Biology: Alternation of Generations: Angiosperms

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So what is alternation of generations, just one more time? Do you know about alternation of generations? Let's see if you do. Alternation of generations is something that has been consistently moving through plant phylogeny. When we take a look at it we see that it's very consistent across. We have a sporophyte which gives rise to spores. The spores are 1N. The sporophyte is 2N, therefore the process of meiosis must be forming the spores. The spores mitotically divide and give rise to gametophytes. The gametophytes, because they're gametophytes give rise to gametes. Wait a minute--because the spores are N, the gametophytes must be N, so they make these gametes by mitosis. See how easy this is? And finally, the gametes are going to do a process called "fertilization," and form sporophytes and the cycle repeats.
Now, we've seen that in mosses, where the gametophyte was the dominant generation. And then we saw ferns, and the ferns, the sporophyte had started to take over with its branching taller pattern. And then we went to the gymnosperms and did an overview of them and saw that holy mackerel, the gametophyte is getting so reduced, we're talking about mobile cells here, and that's important. Going back to the ferns and the mosses, remember something. They needed water to spread their sperm. They needed water. What the pollen grain allowed was the spreading of sperm without water. Now it could be bugs. Now it could be air, wind, anything like that--animals. The pollen grain didn't need the sperm to swim. This is very important and it was a major evolutionary driving force.
Second thing, as we got into the gymnosperms we saw that the female gametophyte stayed home. The female gametophyte stayed within the pinecone and waited there for a pollen grain to approach, to land, to grow a pollen tube, and to fertilize here, and then she formed a seed and it fell to the ground. And as we did our overview of the angiosperms, we see that the sex organs of flower is perfectly adapted not only to grow through alternation of generations, but then to protect its seed by swelling the ovary around it, and covering it with a fruit.
Now let's take this ultimate step and see how the ultimate plants--the angiosperms--not that I'm not a big gymnosperm fan, nor a fern fan, nor a moss fan, but let's take a look at the way these guys have managed to put together alternation of generations and reduce that gametophyte to practically a non-existent entity. That's not a bad thing.
Sporophyte comes up with this specialized structure called a flower. This is not the gametophyte. This is the flower; it's part of the sporophyte. We have a heterosporous situation. We're going to have two different spores formed. One of them is going to be called the "microspore," one of them is going to be called the "megaspore," and they start out in the different parts. Let's start out with the megaspore. Let's start out in the female.
Where's the megaspore going to form? The megaspore is going to form from a cell called the "megasporocyte" in here. Now, what's interesting is in the female portion, in other words, in the carpel itself, you're going to start out with something called a "megasporocyte." Remember, this is in the sporophyte. What's the chromosome number of the sporophyte? 2N. What does the sporophyte do? It makes spores--megaspores. How does it do megaspores? It makes megaspores by meiosis. Once. We get one meiotic division of the megasporophyte and from that we get four cells, and this, you can make an interesting parallel now between this and animals, because guess what? Three of those cells don't survive. One of those cells has evolved to be the big cell to conserve cytoplasm. Isn't that interesting? Very much like the polar bodies on animals.
So let's see where we're going. I`ll come back to this. So we have the megaspores, and here's our megaspores right within what's eventually going to become the seed. We have this colored one in here that's yellow, so to make that we're going to get meiosis. And then we go through a series of mitotic divisions. Why? Because remember, the spore is supposed to give rise to the gametophyte. You knew that. So now the megaspore, 1, is going to give rise to the gametophyte. The spore is N. The gametophyte, each cell will be N. Obviously, mitosis. How big is the female gametophyte? One, two, three, double nucleus, four, five, six, seven, eight. It's literally seven cells, one with a double nucleus. That is one small gametophyte.
Let me tell you about these cells. The cells are important--some of them. The cells I really want to concentrate on is that cell right there, because that cell is the egg, and that's important. I want to concentrate on these two nuclei within this one cell, because those two nuclei are called the "polar nuclei," and they're important. So what's going to happen here? Interesting. We're going to get fertilization. But that is the female gametophyte. I've got to tell you about the male gametophyte before we get fertilization though. The male gametophyte starts from something called the microsporocyte. The microsporocyte gives rise to microspores. How many microspores? Well, we get one division--one, two, three, four, and these all separate. I want to follow one. So we have one microspore. That microspore--remember, it's going to give rise to the what? Gametophyte. How many divisions? Well, remember--gametophyte is N. Mitosis. Ready? One. The male gametophyte is two cells. It can't get any smaller than that and still be called multi-cellular now, can you? And here's the thing. That male gametophyte has two cells in it, and we have to name those. Those two cells are important. Why? You know, to be quite honest with you, it's really one cell called a "tube cell." That's the nucleus of the tube cell. And then there's a nucleus here called the "generative nucleus." So it's one mitotic division, and here's the thing. This will eventually go through one more division and give rise later, in about 30 seconds, to two sperm.
Let's get there. Well, you know the story of the pollen grain. The pollen grain, the male gametophyte, with its gigantic quantity of two nuclei, lands on the stigma, grows the pollen tube down, and penetrates through a hole called a "hilum," in the soon to be seed, and that's going to be right here where we see the pollen tube coming in. And look what it's fertilizing. Now, here's the scoop. In angiosperms and one group of gymnosperms, but in angiosperms you get something called a "double fertilization." One sperm will fertilize the egg, and you get a 2N zygote. The other sperm doesn't fertilize the egg. It fertilizes that cell with the two polar nuclei, and guess what you get? You get a triploid food source called the "endosperm." Here it is right here with its three nuclei. That is a triploid endosperm. What do you think the function of the endosperm is? Now the story just falls completely in place.
Think about it. What have we seen throughout the entire development of plants? We have seen a gametophyte nourishing a sporophyte. We saw the mosses with its archegonium, and the archegonium nourished the sporophyte that came off. We saw the fern with its gametophyte and its bisexual nature where the sperm swam to the egg, and out came the fern. And what nourished it? The archegonium. What are we seeing here? We are seeing the endosperm, the part of the female gametophyte nourishing the growing embryo that will eventually turn into the plant and the sporophyte. Alternation of generations, the key to understanding plant phylogeny, and therefore the key to understanding plant evolution.
The Evolution of Life on Earth
Angiosperms
Alternation of Generations: Angiosperms Page [2 of 2]