Flowers - Online Tutor, Practice Problems & Exam Prep

Angiosperms exhibit an alteration of generations, primarily existing as sporophytes, which produce haploid spores through meiosis. These spores develop into gametophytes that generate gametes via mitosis. Angiosperms are heterosporous, producing microspores (male) and megaspores (female). Pollination involves pollen grains transferring to ovules, leading to double fertilization, forming an embryo and nutrient-rich endosperm. Flowers can be complete or incomplete, perfect or imperfect, and utilize various pollination strategies, including self-pollination and cross-pollination, often facilitated by animal vectors, showcasing coevolution between plants and pollinators.

Alteration of Generations

Hi. In this video, we're going to talk about angiosperm reproduction and angiosperms' reproductive structures, flowers. As you hopefully recall, plants experience what's called an alteration of generations. Essentially, what that means is that during the course of a plant's life cycle, they'll alternate between a sporophyte generation and a gametophyte generation. Now, angiosperms are actually sporophyte-dominant plants, and what that means is they basically spend all their time as sporophytes and their gametophytes are these very tiny, diminutive structures. In fact, when you're looking at an angiosperm, you're basically just looking at the sporophyte. Unless you really dug into the flowers, you've probably never seen the gametophytes. The sporophyte is a diploid multicellular stage of life and it produces spores, hence its name. It produces these spores, which are asexual, units of reproduction, or rather units of asexual reproduction, and they are generally haploid and unicellular. So remember that the process of meiosis will take a diploid cell and generate haploid cells from it. The gametophyte is going to be a haploid multicellular stage of life, and it's going to produce gametes. Shocker, right? Bet you didn't see that one coming. Since it's haploid and since gametes are haploid, it's actually going to produce its gametes by mitosis. The gametes, that's going to be like the sperm and egg, will fuse to form a diploid zygote that will grow and become a new sporophyte.

So looking here, we have a nice diagram of generations. The sporophyte, that's going to basically be like, you know, the main part of the plant, like the tree, whatever, and it's going to have structures that produce spores. These spores will turn into gametophytes and these gametophytes will produce gametes. And of course, the gametes will fuse together to form the zygote, which grows back into the sporophyte, and we have this continual alteration of generations. Now, you might notice here that these gametes are not distinguished from each other. They are, you know, they look interchangeable. The thing is, angiosperms actually have what's called heterospory. Basically, what that means is they produce two distinct types of spores. They produce microsporangium and megasporangium. Now, the microsporangium will produce what are called microsporocytes that will become microspores. And these microspores will develop into the male gametophytes. So, micro is going to be male. Micro, male. Mega, is going to be female. And the megasporangium will produce megasporocytes, which are a type of cell, and these megasporocytes become megaspores and those megaspores will develop into female gametophytes, or like the egg. Right? So here in this chart you can see what this alteration of generations looks like when you have headers for you. So I'm going to get my head out of the way for a second, and you can see that we have the microgametophyte and megagametophyte, and those are going to produce the sperm and the egg, which will eventually result in a structure with a microsporophyte, and a structure with a megasporophyte. Now, the thing about angiosperms is often, you'll actually see the microsporangium and megasporangium on the same plant. I'm not going to get into the breakdown of that just now, but for now, I just want you to realize that angiosperms are going to basically produce sperm and egg. Their gametophytes aren't all the same. With that, let's flip the page.

