 Okay, so I hope you all did well in the exam and it's good to be able to just focus on one part of the course from now on. I know you've all met with your lab at least once or once now with regard to the botany part of the course and seen some of the organisms that we're going to talk about today and just mentioned briefly last time as well as some we talked about earlier. And this week we're going to focus more on vascular plants, get into the vascular plants which we'll start to talk about in today's lab or today's lecture. So first off I'm going to review just a bit about land plants, some of the things I had to say at the end of last lecture. So we're all on the same page here now that you can focus just on this part of the course. And then we'll eventually get into introducing vascular plants today which are the plants that we're probably most familiar with. So first I just want to briefly recap life cycles because we've seen a number of different types of life cycles in the last week and a half or so. And from now on we're just going to be looking at land plants so it's important to not get their life cycle confused with some of the ones we talked about earlier. Okay so first of all of course humans and also diatoms among some other algae we didn't speak about have a life cycle where we just have one multicellular generation, the diploid generation, you and me. And the only haploid stage then is the gamete so there's no multicellular haploid generation. The fungi and actually also the closest relatives of the land plants have a zygotic life cycle where the only diploid stage is the zygote. And the only multicellular organism in that life cycle has haploid nuclei. And of course in the case of fungi there are also some fungal groups, majority of the diversity in the fungi that have this dichariotic stage but we won't be seeing that again. That's exclusively found in some of the higher fungi like ascomycota, basidiomycota, those two groups where we have two haploid nuclei and the same cellular partition. So that's sort of an anomaly for the fungi and we won't be seeing that any longer. Now in all the land plants and I also mentioned in the sea lettuce, the ova, one of the green algae which has independently evolved this life cycle from the land plants. We have multicellular generations that are both diploid and haploid. So we have this alternation of generations between diploid and haploid multicellular organisms of the same species. And so just to recap it here and it's in the actual cycle format here you can see a multicellular haploid organism which we call the gamete producing plant or gametophyte. So the haploid generation produces the gametes by mitosis because they're already haploid. The gametes then fuse together to form the zygote and then we have mitoses to form a multicellular diploid organism. The zygote's diploid obviously we get a multicellular diploid organism that's always called the sporophyte which is the spore producing plant. Fight just means plant. And then we have mitosis occurring to produce spores. So we go from diploid to haploid here in mitosis and then mitosis to give us another multicellular haploid organism. So it just goes back and forth or goes around and around here between the diploid and the haploid organisms. And the structures where the gametes are born on the gametophyte are called gametangia. Those are just hollow sac-shaped structures that bear gametes. And similarly in the sporophyte the structure that bears the spores where myosis occurs is called the sporangium. So those are really simple clear steps and they're consistent throughout the land plants but there's elaboration in these that you really need to... You need to have this down in terms of the main steps and this alternation between these steps to really understand these subsequent life cycles. So please, as I mentioned earlier, try to embrace this and memorize it as soon as possible and it'll help you a lot. Okay, so now we already talked about how the land plants are a monophyletic group. So the land plants appear to have evolved from all the modern land plants and some land plants that have actually gone back to aquatic situations, of which there are several. Those are a monophyletic group that descend from part of the green algae. So the organisms we were calling green algae earlier are actually perophyletic. That is, some green algae are more closely related to land plants than the other green algae. So this is the base of the tree here. These are the ancestors of this group and the tips here. You can see that the most recent common ancestor here of all the green algae is also shared with the land plants. So the term green plant has come to be used for all of the green organisms, including the land plants, since we know now that the green algae don't constitute a natural group without including the land plants. Okay, and so the closest relatives of the land plants are freshwater green algae, not saltwater green algae. So the colonization of land happened not directly from the ocean or from marine situations to land, but with an intermediary step into freshwater before colonization of land. So that's something that's become well recognized now with a lot of recent evidence, both morphological and molecular. Okay, so these are the seven major groups of land plants that we're going to focus on in this course. There are others, but these are the seven major lineages or clades. And here's the most recent common ancestor of all of the land plants now extinct. And you can see that this, based on molecular and fossil data, we think this is back in the early Paleozoic. So right around the time we have a lot of multicellular life evolving. We have the land plants evolving. And then about 50 million years later, we get the origin of the vascular plants, which