hi everybody and welcome back today we're going to be looking at plant diversity and i'm going to start off with bryophytes please note that i'll be making a video for each of the plant diversity subsections and i will be doing a refresher in each of the videos on what alternation of generations are in case you didn't watch any of the previous videos so in order for us to have a really good understanding of plant diversity we need to start off with the basics and we need to look at some characteristics about bryophytes before we get into their life cycle so bryophytes are our simplest plants that we are going to be looking at and there's a couple of reasons why they are the simplest these defining qualities would be something like the fact that they do not have any vascular tissue remember vascular tissue refers to things like xylem and phloem it means that these plants can't grow very big at all they also have no cuticle which means that they cannot prevent drying out so often you will find that bryophytes will be dry will be growing in areas that are very moist and damp often in the shade that's often where we find moss growing which is an example of a bryophyte a really a critical one is that they rely on water for reproduction which yet again doesn't actually make them the best terrestrial plant i'm considering that earth is um quite dry and it means that these plants have to be in a space where there is water to reproduce if not they die without any reproduction taking place and last but not least bryophytes have no true roots no true stems or any true leaves i can understand why when we look at them from a distance they seem to have these structures but because they don't have vascular tissue and because they don't have any um specific very um specified tissues and specialized tissues like other plants do they don't actually have what we would call a leaf or a root they have some other structures for example they don't have roots they have rhizoids which we know is a structure that we also find in fungi that essentially anchors the structure or the plant to any kind of wall or brick or any kind of surface next we need to look at alternation of generations and essentially what that means is plants exist within two generations and depending on what they're doing are they reproducing and making more of themselves or are they adults and they're growing and they're living now there's some very important words that we need to be very familiar with before i can go into this explanation i just want to recap and cover them so first of all at this point we should know what the word meiosis means meiosis is a process whereby you take a number of chromosomes let's say for example a cell has 24 chromosomes and you half it and it would become 12 in this instance if we started with 24 that's the process of meiosis and meiosis makes six cells or gametes on the other hand we have a process called mitosis mitosis essentially is cell division it's when you're making more of the same cells so if we're going through mitosis we would start with 24 chromosomes and if we make two more cells we need to make sure that each of those cells also have 24 chromosomes so we don't half anything the easy way to remember the difference is mitosis is for growth meiosis is for reproduction the next two really important words that we need to be familiar with is haploid and diploid when we speak about diploid or dye we're referring to it having two sets of chromosomes and we use the annotation 2n for diploid for haploid or half it means you have half the number of chromosomes and we annotate that as an n so in this diagram everybody that is on the 2nd side is deployed and everyone on the inside is haploid i've just included that now and so that we can keep track of who is haploid and who is diploid so the basics of alternation of generations works like this we are going to start off with our um adult structures and to do so we're going to start down here at the megasporophyte uh and microsporophytes and these represent the male and female the female is the mega sporophyte so i'm just going to draw a female sign there and the microsporophyte is the male side of this um diagram and so what are these these are where we are going to produce our spores whenever you see sporophyte you are going to think of spores now what happens inside of our sporophytes is we are going to take some cells as you can see they've ringed some of these cells and will undergo meiosis now let's not forget what meiosis was that is when i'm going to take a diploid amount of chromosomes and i'm going to half it and essentially what i end up is a spore or in this case we call the omega spores because they're quite big and they are the female counterparts and one of these female megaspores is going to develop and it is going to produce our gametophyte that gametophyte is found inside the archiconum now the archegonium is a structure in the female part of our reproductive systems in plants and that is where we are going to make an egg think of the archegoman as an ovary if that's the easiest way to remember it on the other side on the male side we have the microsporophyte which is going to be our big main structure it's also going to undergo meiosis and produce microspores those microspores are going to develop and they develop inside of the antheridium which is the male counterpart think of it like the equivalent of testes in humans and that's where we will create our sperm cells now that means that in the alternation of generations plants spend two different portions of their life in different generations the main generational difference between the two and this is often the part that we get confused with is that plants will either spend their time as a sporophyte which was the big structure at the bottom here often you imagine a sporophyte as like a mature plant that's what you can see and the other generation that they spend their time in is in their gameto fight period the gametophyte if we cannot remember is where we make gametes and it's often a reduced in size and so what happens is plants go in between these two phases think of them as their adult growing phase and then their baby producing phase and they bounce between the two depending on whether or not they're growing and um getting bigger or are they producing more plants so let's get into the reproductive cycle of our mass plant i have the whole reproductive cycle here but what i'm going to do is i'm just going to zoom in now specifically on the gametophyte generation because that's where we're going to start