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Video transcript

plants you're familiar with their work they turn all that carbon dioxide that we don't want into the oxygen that we do want they're all around us and they've been around for a lot longer than animals the blazes that we see today probably evolved from a single species of algae that nudged itself on shore about 1.2 billion years ago and from that one little piece of algae all of the half-million or so species of plants that we have today evolved but of course all this didn't happen overnight it wasn't until about 475 million years ago that the first plant started to evolve and they were very simple didn't have a lot of different tissue types and the descendants of those plants still live among us today they're the nonvascular plants the liverworts the hornworts and everybody's best friend the mosses mmm see now yeah it's clear that these guys are less complicated than an orchid or an oak tree and if you said that they were you know less beautiful you probably wouldn't get that much argument from me but now I think you've learned enough about biology to know that when it comes to the simplest things sometimes they're the craziest of all because they evolved early in the scheme of things they were sort of able to evolve their own set of rules so much like we saw with archaea and protists in bacteria nonvascular plants have some bizarre features and some kooky habits that seem to us like kind of just like what especially when it comes to their sex lives the main thing to know about nonvascular plants is their reproductive cycle which they inherited from algae but perfected to the point where now it is used by all plants in one way or another and there are even traces of it in our own reproductive systems [Music] usually when we're talking about plants we're really talking about vascular plants which have stuff like roots and stems and leaves those roots and stems and leaves are actually tissues that transport water and nutrients from one part of the plant to another as a result vascular plants are able to go all giant sequoia the main defining trait of nonvascular plants is that they don't have specialized conductive tissues since they don't have roots and stems they can't reach down into the soil to get water and nutrients they have to take moisture and directly through their cell walls and move it around from cell to cell through osmosis while they rely on diffusion to transport minerals another thing nonvascular plants have in common is limited growth potential largely because they don't have tissues to move the good stuff around or woody tissue to support more mass a way for them to win is to keep it simple and small so small that when you look at one of these dudes you sometimes might not know what you're looking at and finally nonvascular plants need water for reproduction this is kind of a bummer for them because it means that they can't really survive in dry places like a lot of vascular plants can but I'll get back to that in a minute other than that non vascular's are true plants they're multicellular they have cell walls made of cellulose and they use photosynthesis to make their food all the nonvascular plants are collectively referred to as bryophytes and who knows how many different sorts there used to be back in the olden days but we can currently meet three phyla of bryophytes in person the mosses in phylum bryophyte the liverworts in phylum Hypatia fighter and the hornworts in phylum anthos era fighter taken together there are over 24,000 species of bryophytes out there about 15,000 or mosses 9,000 are liverworts and only about a hundred or hornworts hornworts and liverworts funny names but are named after the shape of their leaf like structures horns for the hornworts and liverworts with warts stuck on the end there which just means of herb and you know what mosses look like though some things that are called moss like Spanish moss in the southern United States and reindeer moss up in the alpine tundra of Alaska are imposters they're actually lichens and lichens aren't even plants the very oldest fossils of plant fragments look really similar to liverworts but nobody really knows which of the bryophytes evolved first and which descended from which we does know that something very bryophyte looking was first plant to rear its leafy head back in the Ordovician swamps so now I've got these ultra old timey nonvascular plans to provide us with some clues as to how plants evolved and like I mentioned the most important contribution to the kingdom plantae and everything that came after them is their wonderfully complex reproductive cycles the plants vascular and nonvascular have way more complicated sexual life cycle than animals do with animals it's pretty much a one step process two haploid gametes one from the mom one from the dad come together to make a diploid cell that combines the genetic material from both the parents that diploid cell divides the divides and divides a divide to do voila the world is one Marmont or grasshopper richer plants on the other hand along with algae and a handful of invertebrate animal species have evolved a cycle in which they take on two different forms over the course of their lives one form giving rise to the other form this type of reproductive cycle is called alternation of generations and it evolved first in algae and many of them still use it today however the difference between algae and plants here is that in algae both generations look pretty much the same while in land plants all land plant the alternating generations are fundamentally different from each other and the fundamental I mean that the two don't even share the same basic reproductive strategy one generation called the gametophyte reproduces sexually by producing gametes eggs and sperm which you know are haploid cells that only carry one set of chromosomes and the bryophytes sperm is actually a lot like human sperm except that they have two flagella instead of one and they're kind of coyly shaped when the sperm and the egg fuse they give rise to the second generation called the sporophyte generation which is asexual the sporophyte itself is diploid so it already has two sets of chromosomes in each cell and it has a little capsule called a sporangium which produces haploid reproductive cells called spores during its life the sporophyte remains attached to its parent gametophyte which it relies on for water and nutrients once its spores disperse and germinate they in turn produce gametophyte which turn around to produce another sporophyte generation and so on weird I know but that's the fun of it life is peculiar and that's what makes it so great this means that the non vascular plans that we all recognize the green leafy livery or horny parts of the mosque our tour hornwort are actually gametophyte s-- sporophytes are only found tucked inside the females in their super small in heart see so when the gametophyte generation individuals are always either male or female the male makes sperm through mitosis and a feature called the antheridia the male reproductive structure while the female gametophyte makes the egg also through mitosis inside the female reproductive structures which are called the archegonium now these two commuter fights might be hanging out right next to each other sperm and eggs totally ready to go but can't do anything until water is introduced to the situation so let's just add a sprinkle of water and take a tour of the bryophytes sex cycle shall we by way of the water the sperm finds its way to the female and then into the egg where the two gametes fuse to create a diploid zygote which divides by mitosis and grows into a sporophyte the sporophyte grows inside the mother until one day it cracks open in the sporophyte sends up a long stock with a little cap on top called the calypso ah this protective case is made out of the remaining piece of the mother gametophyte and under it a capsule forms full of thousands of little diploid spores when the capsule is mature the lid falls off and the spores are exposed to the air if humidity levels are high enough the capsule will let the spores go to meet their fate now if one lands on a basketball court or something it will just die if it doesn't get water but if it lands on moist ground it germinates producing a little filament called the protein nema that gives rise to buds these eventually grow into a patch of moss which is just a colony of haploid gametophyte s' that generation will mate and make sporophyte and the generations will continue their alternation indefinitely now because nonvascular plants are the least complex kind of plants their alternation of generations process is about as simple as it gets but with vascular plants because they have all kinds of specialized issues things get a little more convoluted for instance plants that produce unprotected seeds like conifers or gingko trees are gymnosperms and it's at this level that we start to see pollen which is just a male gamete that can float through the air the pollen thing is taken to the next level with angiosperms or flowering plants which are the most diverse group of land plants and the most recently evolved so the main different between the alternation of generations in vascular and nonvascular plants is that in bryophytes you recognize the gametophyte as being the you know the plant part the moss or the liverwort or whatever well the sporophyte is less recognizable and smaller but as plants get more complicated like with vascular plants the sporophytes become the dominant phase more prominent or recognizable like the flower of an angiosperm for instance is itself actually the sporophyte now I maybe just stuck a spoon and all the stuff that you've learned and stirred it up to confuse you more but we'll get into this more when we talk about the reproduction of vascular plants but what do they have a big showy sporophyte like a flower or a little damp gametophyte like a moss all land plants came from the same tiny little ancient nonvascular plants per mout they're hoping to find some lady gametophyte they could call their own and I think that's kind of sweet
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