kill it. If you leave just the barest amount of phloem connecting the upper and lower part, the tree will probably live. I mean, it doesn't take too much in order for the plant to recover but this is deadly. Yeah. Well, if you wanted to kill the plant easily that's a good way to do it but yeah, sawing it down would work as well. In most cases, sometimes plants will produce shoots from the base but yeah, so that's, you can starve the plant out though. If you cut it down, it may have enough reserves in the roots to be able to produce new shoots and get enough leaves going again to survive whereas if you girdle it like that, it might still be able to, it might still be able to produce some shoots from the base but in any case, that is one way to kill a plant for whatever reason. All right, so the root system is the next major system I wanna talk about and we're not gonna spend as much time on this because some of the features of the shoot system also apply like secondary growth but the roots as we mentioned earlier function and anchorage of the plant especially important in larger plants but especially in absorption of water and minerals and to some extent in storage and there are modifications of roots that take on a lot of other functions we'll look at in a minute. So I've already mentioned this several times that eudaicots and monocots have pretty different root systems. Monocots don't have a persistent tap root whereas the tap root is persistent in eudaicots and other vascular plants typically. The tap root, remember, is the root that's formed from the radical. It's basically the mature embryonic root and it produces lateral roots from it in eudaicots and in fibrous root systems, the tap root's not persistent and we end up with adventitious roots that is roots formed from near the base of the stem that contribute and we end up with a more diffuse root system that's often shallower. Okay, so here's the apical meristem of a root. So roots also undergo lengthening growth like shoots and they do this from apical meristems like in shoots but unlike in shoots, we don't have leaves produced from roots. There are no nodes and inner nodes. So there's no leaf primordia formed from near the apical meristem but instead we get what's called a root cap which is this whole region here which is enlarged here. And the root cap functions to help penetrate the soil and to protect the apical meristem from abrasion in the soil. So the apical meristem is producing cells toward the tip here that become part of this root cap and it's continually producing them because the root cap as it's pushing through the soil is gonna be constantly abraded. It does produce a slimy mucilage that lubricates the soil as it goes through but it's still being abraded away. And so this serves as protection for the apical meristem and a way to penetrate soil. So a lot of the cell divisions though are in the opposite direction and these are increasing the length of the root. And we have a zone of cell division in this region close to the apical meristem. And then the zone of division ends at about this level and we have a zone of elongation where as you can see the cells get larger and the root will elongate due to that elongation of those cells. And then thirdly we have a zone of differentiation or maturation where the different tissue types, the dermal tissue, the ground tissue and the vascular tissue will become mature in this region. And here's where we start to see the root hairs which are outgrowths of epidermal cells that function in absorption. So they increase the surface area of the epidermal cells and function in absorption of water and minerals. Okay, so one thing you might wonder about is how root cells actually know which way is down. So how does this positive gravatropism work? That is growing in the direction of gravity. So in the root cap, that protective layer that's just at the very tip of the root beyond the apical meristem, you can see in these slides here, there are plastids. So of course roots aren't undergoing photosynthesis. They don't need chloroplasts, but they still have plastids that function in storage of starch. And these are called statoliths. And these statoliths will settle. You can see these dark purple structures here that have settled on the lower surface of the cell. And that settling of the statoliths, at least hypothetically, is associated with inhibition of cell wall growth on the side of the stem, or side of the cell on which the cells have settled. And so the lengthening of the cell walls here will be inhibited by oxen of hormone. And the cell walls here are not inhibited. And so we have a differential elongation here on this side as compared to this side. And the thing will grow downward. And here you can see this process here in time lapse of some root tips growing down in response to that differential growth rate of their cell walls on the lower versus the upper side of the cell. Okay, so that's the way in which we have downward growth of roots. So that's tied up in the root cap as well then, which is functioning not only to protect the apical meristem and lubricate the soil, but also to help the root find its way. So if we look at a cross-section of a eudaicot root then, this is a root that's