Asexual Reproduction

While sexual reproduction is all well and good, many plants are able to reproduce asexually as well. This asexual reproduction is sometimes called vegetative reproduction, which results in genetically identical offspring or clones. One way that plants will do this is by shooting off a stem that will form a new individual. The stem will travel underground and then produce a new individual, as you see there. We call these rhizomes. Their complement, if you want to think of it that way, are stolons, which are stems that shoot off and produce new individuals above ground. So, the rhizomes are underground, and stolons are above ground. These stolons will also produce adventitious roots, which are roots that are generated from the stem. Occasionally, plants can form seeds without fertilization. This means that gametes, specifically the sperm and the egg, do not need to come together to form a seed, which we will discuss later in the lesson. We call this apomixis, so seeds forming without fertilization. This again will result in a clone or a genetically identical offspring. You can also split plants into fragments, and in some plants, these fragments will develop into mature organisms. We call this fragmentation. This is similar to the idea of making cuttings or clippings of plants to grow new individuals. While this is great in terms of efficiency, as the plants do not have to go through the hassle of sexual reproduction, it leaves them genetically vulnerable. Because they are producing genetically identical offspring, there is less genetic variation in the population, and this can have disastrous results sometimes. For example, consider bananas. Depending on how old you are, you may or may never have tasted the world's best banana because they no longer exist. Bananas are grown from clippings. They undergo fragmentation, and we plant the clippings, and that is how you get new banana trees. The problem is, they are all genetically identical. Every now and then, a fungal infection or something like that will come around and wipe out an entire population of bananas. This has actually happened more than once, and that is why the bananas we have today are not nearly as good as the varieties that we used to have decades ago, which have disappeared due to unfortunate circumstances. I like to think of this as the tragedy of the banana. Vegetative reproduction with human involvement, like with those bananas, will be called vegetative propagation. This involves making cuttings, and using these cuttings to grow new organisms. Those cuttings will be able to produce roots because they will have a callus at the wound site. Remember, that is going to be a mass of cells that are able to differentiate and develop into mature cells, and they will be able to produce roots from the site of injury. With that, let's move on from asexual reproduction to the reproductive structures of angiosperms, which are flowers. These flowers produce gametes, embryos, fruits, and seeds. But first, a little flower terminology. Sepals, as you can see in this image, are green leaf-like things that serve as protection for the flower buds. So when you see the flower bud initially, it is completely covered by the sepals. Then, the flower petals will pop out of that. Generally speaking, sepals are green, but not always. When you take all the sepals together, for example in this image, we have five sepals here. All of those taken together, including the green cup-like structure that the sepals connect to, is called the calyx. Petals are modified leaves that surround the reproductive parts of the flower and are usually there to attract pollinators. Just like the calyx is the entire group of sepals, the corolla is the entire group of petals. With that, let's flip the page.

Flower Anatomy

The reproductive structures of flowers can be broken down into stamen and carpels. Stamen, you can think of as the male reproductive parts, and carpels, you can think of as the female reproductive parts. The stamen is going to be the structure that produces pollen, and it is made up of 2 portions, what we call the filament, which is going to be this stalk-like appendage here. So that is a filament. You can also see it pictured here. And then we also have the anther, and the anther is what contains the microsporangium, which will produce microspores, and we'll talk about the fate of microspores in a little bit.

Moving on, we have the carpel, the female reproductive structure, which is this whole thing right here, that is a carpel. And it's also made up of some components, the stigma, which you can see here, that's the tip of the carpel. So if I were to draw this structure out a little bit, the stigma would be the tip there, oops. And that's going to be where the pollen lands, where the pollen is received during the reproductive process. Again, we'll talk more about that a little bit. The style is basically the pathway that leads from the stigma into the ovary. And you can see it pictured here, it's, let me choose a different color and make it more clear, it's going to be like this portion. And then this bottom portion of the carpel is the ovary. And the ovary contains what are called ovules. These are going to be things that receive sperm from the male portions of flowers. We'll get to all that later.

Now, flowers will also have a structure known as the nectary. The nectary is going to be the gland that secretes nectar, which is a super sugary solution, and that's what some pollinators are going to feed on. And this nectary can actually be located inside the flower or outside the flower, it doesn't really matter for your understanding, the difference between all that, just know that it can be inside the flower, sometimes it's outside the flower.

Now moving on to those ovules. The ovule is the structure that contains the megaspore and it's what's going to develop into a seed after fertilization happens. So this ovule right here is, oops, Sorry. This ovule right here is going to have some structures you should be aware of. We have the integument, which is this outer protective layer, and then the micropyle, which is the opening, the apex of the integument, which is going to allow the sperm to enter in and fertilize this structure. Let's turn the page.

Double Fertilization

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.