are these four major lineages that I'll get to in a moment. But these are the ones that have rigid conducting tissues in their sporophytes that allows them to get to very large sizes and have branching growth for the sporophyte. So the groups like the seed plants, the ferns and relatives, and the lycophytes that I'll mention, which have often been called fern allies, these are all the vascular plants that have the major conducting tissue. They have roots. They have a lot of features that thick cuticles or waxy covering on their plant body that allows them to withstand drought. So they have a lot of features that make them well adapted to land. You can see that there are these other three groups here, the mosses, the hornworts, and the liverworts that I started to introduce last time, which are not a monophyletic group, but a basal grade here within the land plants. So you can see how they branch off in a sequential fashion here. The most recent common ancestor of all the bryophytes is also shared with the vascular plants. And the vascular plants are more closely related to some bryophytes than to others. And the interesting thing here is the bryophytes have long been considered a natural group, but in fact they're actually a grade paraphyletic group. So the features that they share in common that led to them being called a group of plants, the bryophytes, given that they're paraphyletic, we'd infer that those features were found in the common ancestor of all the land plants, including the vascular plants. So understanding the bryophytes really allows us to understand the ancestor of all the land plants, the most recent common ancestor. So that's a really important finding, and this was discovered back in the 1980s based on morphology. Actually Brent Mischler is one of the faculty in IB made that discovery initially, and it's been reinforced with molecular data. The precise branching order of these three major groups is a little bit unclear still, but it's pretty clear that they are paraphyletic, and that the liverworts shown here are the earliest lineage to branch off and are sister to all the other land plants. Are there any questions about that? Okay, so one of the things when we look to the bryophytes that you see right away is they have some interesting features that make them pretty well adapted to land. I mean, they're a very successful group. You find them on all the continents, and they occur in a wide variety of habitats, temperate and tropical. One thing you see is that their spores are resistant to desiccation to some extent. That is, they don't dry out readily. So they can disperse spores, and the spores can travel long distances and withstand some environmental extremes. But one of the coolest things about the bryophytes and something that's only become really appreciated in the last few years, well, it's been known for a long time, but it's been studied more intensively, is that even their main body, the main plant body, the gametophyte in this case, we'll get to the life cycle in a moment, is desiccation tolerant. And so not only can the spores withstand desiccation, but the actual vegetative body of the plant can dry out essentially completely in the bryophytes that at least occur in seasonally dry environments or perineally pretty dry environments. And they look for all practical purposes to be lifeless when they're in that state. You can't detect any metabolic activity. But all you have to do is add water, and they're right back. They rehydrate very, very quickly, and they're photosynthesizing again in no time. So this is something that's been really interesting, and a lot of crop scientists, for example, really wanted to understand this and see if they could get genes from bryophytes into crops, because the vascular plants, as successful as they've been on land, mostly don't have this capability. They have a lot of ways of preventing themselves from drying out. They have great conducting tissue in root systems, thick cuticles, these waxy coverings. But once they actually do dry out, most of them are toast. They're not going to revive. There are some that will, and I'll show you a couple. But this is something that the bryophytes have down. They don't have root systems. They don't have desiccation-resistant leaves, but they survive desiccation or drying out really well. Okay, so the life cycle of bryophytes is interesting. And remember that the closest relatives of the land plants are algae that didn't have an alternation of generations. They just had a haploid organism, and the only diploid stage was a zygote. Now what you see in the bryophytes, and presumably the ancestor of all the modern land plants, is that the dominant generation is the gametophyte, the haploid generation, which makes a lot of sense considering that the closest relatives only had a haploid generation. These have a dominant haploid generation. And the sporophyte generation, which is shown here in this pink part of the life cycle, is a parasite on the gametophyte, on the maternal gametophyte. So the sporophyte is an extremely small and ephemeral, typically pretty short-lived organism that's parasitic on the more often more long-lived gametophyte. And it basically is pretty much an organism that produces spores by meiosis, and it doesn't have a lot more going for it. It's been interjected into this life cycle. I mean, we know that a haploid organism without a diploid generation was ancestral to all the land plants, and we basically had the interjection of a sporophyte generation here that is parasitic on the gametophyte. Okay, so the sporophyte does produce spores, and these spores I mentioned are somewhat resistant to desiccation. They're covered with a substance called sporopollenin that makes them resistant. And it also was present in the zygotes of the