just undergone primary growth, not secondary growth. We're just gonna be concerned with primary growth in the root. There's an epidermis, of course, which you already mentioned that has root hairs. And these are actually extensions of epidermal cells. So these little walls here shouldn't be showing up in this diagram. And then to the inside of the epidermis is the cortex ground tissue, just as we saw in a eudaicot stem during primary growth. And then here's something we haven't seen before. And this is the innermost cell layer of the cortex of the ground tissue. It's called endodermis as opposed to epidermis. And as we'll talk about in a moment, it actually functions as an outer protective layer for the vascular tissue, not for the whole root, but just for the vascular tissue of the root, the conducting tissue. So here's the outermost layer of the vascular tissue, the conducting tissue. It's called pericycle. And we'll get to it's function in a minute. And so the xylem of eudaicots is typically forming the core of the root. So it's a central core of water and mineral conducting tissue that typically has is a multi-armed core that looks somewhat star-like. And in between those arms, we find the phloem. So here in blue. So that's the overall confirmation of a eudaicot root. And here's what it looks like in reality in a cross-section. So you can see the epidermis here, all of this cortex, a lot of cortex here. And then if we look a little closer, you can see the endodermis differentiated here. Pericycle and endodermis are a little hard to differentiate, but here you can definitely see that xylem core, that multi-armed core of xylem, these large vessel elements. And here you can see the phloem, which is a little bit easier to see here. So that's how a root looks in cross-section. Well, how do roots branch? This is something we saw that happens in stems from axillary buds. So remember in stems, at the time that the stem is undergoing its initial primary growth, there are bud primordia that are formed close to the apical meristem that ultimately become axillary buds that have a vascular connection to the overall vascular tissue of the stem. So when buds break and are removed from inhibition, we can get the formation of branches. That's not what happens in roots. Roots don't have buds. There are no nodes in roots. And the only kind of budding that we see happens from the paracycle, that outermost layer of the vascular tissue just inside that endodermis. And we start to see the production of a lateral root here. And as it basically grows outward, disrupting the cortex and the epidermis of the root as it grows. Although the endodermis remains continuous around the vascular tissue of both the lateral root and the vascular tissue of the main root and the lateral root. And we'll get to a minute to why that's so important. So this is the way in which lateral roots form. And they have a vascular connection here that's maintained. All right, but first I wanna talk about some modified roots. Like there were modified leaves and modified stems that have different functions. There are also roots that have, in different species of plants, that have taken on new functions. And one of these is prop roots that you may have seen in old corn plants, old maize plants. These are adventitious roots that are actually formed from a place other than the normal origin from the stem, but not right at the very base of the stem like we saw in the case of a normal monocot, but actually on higher nodes of the stem here where they form props that help to support top-heavy plants like a corn stalk. There are also storage roots that are really common in perinating plants that live for more than one season or basis plants that die back to the ground each year. And prior to flowering, they build up reserves in their roots. And these are tap roots, typically. Carrot is a tap root. Beet is a tap root, et cetera. And these store food and water in preparation for flowering where that'll be used. There are also, in tropical trees, there are some really amazing roots that you might remember from Jurassic Park. So figs and siebas that produce these large buttress roots that are found in soils that are very shallow and help to support these plants in these shallow soils and can become pretty huge and intricate buttresses. There are also roots that are called strangling roots, well-named, especially in the strangler fig, a tropical plant that germinates from a seed that's been deposited, say by a bird, typically a bird, up on a branch of the plant of another species. And it'll germinate, send roots down to the soil, and then it'll produce roots that end up completely encompassing the stem of its host tree. And eventually, it'll completely surround the stem. It can even send roots directly into the stem of the plant and girdle it, killing the tree. And in the end, you just end up with the fig, basically forming a hollow, a cylindrical plant surrounding this rotting or hollow core where the tree was before. So the tree will be shaded out or girdled and killed eventually. And so that's one of the more devious strategies. And then there are arrow roots that we get in things like mangroves that grow out in saturated soils where there's submerged