Hi. In these videos we'll be looking at the various tissues found in plants. Tissues, you might recall, are special collections of cells that usually are from a similar origin and have similar function, and basically unite to carry out some specialized function of an organ. Now, the first type of tissue in plants we're going to look at is vascular tissue, and we've actually talked about vascular tissue before, when we talked about general plant biology. Now, vascular tissue transports water, nutrients, and the photosynthetic products around the plants. And sometimes these can be massive distances. I mean, think about a redwood tree, those huge giants. Those plants have to carry water up from their roots in the ground, 100 of feet in the air. I mean, it's mind boggling, the distances that some plants have to transport various materials. Now vascular tissue is grouped into what are called vascular bundles. You've probably actually seen these before, for example, in a stalk of celery when you cut it, you can actually see the vascular bundles, there are these little dots that you can see in the plant. And these vascular bundles actually run the length of the stem. Vascular tissue is usually broken into xylem and phloem. Xylem is the water conducting tissue. It brings water and dissolved nutrients up from the roots to the shoots. So this is unidirectional, it's bringing this stuff up, and only up. It does not flow bring water down, for example. There wouldn't really be a point in that though, if you think about it. Water is absorbed in the roots, it's needed up in the shoots and other parts of the plant, so the xylem only has to worry about bringing it up. Now, xylem will be composed of what are called trachids, and these are long thin water conducting cells that are found in all vascular plants, and they have what are called pits. Pits are openings in the secondary cell wall that allow water flow, and there's only primary cell wall present in these pits. So in our diagram, let me actually jump out of the way here, you can see we have some pits labeled, in our trachids, and you can see them right here, there's another one right here, and these allow for water flow again. And in those, pits there is no secondary cell wall. Right? None of that. Only primary cell wall at the pits. Now, some plants have, vessel elements. Angiosperms, specifically, have vessel elements. Some other plants do too, but mainly they're found in angiosperms. And these, compared to trachids, are much shorter and wider. And they also have pits, but they have these special openings as well called perforations. So trachids are kinda like the long thin tubes, vessel elements are shorter and wider. So think of it as a trachid as a thin pipe, and the vessel element is a very wide pipe. And these vessel elements have what are called perforations, and they're basically just openings in the cell wall, that you can see here that allow, for water conduction, and actually because these perforations and the morphology, the shape of the vessel elements, Vessel elements are actually better able to conduct water than trachids, or I should say they're able to more efficiently conduct water than trachids. Now, xylem also contain, what are called fibers, and we're gonna learn more about this later, which are a certain type of cell called a skelerenchyma cell, we're gonna learn more about these later too. They also contain parenchyma cells, for lateral transport of water. Right? Xylem are, their main job is to get water to flow up. It's gonna be different cells that transport water, laterally around xylem, And we're gonna learn more about these 2 types of cells later on, and don't worry we're gonna bring it all back together. So, moving on, let's take a look at phloem. These are the other main type of vascular tissue, and phloem conduct sugars, amino acids, as well as chemical, chemical signaling molecules like hormones, and they are bidirectional. Right? They can, they can conduct these things, between the roots and shoots. They can go up and down. Phloem are made up of what are called sieve tube elements. These are specialized parenchyma cells, that transport sugar and other elements, and they have what are called, sieve plates. If we look at our sieve tube element here, which is gonna be this cell, you can see the sieve plate at the bottom has a bunch of holes in it. Right? That allow for, the transport of various materials between cells. You also have what are called companion cells and these support the sieve tube elements, both metabolically and physically. The sieve tube elements lack, for example, mitochondria. So these companion cells are gonna keep them alive so they can keep doing their job. They, the companion cells also help produce membrane for the sieve tube elements. So these sieve tube elements really rely on these companion cells in order to carry out their function. With that, let's turn the page.
