Tissues - Online Tutor, Practice Problems & Exam Prep

Plant tissues are categorized into three main types: vascular, epidermal, and ground tissue. Vascular tissue, comprising xylem and phloem, facilitates the transport of water, nutrients, and sugars. Xylem conducts water unidirectionally from roots to shoots, while phloem transports sugars bidirectionally. Epidermal tissue protects the plant and minimizes water loss through a waxy cuticle. Ground tissue, including parenchyma, collenchyma, and sclerenchyma cells, supports the plant structurally and stores essential molecules. Understanding these tissues is crucial for grasping plant biology and their functions in ecosystems.

Vascular Tissue

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 a 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 contains, what are called fibers, and we're gonna learn more about this later, which are a certain type of cell, a sclerenchyma cell, we're gonna learn more about these later too. They also contain parenchyma cells, for lateral transport of water. Right? Xylem is, their main job is to get water to flow up. It's going to 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 conducts 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 is 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 going to 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 going to 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.

Epidermal and Ground Tissue

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 water loss. And you can see 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. Sometimes on the epidermis of plants, we'll find what are called trichomes, 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, 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 third type of tissue found in plants. So, we have epidermal tissue, vascular tissue, and ground tissue. 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 going to be responsible for producing and storing important molecules for the plant. Generally, we can separate the ground tissue into two regions, the pith and the cortex. The pith is going to be this inner region here, and it is basically ground tissue that's surrounded by the vascular bundles. Now, what we are 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 going to find the cortex actually all the way out here, the layer marked 6, this kind of 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 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, 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 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.

Ground Tissue Cells

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, and that's why they're involved in healing, as well as asexual reproduction. In fact, speaking of healing, sometimes you'll see what's called a callus. This is essentially a mass of unorganized parenchyma cells covering a wound. These cells rush to the scene, develop into the necessary type of cell, and help seal up the wound. Parenchyma cells also act as rays, which were mentioned previously. These radiate through the vascular tissue to transport water and nutrients laterally. Remember, it was said that xylem is responsible for vertical transport, and these cells help out by conducting lateral transport.

There are also collenchyma cells. These are primarily there for structural support and really help in the growing parts of the plant, such as growing shoots and leaves. These collenchyma cells provide structural support. They are favored in growing areas because, despite having thick cell walls and being structurally sound, they remain stretchy and flexible. This flexibility allows them to provide structural support without hindering the plant's growth by being too rigid.

Sclerenchyma cells provide structural support similar to collenchyma cells, but they are found in areas where growth has ceased. Generally speaking, sclerenchyma cells are dead at maturity. These cells have a thin primary cell wall and a very thick secondary cell wall made of lignin and cellulose. You'll find sclerenchyma cells as fibers, which have been mentioned previously. These fibers run along the xylem, for example, and help provide structural support. You'll also see sclerenchyma cells called sclereids, which have really thick lignin walls and form a protective coating on seeds and the shells of nuts, giving them a hard exterior coating on plant structures. The fibers are more involved in structural support within the tissues of plants.

Now, putting this all together, here we have all 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 that could be acting like a ray providing lateral transport of water and nutrients. We also, of course, have our collenchyma cells that are going to provide structural support in growing sections of the plant. That's all I have for this lesson. See you guys next time.

Here’s what students ask on this topic:

What are the main types of plant tissues and their functions?

Plant tissues are categorized into three main types: vascular, epidermal, and ground tissue. Vascular tissue, which includes xylem and phloem, is responsible for the transport of water, nutrients, and sugars. Xylem conducts water unidirectionally from roots to shoots, while phloem transports sugars bidirectionally between roots and shoots. Epidermal tissue acts as a protective layer, reducing water loss through a waxy cuticle. Ground tissue, composed of parenchyma, collenchyma, and sclerenchyma cells, supports growth, storage, and healing. Understanding these tissues is crucial for grasping plant biology and their roles in ecosystems.

How does xylem transport water in plants?

Xylem transports water unidirectionally from the roots to the shoots. It is composed of tracheids and vessel elements, which are specialized cells that facilitate water conduction. Tracheids are long, thin cells with pits that allow water flow, while vessel elements are shorter and wider with perforations for more efficient water conduction. Xylem also contains fibers for structural support and parenchyma cells for lateral water transport. The primary function of xylem is to move water and dissolved nutrients from the roots to other parts of the plant, essential for its growth and survival.

What is the role of phloem in plants?

Phloem is responsible for the bidirectional transport of sugars, amino acids, and chemical signaling molecules like hormones between the roots and shoots. It is composed of sieve tube elements and companion cells. Sieve tube elements are specialized parenchyma cells that transport sugars and other elements, facilitated by sieve plates with holes for material movement. Companion cells support sieve tube elements both metabolically and physically, as sieve tube elements lack mitochondria. This bidirectional flow is crucial for distributing the products of photosynthesis and other essential compounds throughout the plant.

What are the functions of epidermal tissue in plants?

Epidermal tissue serves as the protective outer layer of the plant, safeguarding it from pathogens, physical damage, and water loss. Epidermal cells secrete a waxy film called the cuticle, which is hydrophobic and prevents water from leaving the plant. Additionally, the epidermis may have trichomes, which are hair-like structures that can reduce water loss, defend against herbivores, reflect sunlight, and even trap insects in some carnivorous plants. Overall, epidermal tissue plays a vital role in maintaining the plant's integrity and health.

What are the different types of cells in ground tissue and their functions?

Ground tissue is composed of three types of cells: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are the most abundant and are involved in storage, photosynthesis, and healing. They are totipotent, meaning they can develop into any cell type. Collenchyma cells provide structural support in growing parts of the plant, such as shoots and leaves, due to their thick yet flexible cell walls. Sclerenchyma cells offer structural support in mature regions where growth has ceased. They have thick secondary cell walls and are typically dead at maturity, forming fibers and sclerids for added strength and protection.