Hey guys, we're going to get into a little more detail on the life cycle of the angiosperms. In this video, we'll start with the flowers and take a closer look at variation in flowers and the formation of gametophytes. So here's a generalized version of the angiosperm life cycle.
You can see that we have the adult mature sporophyte, which is the dominant portion of the life cycle. At some point, that sporophyte is going to produce flowers for sexual reproduction. On the flowers will be gametophytes, which produce gametes that will allow for pollination and sperm transfer for fertilization.
After fertilization, we get the diploid zygote, which develops into the diploid embryo in the seed. Around the seed, the fruit matures and allows for dispersal of the seed to develop into a new sporophyte. Flowering occurs on the sporophyte and really is just going to develop from modified leaves. The first thing that we do is we take the leaves. Just like cones are made of sporophylls, flowers are also made of sporophylls, which are simply specialized leaves.
When we say specialization, we're talking about changes in gene expression there. So once the sporophyte is ready, it will change gene expression a little bit in order to grow flowers as opposed to just growing regular leaves. That cue for beginning flowering is controlled by a number of pieces of information.
There will be environmental signals that relate to flowering. Things like temperature and sunlight will be cues that the flower uses to know that it is the right time for flowering. Flowering is also dependent on the developmental stage of the plant. You don't want to flower too early before you have the resources to support reproduction, so flowering will only occur at a particular age, late enough but not too late, where the plant has the resources to be able to reproduce.
Hormones are the ultimate trigger for flowering. So when we say that leaves are going to develop into flowers instead, that's triggered by the release of hormones which will travel through the plant and affect gene expression in order to tell those leaves to develop into flowers instead. Now on the flower we do have four floral organs which are arranged in whorls around the center and we learned last time the names of the organs and this time we're going to add in the names of the whorls.
So the outermost whorl, everything on the outside of the flower, is termed the calyx. Moving inward from the calyx you have the corolla. Moving inward from the corolla, you have the whorl of the andreesium, and then the center-most portion of the flower is the gynecium. Now since we did learn the floral organs previously, see if you can figure out what are the parts that are included in each of these four whorls, and what are their functions? What are the jobs that are done by each of these four rings within the flower?
There is quite a bit of variation in the structure of flowers since they are designed to be attractive to specific pollinators. So they have specific functions and will have specific appearances as a result. For example, many flowers are going to have some of the parts fused together or reduced in size.
So here, for example, you can see the top flower has two petals that are fused together and smaller. two petals that are separate and longer, and that gives it a bilaterally symmetrical appearance. This flower has a reduction in the number of petals so that it has a very simple appearance. This flower has all of the petals fused together into a single layer, so we can take the basic floral parts and tinker with them a little bit to produce different kinds of shapes.
That can go to an extreme and cause one entire whorl to be removed from the flower. These are grass flowers and although they are flowers they are missing one entire whorl of floral organs. So see if you can figure out by looking at the flower which whorl is missing and what does that suggest to you about the functioning of these flowers. So the flowers that have all four whorls are termed perfect flowers.
That means that they are going to develop both male and female parts, both stamens and carpels. So a perfect flower is one that will be a hermaphrodite. It has both male and female parts on it. They don't necessarily have to appear at the same time, so sometimes the carpels could appear earlier and the stamens could appear later or vice versa.
but if a single flower produces both stamens and carpels, that is termed a perfect flower. An imperfect flower will be only one sex, so it will produce either a stamen or carpels, but never both. If you have imperfect flowers, you essentially have male and female flowers. Those male and female flowers can be on a single plant, which is termed monoecious, Or they could be on entirely separate plants, which is termed dioecious. The ecious in these words means house.
This is the same root as economics, so mono-ecious means one house. Males and females live in the same house. The male and female flower are on the same plant.
A dioecious plant is one where males and females live in different houses, so they are on entirely different plants. In other words, not only do you have male and female flowers, you actually have separate male and female plants. So you can have perfect flowers where one flower is both male and female.
You can have imperfect flowers where the flowers are either male or female. You can have a single plant with both male and female flowers, in which case it is monoecious, or separate male and female plants, in which case it is dioecious. We also see variation in the symmetry of the flower. So some flowers are radially symmetrical and pretty much look the same from every angle. Other flowers show clear bilateral symmetry, where one side of the flower is quite different in shape or possibly color from the other side of the flower, so that there is a clear bilateral plane of symmetry.
Overall, within the angiosperms, we see a general evolutionary trend towards increased bilateral symmetry. If you think about the early angiosperms that I talked about last time, things like the water lilies and the magnolias, those are radially symmetrical flowers that are pretty simple and open in shape. As we go through angiosperm evolution, we see increasingly specialized flowers with increasing bilateral symmetry.
Bilateral symmetry has probably evolved many, many times. within the angiosperms. If that is the case, that suggests that there is a major benefit of having bilaterally symmetrical flowers. So what do you think that might be?
