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Fertilisation, zygote, embryo, germination

Let's explore how fertilisation takes place in flowers. Created by Mahesh Shenoy.

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Video transcript

pollination is the act of transferring the pollen grains on to the stigma great we have transferred pollen grains onto the stigma but what happens next that's basically what we're gonna talk about in this video in this video I'm gonna see exactly how the sperm cells are gonna reach the eggs and eventually have a new baby plant is born so let's zoom in on this female part and see what happens after pollination so here is the pollen grain which has landed on the stigma now a couple of things is first of all there will be a lot of pollen grains and pollen grains are usually much smaller than what I've shown over here but I've shown them big and I've shown one so that we can clearly focus on and see what's going on so the sperm cells are going to be produced inside this pollen grains and the egg cells will be formed inside the ovules over here so we need to get the sperms from here to there but how does it happen it is for this reason the pollen grain starts germinating so what's gonna happen is the pouring rain starts absorbing some of the secretions that are done by the stigma the sticky stigma and then the pollen grain starts germinating and a tube starts growing from this grain so let me show you so tube starts growing from this pollen grain and the tube will eventually the tube will eventually go all the way into the ovary this is the ovary it'll go all the way into the ovary and all the way in to the overview so it makes a pot for the sperms and leads them all the way to the ovule where the egg cells are lying now at first you may be wondering well how does how does the pollen tube know exactly where the ovules are well it turns out that the ovules over here starts secreting some chemicals and the pollen tube basically senses the chemicals and starts growing towards it and we've talked about such motion before we call this the chemi tropism tropism means movement and chemi means movement due to chemicals anyway so don't worry too much about that a pollen grown a pollen tube has now grown and now we will see some sperm cells which I'm drawing over here we draw that over here a couple of sperm cells they will start moving through the pollen tube and they will enter into the Oh view great what's gonna happen next well let's zoom in on this overview and find out so here is the zoomed in version of the row view this is that same pollen tube and this is that of view a single of you'll have zoomed in over here now if you look inside the overview you'll find the egg cell that's where the spawns need to reach but here's the thing the ovule doesn't only contain X cells it also contains some other cells as well in fact if you count it turns out there are one two three four five six and this whole big total seven cells this big cell has two nuclei inside of it all right so it turns out there are total eight nuclei inside this of you but don't worry about it we don't have to worry too much about it right now so let's not worry about these other cells and so now from this pollen tube the sperms are coming in so let me just draw those sperm cells so these are the nuclei of this pumps they will move they'll move and eventually the pollen tube guides them into the overview now what happens well one of the sperm will unite with the excel so one of those pumps unites with DX cells and the other sperm not this one the other sperm cell will go and unite with the two nuclei remember I showed that there are two nuclei over here sharing one entire cytoplasm so that other cell other sperm is going to unite with that now the reason why I'm telling you this even though that's not really important the reason I'm telling you this is because because there are two sperms uniting one uniting with the egg and one uniting with the other cells we say there are two fertilizations happening in this plant and that's why usually in flowering plants we say double fertilization space fertilization is basically the Uniting of the sperm cells with one of these cells okay but anyways we don't have to worry about that other sperm and that other thing so let's get rid of that we will only consider the fusion of the sperm cell with the excel so when that fusion happens we now get a new cell and this new cell which is basically the fusion of the two we give a name to that we call this the zygote zài good so what exactly is a good zygote is the fusion of sperm sperm and the egg great we now have a zygote what happens next well next a lot of changes start happening first of all the pollen tube has done its job so it'll start disintegrating then we will see that the zygote will start dividing from 1 to 2 2 to 4 4 to many so let me show you that so here it is we can now see the zygote is multiplying it's going into dividing into many many many more cells and eventually you will find that the zygote would eventually have divided into a lot many cells to give us this kind of structure this structure is now ready to grow a future baby plant so this now is called let me write that down this structure is now called the embryo so you can think of embryo as the future future plant it's going to grow into the future plant in fact we can