root systems in mud that's where there's very little in the way of aeration to those roots. They'll send up roots that exhibit negative geotropism and grow up through the water and form these little, what look like little siphons sticking out of the water and are ways in which air can be transported down to the roots so that they can acquire the oxygen they need for respiration. So these are just a few of the modifications we see in root systems. Okay, so the actual way that water and minerals get into the xylem of roots is pretty interesting. One way in which you can imagine water and minerals getting into roots is through a normal channel by crossing a cell membrane and then running, going between cells staying in the cytoplasm the whole time. Which is possible because of what are called plasmodesmata. And these are just holes in the cell walls of all plants, plant cells, through which the cytoplasm has continuity between adjacent cells. So water and dissolved substances or solutes can make their way through plants in the cytosol through the cytoplasm. If these solutes and water pass through these pores where we have cell membranes lining these interconnecting the cells, that's a possible pathway for water and dissolved substances to come in. There's another way though, that's called the simplast. So simplast just literally meaning within the plasm, within the cytoplasm or within the protoplast. That's one mechanism for water and minerals. Or one route. Another route would be to come in through the cell walls or in the intercellular spaces. Remember in a root unlike in a chute, we don't have this resistant waxy layer on the outside. The plant has root hairs that promote the movement of water into the stem. And that water, once it gets into the stem, it can move along through the cell walls or in the intercellular spaces between cells carrying with it all of the dissolved substances from the soil. So those substances can actually make their way through the cortex of the root without ever being subject to any selectivity by the plant. So the plant can't keep those things out of the cell walls and intercellular spaces. It can keep them out of the cytoplasm because in order to cross a cell membrane, there's selectivity exerted by the plant at that level that will keep out some substances. Of course, that's a crucial thing for the plant in order to control the cell contents. So what's gonna happen though to prevent the dissolved substances of the soil from moving into the xylem? If it's coming in through the cortex, it could work its way right into the xylem. Those trachea and vessel elements are dead. There's no protoplast. There's no cell membrane there. How do these plants keep their xylem from becoming fouled up with all the crud that's in the soil solution? They have a really elegant way of doing this and it's a simple mechanical way and it's called the Caspian strip which is named for a botanist named Casperi who discovered it back in the 1800s. And this is in the endodermis that innermost layer of the cortex just outside the vascular cylinder. This is an endodermal cell looking at it from the top in cross-section and you can see it has this reddish strip which is basically the cell wall that's impregnated with a waxy substance, subarin, the same waxy substance that's in the cork cells that are formed by the cork cambium and the secondary dermal tissue, the paraderm. And if you look at it from the outside of the stem toward the center, toward the vascular cambium, you can see that this strip runs all the way around the cell. This is the transverse or upper side and these are the lateral sides. And if you look at a bunch of these, this is that hole, this is the endodermis taken as a hole. You can see that these cells are tightly packed relative to one another and here you can see their strips all in line with one another, casperian strips. And if you look at the, from the outside to the inside, we basically, if we're any kind of solute that's dissolved in the water that's flowing through the apoplast in those intercellular spaces or in the cell walls that hasn't crossed the cell membrane, this is as far as we're gonna get because that waxy layer is gonna prevent any further movement through the cell walls. The only way into the vascular cylinder is by penetrating the cell membrane and getting into the actual cytoplasm where the plant has some ability to discriminate and keep out substances that don't belong in. So again here, the apoplast movement can only get that far before it runs into the casperian strip and here's a cartoon of that. So you can see in purple here the route that water and solutes could take without ever crossing a cell membrane. So this could be just whatever's out in the soil solution. It can only get that far and at that point the blue route is basically through the cytoplasm, through the symplastic travel and you can see that can go right on through into the vascular cylinder but at this point you have to penetrate and end a dermal cell through the cell membrane in order to get in. But that's how plants can keep out substances from their xylem and it's also how the plant can keep from leaking concentrated minerals that the plant actively is concentrating in the xylem by