 It's not that hard to wonder about plants and their lives. Every time I step onto my balcony, I wonder. I can't even begin to describe the impact that plants have on us. They are literally responsible for keeping us alive. But what about their lives? How do they grow and reproduce? Do they have a life cycle just like ours? Let's find out. A plant's life cycle consists of two prominent multicellular stages or generations. A haploid generation and a diploid generation. And every plant alternates between these two generations. That means a part of their lives is haploid and then diploid. This is why this phenomenon is called the alternation of generations. The haploid stage or generation where the cells have only one set of chromosomes is called the gametophyte. The gametophyte. And the diploid stage or generation with the cells having two sets of chromosomes is called the sporophyte. Sporophyte. The gametophyte is the stage where haploid gametes are formed by mitosis. That means the gametophyte itself is also haploid in nature. The male gametophyte produces male gametes and the female gametophyte produces female gametes. When these gametes fuse together, they undergo something called fertilization to form a diploid zygote. This zygote undergoes mitotic divisions several and mitotic divisions to grow up to become the diploid sporophyte. This sporophyte will eventually produce spores via mitosis. That means these spores are going to be haploid in nature. Each of these spores then give rise to a new gametophyte via mitosis and the cycle continues. Now generally this is how alternation of generation really works. But the specifics changed as the plants evolved. The simpler more primitive plants ended up exhibiting a dominant gametophyte which was more prominent and long-living. But this dominance reduced like crazy as the plants became more and more complex. Now if we check out the bryophytes which are the simplest plants of the kingdom then we'll see that they show a dominant and independent gametophyte. That means the haploid stage occupies a major portion of their lives. The main plant, the one with the leaf-like structures or the root-like structures or the stem-like structures, this main plant belongs to the gametophyte generation which is this portion right over here in the picture. This is a moss plant by the way if you're wondering. Now this gametophyte generation also has the sex organs, the male and female sex organs which are going to produce the male and female gametes respectively. So we have a gametophyte which is haploid in nature and it's going to give us haploid gametes. When these gametes fuse together they undergo fertilization and we end up with a diploid zygote. This zygote will undergo several mitotic divisions and give us a diploid sporophyte. Now this sporophyte is short-lived, barely noticeable and it is heavily dependent on the gametophyte for pretty much everything. Food, shelter, water, everything. And it gets all of these things by staying attached to the gametophyte. So that means this long stock-like thing that you can see, this is the sporophyte. Once the sporophyte matures, it is going to produce spores via meiosis. That means the chromosome numbers are going to get halved and these spores will end up being haploid. Let's change the color so these spores are haploid. These haploid spores are then released into the environment and they will give rise to new gametophytes via mitosis when the conditions are ideal. And this is how the entire cycle continues. However, this is not the case with more complex plants. In teritophytes, the sporophyte is dominant and independent. That means the main plant that you see, the one with the leaves, roots and stems, that belongs to the diploid sporophyte generation. Now when the sporophyte matures, it's going to produce haploid spores via meiosis. Each spore will then mitotically divide to give rise to a new haploid gametophyte, which is so tiny that it is barely one centimeter in size, barely that tiny. But despite its size, this gametophyte is also independent, just like the sporophyte. It can even gather raw materials like water and minerals from the soil. Using these thread-like structures, I believe you can see it right over here. Let's zoom in a little bit. So do you see these thread-like structures over here? These are called the rhizoids. And using these rhizoids, this gametophyte can easily gather raw ingredients like water and minerals from the soil. The gametophyte also bears the sex organs, which are going to produce the male and female gametes. These gametes are going to fuse, undergo fertilization, and give us a diploid zygote. This zygote will develop into an embryo, which is again also diploid. And this embryo will further differentiate into root stems and leaves and we will end up with our beloved diploid sporophyte. Now, this process basically remains the same in gymnosperms and angiosperms too, except those plants also produce seeds. In the seed plants, the sporophyte continues to remain dominant and independent. That means the huge pine tree that you see, or that plant with the pretty flowers, that's the diploid sporophyte. But unlike the pteridophytes, this sporophyte produces seeds, which keep the embryo protected and nourished inside. When a seed gets the right conditions, it will germinate into a new sporophyte. Then, where's the gametophyte? The gametophyte generation in gymnosperms and angiosperms is extremely reduced, so much so that you will find them inside the sporophyte. In gymnosperms, the gametophyte generation takes place inside these hard structures called cones, which are born on the sporophyte itself, the diploid