actually identify the parts of the embryo if you look at these two leaf like structures they are called the cotyledons let me just down so these two leaf like structures are called the cotyledons and their main job is to provide the embryo with all the nutrition all the food that they need once it starts growing it'll require a lot of food and remember we don't have leaves yet so until the new leaves get formed the cotyledons are the one that's gonna provide them with nourishment when it starts growing eventually we look at that in a second and then you can see this part over here this and important to see them and of course over here I have drawn two cotyledons but not all plants will have two cotyledons some will have two which is called dicots and sometimes we'll have just one cotyledon we call them as mono cause you may have learned about that previously anyways this part as you can see in between the two cotyledons this is the part that eventually in the future will grow into a new chute and that part is called the plume you fumu this will grow into a new chute and finally this part over here let me write that in a different color this part this is something will eventually come out from here and grow into a new root so this is called the radical so the radical grows into a new root the plume mole grows into a new chute and the cotyledons kind of act like leaves they're not really leaves but the a collectives in the sense they give them all the nourishment needed once it starts growing and a lot of other changes happen as well for example the outer wall starts to become a little hard and now we no longer have an overview we are gonna call this as a seed so the fertilized oval has now finally got converted into a seed great but what happens to the flower well let's see so here is the flower here is our flower we are zooming back out now alright here is the flower here is that particular overview we're talking about what are the changes that are happening first of all we will see now a lot of Oh mewls would have gotten fertilized remember I just showed one Poland grain but there are multiple pollen grains mud which are Cana fertilized multiple ovules so all these ovals have now gotten converted into seeds they're now seeds and the unfertilized ones are eventually going to disintegrate what else is gonna happen well we no longer need the stamens so that stamens will start drying off we no longer need the petals so though you will see the petals fall off and we will now find eventually the ovary this already starts swelling and starts becoming bigger and bigger here it is the old starts becoming big and big and eventually it's the Orry that's going to grow into a fruit and there will be seeds inside that particular fruit so over here I've shown a tomato as an example but it can be any fruit with seeds in them then you can eat that fruit and then sow the seeds so maybe you can't do that for tomato but for some other fruit you may be able to sow the seeds so the next question is what happens once you sow the seeds well once you start sowing the seeds you will now see that first root starts developing from there radical radical is the part where the root develops right then we will find the chute starts developing from the plum you remember we just talked about that let me show you that so from the radical the roots are coming from the plumie all the chute is coming and the cotyledons these cotyledons these two things are the quarterly the cotyledons become massive they become this swell up and they almost take up the entire seed these are other cotyledons it's the cotyledons are providing them with all the nourishment needed to grow they are giving them all the food to grow and then as time passes by we will now find that eventually that chute starts growing into new leaves and now the leaves the first leaves can now true leaves we call them can now start photosynthesizing and start and can start making the new food and now no longer cotyledons are needed so the cotyledons eventually get drain they will now start shrinking actually the cotyledons go away they will shrink and finally a new baby plant is born the roots can keep on growing the plant can keep on growing and once the plant becomes mature enough the plants can grow new flowers and the flowers can get pollinated and now the cycle can repeat this process of new plants growing from a seed by the way is what we call germination and so this in short explains how fertilization happens in plants so to quickly summarize the important points after pollination a pollen tube starts growing and it directs the sperm cells all the way into the overview once the sperm cells reach the ovule one of this mom fuses with the excel this process of fusion is what we call fertilization and the new fuel cell is what we call a zygote the zygote can now multiply divide into many cells and eventually forms what we call the embryo the embryo is the future plant the cotyledons of the embryo are going to nourish it once the embryo starts gender once the seed starts germinating the radical is Weathers root grows and the pump loomio is where the shoot starts growing at the same time the flower loses all its other parts their ovaries swells and becomes the fruit and eventually as the plant grows the cotyledons shrink and now the sleeves take care of all the nourishment and the food requirement of the plant