Land Plants - Online Tutor, Practice Problems & Exam Prep

Land plants evolved from green algae, adapting to terrestrial life around 850 million years ago. Key adaptations include the cuticle for water retention, stomata for gas exchange, and vascular tissue (xylem and phloem) for nutrient transport. Nonvascular plants, like mosses, are gametophyte dominant, while seedless vascular plants, such as ferns, are sporophyte dominant. Seed plants, including gymnosperms and angiosperms, feature seeds and pollen, allowing reproduction without water. These adaptations enable plants to thrive in diverse environments, showcasing evolutionary success through structural and reproductive innovations.

Land Plants

Hello, everyone. Today, we are going to be talking about the different types of land plants, and how they evolved over time. Alright. So, land plants originally evolved from algae that lived in the water, specifically, green algae because there are different lineages of algae. There's brown algae, red algae, and green algae. Land plants only came from green algae. So, they evolved from these aquatic organisms, and they evolved to live on land, which means they had very specific adaptations to live on land because their environments drastically changed. Now, this change happened around, I believe, 850,000,000 years ago is when plants began to colonize dry land, which is a really long time ago, and they came from these freshwater green algae. And they're gonna have very specific adaptations because, as you can imagine, moving from an aquatic environment to a terrestrial environment has its issues. And some of the adaptations, which you will all learn more about in later lessons, are going to be things like the cuticle, which is a waxy covering of the plants, which helps them retain water because they're no longer living in the water, they're now living in the air. They have the possibility of drying out. They also have things like seeds, vascular tissue, some of them even have pollen, which allows them to reproduce without water, which is very interesting. So, there are many adaptations, which we will learn more about in later lessons. But right now, let's talk about the lineage and the evolution of these land plants. So, land plant, as we normally call them, is a very informal name, which is talking about all of the plants on land. But this is a very informal name, and it's actually very interesting. Land plants also encompass some plants that went back to an aquatic ecosystem, that are now aquatic again. They went from aquatic to terrestrial to aquatic through their evolutionary processes. So land plants, if you wanna be more specific, you're going to call them embryophytes. Embryophytes are a specific type of organism that holds the embryo or the developing offspring inside the tissues of the parent. Don't worry, you'll learn more about that specific quality in later lessons when we talk about plant reproduction. I was just telling you why they have this particular name. Now, there are three types of land plants: nonvascular, seedless vascular, and seed plants. And they did evolve in that particular order. Nonvascular plants came first, then seedless vascular, and then seed plants. The majority of the plants that you think of are gonna be seed plants and they're gonna be the youngest and generally the most specialized plants that live on dry land. But first, let's talk about nonvascular plants. These are gonna be the first plants that actually colonized land, and they're gonna be very similar to the green freshwater algae that they evolved from. So these were the first land plants, and they lacked these particular structures called tracheids. Now, tracheids are going to be a type of plant cell, and tracheids are cells with very thick walls made of lignin. Lignin is like cellulose. It is a very important structural component of most plants, except for nonvascular plants. You'll find that seedless vascular and seed plants have this lignin cell wall component to their anatomy, and this is gonna be very important for structure. This is gonna give them the ability to grow taller and larger. But nonvascular plants, the first land plants didn't have tracheids; they didn't have lignin, so they were not able to grow to great size. So, they could not support large vertical growth. They couldn't grow really tall. Now, you would be knowing this now, you're probably not surprised to hear that nonvascular plants are gonna be things like moss and liverworts, which sound awful, but they're actually not as awful as they sound. So, moss, liverworts, I also believe hornworts are nonvascular plants, and these are gonna be plants that are also called bryophytes, which we'll learn more about in their own lesson. There's a whole lesson on bryophytes. But these are very short plants, very small plants. Moss is kind of like little little tufts of hair or grass on the ground. They don't get very tall. And the reason is, is because they did not evolve tracheids. They're the first land plants, they didn't have this adaptation as of yet. Now, these plants are a gametophyte-dominant life cycle. You will find that the other two types of plants, seedless vascular and seeded plants are not gametophyte-dominant life cycle, they're sporophyte-dominant life cycle. And this means, because they're gametophyte-dominant life cycle, that they are haploid. Most of their life, they only have one set of chromosomes, the majority of their life, their dominant part of their life. Now, they do have some sort of internal water conducting tissue, but they don't have vascular tissue that the larger plants of land today actively have. So they don't have as specialized water and nutrient conducting tissues. So that's also why they're very small. Now, let's go on to the next type of plants. We have the seedless vascular plants. These are going to be the ones that evolved next. Seedless vascular plants are a paraphyletic group, which I'll explain more when we look at the phylogeny down here in just a second. Basically, what that means is seedless vascular plants, that term, that group name encompasses the ancestor of vascular plants, but it doesn't encompass all of the descendants of that ancestor, because the other descendants of that ancestor are gonna be seeded plants and, obviously, they're not seedless vascular plants. They're seeded vascular plants. So, but don't get too much into the specifics. Phylogeny in just a second. Okay. So, what's new about these plants is that they have vascular tissue. That means that they have specialized water conducting and nutrient conducting tissue that acts a lot like our blood vessel does. It is able to transport water and nutrients to the extremities of the plant. This allows the plants to get a lot bigger because they are able to move water and nutrients farther. So, these are also gonna be plants that have a sporophyte-dominant life cycle. Remember that nonvascular are gametophyte-dominant life cycle, seedless vascular plants are sporophyte-dominant life cycle, meaning that most of their life they're gonna be diploid, and they're gonna have two sets of chromosomes in their cells. Also, they evolved lignin like we talked about earlier. And this allows them to have strong vascular networks, and it allows them to support more vertical growth. So these are gonna be larger plants. These are gonna be things like ferns and horsetails are also seedless vascular plants. So, they have vascular conducting tissue. They are able to grow taller because of their lignin, but they don't have seeds as of yet. So, now, let's move on to the seeded vascular plants. So, we just call these seed plants, but these are most of the plants that you think of today whenever you think of a plant. And this is going to encompass the gymnosperms and the angiosperms, two different types of seeded plants. Gymnosperms have naked seeds. These are gonna be things like conifers. Basically, what this means is they don't have an active protection around their seeds, and they don't have any parental part around their seeds. So the opposite of that is gonna be angiosperms. Angiosperms are enclosed seeds. So, you'll...