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concept
Epidermal and Ground Tissue
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5m
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Epidermal tissue is kind of like the skin of the plant. It protects the plant from pathogens, physical damage, and also helps prevent water loss. Now epidermal cells will actually secrete this waxy film called the cuticle that helps prevent with, helps prevent water loss. And you can see the, in this image, the water beading up on the surface of this plant because of the cuticle. It's hydrophobic and causes the water to bead up on the surface. It also prevents the water from leaving the inside of the plant. Now, sometimes on the, epidermis of plants we'll find what are called trichomes, that are little hair like structures made of, specialized epidermal cells, and they carry out a very wide range of functions. They can be involved in water loss. They can help defend against herbivores by containing some sort of nasty chemicals. Like, these are chemical trichomes here that are meant to prevent herbivores from eating the plant they cover. They can also help reflect sunlight, and in fact they can even be involved in eating animals like we see here. This, stalk of the plant is covered in these trichomes, which are the little hair like things with the beads on the end. And you can see that these trichomes have wrapped around this hapless insect here, which is now destined to become plant food. Ground tissue is the 3rd type of tissue found in plants. So we have epidermal tissue, vascular tissue, and ground tissue. And ground tissue is basically everything that isn't epidermis or vascular tissue. So it's kind of the grab bag, but generally speaking, ground tissue is gonna be responsible for producing and storing important molecules for the plant. Generally, we can separate the ground tissue into 2 regions, the pith and the cortex. The pith is gonna be this inner region here, and it is basically ground tissue that's surrounded by the vascular bundles. Now what we're looking at, just to be crystal clear, is the cross section of a plant stem right here. So basically, if we had a plant stem, and I don't know, like here's a little leaf coming off of it. You know, if we cut that across, and then took that piece and were to look at it, look down on that cross section, that's what we're looking at right here. So that interior layer is the pith. You're gonna find the cortex actually all the way out here, the layer marked 6, this kinda dark layer on the outside, that's the cortex. It's the ground tissue outside of the vascular bundles. So this whole region in here, those are the vascular bundles. This region marked 3 is the xylem. Oops. Spelling it the French way there. Put an extra e on the end. Kidding. And then 4 here, this is the phloem. And these are the vascular bundles. So the pith is the inside, 6, that's the cortex on the outside. And 7, what's labeled 7 here is actually our epidermis. So, the cortex can actually be broken down into, certain layers itself. We have the endodermis, which is the innermost layer of the cortex. So it's going to, form the cell boundary between, the the cortex and the vascular tissue of the plant. On the other side, we have the pericycle, which is a thin layer of tissue between the endodermis and the, phloem. So essentially, the, if we were to kind of zoom in on this image right here, we'd basically have the phloem, which is layer 4, and then we'd have the, pericycle, and then we'd have, which is, you can't really see here. It's not layer 5. Layer 5 is something else, but you don't need to worry about it. So basically, just to make this crystal clear, let me jump out of the image and write this in. You have the phloem, the paracycle, and then the endodermis. And we're going deeper into the plant in this, as we go along this way. Alright. Let's flip the page and talk about some of the specialized cells of the ground tissue system.
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concept
Ground Tissue Cells
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4m
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Ground tissue is made up of 3 special types of cells. Parenchyma cells are the most abundant cells in plants. They form the pith in the cortex of stems, the cortex of roots, and the mesophyll of leaves. Many of these parenchyma cells are totipotent, which means they can develop into any type of cell at any time basically, and that's why they're going to be involved in healing, as well as asexual reproduction. In fact, speaking of healing, sometimes you'll see what's called a callus. This is basically a mass of unorganized parenchyma cells covering a wound. Right? So these guys are gonna rush onto the scene and, develop into the necessary type of cell and help seal up that wound. But you'll also see parenchyma cells act as rays, which we mentioned previously. These radiate through the vascular tissue to transport the water and nutrients laterally. Remember, we said that the, xylem are responsible for vertical transport, these guys are going to help them out by doing lateral transport. There's also colon chymus cells. These basically are just there for structural support, and they really help out in growing parts of the plant. So growing shoots and leaves, you'll have these colon chymosomes providing structural support. The reason they're favored in growing areas is because even though they're, you know, they have these thick cell walls, they're very, structurally sound, they're actually still very stretchy and flexible. So while the plant is growing these can provide structural support, but not hinder the plant's growth by being too rigid, for example. Sclerenchymic cells, as a mouthful to say that, provide structural support similar to cholankymic cells, but they provide structural support in places where growth has ceased. And generally speaking, sclerenchyma cells are dead at maturity. So these basically have a thin primary cell wall and a very thick secondary cell wall made of lignin and cellulose. You'll find sclerenchymal cells, as fibers, which we previously mentioned. These fibers will run along the, xylem, for example, and help provide structural support. You'll also see sclerenchymo cells called, sclerids, which basically have really thick lignin walls and form, protective coating on like seeds and the shells of nuts, for example. So these guys have, like, extra beefed up to form a really hard exterior coating on plant structures. Whereas the fibers are more involved in structural support on in, the tissues of plants. Now putting this all together, here we have, all of the, types of cells we've talked about, the xylem, the phloem, all next to each other with their support cells, like the sclerenchyma fiber, which again is going to provide structural support. We've got a parenchyma cell, you know, that could be like a ray that's providing lateral transport of water and nutrients. We also, of course, have our colon chymosales that are going to provide structural support in growing sections of the plant. Alright. That's all I have for this lesson. See you guys next time.