closely related green algae, the carophytes. But these are land disperse spores, and they germinate, mitosis occur. We get a filamentous gametophyte initially in the mosses. This is a moss life cycle here. And eventually we get the mature gametophytes forming, which look leafy. And at their apex we have gametangia produced. Remember, these are the sac-like structures that bear the gametes on gametophytes. And in a lot of mosses, like the one represented here, we have unisexual gametophytes. There are two different types of gametophytes, one that produces sperm, one that produces eggs. But some of the mosses have bisexual gametophytes or hermaphroditic gametophytes that produce both types of gametangia. And we call those anthiridia. Singular would be the anthiridium, which produces the sperm. And the archegonia, or the archegonia is singular. That's the type of gametangium that produces an egg. So one of the things that pretty much locks the mosses into at least seasonally wet environments to some extent, if they're going to be sexually reproducing, is that they have free swimming sperm. And the sperm has to leave the anthiridium and swim to the neck of the archegonia and swim down the canal of the archegonia, which is a flask-shaped gametangium, with an opening at the tip and then a long neck, and then at the base it's swollen and there's an egg there. That's a typical shape of an archegonia. And the sperm has to swim down there to fertilize the egg. But not only does the female gametangium produce a gamete, the egg, but it's also where fertilization occurs and where the embryo develops. So it's analogous in a lot of ways to female reproductive tracts and animals, but it's a completely independent evolution of these features. And so we see fertilization occurring here, the zygote down at the base of the archegonium diploid now. And then this undergoes mitosis to form an embryo. Now an embryo in plants basically just refers to an immature sporophyte that's at the stage where there can be a resting stage, and it's elaborated to a greater or lesser extent in different groups of land plants. But they all have embryos, and so all the land plants, that is, the bryophytes plus the vascular plants, are often called embryophytes or embryo-producing plants. And the embryo becomes really important when we get to the seed plants. I'll talk about them shortly. So here's the young sporophyte then developing. And as I mentioned, it's parasitic on the gametophyte, the maternal gametophyte. So it has a foot here, what they call a foot that's actually embedded in the tip of the gametangium, or in the tip of the gametophyte here. And it also has a stalk in mosses, which we call the ceda. And at the tip of it is basically just a sporangium, which the moss specialist, the bryologists, call a capsule. And here you can see the sporangium or the capsule magnified. And here you can see the tip of it magnified using scanning electron microscopy to show this elaborate structure at the tip called a peristome. And the main point here is that the mosses can regulate their spore dispersal by means of this specialized structure that responds to atmospheric humidity such that spores will tend to be released when it's dry out. So it's more easy to disperse spores into the environment to get them away from the parent gametophyte or the parent sporophyte here. If it's dry, if we have a dry climate. And under wet conditions it would be enclosed. So the spores have some regulation of their spore dispersal that's a nice adaptation to land. We don't see this structure in the liverworts or the hornworts, the other two groups of bryophytes. Also some of the mosses have the ability to conduct water and nutrients in their tissues by means of specialized conducting cells that don't appear to be homologous with those invascular plants and they're not strongly reinforced structurally so they don't allow mosses to get very large. The largest bryophytes are typically only about 15 centimeters tall. So under a foot tall, not very big plants, some of the mosses can get up to a couple of meters tall but those are real exceptions. Okay, so just to... so are there questions about the life cycle of bryophytes or some of these? Yeah, they have a similar life cycle. In the liverworts, there are some leafy liverworts that have this kind of leafy appearance. The life cycle is pretty much the same except that most of the liverworts and hornworts don't have any elongated stalk that bears the capsule. So the mosses get their capsule up away from the gametophyte pretty well. And some of the liverworts do actually have a stalk but they basically, when the spores are ready to disperse, the capsule just completely opens up and all the spores are released. They don't have this regulation of spore dispersal. But the general steps of the cycle are the same. The gametophyte is dominant. And the sporophyte, if anything, is even less conspicuous in the liverworts definitely. I'll show you a hornwort sporophyte in a moment. So here are the three major groups of bryophytes just briefly. The mosses, again, worldwide about 9,000 species, really pretty diverse. Probably a lot more species than are recognized because mosses seem to evolve fairly quickly. I mean, I should say their morphology evolves really slowly, but it's clear that they're speculating or diversifying much more rapidly than they're changing in conspicuous ways morphologically. And that's clear from molecular data. So they're probably a more diverse group than we recognize. But they have this green gametophyte. It doesn't look very green in this shot, but trust me, these are photosynthetic, this darker area. Here you can see a gametophyte here that's green. And here's the sporophyte with the long cedar embedded in the tip of the gametophyte. The sporangium or