Growth - Online Tutor, Practice Problems & Exam Prep

Plants exhibit indeterminate growth through meristems, which are stem cells that differentiate into various tissues. Primary growth occurs at apical meristems, extending roots and shoots, while secondary growth, mainly in woody plants, involves lateral meristems like vascular cambium and cork cambium, expanding the plant's circumference. Vascular cambium produces secondary xylem and phloem, contributing to growth rings in trees, while cork cambium forms bark. Root growth occurs in zones of cellular division, elongation, and maturation, with root hairs essential for nutrient absorption.

Primary Meristems

Hi. In this video, we'll be talking about plant growth, and we'll be focusing 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. 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 extend their roots through the soil or grow their shoots to increase light absorption. Sometimes this can mean sending the roots down vertically, and sending the shoots up vertically. However, sometimes it's about creating a more diffuse root system, or shoots. 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 creatively, shoot apical meristem. This gives rise to things like leaves, flowers, and of course, 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, which 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 going to 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 into, 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 three 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 going to 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. 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, the exterior layer of cells over here is our protoderm, and that's going to give rise to our epidermis. Now, the procambium, which is going to give rise to the vascular tissue, is found inside the protoderm. Right? So the protoderm is going to 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 core area here. And in our shoot it's actually going to be this, you can kind of see this darkened line that kind of 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 have the 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 shoot it's actually going to be found in this region, as well as this region in here. The ground tissue in the shoots, we have the ground tissue divided up as pith and cortex. Right? The stuff that's inside the vascular bundles, and the stuff that's exterior to the vascular bundles is found on the inside and the outside of our procambium there.

With that, let's Turn the page. My gosh, let me get my 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 head, 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 clear, let me put some green over here, and just all in there. That's all ground meristem. Cool. Now, let me get my head in the picture and say, let's turn the page.

Root Growth

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, 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 going to become, you know, like dermal, vascular, ground tissues, whatever. And 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 area in here. Notice all of these dead cells sloughing off the root cap, in this last external area that's marked by the number 4 there. Right? As the root grows and extends, the root cap is actually going to slough 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 apex 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 phloem 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 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.

Secondary Growth

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 going to focus on woody plants as our example. Now, the meristem responsible for secondary growth is called a lateral meristem, and we're actually going to 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. 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 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. 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, basically like the outer layer of the tree. The vascular cambium is going to 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. First, I want to 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 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 cork 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 want to 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 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 going to 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 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 transporting xylem in them. Now, we also said that bark is formed from those cork cells. It actually is also made in part from the secondary phloem. Remember, 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 cork cells. So this layer, this bark because it has these cork cells in it, it's super impermeable to water and gas. But, trees have evolved, or say, woody plants have evolved, a way of dealing with that impermeability. They have these things called lenticels 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 lenticels. So these horizontal lines are our lenticels. Alright. That's all I have for this video. I'll see you guys next time.

Here’s what students ask on this topic:

What is the difference between primary and secondary growth in plants?

Primary growth in plants occurs at the apical meristems, which are located at the tips of roots and shoots. This type of growth is responsible for the elongation of the plant, allowing it to extend its roots deeper into the soil and its shoots higher into the air. Primary growth involves the differentiation of apical meristems into primary meristems, which then form the epidermis, vascular tissue, and ground tissue.

Secondary growth, on the other hand, occurs in woody plants and involves the lateral meristems, such as the vascular cambium and cork cambium. This type of growth increases the plant's girth or circumference. The vascular cambium produces secondary xylem (wood) and secondary phloem, while the cork cambium forms the bark. Secondary growth results in the formation of growth rings in trees, which can be seen when a tree is cut in half.

What are meristems and what role do they play in plant growth?

Meristems are regions of undifferentiated plant cells that have the ability to divide and differentiate into various types of specialized cells. They are essentially plant stem cells. Meristems play a crucial role in plant growth by enabling both primary and secondary growth. Primary growth occurs at apical meristems, which are located at the tips of roots and shoots, and is responsible for the elongation of the plant. Secondary growth occurs at lateral meristems, such as the vascular cambium and cork cambium, and is responsible for increasing the plant's girth. Meristems ensure that plants can grow continuously throughout their lives, adapting to their environment by extending roots and shoots or increasing their structural support.

How do root hairs contribute to plant growth and nutrient absorption?

Root hairs are tiny extensions of epidermal cells located in the zone of cellular maturation in roots. They play a critical role in plant growth and nutrient absorption by significantly increasing the surface area of the root system. This enhanced surface area allows for more efficient absorption of water and essential nutrients from the soil. Root hairs are particularly important for the uptake of minerals and water, which are vital for various physiological processes within the plant. By facilitating better nutrient absorption, root hairs contribute to the overall health and growth of the plant, enabling it to thrive in its environment.

What are the different zones of root growth and their functions?

Root growth occurs in three distinct zones, each with specific functions:

1. Zone of Cellular Division: Located just behind the root cap, this zone contains the apical meristem and primary meristems. Cells in this zone are actively dividing, contributing to root growth.

2. Zone of Elongation: In this zone, cells derived from the primary meristems elongate by absorbing water. This elongation pushes the root cap through the soil, allowing the root to extend deeper.

3. Zone of Cellular Maturation: Here, cells complete their differentiation into specific tissue types such as dermal, vascular, and ground tissues. This zone also features root hairs, which are crucial for water and nutrient absorption.

What is the role of the vascular cambium in secondary growth?

The vascular cambium is a type of lateral meristem responsible for secondary growth in woody plants. It produces secondary xylem (wood) and secondary phloem, contributing to the plant's increased girth. The vascular cambium forms a continuous ring of meristematic tissue around the stem or root. It generates new cells both inward and outward: secondary xylem cells are produced towards the inside, while secondary phloem cells are produced towards the outside. This process results in the formation of growth rings, which are visible in cross-sections of trees. The vascular cambium thus plays a crucial role in the structural support and nutrient transport of mature woody plants.