So what about the phloem? As I said, the job of the phloem is to transport sugars from one side of the plant to the other, usually from the leaves to either the flowers or to the roots. The structure of the phloem is much more complex though than the structure of the xylem. The phloem is actually comprised of two types of cells. One large cell, which is called the sieve tube member or sieve tube element, depending on what type of textbook you're using and a smaller cell called the companion cell. The sieve tube member is a large cell comprised of a cell wall and of the protoplast, but the protoplast is empty. It doesn't contain a nucleus, it doesn't contain a vacuole, it doesn't contain any type of plastid or mitochondria or any other type of organelle. The only thing the sieve tube member contains is the cell membrane that has proteins for transporting sugars and the sugars accumulate within the protoplast. The balloon of the sieve tube member. The companion cell, on the other hand is a much smaller cell, but it's a complete cell with all the organelles, with a nucleus, with plastids and with a central vacuole and the job of the companion cell is really to act as a companion for the sieve tube member. It provides the sieve tube member with all the proteins, with all of the energy, with all of the membranes that it needs to function. So the companion cell is connected to the sieve tube cell by numerous, numerous plasmodesmata along the common wall. And on the top and the bottom of the sieve tube members are open pores, which connect various sieve tube members one to the other, so the sugar can go from one sieve tube member across its membranes into the next sieve tube member. And that way, transport the sugars from one side of the plant to the other. So how do we get this complex structure of the phloem, of two cells? One with only a protoplast and one with a complete protoplast? How do we get two cells, one large and one small? To understand this process, we first have to also understand something that's called cytokinesis, which is the process of cell division. During cell division, the first thing that happens, of course is that the nucleus replicates itself and then migrates to two sides of the cell. This is called mitosis. You can learn more about mitosis in many classes about genetics. But once the mitosis has occurred in a plant cell, then a new cell wall has to be formed. The cell wall is formed in the cytoplasm between the two nuclei such that in the end, we have two daughter cells, each with its own nucleus. Now the plain where the new cell wall is formed will then determine what is the structure of the two cells, because the two cells are bound to each other by the developing cell wall and the cells themselves can no longer migrate. So let's look at an example here of this yellow cell. This yellow cell goes through mitosis and then it decided to divide right and left. When it's divided right and left, it's added new cells to the same cell layer. In other words, it's elongated that cell layer. On the other hand, the same cell could have divided up and down. When it's divided up and down, it's made new cell layers. It's made the same tissue thicker, whether the cell divides in this example right or left or whether it divides up and down will determine the structure of the tissue. Now this structured cell division actually has names. When it's adding cells to the same layer, it's called an anticlinal cell division. And when it's making new cell layers, it's called a periclinal cell division and the decision of whether to have an anticlinal cell division or a periclinal cell division will then affect the overall structure of the plant. Now this is a very difficult concept for some of you to understand and there's no such thing actually as right or left or up or down. Let's, for example, look at this example going through a stem up through the leaf. If we're looking up at the leaf here, these two yellow cells have had an anticlinal cell division. The cell division is perpendicular to the plane of the leaf, we've added two new cells to the cell layer and so the cell layer is elongated. Well, we're looking at these two yellow cells here in the stem, they've divided up and down. But when they've divided up and down, it's still perpendicular to the plane of the cell. It's added new cells to the cell layer, it's also caused it to elongate. But in this final example here when it's divided right and left, here it's divided actually parallel to the cell layer. It's added new cells, it's made us a thicker tissue, it's made it a fatter tissue. So whether the cell divides anticlinal or periclinal will then decide what the structure of the plant is. So if we go back and look at the development of the phloem, what has to happen for us to develop these two types of cells? So if we start from a cell early in embryo genesis, and again, this cell knows when it grows up, it wants to be floum. In this case though, two things have to happen. The first thing is it has to go through a cell division, a structured cell division. You're gonna get mitosis, two nuclei form and then a new cell wall forms that's gonna separate it into two cells. In this case, it's an imperfect division where you're gonna get one larger cell and one smaller cell. Once we now have two cells formed, the larger cell is gonna go through partial apoptosis, very similar to what happened in the development of the zygote. The nucleus is gonna break down. The vacuole is gonna break down. But here, it's gonna end. So we're gonna be left with a cell that has a cell membrane, the protoplast and the cell wall, but it's still a partially viable cell. The second cell, the smaller one is gonna become the companion cell, which will then provide the larger cell with all of its needs. So what type of cell division did we have here in the beginning? Was this first cell division an anticlinal cell division or a periclinal cell division? As you see, what's happened here is that we've had a periclinal cell division. We've taken a layer of cells and now rather than having one layer, we have two layers. We've made the whole tissue fatter. What would have happened to the flow of development if had been an anticlinal division? If the first division is an anticlinal division, you would see then that the companion cell would be above the sieve cell. Therefore, there would be no clear connection between one sieve cell to the next one above it. This division of a periclinal division is genetically programmed and is necessary for the proper development of the phloem, which brings me to one last type of cell. We've been talking about in the embryo. Actually, the embryonic tissue, which we call stem cells. You may have heard of stem cell research, this is not the cell as in the stem of the plant, but these are the stem cells of the plant. The embryonic cells that have the potential to form any other cell. The stem cells of a plant are actually called the meristem cells and a single meristem cell has the potential to either become part of the ground tissue. It could become, for example, xylem. It could become, for example, phloem. And in the phloem, it could either be the companion cell or the sieve cell. What determines whether a meristematic cell becomes ground tissue or vascular tissue is its positioning within the plant and the timing of its development, but this meristem cell has the potential to do anything. Where these meristem cells found within the plants? Whether they're actually found primarily at the tips. These are called the apical meristems. They're found at the very tips of the roots and at the very tips of the shoots. This is the only area in the plant or basically, the only area in the plant where there is cell division. This is basically the main area of the plant where there's cell differentiation and this is where the plant is going from. From the tip of the shoot up and from the tip of the root down. The cells in the meristem are not differentiated. And only later in development, do they get their final development, their final differentiation, their final purpose in their plant physiology.
