hi everyone before we get into the details of the seed plants i'd like to look at how seeds actually evolved okay so we were looking at the ferns before and we talked about the spores being produced in ferns and the spore being the haploid cell that divides into a gametophyte by mitosis which produces gametes etc etc when we did that life cycle we only saw one type of spore so we refer to the ferns as being homosporous having one type of spore a few of them are actually heterosporos a few ferns a heterosporous in other words they produce two different kinds of spores and so what i'd like to do now is contrast the homosporus lifecycle which is the masters and most ferns with a heterosporous life cycle and then we'll look at how that led to the evolution of the seed plants so let's first look at a homosporous life cycle i'm just going to draw out what we have seen already many times so remember that we have the sporophyte and the sporophyte is the multicellular diploid stage of the life cycle the sporophyte produces spores and those spores are haploid okay and remember that the spores are produced i'll write a little note on the side here the spoils are produced when spoon mother cells divide by meiosis in sporangia okay so they're produced when the small mother cells divide by meiosis inside the sporangia and we've seen sporangia already right okay so now let's take a look at what happens sorry about this what happens with those spores so they develop into they divide by mitosis remember and they develop into the multicellular haploid stage of the life cycle which is the gametophyte and the gametophyte produces gametes and those gametes of course are produced by mitosis so eggs and sperm and then those eggs and sperm fertilize cross fertilize from different plants to produce a zygote and the zygote divides into the sporophyte and so what you have there is the sporophyte producing one kind of spore that spore produces one kind of gametophyte and that gametophyte produces eggs and sperm but as i said before some ferns and actually all of the seed plants as you can see there are actually heterosporos they've got two types of spores and the thing i want to emphasize is that the life cycle is exactly the same as what we just saw in terms of when does meiosis happen what does meiosis mitosis produce etc um i'll just show you the slight variations so you have your sporophyte stage and instead of producing one kind of spore the sporophyte produces both megaspores and microspores right and so the megaspores are produced in what are called mega sporangia the microspores are produced in microsporangia so the megasporangium would have a megaspore mother cell that divides by meiosis to produce megaspores and so on the megaspore then and so remember spores develop into gametophytes so the megaspore is going to become what we call the mega or female gametophyte okay so it's the excuse me it's the female gametophyte okay and the microspores divide by mitosis into the microgametophyte right the microgametophyte is going to produce sperm and the micro is the male and the mega gametophyte is going to produce eggs and then sperm and egg will fertilize from different plants to produce a zygote and psychic will divide into a new sporophyte generation which is diploid and the mega meter fight and microgametophyte are haploid like all gametophytes okay so when you compare it to the homosporus lifecycle it is the same sequence of events it's just that in the homospores we've got one kind of spore and in the heterosporus we have two different types of spores okay so we're going to look at the heterosporous life cycle in detail in the gymnosperms and the angiosperms but what i'd like to do now is just take a look at how the seed plants evolved and as i say here they evolved when a heterosporous plant retained its megaspores on its sporophyll so i know that's saying a lot that's not that's a lot to digest and there's a term sporophyll there that you haven't seen before so let me explain it we're going to start with a fern that we're familiar with now and i'm going to draw a fern frond actually has a vein going all the way through okay so this is my fern frond excuse my friend okay there it is now remember that the ferns have on their leaves on the underside of their leaves they develop clusters of sporangia so i'm going to say just imagine that this is one sporangium over here and i'm going to actually call this one here a mega sporangium because it's going to produce megaspores the fern leaf itself is it contains the sporangia right so it's holding the sporangia and because it's a leaf did not mean to do that because it's a leaf that holds a sporangium of cephalosporangie we refer to it as a spore leaf or sporophil okay so sporophyll is just a leaf that holds sporangia and in this case because it's specifically megasporangia we'll say that it is a mega sporophyll okay so i say that the seed plants evolved when a heterosporous plant retained its megaspores on its sporophylls so when we looked at the fern life cycle last time and the most life cycle the last time we talked about how the sporangia dry up they open up and the wind will carry those spores out of them what i'm saying here is that seeds evolved when the megaspores were never released from the sporangia the the megaspores were not released from the sporangia okay so i'm going to draw one more diagram over here and what we're going to do is we're going to be looking at a close-up of one mega sporangium okay on a megasporo film and draw a slightly different drawing so now we're going to make our fun quote-unquote fun look a little bit different and i'm going to draw it like this so we're going to call these leaves or modified leaves okay and each of these is going to have on it to megasporangia and here's another one over there okay and each megasporangium is going to have inside it a megaspore mother cell your textbook by the way remember calls for mother cell sporocytes i'm calling them megaspore mother cells and we already know that it is the spore mother cells that divide by meiosis to produce the spores okay so in this case one of these megaspore mother cells so i'm going to draw this structure a little bit closer up okay so this is now one mega sporangium we're specifically going to refer to it as an ovule okay and so the megaspore mother cell divides by myosis and it's going to produce four megaspores but three of them die they start to degenerate they start to get smaller and so i'm going to call this the surviving megaspore okay so that's all taking place right here on the sporophyll which i will label as well so each of these is a megasporophyll and so each one of these has got two of these megasporangia or ovules okay so now just to remind you i'm going to scroll back up here remember megaspores here on the right divide and produce mega gametophytes so this megaspore this surviving megaspore is now going to divide by mitosis it's haploid it's going to divide by mitosis into a mega gametophyte which is going to produce eggs all right so we'll say the next stage is that we have something that looks like this i shall draw it like that okay so i'm just saying that the megaspore produces the mega gametophyte when it divides by mitosis okay and so right here and i'll just switch colors um so that you can see a little bit more clearly we're going to call all of this the mega gametophyte and then this is an egg okay and then remember that the whole structure the mega sporangium the whole structure is called the ovule right now the next stage is that one of the eggs is going to get fertilized by sperm and that sperm we'll talk about how that happens basically the sperm is going to be carried by a pollen grain over to this to each of these ovules and only one of the eggs will successfully get fertilized by a sperm okay so i'll have to fit it in over here so we'll say that this egg is fertilized by a sperm okay and when it's fertilized by that sperm we're going to end up let me just go back to my green ovule okay it's going to become a zygote inside there and then that zygote so we'll save by sperm to produce the zygote and remember zygote becomes the embryo when the embryo divide when the zygote divides by mitosis it becomes an embryo okay and so i'll just show this let's just call this the whole embryo inside okay and so here's the embryo and we'll say that the stuff on the outside of the embryo is food for the embryo and now that we have an embryo inside it now that we have an embryo we call it a seed no longer do we call it an ovule we now call it a seed that seed used to be an ovule okay so basically what you had then was the mega sporophylls which are these leaves holding the megasporangia okay so just like in a fern so they're holding the megasporangia the megaspore mother cell inside devised by meiosis it produces four megaspores only one survives here's the surviving megaspore that surviving megaspore divides by mitosis to produce the mega gametophyte here's the purple megagametophyte inside here with some eggs inside and then that mega the egg one egg gets fertilized by a sperm as i say over here develops into a zygote she develops into an embryo and now you have an embryo with food on the outside and you've got this protective seed coat around it okay that's what a seed is that's why