In what ways would a bilaterally symmetrical flower potentially be better than a radially symmetrical flower? Think about the job of the flower and what bilateral symmetry does in the shape of the flower. You can also have a variation in the ways in which the flowers are attached to the stem.
A lot of flowers are just one simple flower by itself, so these are examples of flowers where it is one flower attached to the stem at one spot. But a lot of plants may also produce inflorescences. An inflorescence is where you have many flowers tightly grouped together to give the appearance of one flower. So when you look at a grass flower, for example, and you see this nice feathery plume, our eyes see that as kind of one flower, but in fact it's a very large number of really tiny flowers.
So inflorescences are usually made up of many very small flowers which together give the appearance of one larger flower. This does include things like daisies and sunflowers. And looking at the sunflower, this looks to your eye like a single flower with a number of petals around it.
But if you cut it open, you can see what's really going on here, which is that in fact, the sunflower is an inflorescence made of many very small flowers. There are two kinds of flowers in the inflorescence. There are the ones in the center, which are called disc flowers, and those ones actually don't produce petals at all.
and then there are the ones around the edge called ray flowers, each of which produces one petal. So every petal that you see on the sunflower actually comes from a separate different flower, but all of the flowers work together to produce a single kind of unified appearance, and that's how an inflorescence works. Since an inflorescence contains multiple flowers, it will end up developing into a multiple fruit. such as the pineapple that we talked about last time. Now the whole point of the flower is to produce spores which will develop into gametophytes.
So I want to take a slightly more detailed look at the formation of the gametophytes on the flower. On the female side we have the carpal. At the base of the carpal is the ovary.
The ovary may have several chambers to it. So this is a cross-section slice through the ovary. And in this particular ovary of a lily flower, you can see it has three chambers to it.
Within those chambers are ovules. Inside the ovule is megasporangium. The megasporangium will contain diploid megasporocytes, which undergo meiosis to give you four haploid megaspores, only one of which will survive.
The other three will degenerate. The surviving megaspore will go on to develop into a haploid multicellular gametophyte. In order to do that, the megaspore will develop through mitosis, and it's going to do a total of three rounds of mitosis.
If the megaspore does one round of mitosis, that gives you two cells. If it does a second round of mitosis, that gives you four cells. A third round of mitosis would give you eight cells, but we're going to not do cytokinesis in one of them.
So we will get seven cells, one of which contains two nuclei. Three rounds of mitosis, eight nuclei, but one cell doesn't do cytokinesis, so only seven cells. This eight-nucleate, seven-celled structure is the megagametophyte.
which in the angiosperms is also termed the embryo sac. So if you count up the number of cells in here, you'll see there is one big one in the middle. That's the one that didn't do cytokinesis, so it has two nuclei in it, which are termed the polar nuclei, and then there's three little cells on top and three little cells on the bottom, so a total of seven cells.
Out of these cells, the important ones for you to know are that big central cell with the two polar nuclei in it, and then the small ones are the small cells. and the egg cell. So our megagaminophyte is seven cells, technically multicellular, but just barely, and it's only done three rounds of mitosis to develop from the megaspore.
And that will eventually produce an egg. On the male side, we have stamens with the anther at the top. If you take a cross-section through the anther, you can find inside the microsporangia.
Inside the microsporangium are microsporocytes. which are diploid cells that undergo meiosis to give you four haploid microspores. All four of those microspores will go on to survive for the male side.
The microspores are coated in sporopilenin because they are going to disperse. The microspores will develop into a haploid multicellular microgametophyte, but they're only going to do one round of mitosis. You do one round of mitosis, you get two cells. so the microgametophyte is only two cells. Those two cells will be the generative cell, which is going to produce sperm, and the tube cell, which is going to produce the pollen tube.
The entire microgametophyte is still retained within the sporopalenin coat of the microspore, and that will be the pollen grain. So pollen is the microgametophyte containing two cells with the sporopalenin coat left over from the microgametophyte. the microspore.
So here's what they look like a little bit closer up. Here's the megagametophyte inside the ovule or embryo sac, and inside the megagametophyte we have eight nuclei, seven cells. A couple at the top that are not too important. One big one in the center containing two haploid polar nuclei, and three little ones at the bottom, the important one of which is the egg. There are also two other smaller cells surrounding the egg which are called synergids.
It is thought that these guide the pollen tube into the ovule to help deliver sperm to the egg. But again the cells you need to worry about are the egg itself and then this big central cell with the two polar nuclei in it. On the male side we have the microgametophyte or pollen grain. Notice the sporopalenin wall around the outside which is going to reduce desiccation of the microgametophyte.
And inside we have two cells, the tube cell which will grow the pollen tube, and the generative cell which will produce sperm to eventually fertilize the egg. At this point, once the gametophytes have formed, we are ready for pollination to enable fertilization. We'll take a closer look at pollination in the next video.