 So, let's talk about alternation of generations, which is something that is unique to plants and some protists. So with alternation of generations, it's very important that you remember how mitosis and meiosis work, so here's a quick review. Mitosis is a type of cell division in which one cell divides and produces two genetically identical daughter cells. So this is what growth and repair is all about. Now meiosis is different in that one cell will divide and it will produce a cell or cells, but the daughter cells have half the number of chromosomes. Now it's not just any half the number of chromosomes though. In fact, we take a diploid cell and go through this process of meiosis and we wind up with cells or a cell that is haploid. Diploid cells have two copies of each chromosome, whereas haploid cells have only one copy of each chromosome. So alternation of generations uses both mitosis and meiosis. Mitosis, one cell divides, produces daughter cells that are genetically identical. Mitosis, when the cell divides, the resulting cell is haploid. So let's look at alternation of generations in more detail and see where the different types of cell division occur. Let's start out with, it's a life cycle, so where we start is really arbitrary, but we'll just start with the sporophyte generation. Sporophyte plants have a dominant sporophyte generation, so if you look out your window and you see a bunch of trees and grass, then you're looking at the sporophyte generation of plants. So think about an oak tree, for example, that oak tree represents the sporophyte generation. That sporophyte generation is diploid, meaning that it has two copies of each kind of chromosome. Now in the sporophyte generation, there are special structures where meiosis takes place. And when meiosis takes place, we have a diploid cell that divides by this process of meiosis and produces haploid spores. Now what this looks like depends upon the kind of plant that we're dealing with, but just in general, remember that sporophytes produce spores by meiosis. These spores are haploid. And so this haploid spore will then divide by the process of mitosis and that results in the gametophyte generation. And the gametophyte generation of course is haploid. Now sometimes this is confusing because this is the first time that you've probably heard of haploid cells dividing by mitosis and producing more haploid cells. But remember, with mitosis, one cell divides and produces daughter cells that are genetically identical. So this haploid spore will divide and produce two cells, then those two will divide and then those will divide and those will divide and will get a gametophyte. Now in some plants, this gametophyte is going to be microscopic and you would never notice it most probably. In other plants such as mosses, the gametophyte generation may be dominant. But for most plants, the gametophyte generation is something that you're not going to be aware of. Now this haploid gametophyte, as the name suggests, produces gamete, sperm, and egg. If you think about pollen, pollen represents the male gametophyte for plants such as trees, pine trees, oak trees, etc. So the sperm and the egg are both haploid. During fertilization, the sperm will fertilize the egg and we get a diploid zygote. This diploid zygote will then divide by the process of mitosis and we get a sporophyte generation. So this is why it's called alternation of generations and if I drew a line between the across, you can see, if I can actually make the line work, you can see that we alternate between the sporophyte generation and the gametophyte generation.