capsule, and there's a little peristome at the tip that regulates spore dispersal. Okay, now the liverworts are about 6,000 species recognized. It's still a pretty diverse group, also found worldwide. And they have a variety of different forms. The ones you most commonly encounter and are easily recognizable as liverworts have these what are called thaloid bodies, which are basically just flattened bodies flat on substrate like soil and can become lobed like this. And as I mentioned, the sporophyte is pretty much embedded usually inside the gametophyte, although sometimes on a stock. But the stock is pretty fragile by comparison with the one that we get in mosses if there is one. And the hornworts are a pretty low diversity group. They're just a couple hundred species of these. And they also have a thaloid body like some of the liverworts that's just basically lying flat on substrate. And here you can see the sporophytes of hornworts, which are photosynthetic. And they actually have stomata, which are pores that regulate gas exchange, which is interesting. And they also have basically this whole elongated horn shaped structure, which is the basis of the name hornwort, is a capsule. It's just an elongated sporangium. It doesn't have a stock. It splits open at maturity. And it's unclear whether the hornworts are the closest relatives of the vascular plants or the mosses. There's some uncertainty about that. But anyways, there's some considerable diversity in the bryophytes. And these three groups are definitely natural groups. And wart, you'll see the name wart used a lot in botany. It's just an archaic term for an herb, basically a non-woody plant. So that's what the basis for wart is. Okay, so those are the three major groups of bryophytes. And I should mention that they began to diversify from a common ancestor prior to the origin of the vascular plants. Remember, they're a paraphyletic group. Alright, so economically and ecologically, the mosses are really important. And although they don't produce food items like vascular plants for us, they are important in many ways. Some of the most conspicuously important mosses, and mosses are where we find a lot of economically important bryophytes, are the peat mosses, members of the genus Fagnum. It's a very distinctive moss group. And these are often found in boggy environments, saturated environments. And they impart acidity, low pH, and certain chemical properties to these bogs due to these phenolic compounds they produce that result in very low rates of decomposition in these bog environments. And so the carbon that's fixed by the peat moss and other photosynthetic organisms in these situations, will tend not to decay rapidly, will tend to be deposited and remain in the soil environment, or remain out of the atmosphere. And so bryophytes are really important for fixing carbon and keeping it out of the atmosphere. And unfortunately now, well, the peat is accumulated over the ages to such an extent, especially in some northern climates where we have cool temperatures and anaerobic conditions. That's typically where we find sphagnum, are in cool environments. That now this is being extensively used as fuel. And here you can see it's one of these industrial peat harvesting operations in the UK. And so a lot of this carbon is being returned back to the atmosphere. But even more disconcerting is that a lot of the peat that's been deposited in soils over the years, a lot of these peat environments that are still intact in northern climates, are starting to dry out with global climate change and rates of respiration are increasing and this carbon is being returned to the atmosphere. So there's a real concern about some positive feedback loops that might result in a lot of carbon returning to the atmosphere from these kinds of environments. In any case, peat moss is also useful economically. It holds water really well and the acidity of it. And as I mentioned these phenolics make it an antimicrobial environment that's good for say wrapping up plant cuttings or roots for shipment. It's also good as a soil conditioner, etc. So there are a lot of uses for peat moss. There are some mosses too that are medicinally important. Minority but in some Asian medicinal traditions there are some useful mosses that are being actively studied. Now another really important thing scientifically about these peat bog environments and boggy environments in general is that there are anaerobic conditions, lack of oxygen in the cool environment. The low pH has not only led to these big carbon deposits but also preservation of fossils and a lot of what we know about the last several thousand years of history from the fossil perspective comes from these kinds of environments. And not only plant remains are preserved in these situations but also animal remains are well preserved including human remains. And in Northern Europe there are these so-called bog people who have been excavated usually inadvertently from bog environments and some of these individuals like this one look like they're sleeping. They show no decomposition. You can make out their facial features perfectly and they're basically just deeply stained by the phenolics. This is a level of preservation for corpses thousands of years old that you can't see in any other conditions, any other situations. There's over a thousand of these people that have been found and you can see the rope around this person's neck. These people almost inadvertently, I mean almost always appear to have been executed and they may have been ritual sacrifices, they may have been executed criminals, it's unclear but in any case there's a lot that can be learned about. These actually date back to the Iron Age so there's a lot of important human remains that have been found in these situations for