To understand how a plant maintains growth throughout in its entire life cycle. We need to go back and revisit the last type of tissue we learned about in the previous class. And these were the plant's stem cells which I define as the meristem cells. The meristem tissue, the meristem areas of a plant are perpetually embryonic tissue, which continue to divide throughout an entire plant's life cycle. Now these meristem cells, the meristematic tissue is found in two areas in the plant. At the tip of the roots, and this is called the root meristem. The root meristem continues to divide and form new roots. And at the tip of the shoot which is called the shoot meristem, which continues to divide and form new shoot tissue. So to understand how a meristem works in general, let's look at a theoretical model. Let's say we have here a theoretical model of an apical meristem that has nine cells. This could even be the apical meristem of the root. Within this nine-cell apical meristem, we are going to define the center cell as the quiescent center. This quiescent center does not do anything. This is a cell that doesn't divide. It is the forming the base or the center of the meristem, it knows that around that the whole meristem will form. Directly above the quiescent center, we have three cells, which now are going to divide. Once these three cells divide, it leads to two new types of cells. The first cell, the lower layer of cell that remains next to the quiescent center is still part of the apical meristem. But these three new cells which I've given colors here, we're going to call these derivatives. Why are they called derivatives, because they derived from the apical meristem. So now, we have 12 cells. 9 cells of the apical meristem, and three derivative cells. What's important now is that these three derivative cells can now start to differentiate. They can now start to get their final purpose in life. This cell will become a blue cell. This cell will become a red cell, and this cell will become a green cell in this model. What's important here to see is that the original apical meristem has renewed itself. It's always maintaining it's size, and again this is just a theoretical model here of nine cells. Out of these nine cells, the top three divide. When these top three divide, it pushes the original nine down. If this was the root, it's growing down into the ground. And now these new cells can then differentiate into what will be adult root cells. When we're talking about differentiating, two different things are happening. One, the shape of the cell's changing, for example, these three cells have elongated. Now think for a second, what has to happen for a cell to elongate. Well, if you remember from our first lecture. One of the easiest ways for a cell to elongate is to increase the size of its vacuole. And that's exactly what happens in these cells. Once it's time for a cell to elongate, the cell pumps water into the vacuole. The increase in water pressure in the vacuole puts pressure on the cell wall. And then if enough pressure rises, the cell can elongate. This is part of its differentiating and then there also, there's a change, will be a change in its genetic makeup so that this cell will be a blue cell. Let's say that this will be the epidermis, the red cell will be, let's say, the cortex, and the green cell will be, I don't know, part of the vascular tissue. But what's key here is that the apical meristem always renews itself. And gives rise to cells that can differentiate. So as I said, these apical meristems are always found at the ends of the roots and at the ends of the shoots. The root apical meristem will always make roots, and the shoot apical meristem will always make shoots. And there's one other type of meristem which we're not going to have really much too much time to go into. And this is called the secondary meristem or lateral meristem. And the secondary meristem is found throughout the entire length of dicotyledon plants. If the secondary meristem which one it divides, increases the girth. Increases the width of the stem. The secondary meristem is actually in this ring of vascular tissue that I talked about. And this is what leads to the rings that we see in wood. So this is also an embryonic tissue that continues to divide, increasing the girth of the stem.