D3.1 Plant Reproduction [IB Biology SL/HL]

[0:00]This is D3.1 reproduction and we'll be focusing on standard level content related to how plants reproduce. Not all plants reproduce sexually, but for those that do, there are three distinct parts in that process. So producing gametes,ation and fertilization, and we'll talk about all three of those. Seed dispersal would be something that's happening after fertilization takes place. There are multiple um types of plants that produce sexually. So you could have one flower that has both male and female parts, that's what we'll be drawing later. You could have one plant that has male flowers and then separate female flowers, or some plants either even have separate male and female plants. So we'll be focusing in um our drawings and our discussions on flowers that have both male and female parts just for simplicity, but it's important to know that there are other arrangements for sexually reproducing plants. Here is an example of a flower that has both male and female parts. And we'll discuss the male parts first and I'll try to circle those here in blue. So when we're talking about the male parts, we're talking about the anther and the filament and together that makes up um a structure called the stamen. And it's the anther in particular, this bit right here at the end and you can see there's a few of them on this flower that produces the male gametes. The anther is the site of meiosis. Meiosis is going to produce four haploid cells and each of these haploid cells is called a pollen grain. Each of these haploid cells is going to undergo meiosis, sorry, mitosis and produce three haploid nuclei. So three haploid nuclei, three, three, three, so on and so forth. Of these three haploid nuclei, two of them are going to turn into male gametes and then one is a helper during the fertilization process. So, the point here is that the anther is the site of meiosis and that this is going to produce pollen grains, inside of each grain of pollen, we have two male gametes and another haploid cell that helps in fertilization. Now, let's talk about the female structure. This is called the carpel and it consists of the stigma, the style and the ovary and inside the ovary, there is an ovule and the ovule is the sight of meiosis. Meiosis produces four haploid gametes, but unlike in the male gamete production, here we're going to see an unequal division of the cytoplasm to where three of those haploid gametes do not receive the cytoplasm um or organelles that they need and only one of those will end up developing into a mature ovule. So this one cell that receives more of the cytoplasm, it's going to undergo several rounds of mitosis. So one will turn into two and those two cells will turn into four and then those four cells will turn into eight. So I've got eight cells, seven of them are only going to be helper cells. They will help with fertilization and embryo development, only one of them will develop into the haploid egg. Now, the anther is the site of pollen production and that pollen needs to be transferred to the female organs. So the anther is male, it needs to be transferred to this area of the carpel called the stigma.

[3:41]So that act of transferring pollen from the anther to the stigma is called pollination. This can happen either by wind or by animals, which is what we're seeing here. Um and it's very it's not great to have pollen moving from the anther to the stigma in the same flower. We'll talk about that later, but just pollination is a strict definition, the transfer of pollen from the anther to the stigma. So we've talked about gamete production, pollination and then the last bit is fertilization. It's important to note that pollination and fertilization are not the same thing. They are separate events. So pollination is the transfer of pollen, once it lands on that stigma, the pollen or the fertilization process can ensue. Remember that each pollen grain contains two male gametes and then a helper cell. And this pollen grain is going to build its own tube. It builds a pollen tube that extends into the ovary and towards an ovule where the female gametes are. So that pollen tube will extend into the ovary towards the ovule where the female gametes are. The pollen grain releases those male gametes, they travel through the pollen tube, and then they fuse with the female gametes to create that fertilized um egg or I should say just a zygote, right? We know what that is by now. That zygote will undergo mitosis to create an embryo and that embryo will develop into a seed. So we're going to be drawing an insectted flower and there are lots of iterations of this. We're only going to draw one very general one and it's important to note that it's insectted. So we want to be drawing and including features that are going to attract insects um that can act as pollinators. So first I'll start with the female um reproductive organ and that is the carpel. So the carpel has the stigma, whoops, let me just draw this here. The carpel has the stigma, this long part here, the style and the ovary. Inside of the ovary is a structure called the ovule. Okay, and that's the carpel, the female reproductive organ. The male reproductive organs called the stamen consist of two parts. So we have a filament and we have an anther, and it's the anther that is the site of meiosis. There are usually several, so I'm just going to draw a couple on either side. Again, some flowers might have 400, okay? So there's a large variation in that number and arrangement here. All right, so now that I've got the reproductive organs, I need to think about the parts that are going to help um develop or attract these insects and those are, of course, the petals. So I will draw three petals. I'm going to draw one here, here, and here, and those are the petals of my flower. Those are going to sit on kind of like a receptacle of sorts. And before this flower blooms, it's going to be protected by a structure called the sepal. So I'm going to draw that in here, okay, and then we'll take a look at some pictures another time, but that will be the sepal. So let's label these and annotate with the functions. I'll start with the sepal, so the sepal is this structure here, and this is going to protect the developing flower before it blooms. These large petals are here to um attract pollinators to guide those pollinators in. And the pollinators need to be able to brush up against the pollen. So this pollen is produced at the anther. So this is the sight of meiosis for the male part of the flower. It's producing pollen and we want that to brush up against the