sporophyte. Inside a cone, haploid spores are formed, which develop into haploid gametophytes. A male cone will have male spores that will develop into the male gametophyte with the male gamete. And a female cone has the female spores, which will develop into the female gametophyte with the female gamete. Now, the female gametophyte with the male gamete is transferred inside the female cone where the male and female gametes will finally come in contact with each other and they will fuse. And that will lead to fertilization. So, the male and the female gametes are haploid, by the way. And they will undergo fertilization and we will end up with a diploid zygote. This zygote will develop into an embryo encased inside of a seed. So, this embryo is also diploid. And you'll find multiple seeds like these with the embryo inside on the scales of the cone. Now, I do have a picture here. So, let's take a closer look. So, do you see the shelf-like places right over here? These are the scales of the cone and inside it you can see several seeds placed. And each of these seeds will germinate into a new sporophyte once the conditions are ideal. And then, the entire cycle will continue all over again. In angiosperms, the gametophyte generation takes place inside the diploid flowers which are born on the diploid sporophyte. So, the flowers are also diploid. And these flowers, they have sex organs which produce haploid spores via myosis. So, this small purple color M means myosis and the capital blue color M means mitosis. So, via myosis, you are going to get the haploid spores which will then develop into haploid gametophytes. So, the male sex organ is going to produce the haploid male spores which will develop into the male gametophyte with the male gamete. And the female sex organ is going to produce female spores, haploid spores. And that, those haploid spores will develop into the female gametophyte with the female gamete via mitosis or via mitotic divisions. Now, when these gametes come in contact with each other, they will fuse together and they will undergo fertilization and we will end up with a diploid zygote. This diploid zygote, just like how it was happening in the gymnosperms, this diploid zygote will develop into the embryo within a seed via several mitotic divisions. And this seed, when it gets the right conditions or the ideal conditions, it will germinate into a new sporophyte and the cycle will continue. Now, if we had to name all of these life cycles, then it would go something like this. All plants have a haplodiplontic life cycle, haplodiplontic life cycle. Now, in this type of life cycle, myosis doesn't directly produce the haploid gametes. In haplodiplontic life cycle or in plants, what we have seen is that the diploid sporophyte produces haploid spores via myosis. Then these spores undergo several mitotic divisions to give us a haploid multicellular gametophyte. This gametophyte will then produce the male and the female gametes. So, this gametophyte right over here, it is multicellular, multicellular and haploid. On the other hand, the sporophyte is multicellular and diploid. And this happens in the seed plants too. But because of their reduced gametophytes, you'll find some scientists and textbooks saying that gymnosperms and angiosperms have a diplontic life cycle. A diplontic life cycle is characterized by unicellular haploid gametes that are formed directly via myosis. And only the diploid stage is multicellular in nature. That means these male and female gametes that I have on this picture over here, they are haploid, definitely haploid. And they're also unicellular, unicellular in nature. And they are formed directly via myosis. Meanwhile, this diploid organism is the one whose multicellular and diploid. So, it's multicellular. And I've already didn't diploid, but we'll also write 2n here. So, these are the characteristic features of a diplontic life cycle. Actually, we have a diplontic life cycle, like us human beings. Our sex organs produce unicellular haploid gametes directly through myosis. And from the zygote onwards, we are multicellular and diploid. But technically, the gymnosperms and angiosperms aren't like that. They don't have this direct gamete production through myosis. Both of them produce haploid spores via myosis that develop into haploid multicellular gametophytes with the help of mitosis. Now, plants aren't the only ones with such life cycles. Another type of life cycle called the haplontic life cycle also exists. But you will find this one mainly in algae. Here, the haploid stage is always multicellular. And only the haploid stage is multicellular. The main organism is haploid in nature and it produces haploid gametes via mitosis. These gametes fuse together to form a diploid zygote, which immediately undergoes myosis and gives us haploid spores. These haploid spores then undergo several mitotic divisions and develop into the haploid organism that we started with. So the diploid zygote is unicellular because it never gets a chance to become multicellular. And meanwhile, the haploid spores, they undergo so many mitotic divisions that they end up becoming multicellular. So the organism is haploid and multicellular. The zygote, which is the diploid phase, is unicellular. In a way, you can say that the haplontic life cycle is kind of like an opposite of the diplontic life cycle. In the diplontic life cycle, the main organism is always diploid and multicellular. And in haplontic life cycle, the main organism is always haploid and multicellular. And that's all about plant life cycles. Every plant with their unique styles of reproducing. Probably that's what makes this kingdom so fascinating.