Pollination

Up to this point, we've been talking about all flowers like they're the same. But as with everything in biology, there are exceptions, and there is variety and diversity, and that's a good thing. And we need to go over some of that terminology. So we've actually been talking about what you'd call complete flowers. That is flowers that contain sepals, petals, stamen, and pistils. All parts. Some flowers, though, are considered incomplete, and that's because they are missing one of those components, maybe more than one of those components, in fact. Now, you can also have what's called a perfect flower, which is going to be a bisexual flower that has stamen and pistil structures within the same flower. That's as opposed to what's called an imperfect flower, which has either stamen or pistils, and is unisexual. Now amongst unisexual flowers, we can get another dichotomy, and that is, the difference between monoecious plants and dioecious plants. Now, monoecious plants and dioecious plants both have unisexual flowers. But in monoecious plants, the male and female flowers will be found on the same plant. So one plant will have some flowers that have male parts, some that have female parts, but it's all on the same plant. Dioecious plants, on the other hand, will have just male flowers or just female flowers on the same plant. So the male and female floral organs are separated by plant. And here you can see we have some images to represent all this stuff. So here we are looking at incomplete flowers, and this one is male, this one is female, and here we have a perfect flower. Actually, I'm sorry. This is supposed to be imperfect, not incomplete. Well, actually, it could be incomplete. It's both incomplete and imperfect actually. So imperfect perfect, but over here is where I wanted to show our incomplete-complete dichotomy. Let me get my head out of the way. So this is a complete flower, It's, got the, you can see the stigmas up at the top there, here are the anthers, you know, this is the style, the ovary is going to be down in there, you can't really see it. It's got, petals, you can't see it, but it has sepals under there. It's a complete flower. This is an incomplete flower. Oh, if you're curious, this is a, on the left here we have a hibiscus flower, on the right is a calla lily. And while this might actually look like a petal, it's not, it's a leaf. It's a leaf that has had its color altered to appear as a petal to fool pollinators, so to speak. And this structure is actually, covered in pistils. Though you'd have to look through a zoomed-in lens to really see it, but the point is, it's incomplete. Now, if we move on, we need to talk about how pollen gets around. As I'm sure you may have thought, you know, if a flower has the male parts and the female parts, well then can't it pollinate itself? The answer is sometimes. Some plants can self-pollinate, but others can't. Others will cross-pollinate, which is when pollen is transferred from the anther of one plant to the stigma of a different plant. And some plants are prevented from self-pollinating by genetic mechanisms. We call this self-incompatibility. So that is when genetic mechanisms prevent self-pollination, and this encourages what's known as outcrossing, which is when you breed genetically unrelated individuals. Now there are other methods that plants use to prevent self-pollination. One of these is known as temporal separation. Essentially, the male and female gametophytes on a plant will mature at different times, so that neither one is mature at the same time, meaning it can't pollinate itself. However, other plants will have their male and female gametophytes maturing at, you know, on a different schedule, and so those guys can get together. There's also spatial avoidance, which is basically spatial positioning of male and female flowers or floral organs to avoid self-pollination. So if pollen has to get to stigma, for example, maybe all your female flowers are somewhere where the, male flowers can't get their pollen to them, or, for example, you put your, stigma and anther within the same flower in a configuration where the anther can't hope to get its pollen up to the stigma, for example. Now, lastly, we need to talk about pollination syndrome, which are flower traits that have evolved in response to pollen vectors. So these are going to be traits that influence a plant's ability to pollinate by wind, or by attracting bees, or by attracting birds, for that matter. And, the reason that you have organisms that will pollinate these plants is because of mutualism. Right? They're not doing this for free, it's not charity. There's something that they get out of it. The animal pollinators, like this dusty bee covered in pollen here, or this bee slurping up its meal, or this hummingbird behind my head here slurping up its meal, they're getting food. They're not there thinking, alright, I'm going to take some of this pollen, bring it to this other plant later. See? No. They're like, I'm getting a meal right now, and, you know, it's I've got to get a little dirty to get this food. Right? I get dusty in the process. I don't really care, I'm getting a delicious sugary snack. And when I go to my next meal, I might rub off some of that dust, that pollen on that plant. Right? So the plants get to spread their pollen around, the pollinators get a meal out of it. And what we see often with these pollination syndromes is coevolution, which is when, one species' evolution influences the other species' evolution. So a lot of these pollinators and flowers have co-evolved. In fact, when Darwin, the father of the idea of natural selection and all that good stuff, when he went to the Galapagos and he saw this particular flower that had this really long tube down to the nectary, he said, I bet you that there is some organism that has evolved to have a feeding tube that will fit into that flower. He was not wrong. He didn't discover that moth, but he was right in predicting that it existed, and that is just one eloquent example of coevolution, and you're going to see that with these pollinators and the organisms that they're pollinating for, because they rely on each other. Right? This is the food source for those animals, and this is how these angiosperms are going to spread their pollen around. Now, I should make note that not all vectors are animal vectors, you know, there's other stuff like wind, but the, you know, the really cool examples of coevolution come out with those animal pollinators. Alright, that's all I have for this lesson. I'll see you guys next time.