living cells around the tracheary elements, keep them from leaking back out into the soil solution. So it goes both ways. It keeps out things that don't belong in the xylem and the casperian strip also keeps minerals that are being actively concentrated in the xylem from leaking back out. Okay, there are also some functions of roots that are found in just a selection of plants and in particular in the legumes, the members of the bean and pea family, things like peanuts, beans, peas, alfalfa, that huge family that we call the legume family and some related families that have independently evolved the same sort of ability, there's nitrogen fixation. So nitrogen is limiting in the soil typically and plants are in some cases able to have a mutualistic symbiotic relationship with bacteria in these little swellings called root nodules. And here's a section of a nodule. And inside these nodules, particular strains of bacteria, typically this genus rhizobium can live under anaerobic conditions, oxygen-free conditions, because the plant actually produces a type of hemoglobin, what we call leg hemoglobin for legume hemoglobin, that binds oxygen, just like the hemoglobin in your blood binds oxygen. And if you cut open these nodules, they stain red, or they'll stain red, they are red. And that's, when you're exposed to air especially, they turn red. And that anaerobic environment then allows nitrogen fixation to take place. The bacteria can only fix nitrogen in the absence of oxygen. And so the bacterium gets a nice little home here where it actually, the plant root provides it with carbohydrates, and then the bacterium provides the plant with nitrogen. And that nitrogen can end up going back into the soil when the roots die, so other plants can utilize that nitrogen as well. And in fact, a rotation of crops generally involves legumes. So people would grow a crop of grain, then grow a crop of legumes, like beans, to enrich the soil of nitrogen for the next generation of grain, for example. And the amount of nitrogen fixation that occurs from roots in legumes is greater than the amount that's deposited in soil from industrial fertilizer on an annual basis. There's a huge amount of this nitrogen fixation going on. We already talked about mycorrhizal associations, so I'm not gonna mention it much again, except just to remind you that plant roots also have associations with fungi that increase their ability to absorb water and nutrients. And again, it's a mutualism where the fungus is getting carbohydrates from the plant roots, and it in turn is allowing the plant improved ability to absorb water and nutrients. Now in the case of these are muscular mycorrhizal fungi, like we showed you earlier, you still have root hairs produced that are used. But in the case of some of these ectomycorrhizal fungi that completely clumped the root with hyphae, there's no point in having root hairs. The root is completely covered in these hyphae. All right, so as far as nutrient uptake goes, just a few points I wanna make is that originally it was thought that plants actually ate the soil, and that's how they acquired their nutrients. That's not a surprising inference that Aristotle made because plants are rooted in the soil. They don't obviously get nutrition any other way. And but then an experiment was done in the 1600s by Von Hellmont and showed that the amount of soil actually didn't decrease appreciably through time as the plant got bigger. And so he postulated that instead the plant growth was largely due to taking up more and more water. And then it was about a century later that Hale's British scientist came up with the idea that the plants actually were consuming air, something in the air they were consuming. And so all of those ideas have a bit of merit, about 80 to 90% of the fresh weight of plants is water taking up from roots. But as far as dry weight goes, most of this is attributable to carbohydrates which derived from CO2 that is taken up through the air. And a bit of water from the roots in the photosynthetic reaction. And the remaining dry weight includes both organic and inorganic chemicals that include essential elements and essential elements that are required for survival and reproduction that are taken up by roots from the soil. So some of the elements required for plants to survive. And this is established from experimentation under hydroponic conditions that this is, some of these have to come from the soil. All right, and I'll just finish by mentioning here for today that there are essential macro nutrients required in relatively large amounts. And then micronutrients that are required in just trace amounts, carbon, oxygen, hydrogen, nitrogen, phosphorus and sulfur are all components of organic compounds. So really important in terms of making up the plant body. These three other macro nutrients, potassium, calcium and magnesium function in part as components of other critical compounds like chlorophyll in the case of magnesium or important in cell wall or membrane formation in function like calcium or in regulating water balance in the case of potassium. That's all we have time for today, but next time we'll get into more about function from a physiological standpoint in plants, which is pretty cool.