insect or whatever is pollinating there and that's really the job of the filament, not the style, the filament. Um is to hold up that anther um so that it positions the anther in a way that the pollinator will come in contact with that pollen. There's a similar kind of function here with the stigma. So the stigma is this top part here, and this uh has the job of capturing the pollen. So we want that to be kind of like sticking out where it's likely that the pollinator um who's coming into the flower brushes up against there and that is the job of the style. So the style is like this long tube, it's going to position that stigma in a way that allows that to happen. It's also where the pollen tube will extend into. Then we're in here to this ovary and the ovary's main job is to hold the ovules. So let's talk about the ovule for this I'll draw a line maybe because I'm running out of room. Um this structure is the ovule and that ovule is the sight of meiosis. So producing the female gametes and that's where the seed would develop if fertilization takes place. And here's what that looks like in reality. So I see these nice petals and I can see these anthers here that are going to brush up against the pollinator and this bee will end up with pollen on its little bee fur. When it goes to another plant or maybe it was already carrying pollen when it came to this plant and it's brushing up against this stigma here. And so um that is very helpful in the transfer of pollen. It's worth noting, um and we'll talk about this later, that flowers that are not insectted but rather windted, aren't going to have petals because they are not trying to attract those pollinators. Now, it is possible that flowers can self-pollinate, right? That pollen from the anther can land on the stigma in the same plant, but that's really not desirable. What is better is something called cross-pollination. So this is the transfer of pollen from the anther of one flower to the stigma of a different flower of the same species, but a different individual. So instead of having pollen go from here to here, we really want it going from here to another plant, so this is much better because we're going to have more genetic variation this way. So if I'm sending male gametes and female gametes from the same individual, not a lot of variation in my offspring here. But um from different individuals that have different genes and different alleles, um this is much better. If I want to promote cross-pollination, then what I really need to do is prevent self-pollination. And there are several strategies there. One is to either have like structures that allow for wind pollination, or attracting animal pollinators to carry the pollen to a different plant. Another option is having separate male or female flowers or even better separate male or female plants. In the case like this, like the one that we drew, that has both female and male parts, one strategy could be that the male parts develop at a different time than the female parts. So that when this male part is releasing its pollen, the female part is either um not mature yet or has already passed maturity and that prevents self-pollination. Despite those mechanisms, anytime you have a flower that has both male and female parts, it's likely that some pollen from the anther will land on the stigma of the same plant. So plants have evolved this mechanism called self-incompatibility, and this is when a pollen grain fails to develop when it reaches the ovary of the same plant, and it works um with these different versions of genes called alleles. So I'll show you what that looks like um as an example. So let's say, and I want to use the female in green. Okay, so let's say that I have a female and she has two alleles for this self-incompatibility gene and she has S1. Okay, so that'll be the same in all of these plants that this female is homozygous for this S1 allele. Now, let's consider three different options for a male. Okay, let's say that the pollen is coming from this same plant. Well, if it's coming from the same plant, then the male will have the same alleles. Self-incompatibility means that the same alleles are not compatible with one another. And if these alleles match, then fertilization will not take place. The pollen tube will fail to develop or the pollen won't release, something will happen. Let's say that we have a male that has one of these genes and a different gene, okay, sorry, not a gene, a different allele. So one of these S1 alleles and then a different allele. This is going to lead to partial compatibility. So in this case, this allele, well, this allele is not compatible, but this one is. So pollen grains, remember there are haploid, pollen grains containing this allele will be able to fertilize the um female gametes down here, but not so much any pollen grains that contain this allele. Okay, so it's partially incompatible, whereas this first one was incompatible like totally. Let's consider a third male that has very different alleles. So this has S2 and let's say S3 alleles. Well, it can't be from the same plant, right? So because the same plant would have these alleles. That means that both of these are going to be compatible with this female. So female gametes can only be fertilized by male gametes that carry different self-incompatibility alleles. And so that's the whole mechanism for preventing um pollen from that plant from fertilizing um any of those female gametes. And the last bit here um in regards to plant reproduction is the dispersal and germination of seeds. So dispersal means spreading those seeds elsewhere. And plant parents do not want their offspring growing up anywhere near them because that would foster competition. So by dispersing those seeds, we can reduce competition between the parent and the offspring. There are different mechanisms for dispersing seeds. Some plants grow fleshy fruits around their seeds, and then they're nice and tempting for animals to eat them. They eat the whole thing, and then they disperse those seeds in their stool elsewhere. Some seeds are covered in hooks and these hooks tend to stick to like animal fur, so then the animals carry them elsewhere on their fur. And some seeds are covered or not covered, but have these almost like wing-like attachments to them so that they can be carried by the wind. So again, the goal is to get those seeds away. I just want to end this video with talking about the fact that pollination, fertilization and seed dispersal are separate processes. And so it's good to have an idea um of the different mechanisms that allow each one of those um to work.