Land Plants - 2

Hello, everyone. In this lesson, we are going to be talking about the specific adaptations that land plants acquired in their transition from their aquatic environment to their new terrestrial environment. Now, plants had to come up with many complete aquatic environment, to moving to living in a terrestrial environment would be quite dramatically different. So, plants evolved many structural features that allowed them to live on land. Now, what do you think the majority of these new adaptations are going to do? Well, if you go from living in water to not living in water, you're going to find that the majority of these new adaptations that these plants have is going to help them retain water, going to help them not dry out from living in very dry air. So the majority of these adaptations help them retain water. And some of the major adaptations for retaining water are going to be the cuticle, the stomata, and the guard cells. So, the cuticle is going to be the main line of defense against desiccation or drying out. This is going to be a waxy film covering the epidermis, or outer layer of cells, of many plants. You can actually see the cuticle right here. It actually looks like this clear layer of wax. And that's pretty much exactly what it is. It's a clear layer of wax that goes directly over the outer layer of plant cells. It's kind of like a raincoat, But instead of keeping the rain out, it wants to keep the water inside of the tissues of the plant. So the cuticle's main job is to keep water in. So, it keeps the plant cells from losing water. Also, the cuticle is very important for keeping out pathogens, but we'll learn more about that whenever we learn about the immune system that you find inside of plants. All right, so now let's talk about the stomata. Stomata are going to be pores in the tissues of the cells, generally the leaves, that are going to control gas exchange, and they're going to regulate water loss. Stomata are basically pores or holes in the tissues of the plant that are able to open and are able to close. You can actually see a stomata, or a stoma. That is the singular version. Stomata is plural. You can actually see a stoma right here. This is a stoma. This hole in the surface of the leaf. This is a cross-section of a leaf. If you guys were wondering, here's the leaf. Here's the cross-section of the cells of the leaf. And we have a pore in the bottom of the leaf right there, which is a stoma. Now, the stomata are actually able to open and close. And the way that they're able to open and close is because they have these specialized guard cells. And these guard cells are able to change their shape based on their turgidity, which is going to be the amount of water pressure that they have inside of them. And they are able to open and close the stoma. Now, why would a stoma need to open and close? Well, plants need access to the air to do photosynthesis. Photosynthesis requires CO2, and it gets rid of oxygen. So it needs to interact with the gases that are in the air. So the stomata need to open for that particular reason, for gas exchange, for photosynthesis. But because the internal tissues of the plant are interacting with the air, that means that water is actively evaporating out of the leaves, which is bad because we don't want these plants to lose too much water. So, this is why stomata open and close. They open so that the plant can do photosynthesis and exchange gases. But they close so that the plant doesn't lose too much water when it's not doing photosynthesis. In most plants, they're going to close at night when they're not doing photosynthesis because the sun is not out. There are some specialized cases that we will learn about later. Now, I wanted to show you guys how these are actually going to work. So, whenever you have these guard cells, whenever they have low water pressure, so low pressure inside of them, they are going to be closed. So, these are closed guard cells. Meaning that the stomata is closed. So, these guys down here are guard cells. Now, whenever water is actively pumped into these cells they're going to become turgid or have a very high water pressure inside of them. And, they're going to form this, kind of, like, macaroni shape. And this is going to allow the pore to open. So this is going to have high pressure, and this is going to allow the stoma to open. So now we can have these gases, so we can have CO2 entering the plant, and we can have O2 leaving the plant because this stoma is open. And that's high water pressure versus low water pressure. So this is going to be how the stoma actually opens and closes. And, generally, the stomata are going to be found on the bottom of the leaf, as you guys can see right here. So, those are going to be 3 major adaptations for living on land, generally to allow these plants to do gas exchange, but mostly to allow them to retain water. Now, also, we learned that most plants, not all land plants, have vascular tissue. But, vascular plants did, in fact, evolve vascular tissue. Vascular tissue is very, very important because it allows these plants to transport water and nutrients throughout the entire body of the plant. This allows plants to grow quite large because they can transport water and nutrients to their extremities. You can kind of think of this particular system, as an analogy to our blood vessel system. Our blood vessel system actually transports water and nutrients as well to our extremities. Same thing with a plant, except it's a little bit different. They have Vascular Tissue. And, they have one type of Vascular Tissue that is specific for water and one type of Vascular Tissue that is specific for nutrients. And these two types of Vascular Tissue are going to be Xylem and Phloem. Xylem is going to be the transport vascular tissue that transports water and minerals. This is going to be the one that transports for water. Now, wood is very interesting. It is a form of Vascular Tissue, specifically xylem. And, you can look at the Greek word that Xylem came from, so Xylem is the English word. The Greek word is actually Xylon,

