Hi. In this video, we'll be talking about plant growth, and we'll be focusing in on the various meristems that plants use. Now most plants experience what's called indeterminate growth, that is they grow throughout their entire lives, and that growth is not terminated. Now, plants grow using what are called meristems. This is a collection of plant stem cells that is able to produce daughter cells and differentiate into any of the necessary mature cell types. Meaning that these, stem cells can form any of the necessary tissues and structures within the plant. Now primary growth, as opposed to secondary growth that we'll talk about a little later in this lesson, is when the plants, when the plant extends its roots through the soil or grows its shoots to increase light absorption. Sometimes this can mean sending the shoots down vertically, or the stems up, I'm sorry, sending the roots down vertically, and sending the shoots up vertically. However, sometimes it's about creating a more diffuse root system, or shoot For example, if there's nothing competing for light above the plant, it might actually send its shoots out to create a wider surface area for sunlight absorption, almost like creating a bigger solar panel, you could think of it as. Now the apical meristem is going to be the meristem located at the tip of each root and shoot, And we call the apical meristem in the shoot, very creative name, shoot apical meristem. This gives rise to things like leaves, and flowers, and of course, we'll see new stems coming from this. And the root apical meristem gives rise to, spoiler alert, roots. Here you can see an example of a shoot apical meristem. We have this little nub right in here, that is our, technically, this is our shoot apical meristem, and you can see it's surrounded by these young leaves right here. And if we actually take a look at the image over here, you can see that we have our little apical meristem in here. Right? That's gonna be our shoot apical meristem. And then over here, let me get my head out of the way, you can see that, these little regions in the center, those are our shoot apical meristems, and surrounding them are these young leaves. So that's actually a young leaf. So, moving on, let's actually go ahead and talk about what are called the primary meristems. You see these apical meristems will actually differentiate in, or to let me say that a different way. These primary meristems that are going to form the various tissues of plants differentiate from the apical meristems. And these primary meristems are the meristems responsible for primary growth. They come in 3 flavors for our three tissue systems. Right? We have the protoderm that's going to give rise to the epidermis, the procambium, that's going to give rise to the vascular tissue, and the ground meristem, which you guessed it, is gonna give rise to the ground tissue. So let's take a look at these two figures here. On this side I have a shoot, and on this side I have a root, and let's actually take a look at some of these meristems. So the protoderm, since it gives rise to the epidermis, it should come as no surprise that it's found on the exterior of these structures. So, this exterior layer of cells here, and also on this side, Add exterior layer of cells and the exterior layer of cells over here is our protoderm, and that's going to give rise to our epidermis. Now, the pro cambium, which is going to give rise to the vascular tissue, is found inside the protoderm. Right? So the protoderm is gonna be the most external layer, and the procambium is going to be a more internal layer. And we can see our procambium, in our root as the most central area, this like core area here. And in our shoot it's actually gonna be this, you can kinda see this darkened line that kinda goes through the middle of the structure here. That's our procambium. Right? That's going to develop into the vascular system. Now, lastly, we, Oh. Actually, before I get to the ground meristem, really quick, I do wanna say that the procambium will actually also develop into what's called the vascular cambium and cork cambium. These are actually other types of meristems. So the pro cambium, directly develops into vascular tissue, and it also develops into these other types of meristems that are technically lateral meristems, and we're gonna talk about those in just a moment. So don't worry, we'll get to that. So let's finish up here with our ground meristem. And remember that the ground meristem will give rise to the ground tissue, and this ground tissue is basically the tissue that's, not vascular tissue and not dermal tissue, so it's the everything else stuff. And in our root it's going to be, the stuff found in this internal area here, of course also over on this side, I'm just not drawing it all the way across. And in our chute it's actually going to be found in this region, as well as this region in here. The, you might remember that in the shoots, we have the ground tissue divided up as pith and cortex. Right? The stuff that's in, inside the vascular bundles, and the stuff that's exterior to the vascular meristem is found on the inside and the outside of our procambium there. Alright. With that, let's actually Oh. My gosh. Let me get my big head out of the way so you can see where I drew. I'm sorry guys. Yeah. So our ground meristem in the roots, pardon my fat noodle, you can see that it's, this internal tissue here, and I was just drawing it in over here on this side just to be clear that, you know, these things are occurring on both sides. So just to be super crystal clear, let me put some green over here, and just like all in there. So That's all ground meristem. Cool. Now, let me get my big fat head in the picture and say, let's turn the page.
