Crash Course: Biology
The sex lives of nonvascular plants
- 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 plants that we see today probably evolved from the single species of algae that nudged itself onshore 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 of this didn't happen overnight, it wasn't until about 475 million years ago that the first plants 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, fuzzy. 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 less beautiful, you probably wouldn't get that much argument from me. But by 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 and 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. (upbeat 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 in 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. The 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, nonvasculars 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 Bryophyta, the liverworts in phylum Hepatophyta, and the hornworts in phylum Anthocerophyta. Taken together, there are over 24,000 species of bryophytes out there, about 15,000 are mosses, 9000 are liverworts, and only about 100 are hornworts. Hornworts and liverworts, funny names, but are named after the shape of their leaf-like structures, horns for the hornworts and livers for the liverworts, with wort stuck on the end there, which just means 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 impostors! 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 just know that something very bryophytic-looking was the first plant to rear its leafy head back in the Ordovician swamps. So now I've got these ultra old-timey nonvascular plants to provide us with some clues as to how plants evolve, and like I mentioned, the most important contribution to the kingdom Plantae and everything that came after them is their wonderfully complex reproductive cycle. See, plants, vascular and nonvascular, have a 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 combined the genetic material from both the parents. That diploid cell divides and divides and divides and divides until voila, the world is one marmot 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 plants, the alternating generations are fundamentally different from each other. And by 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 sperms, which you know are haploid cells that only carry one set of chromosomes, and the bryophyte sperm is actually a lot like human sperm, except that they have two flagella instead of one and they're kinda coily-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 gametophytes, 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 nonvascular plants that we all recognize, the green leafy, livery, or horny parts of the moss, or liverwort, or hornwort are actually gametophytes. Sporophytes are only found tucked inside the females and they're super-small and hard to see. So in the gametophyte generation, individuals are always either male or female. The male makes sperm through mitosis in 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 archegonia. Now these two gametophytes might be hanging out right next to each other, sperm and eggs totally ready to go, but they 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 bryophyte 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. (creaking) The sporophyte grows inside the mother until one day, it cracks open and the sporophyte sends up (slide whistle rises) a long stalk with a little cap on top called the calyptra. 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 protonema, that gives rise to buds, these eventually grow into a patch of moss which is just a colony of haploid gametophytes. That generation will mate and make sporophytes, 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 tissues, things get a little more convoluted. For instance, plants that produce unprotected seeds like conifers or ginkgo 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 difference 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, while 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 in all the stuff that you 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 whether 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 plant who just put their sperm out there hoping to find some lady gametophyte they could call their own, and I think that's kinda sweet.
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