archaeologists. Okay, so also another thing that Pete Moss used to be really important for, here you can see sphagnum moss for the wounded on the front being harvested up in Canada, British Columbia and Prince Rupert. Pete Moss was extensively collected and shipped to the front during World War I to use a sterile bandaging for soldiers that were involved in trench warfare and these really unsanitary conditions and it saved a lot of lives at a time when antibiotics and a cylinder in particular wasn't available. So that's not such a common use anymore of course but that's an interesting historical footnote. Okay so now we're going to move on to vascular plants which as I mentioned these are the organisms that dominate the terrestrial environment today and that have extensive conducting tissue to conduct both water and nutrients in their plant body. They also have well-developed root systems for conducting water and well-developed cuticles that is waxy coverings on their sporophyte generation. They show a number of important features for survival on land and one of the most interesting fossil findings about ancient vascular plants which are a monophyletic group is that the earliest vascular plants appear very different from our modern vascular plants and these arise about 50 million years after the bryophytes first appear and these things are only about the size of a matchstick. These are really small and they don't have much in the way of conducting tissue. They also don't have any leaves. This artist's rendition is actually inaccurate. They shouldn't show leaves here, they actually aren't there in the fossils. There are no leaves, there are no roots and they have dichotomous branching which means there's no apical dominance of any particular branch. The apex of the chute just splits into two equal branches and you get this Y-shaped branching throughout the plant and the tips of the branches are terminated by these reproductive bodies either by a sporangium in the case of a sporophyte or a gametangium in the case of a gametophyte. In the gametophyte generation and the sporophyte generation the haploid and the diploid generations are very very similar morphologically. So we've gone from a gametophyte dominant generation or I should say gametophyte dominant life cycle in the bryophytes to a life cycle where the gametophyte and the sporophyte share dominance where they're essentially on equal footing as far as the degree of elaboration of these two generations. So that's a very simple vascular plant organization and there are no members of this group left today. They're extinct. Rhineophyte is the name named after the Rhine chute where they were found in the UK but these aren't around anymore so we won't be seeing these. The modern vascular plants include the four groups that I mentioned earlier here. Here you can see vascular plants. The lycophytes which have often been called the fern allies the true ferns and relatives and then the seed plants of which there are gymnisperms and angiosperms. We'll be talking about these a lot. So first it's easiest to introduce you to two of these lineages which we call the free sporing vascular plants and these really should be in quotes because this is not a monophyletic group but includes two monophyletic groups the ferns and relatives and the lycophytes. Lycophytes have often been thought to be very, very closely related to the ferns that's why they're called fern allies commonly but in fact the ferns and relatives are more closely related to the seed plants than they are to the lycophytes and this has been reinforced by many lines of evidence this is a bullet proof finding at this point the lycophytes are sister to all the other vascular plants and the lycophytes these so-called fern allies plus the ferns they both freely disperse their spores into the environment so they don't have seeds these are the seedless vascular plants as opposed to the vascular plants that have seeds the seed plants here well the absence of seeds is a primitive or ancestral feature so it shouldn't be a surprise that these aren't a monophyletic group taken together but these are two well supported clades, these two groups and here they are the seedless vascular plants lycophytes on the right and ferns, horsetails and whisk ferns which are all basically the fern group on the left now the lycophytes which are sister to all the other vascular plants are interesting because there are basically three major groups that are still alive two of them are shown here and you might look at these and say wait a minute he mixed up the slides those are mosses well they look like mosses but these are sporophytes not gametophytes so the dominant generation in all the modern vascular plants including the lycophytes is the sporophyte generation the gametophyte generation is highly reduced so we've gone from only having a gametophyte generation in the case of the freshwater green algae that are closely related to land plants to having a gametophyte dominant alternation of generations with a parasitic sporophyte in the bryophytes to this sharing of dominance in the first vascular plants that are now extinct sharing of dominance between the gametophyte and the sporophyte in all the modern vascular plants the sporophyte is dominant it's the major photosynthetic organism the big branching organism and the gametophyte is highly reduced so in lycophytes here we see another way you can tell these aren't mosses if you look closely you can see that there are actually well it's hard to make out but there are cones on these actual strobolye so in a lot of the vascular plants we have what are called cones or a strobalis or strobolye plural which are basically just a structure made up of modified leaves that are in association with sporangia so it's a structure that bears sporangia and