our journey and so essentially to keep it really simple i'm going to begin our journey with a spore so a spore is released there is one over there and it starts to produce what we call a protonema a protonema essentially is a very underdeveloped very very small um vegetative structure that's going to grow into the bigger plant later on slowly but surely it's going to grow rhizoids and rhizoids are the roots it anchors this whole structure down and you'll notice growing on the top of the protoneeda are emerging gametophytes and we must remember that gametophytes make gametes so they're the ones who are going to start making the eggs and the sperm now in moss which is what is so unusual about moss is that they actually spend the majority of their time as gametophytes so when you often see moss it's in its gametophyte period or its generation and so when you look at moss you actually have two different gametophytes growing next to each other you will have a female gametophyte growing right next to a male gametophyte and remember their gametophytes which means they are haploid they have half the number of chromosomes if we were to zoom in on their reproductive structures we often find these at the tips of their bodies or stalks at the very top so if we zoom in on the male 1 the male one is called the antheridium and inside the antheridium inside these little sectional pieces we are going to produce our sperm cells at the end of the female gametophyte we are going to find the archegonium and in the archegonium is where we find the eggs now remember i said that bryophytes rely on water to reproduce well essentially what happens is maybe rain falls or water washes over the moss plant and essentially it washes our sperm cells over to the female antheridiums and essentially those sperm cells are going to try and um fertilize each one of these egg cells waiting to be fertilized so now let's look at what happens after fertilization and what starts to grow so after fertilization we are going to move into the second part of the generation that we find in our bryophytes and that's what this dividing line is sitting up here it divides between the gametophyte generation which is haploid and the sporophyte generation which is deployed now after fertilization which i want to remind you is when we take one sperm cell which would be n because it's haploid plus an n which is the egg cell and that gives us a diploid number of chromosomes that was the goal remember that's what we want to do we want to bring those two numbers back again towards each other and to create a zygote a zygote is deployed it means it has its full set of chromosomes and that is what's going to grow into our future sporophyte generation so this is all inside of the female gametophyte this is what's happening and as the zygote grows it's going to develop into our sporophyte and you can see it's developing and it's growing here now if we zoom out a little bit so we can see what's happening at the top of the female and it's only at the top of the female gametophytes we will start to see a sporophyte growing out of the top and so what we have here is this long stalk with the capsule at the end now this capsule is really important because that is where we are going to make the spores to start this whole process all over again so if we were to zoom into it that capsule at the end is called the sporangium and inside the sporangium is essentially where we find all of our sporocytes those are the cells that can turn into spores and so what happens is inside our sporangium we are going to take our diploid cells we are going to go through meiosis and we're going to produce haploid spores so essentially we need to half that chromosome number and if we zoom out a little bit so we can see this once more put it into focus here is our meiosis step moving from meiosis there are our spores our spores are released and eventually the spores burst because of the heat and and when it's ripe and ready all the spores land on the ground and we start this whole process again beginning once again with a developing gamito fight so that is the life cycle of moss it alternates between these two generations and it's important to know that moss does this weird thing where the sporophyte grows out of the gametophyte and just for some quick clarity on what i've just said now about one growing out of the other this is a diagram of our moss plant in both cycles or both generations at the same time this is the gametophyte generation sitting at the bottom here is the sporophyte generation on the top and so technically the gametophyte which is in was fertilized by a male gametophyte and now we have produced a sporophyte that grows out of the top and so that's the weird part about it is that normally it's the other way around in plants in other plants but in moss they do this thing where they grow their um their sporophyte out of their gametophyte and so what you actually have here is if i did like a dividing line you have the two generations growing out of one another so to finish off this lesson let's do a quick terminology recap so we briefly spoke about vascular tissue where bryophytes lack this spatial con connecting tissue which is xylem and phloem they also don't have a cuticle which means they cannot prevent drying out and we looked at the alternation of generations where our plants either exist in a sporophyte form or a gametophyte form we found out that bryophytes tend to do these two things at the same time which is what makes them quite unusual we also looked at meiosis which is when you take a chromosomal number and you half it and that's when you make gametes mitosis on the other hand is when you want to maintain your chromosome number and that's just when you're growing and getting bigger we also looked at what a sporangium is a sporangium is the structure where we make our spores then if we looked at the specific reproductive organs of plants they are called the antheridium in males which is where we make sperm the archigona which is where we um find this in females that's where we find the egg cells it's where fertilization takes place and once um we have a fertilization we can produce a spore spores are often released into the wind and they can float on water or in the wind to their desired locations to grow again and lastly we spoke about zygotes a zygote is when you take a sperm cell and an egg cell and you combine the two together and it is the resultant cell once you fuse them together i hope that you've enjoyed this lesson it has been useful and i will see you again soon bye