 We all love flowers, don't we? Flowers are beautiful, they are just so appealing to our eyes. And the pictures that you see on the screen, these are from my own flower garden. Every year in the winter, me and my father try our hens on gardening. And these are the pictures of last winter. And if you are someone who loves gardening and loves flowers just like I do, you must have noticed a lot of pollinators in the flower gardens. And the pollinator that I see the most in my garden are the bees. So this is Mr. V doing his regular job. And while doing his own job, that is collecting nectar and collecting honey, he is also doing the pollination thing. That means when he hops from one flower to the other, he transfers pollen. Now we have a whole different video that talks about pollination and all different pollinating agents. I won't discuss about it much in this video, but we are going to talk about two very specific situations. The first one is, what would happen if, let's see this bee, hops on from one species of flower to another species of flower. And in the process, it might leave the pollen of one species to some other species. And the next situation we are going to talk about is what would happen if this bee transfers pollen between the flowers of the same species. So before we even start discussing, how about you take a moment to think about what might happen. Alright, let's begin by talking about the first situation when the pollen from, let's say a flower X falls onto another flower Y. Okay, these two flowers are of different species and this bee, the pollinator in this case, transfers pollen from this flower here to a different species of flower. Let's say this is the pollen, this one here, and it is able to stick to the stigma because this stigma secretes some chemical substances that helps pollen to stick onto it. And those sticky substance on top of stigma also help recognize whether this pollen is from the same species or not. So it's the chemical interaction between the two that does the identification. So if this pollen is identified as the one from the same species, the stigma will allow this pollen to go all the way down to where the egg is or we can say to where the embryo sac lies. And if the stigma through the chemical interaction realizes that it is not of the same species, it will reject this pollen. It is very much similar to how you would react if a stranger is at your door. So will you open the door and allow him in? Probably not, right? But let's say it's your parents and your brother standing outside. You will immediately let them in. Similar is the case with the pollen here. So we can say that if the pollen is from the same species, the stigma will let it in and if the pollen is from a different species, the stigma will not let it in. And in biology, we use the term compatible or not compatible to mean the two. And this compatibility is checked by the chemicals that's present or that is secreted by the stigma. So it basically happens through chemical interaction. And now that we're talking about compatibility, have you heard of something called self-incompatibility? It is seen that sometimes the pollen of the same flower cannot fertilize the egg of its own flower. The pistol will not let the pollen tube to form. And it is seen that 30% to 50% of the angiosperm shows such behavior. Can you think of a reason why? Why would nature adopt such a thing? Well, the reason is very clear. Imagine a plant that is allowed to self-reproduce over and over again and that plant has a faulty gene. It will transfer that faulty gene to its next generations, right? And since they are self-reproducing, there is no chance that a new gene will ever be added into the gene pool that it already has, right? And therefore, we will never see variation. And that do not favor evolution. And that is the reason nature seems to favor self-incompatibility. All right, now let's talk a little more about what happens if a pollen is compatible. We discussed already that it will be allowed to go all the way down to the embryosack somewhere here, but how exactly does things work? And to understand that better, let me enlarge the female reproductive part that is the pistol here. And also for the ease of understanding, I have made just one ovule instead of so many in the ovary. And let us begin by placing a compatible pollen on top of the stigma. And before we even discuss what happens next, let us focus a little bit on the structure of pollen. We have discussed about the structure of pollen in great detail in the video called Microsporogenesis, but there is no harm in having a quick recap. So the outer layer that you see, this violet color layer here, this is called the exyne. And this exyne is made up of sporopollenin, which is the hardest organic substance known. So this exyne or the outer coating provides ultimate protection to the pollen and the content inside. And the yellow color inner lining that you see, this is another layer which is called the intime. And it is similar to the cell wall of any plant cell. And almost 60% of these pollens have two cells in it. One large cell called the vegetative cell and the other one is a small spindle-shaped cell called the generative cell. And in this two-cell stage, the pollen is transferred from the enter to the stigma. But the pollens are also transferred in the three-cell stage. Now what is that three-cell stage we will get to know as we proceed in the video. Alright, now coming back to the pollen that has two cells, when such a pollen falls onto the stigma and the stigma recognizes it to be a compatible one, then it secretes another chemical substance. And the main constituent of this chemical substance is water and sugar. And the pollen absorbs this liquid through the small small pores on the exyne. This pores are called germ pores. And as the water or the liquid enters into the pollen, the intime stretches. But it cannot stretch too much because of the hard layer of exyne outside. And when the pressure inside increases, this intime pops out through one of the germ pores and we call it the pollen tube. And this is what we call the germination of pollen tube onto the stigma. But before the pollen tubes become this long, there are a number of important even that takes place. And the first one is the division of the spindle-shaped generative cell. At this stage, the generative cell mitotically divides to form two male gametes. Now with this division, the pollen which was a two-celled structure became a three-celled one. And this is the three-celled structure that I was talking about. Two male gamete cell and one tube cell. And it is seen that while transferring the pollen from the anther to the stigma, it is usually in the two-celled stage. Almost 60% of the pollens are seen to be in the two-cell stage and they attain this three-celled structure only once they fall on the stigma. But there are also some plants in which the pollen matures to this three-celled stage in the anther itself and it is transferred to the stigma in the three-celled stage. All right, now that we have got our two male gametes which are haploid because the generative cell divided mitotically to give rise to two male gametes and the generative cell was haploid. Now if you are wondering how is the generative cell even become a haploid one, then I would recommend you to go back and watch the video of microsporogenesis where we discussed in detail about the ploidy of each cell. Okay, coming back to our male gamete and pollen tube, the next thing that we will get to see is that our pollen tube will grow even longer and the cytoplasm and the two male gametes will flow along with it. So let me show you how. This is how things will travel all the way down in our pollen tube. It will also have the cytoplasm in here. This vegetative cell nucleus which is also called the tube nucleus along with the cytoplasm helps this tube to grow longer and leads the two male gametes all the way down to the embryosac and this movement of pollen tube is mediated by chemicals that are secreted by the cells of the embryosac and since the movement of pollen tube is chemical mediated therefore this movement is called camotexis. And when the pollen tube becomes long enough it is the filiform structures in the synergies of the embryosac that guides the pollen to exactly the location where the fusion will take place. So to get a clear picture, let me enlarge the ovule and the embryosac. So these two cells that you see are the synergies and this is the egg cell and the pollen tube fuses with one of the synergies. So if we consider this yellow line here to be the pollen tube it will fuse with one of the synergies first and this becomes possible because this filiform structure or the filiform apparatus that we call helps and guides the pollen tube with its secretions to the exact location of fusion. Now as the pollen tube reaches the synergies cells the tube nucleus or the vegetative cell nucleus disintegrates. The main purpose of this nucleus was to elongate the tube cell and as the pollen tube reaches the synergies we no longer require further elongation and therefore the tube nucleus degenerates. So as this tube nucleus is no longer there the pollen tube is left with only two male gametes. Let me draw those male gametes here as well and after this one of the synergies cell it ruptures and it paves way for these two male gametes to enter into this embryosac. So everything that we discussed in this video starting from a compatible pollen lending on to the stigma and this in time making a long pollen tube and allowing the male gametes to come and fuse with this embryosac all these processes together is called pollen-pistol interaction. So in a future video we will see what this male gamete does after entering into this embryosac.