these include in this particular group what are called the spike mosses these are the club mosses these are not mosses though they are vascular plants in this group we have desiccation tolerance in some of them that can dry out completely and revive so some of these early branching lineages of vascular plants do have this ability to desiccate and revive but it's very limited within the vascular plants in general okay so the lycophytes are widespread they're only about 1200 species worldwide it's not a very diverse group but it used to be really diverse and I'll show you some ancient lycophytes in a moment back in the Carboniferous period in the Paleozoic they were really big trees now they're just small plants that don't typically get more than about yea big so it's not a particularly conspicuous group of plants even though they're found from tropical to temperate environments okay the ferns in relatives are much more diverse there are about 12,000 species of these about 10 times more than there are the lycophytes and probably a lot of undescribed diversity too now we single out the horsetails here and the whisk ferns here because it was only fairly recently within the last 10-15 years that it was realized that these are not that these are actually ferns these don't look too fern-like they have aerial stems that are photosynthetic and impregnated with silica, the horsetails these whorled branches with many branches at a node it's very bizarre looking plant they're only about a dozen species worldwide but they're widespread in northern temperate areas in any case this is a group that was thought to represent a completely different lineage but in fact turns out to be a fern group and this thing may look familiar as similar to that earliest vascular plant that I showed you with that Y shaped branching and no leaves and it doesn't have roots either it's one of the few vascular plants that doesn't have roots and it was thought to be one of the last surviving remnants of that earliest group of vascular plants until molecular data came along and showed that it's just a highly modified fern so the guy that actually used to push the idea that this was the last of the rhinoophytes he was actually present when the molecular data were presented showing that the whisk ferns are ferns and not ancient rhinoophytes and he stood up and said he's an old man at that point he stood up and said you just ruined my life to the guy that made that finding so it was a pretty dramatic moment and for the speaker who was just a young graduate student and the guy that said that that he wrecked his life was considered the world authority on seedless vascular plants okay so the life cycle of ferns yeah so basically you can't get too attached to your ideas in science they're always subject to being overturned with additional data so life cycle of ferns is similar to what we saw in the mosses but as I mentioned the haploid generation is relatively reduced and the sporophyte generation is dominant in terms of being the conspicuous and typically much more long-lived organism but here you can see the sporophyte generation in the lower half the haploid generation in the upper half so again meiosis resulting in spores haploid spores and here you can see a typical fern gametophyte and this is usually an extremely tiny organism that you would easily overlook unless you're down on your hands and knees digging around in the soil and oftentimes the fern gametophyte is heart shaped like this that's not always true and they're often photosynthetic some of them are actually found underground and they don't photosynthesize at all they're in association with mycorrhizal fungi that I mentioned earlier they gain their nutrition that way but they're tiny organisms but they do produce gametangia and they're commonly in the case of ferns they're commonly bisexual or hermaphroditic producing both antheridia the male gametangium or archegonia the female gametangia so we have sperm which again in the ferns even though this is a really widely widespread and diverse group of plants they have free swimming sperm the sperm have to swim through the open environment to the neck of the archegonium and swim down the canal down to the egg and fertilize the egg now there are ferns that you find in desert environments and some of these are desiccation tolerant their leaves can dry up in fact that's common in ferns that occur in desert-like environments but in general those desert ferns are strictly asexual in their reproduction this is a really limiting biology for being able to have sexual reproduction in dry environments okay so then we have fertilization down inside the archegonium again like in the mosses we get a development of an embryo again and here's the young sporophyte terminating out of the archegonium and it's actually parasitic on the gametophyte initially but that isn't last long it develops its own root system the gametophyte doesn't have a root system it just has rhizoids these filaments of cells that associate with the upper substrate but are not really particularly important in conduction but the sporophyte does have a good root system as you can see here and the sporophyte of ferns typically has its stem underground as a horizontal stem or a rhizome a rhizome is just a horizontal underground stem and stems produce leaves as well as in this case adventitious roots or roots that form where they normally wouldn't from the side of the stem and the leaves emerge from the ground and the main fern body that we see above ground are the leaves typically there are some tree ferns that produce long stems aerial stems that can get up to several meters in height but they're a minority of the ferns and also there are those horsetails I mentioned which have aerial stems that are pretty bizarre things that were