But getting back to apical meristems, let's use the root apical meristem as our example. It's much simpler in structure that the shoot apical meristem. But in order to understand how it functions, we have to understand a little bit more about root structure in general and so let's take a look at a typical root. For example, the root of an onion and we're gonna define this root, both horizontally from the outside in and vertically from the bottom up. When we're looking horizontally from the outside in, we're gonna look at the different tissues that comprise the root. The most outside tissue as we talked about in the first class is the epidermis. This is the skin of the root, which surrounds it. It's responsible for the absorbing of water and nutrients from the soil. Underneath the epidermis, we have the ground tissue, which forms most of the, excuse me for using the word, the meat of the root and this is made from one main type of tissue, which we call the cortex. Inside of the cortex, there's one layer of ground tissue, which is very important for the root function. I'll talk about that in a second. This is called the endodermis and inside of the endodermis, we have the vascular tissue. We have a cylinder of vascular tissue and this is an experiment actually, you could do at home. If you just go and take a carrot right now and cut it vertically, you'll see all three of these tissues. You'll see the epidermis, which is what normally we peel off when we're peeling a carrot. You can see the ring of the ground tissue that's surrounding the vascular tissue right in the middle of the root. The vascular tissue, of course, contains both the xylem and the phloem. So this structure of the root is needed for the root to selectively uptake water and minerals from the soil to transport it into the vascular tissue and for then, it's transport up to the leaves. This uptake of water and minerals by the roots is not a passive process, but an active and selective process. What happens first is that there's a transfer of water and minerals from the soil into the epidermis and you remember the epidermis has a very long structure, because some of them have root hairs that are coming out of the epidermal cells. Once the water has absorbed by the epidermis, it can then transfer into the cortex by one of two routes. It could either go straight into the protoplast and this is being transporting through the plasmodesmata or the water and minerals can actually go around the cells through the cell walls. Regardless of what root is taken, the water and minerals then go through the cortex towards the vascular structure. But before it reaches the vascular tissue, before it reaches the xylem, it first encounters the endodermis. And the endodermis has a slightly different structure, because surrounding each endodermal cell is a ring of wax. This wax is a very hydrophobic structure. What this means is that any water and any minerals that are coming around the cell walls or through the cell walls, get stuck at the epidermis. They hit a wall, it can even seem like they hit Hadrian's Wall. They can't make it through into the xylem. In order for water and minerals to get through to the xylem, they must first go in through the protoplasts of the endodermis and then there through multiple plasmadismata into the vascular tissue. In this way, the endodermis is a selected barrier. It allows some of the minerals through, while it rejects some of the minerals and kicks them out of the plant. This way, the plant can allow beneficial minerals such as potassium to enter into the plant, whereas poisonous minerals such as heavy metals get stuck in and around the endodermis and are not allowed through into the other parts of the plant. Now let's look at the plant vertically. We can divide the plant into three different areas. The area closest to the bottom, we'll call this the zone of cell division. This is actually the zone of the apical meristem. This is the only area of the root where the cells actively divide. It's full of many, many small cells that divide and divide and divide. These cells are not differentiated into epidermis, into ground tissue or to vascular tissue. Their sole role at this point is for dividing. Above the area of cell division is the area where we call the cell of zone of elongation. Here the cells have stopped dividing and start elongating. This is giving most of the length of the root. Again, how does the cells elongate? By pumping water into the vacuoles, putting pressure on the cell walls, the cell walls elongate. Once the cells have gotten to their adult size, then they can finish their differentiation, take on their final purpose in life, take on their final function and begin to function either as epidermal cells or xylwm cells, as phloem cells or as ground tissue cortical cells. So first, the cell divides, then it grows. It reaches it's final size and then it finishes its differentiation. We can actually notice the border between the zone of elongation and the zone of differentiation if we look at the formation of root hairs. While the cell is elongating, there is no root hairs. Only once it's differentiated into an epidermis, then do we find the root hairs being formed. Now there's actually a fourth zone, which I didn't talk about right below the meristem, right below the zone of cell elongation and this is called the root cap. Now the root cap is a group of very, very dense cells, whose role it is to protect the apical meristem. Now think about it. The apical meristem at the tip of the plant. These are the stem cells. This is the embryonic tissue. This is the source of all cells that will form all future roots. These cells are surrounded by a violent environment. There's a lot of friction with the ground. If these cells are harmed, the roots will start to grow. The way that the plant protects its embryo cells, this embryonic cells is by surrounding it by what's literally called the root cap, which surrounds the meristem and provides protection from the friction of growth down into the soil. In addition to protecting the root apical meristem, the root cap also secretes a lubricant called muscilage and what this lubricant does, what the muscilage does is it helps the root get through the soil as it's growing down into the dry soil. Now let's look closer into the meristem. These cells are dividing and are not yet differentiated. From a structural point of view, there's no difference between this cell, this cell and this cell. This cell does not know if it's gonna be an epidermis cell when it grows up, whether it's going to be a xylem cell or whether it will be a endodermal cell. All these cells have the exact same potential. What will determine the fate of these cells is their position within the meristem. As these cells divide and they are pushed out or in or somewhere in between, this determines what will be their eventual cell fate. This cell, as it divides and pushed to the periphery of the meristem. It now knows as it divides, it will eventually become an epidermal cell. These cells in the middle as they divide up will eventually become part of the vascular tissue, but at this point, they don't know. Again, that whether they will become xylem cells or phloem cells and these cells between the two will become part of the ground tissue eventually, either cortical cells or endodermal cells. So at one point in the meristem, the cells are just dividing and they only get their final determination once they reach their final position within the meristem. So it's the position of the cells, which eventually determines what its final cell fate will be. So I talked about the position of the cell, helping to determine what its final cell fate is. What this is means is that there's communication in exchange of information between the cells and this is clearly seen in the epidermis. Not every epidermal cell will form a root hair. And actually, there is a type of order in determining which cells will have a root hair and which cells will not. Now look at this picture here. These red cells are all epidermal cells and the light blue cells underneath them are cortical cells, cells of the cortex. All of these red cells are epidermal cells and below them is a layer of cortical cells. Only these cells though, designated with lines will develop root hairs. Can you figure out what are the rules that are involved in determining which epidermal cells will have a root hair? As you can see, the only epidermal cells which develop root hair are the epidermal cells which border two cortical cells. An epidermal cell that touches only one cortical cell does not have a root hair, whereas an epidermal cell which borders two cortical cells does. So we clearly see that there is an exchange of information between these two cell layers. That this cell knows that below it are two cortical cells and this exchange for information is determined genetically and the way we know that is that we have in the laboratory mutants that have lost this communication. I just wanna show you what such a mutant would look like. These are normal wild type Arabidopsis roots with one root hair for every couple epidermal cells. Why have we coupled epidermal cells? Because only every coupled epidermal cells touches two cortical cells. And here, we have the roots of a mutant plant, which have lost this communication. Each one of these epidermal cells now thinks that it is touching multiple cortical cells when in reality they're not. And as you can see, each one of these root epidermal cells has a single root hair. These roots have multiple root hairs. So the communication between cells is encoded genetically.