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Root Growth
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5m
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Root growth occurs in 3 areas, which are found behind the protective covering called the root cap. This protects the apical meristem and amazingly, senses gravity, so that the plant knows which direction to grow its roots in. It also secretes a lubricating polysaccharide that allows the growing root to push through the soil more easily. Now these three areas are known as the zone, sometimes called area, you know, it doesn't really matter what term you want to use there, zone, area, whatever. But it's the zone of cellular division, and this is where the apical and primary, meristems are found, and this is where cells are actively dividing. We also have, behind that, the zone of elongation. This is where these primary meristem, tissues, or rather tissues derived from the primary meristems, are elongate, and they actually elongate by swelling with water, and this swelling with water provides force to actually push the root cap through the soil. So the root cap is actually forced through the soil by the swelling of the cells in the zone of elongation. Pretty incredible. Then you have, lastly, what's called the zone of cellular maturation. Basically, this is where the cells go to complete their differentiation into whatever tissue type they're gonna become, you know, like dermal, vascular, ground tissues, whatever. And it's it's actually from this area that we see these epidermal outgrowths called root hairs. These are super important. They're the sites of water and nutrient absorption. And here in our diagram, you can see all of these areas labeled, we have our root cap at the bottom, then behind that the area of cell division, behind that elongation, and lastly the area of cell differentiation or zone of cell maturation, it's the same thing. So this is, also the same as zone of maturation, and this is the area where you'll find the root hairs growing. Alright. Looking over here, we can see some of the particular, primary meristems we covered, and they've been labeled. So of course, as we saw on the last page, that central area is going to be our, here it's labeled vascular cambium. I want you guys to think of this as our procambium. And also we have our protoderm on the outside, and then of course the ground meristem in between those two layers. Looking down at this pink image here, it's a really nice picture of the root cap, that's basically what we're looking at here. This is really just a root cap photo, and what I want you to notice here's the root cap. Right? It's, this this area in here. Notice all of these dead cells slewing off the root cap, in this last external area that's marked by the number 4 there. Right? As the root cap, or rather, as the root grows and extends, the root cap is actually going to slew off the dead cells. Now lastly, I just want to turn your attention to this image behind my head, and get out of the way. Here you can see our root ape meristem, labeled in this area. It's, of course, found behind our root cap, and I also want you to notice all of these internal tissues. Right? So, here in red and blue that's our vascular tissue, that's our xylem, and it's actually the red stuff is the flow in there, so I'll put that in red. Xylem and phloem, and then, of course, we also have our ground tissues in these layers here, that's our ground tissue. So really all I want you to see is how in the root, how the root grows, how those primary meristems, are are how they're found, how they're located in there, and then, of course, the tissues that they're going to develop into. Alright. With that, let's flip the page and talk about secondary growth.
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Secondary Growth
Video duration:
6m
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Primary growth is all about extending the roots and the shoots, but secondary growth actually only occurs in places that no longer grow in length, and secondary growth is all about expanding the circumference of the plant. Usually we only see this in woody plants, there are some other types of plants that experience secondary growth, but we're just gonna focus on woody plants as our example. Now, the meristem responsible for secondary growth is called a lateral meristem, and we're actually gonna look at 2 types of lateral meristems, both of which are derived from the procambium, which you might remember is one of those primary meristems that gives rise to the vascular tissue system. Now, vascular cambium is one of those lateral meristems, and its job is to produce xylem and phloem. Now, you might recall the procambium also produces xylem and phloem. We call those xylem and phloem produced by the procambium, primary xylem, and primary phloem. What the, the vascular cambium produces is known as secondary xylem and phloem. Now, the growth pattern here is kind of interesting. You see, the vascular cambium produces xylem and phloem both to the outside and inside of where it's located. I'm highlighting the vascular cambium as this layer of cells here in blue, that's our vascular cambium. Cells that it produces towards the outside of the plant, here is the outside, here's the inside. Cells that are produced towards the outside are secondary phloem, whereas, the cells produced towards the inside are secondary xylem. Now what this results in is an actually pretty interesting growth pattern. You've probably seen a tree cut in half, and it has those growth rings. Right? Those rings that, those concentric circles that, continue all the way from the outside of the tree to the interior. Well, what's actually happening is the vascular cambium stays on, the, basically like the outer layer of the tree. The vascular cambium is gonna stay towards the outside. It's going to leave behind lots of older xylem on the inside. Right? So these are all from the previous year's growth, as you can see. Whereas, this layer right here is the current year's growth. And again, these are older years seen in here. Well, these xylem that move, or rather, due to the fact that the cork cambium continually produces these cells, but keeps moving towards the outside, or rather keeps its position on the periphery of the plant, we get this build up of old xylem on the inner portions of the rings. We'll get back to those in a second. 1st, I wanna quickly mention the cork cambium, that other lateral meristem, and this one is actually located, even more towards the outside than the vascular cambium. We actually highlight the cork cambium here in red, it's this lateral meristem there, that's our cork cambium, and it's going to produce cells towards the outside that are called, quark cells. These are non living cells that are highly impermeable to gas and water, and actually will form the structure known as bark. Now, I wanna get back to those interior xylem. See, those interior xylem have a name that's much more common that you've probably heard of. It's called wood. Right? Wood is that porous structural tissue that's derived from those secondary xylem. It's it should be noted that it's only the outer layers of xylem that actually transport water. The inner layers accumulate, various gums and resins to resist decay. And we actually have two names for these, different regions of xylem. We have the heartwood, which, I'm gonna jump out of the image here, is this darker internal area. So this is our heartwood. Then we also have the sapwood, which is this lighter colored external layer. That's the sapwood. So, the heartwood is the layer that's accumulated all these gums and resins in order to resist decay. And it's no longer actively transporting anything. You know, effectively it is just structural support at that point. The sapwood, these, lighter outer regions, those will have actively, that layer will have actively transporting xylem in it. Now, we also said that, that bark is formed from those quark cells. It actually is also made in part from the secondary phloem. Remember, that the vascular cambium produces xylem to the inside and, phloem to the outside. Well, some of those phloem, the non active ones, will become bark, in addition with, quark cells. So this layer, this bark, because it has these quark cells in it, it's super impermeable to water and gas. But, trees have evolved, or say, woody plants have evolved a, a way of dealing with that impermeability. They have these things called lenticles in their bark. Basically, it's a porous tissue that creates little openings to allow for gas exchange. And in this birch bark that you see right here, we have these horizontal lines that are very characteristic of birch bark, if you've ever seen it in the wild, those are lenticles. So these horizontal lines are our lenticles. Alright. That's all I have for this video. I'll see you guys next time.