Biology: Genration Alternation: Flower Structure

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I've got to say that you humans have a very strange was of expressing your love. Think about it. What does a guy do when he wants to impress a woman? And sometimes it goes the other way. He gives her a bouquet of sex organs. Now, isn't that just a little bizarre? But, you know, that's exactly what we do, and I'm sure it's not because of the beauty of flowers. You're trying to send a message here. Would you give her a dozen eggs? No. Well, those aren't sex organs; they're like egg cells. But why? Why do you humans do that stuff? I don't know. Let's talk about flowers.
Flowers, indeed, are the sex organs of the sporophyte of the plant. I want to talk to you a little bit about the reproductive nature of flowers and then we'll take a look at how the whole alternation and generation thing fits here. Remember what happened with the plants. You had the seed plants branching off as a vascular plant. The ferns went off on their own because they did not have seeds. The spore was their mechanism of spreading their offspring, and then the seed plants came along, and from the seed plants we have the naked seed plants and the hidden seed plants, or the covered seed plants, and this flower is indeed the sexual organ of the hidden seed plants.
Let's do a basic flower review. Let's go through the steps of plant reproduction before we get into the nitty gritty details. Certainly you know that flowering plants and insects co-evolved, and this whole idea as the petals as an attractant for insects, and the organs. There are so many great studies that have been done on this. Do you realize there's even an orchid that resembles a wasp, and the male wasp comes over and thinks it's a female wasp and tries to mate with it. That's how co-evolved these things are. How is that for incredible? What does the flower do? It gets the insect to spread its pollen while tricking it.
So the point is that the petals are there, generally speaking, to attract insects so that pollen can be spread--that male gametophyte. More on that later. A plant--this perfect flower, if you will--has both male and female organs. Some do not. This one is called dioecious, which means it has both houses, as opposed to a monoecious flower, which this one isn't. So this dioecious flower with both houses has sperm and eggs, and it can indeed self-pollinate in some cases. These sperm are produced in these boxes up here called "anthers." If you know anything about Gregor Mendel you know that Mendel, in order to cross-pollinate his flowers, removed the anthers so that they could not self-pollinate.
This particular structure, this green structure in the center is called the "carpel." The carpel has three structures in it. The first structure is called the "stigma," and the stigma produces a sugary substance, and that sugary substance--again, co-evolution with insects, attracts insects over to it, and this nectar, is what it's called, will indeed stimulate the growth of a pollen grain, but more on that later.
The style is merely a long tube that connects the stigma and the bottom, and the ovary is right here. Inside of the ovary are ovules, which will eventually form the female gametophyte, and even past that will form the seed. They will contain the actual egg, and some of them have merely one and some have many, and if you think about that, you know of some plants that only produce one seed, like, say, a peach, and others that produce many seeds, like a green pepper. Interesting. We'll come back to that later. That's real interesting.
What happens in the life of a flower? How does a flower reproduce? Let's go through the general and generic steps. The first thing, of course, that happens is gametogenesis. You know about gametogenesis. You know that in gametogenesis we're going to form gametes and we're going to plug all of this into our alternation and generations life cycle later. Second, before fertilization can happen in a plant, we have a process called "pollination." Pollination is the transfer of pollen from the anther to the stigma. So if I were to draw a stigma for you on top of a plant, and draw the style down, and eventually show the swelling of the ovary there, pollen grains would land on top of the stigma and stick to them because of the action of the nectar.
Then what happens is something that most people don't know about. The pollen starts to elongate and it grows what is called "a pollen tube." The pollen tube will come down here and grow toward the ovules. Let's just draw a few ovules in here. So the ovules might be down in here. Now, the pollen tube will then insert, and it will insert into the ovula, and we'll call that "pollen tube insertion." What comes next is fascinating, because now the sperm and the pollen can enter the egg. Generally, there are two. They will enter the egg and do a process called "fertilization."
Now the coolest part of all, in my opinion. The adaptation for spreading that seed because once you get fertilized you form seeds. And so we come to the next step, which is the swelling of the ovary, otherwise known as fruit formation. Yes. Fruit is a swollen ovary. Now, that being said, why are these called "angiosperms?" Now you get it. The seed is protected. We call them covered seeds, hidden seeds. They're covered by a fruit. Think of a fruit. By definition, what is a fruit? A fruit is anything that contains seeds. No, it's not something you necessarily find in a fruit salad. You rarely find cucumbers in a fruit salad. You rarely find tomatoes in a fruit salad. You sure don't find pumpkins in a fruit salad. But, you know what? Fruits are anything with seeds.
And that's why we have to kick that definition of fruits--that generic definition of fruits out the door, that it's something that's sweet, because not all fruits are sweet. Fruits are things with seeds in them, so those of you who live where there are maple trees, you ever see those little helicopters that come down? Those are fruits. Inside of those are seeds. Those of you who don't know what I'm talking about because you've never been around a maple tree probably know about grass. Did you ever not mow your lawn and you get these seed heads that come out? Fruits. So you see, fruits are the leftovers from the ovary, the swelling of the ovary.
Now, how does all of this fit into the big picture of alternation and generations? Where is the sporophyte, where is the gametophyte, and how does the picture fit together? I think I'll tell you about that later one.
The Evolution of Life on Earth
Angiosperms
Alternation of Generations: The Structure of a Flower Page [1 of 2]