only recently recognized as ferns now the sporangia of ferns are typically but not always but typically on the underside of a leaf or sheltered on the underside of the leaf in these little clusters called sorai singular would be a sorus here's just one of the sporangia associated with each one of these sorai and in ferns there's often a mechanism that allows for forceful ejection of the spores and most ferns have this catapult like ejection system where the sporangium bursts open in a particular way as it dries out and it forcefully catapults the spores the spores are ejected away from the apparent plant so that's a nice dispersal mechanism and here you can see the underside of a fern leaf this is a sword fern which you find out in redwood forests understories typically and here you can see individual sorai these circular little brown structures here's one of those enlarged and you can see each one of them is associated with large numbers of sporangia so this for example is a sporangium not a spore each one of these contains dozens of spores so there are a large number of spores associated with each one of these sporangia it's protected sometimes by a shield like structure here which is an induzium not all ferns have this but it serves to protect the developing sporangia and it withers away when the spores are ready to disperse gets out of the way of the dispersal mechanism so the shape and size and location of these so-called sorai is diagnostic for different types of ferns so this is something that if you want to identify ferns you usually are having to look for their sporangia and how they're arranged on the leaves okay so we're at the point now where it's really important to introduce this concept of heterospery which just means basically other spores where there's more than one type of spore as opposed to homospery where there's only one type of spore and the fern that we just looked at that life cycle was homosperous remember there was just one type of spore that gave rise to one type of gametophyte that bore both types of gametangia it produced both sperm and eggs on the same individual I should mention that ferns have differential timing of sperm production and egg production on the same gametophyte that tends to limit cell fame otherwise there would be potentially a lot of inbreeding but there are a lot of plants that have unisexual gametophytes and I showed you a case of a moss that did that the case of mosses, we don't have really distinctive spore size differences like we see in the vascular plants for example, salaginella is shown here where we have heterospery leading to unisexual gametophytes so some of the lycophytes and a minority of the ferns, namely the water ferns produce two types of spores this is actually a lycophyte cone that's been sliced down the middle longitudinally right through the middle of the sporangia here on one side it's producing micro sporangia with one type of spore, the microspores and on the other side it's producing mega sporangia with mega spores which are larger obviously you can see there's only four products of meiosis in one of these the four products of meiosis are the only spores in one of these sporangia whereas there are large numbers of microspores here so the microspores germinate to become male gametophytes that produce sperm the mega spores germinate to become mega gametophytes, female gametophytes that produce eggs and that's what I've indicated here the home osprey, the presence of one type of spore is ancestral in both the lycophytes and the ferns and heterospery is derived in both groups independently and in both cases what we see is that the gametophytes that develop from these different spores have all of their development occurring inside the spore wall so they're protected inside the spore wall and it's only late in their development that they emerge from the spore wall so there's some additional protection afforded these heterosperous gametophytes in both the ferns and the lycophytes so heterospery may have arisen in part as a means of promoting out crossing because you can't self with yourself obviously if you're only producing one type of gamete but also it may have been important for protection of the gametophyte generation that vulnerable gametophyte generation may be more readily protected in these particular types in this heterosperous life cycle and we'll see this taken to a real extreme in the seed plants and that's why I'm bothering to mention it it might seem like an esoteric aside right now but the seed plants all of them are heterosperous and the evolution of the seed was possible because of heterospery so getting back to the lycophytes and ferns even though today they're fairly low stature plants for the most part if we go back to the time when the seed plants were just beginning to evolve back in the late Paleozoic about 300 to 350 million years ago the lycophytes were absolutely immense here's a six foot tall person for comparison here's a tree lycophyte and here's a horse tail basically a tree horse tail and these things dominated the so-called coal forests of the Carboniferous which are the basis for extensive coal deposits worldwide and this was a time when there was very low topography on Earth for the most part and temperatures at the beginning of this period were very warm and wet and there were boggy environments where fossilization would have been promoted also these tree lycophytes and tree ferns had a lot of lignin in their tissues which is also found in wood but primarily in their bark they didn't have very much wood and this allowed them to fossilize easily because lignin is very difficult to decompose and there may not have even been the appropriate decomposers at this time okay so we'll get into seed plants next time and you'll start to see these in lab this week