Pollen was a wonderful technological advancement in the evolutionary history of plants. Prior to pollen, plants had to use water to get sperm to the egg. Pollen provided an avenue that allowed for a much more diverse variety of methods of connecting sperm and egg. Now pollen is the male gametophyte, and it's surrounded by a watertight coating made of a material called sporopollenin. It actually contains 2 cells, the generative cell and the tube cell. We'll talk about what those do momentarily. Now pollen forms when a microsporocyte, which is a diploid cell, goes through meiosis and forms a microspore, which is haploid and through the process of mitosis will form microgametophytes. And you'll see in a little bit why I have a 2 right there because, as we'll see, there's actually going to be 2 microgametophyte or 2 sperm involved in angiosperm reproduction. Now the female component is the embryo sac. This is the female gametophyte, and it is contained within the ovule, and it's what will develop into the embryo. It forms when a megasporocyte, a diploid structure, undergoes meiosis to form megaspores, and it is going to form 4 of them. Now 3 of these will degenerate. One will go on to form the megagametophyte. And the reason I have an 8 right here is as you'll see, this megagametophyte is actually going to have 8 nuclei in it. Two of these are going to be what are called the polar nuclei, haploid nuclei that are going to develop into endosperm, which is something we'll talk about in just a second.
Now, pollination occurs when the pollen is transferred to the ovule. It begins with germination where pollen will land on the stigma, resume growth, usually by absorbing some water, and the tube cell in the pollen grain will generate a pollen tube. This pollen tube grows through the style and connects to the ovule, and it will transmit those male gametes or sperm. Now I said I'd get back to that 2 sperm thing, and here we are. The reason for it is something called double fertilization. Basically, the pollen tube releases 2 sperm. One is going to fertilize the egg and form the embryo. That's to be expected. The other does this funky thing; it interacts with those polar nuclei and will form endosperm. This endosperm is a nutrient-rich material or tissue, really, that surrounds the embryo and provides it with things like starch, protein, and oil. In fact, when you eat nuts, you are actually basically eating endosperm. So for example, if you've ever eaten a peanut, you might notice that on a peanut, there's that little kinda nib thing on the end. Right? Or if you crack the peanut in half, you'll see that there's, like, a little, nub structure on one end. That is the embryo, actually. The nut, most of what we're eating, what we think of as the nut is the endosperm, and it's super nutrient-rich. Right? And also delicious in many cases. Now these 2 sperm, not to get too sidetracked on endosperm, but these 2 sperm, these 2 male gametes, will be produced by the generative cell, which divides by mitosis and produces those 2 sperm, sends them down the pollen tube. Now seeds will develop once the ovule is fertilized, and it contains the embryonic plant surrounded by a protective coat. It's also got some endosperm in there. Never forget the endosperm. And just to look at a nice overview of this process, here we have, on one side, our microsporangium producing microspores. The microspore will go through meiosis. Right? That's what's going on here. And then, it will produce the microgametophyte through mitosis because it has to make those 2 cells, the generative cell and the tube cell, and that is our microgametophyte or pollen. So this whole thing right here is our pollen grain. Now on the female side, here you can see those 4 cells that we talked about, 3 of which are going to degenerate, and one is going to go on to go through mitosis and form those 8 nuclei, right, and you can count them off in this image, it's a little small, but we have 1, 2, 3 on top right here. These 3 guys are polar nuclei, so that's 4, 5. And then you can see on the bottom here, we have another 3 cells, so that's going to be 6, 7, and 8. So that accounts for all of those 8 nuclei, and that is all part of the megagametophyte or the embryo sac. Now just to finish it off here, you can see the pollination happening. We have our pollen grain landing on the stigma, so that's a stigma, remember. Here's our little orange pollen grain. The tube cell is going to produce this pollen tube that goes down to the ovule, and here you can see the pollen tube reaching the ovule, and it's going to send the sperm into, the ovule where they, one, will interact with the egg cell right here, and one is going to interact with these polar nuclei to form the endosperm. With that, let's turn the page.