Land Plants - 3

Vascular tissue allowed vascular plants to develop roots, which are organs that generally lie below ground. However, there are certain species that have aerial roots. Don't worry about it; there's always an exception in biology. These roots absorb water and nutrients for the plant; they also root it into the ground. So, they allow plants to grow taller because they provide stability to the above-ground portion of the organism. Vascular tissue also allowed for the development of leaves, which are an organ that is specialized for photosynthesis. And actually, we're going to see leaves come in two flavors. There are microphylls, which are small leaves supported by a single strand of vascular tissue. You can see an example of that here; we have a microphyll. In red, this is our vascular tissue. You can see that in this microphyll leaf, there's only this one strand of vascular tissue. Whereas megaphylls, which you can see an example of here, megaphylls, have a much more branched vascular system in the leaf. Of course, I've included a picture of a maple leaf there. You can see the vasculature going through the leaf. I'm not going to draw in all the little lines, but you can see it pretty distinctly in this leaf. This is an example of a megaphyll. And why did I pick a maple leaf? Because I love maple syrup.

Now, seedless vascular plants, again, show that transition from gametophyte dominant to sporophyte dominant life cycles. We've discussed alteration of generations before. All land plants show alteration of generations, and this is, again, just a life cycle in which both the diploid and haploid stages are multicellular. You have your gametophyte, which is that haploid multicellular stage of life that produces the gametes, and the sporophyte, which is the diploid multicellular stage of life responsible for producing spores via meiosis. Right? And a spore, again, is just an asexual, a unit of asexual reproduction. It's usually haploid and usually unicellular. Plants specifically evolved, what are called sporophylls, and of course, since these are modified leaves, this has to be vascular plants we're talking about. These sporophylls bear what are called sporangia, which are basically just enclosed structures in which spores are formed. You can see an example of sporangia here. All of these kind of reddish-brown dots on the backsides of these leaves are sporangia. This is actually a fern leaf. All of these are sporangia.

So let's actually turn the page and take a look at some other adaptations.

Land Plants - 4

Non vascular plants and most seedless vascular plants are homosporous, which means that they only produce one type of spore. Now, some seedless vascular plants and all the seed plants are heterosporous, meaning that they produce 2 distinct types of spores. And those 2 distinct types are microspores, which are made from microsporangia, and these develop into male gametophytes. So these ultimately lead to the production of sperm. And then you have megasporangia, which produce megaspores, and those megaspores will develop into female gametophytes or eggs. Now, in terms of reproduction, non vascular plants and seedless vascular plants have a big disadvantage compared to later plants. And that is that their sperm requires water in order to get to the egg. Not only does it require water, but it basically requires a continuous path of water to get to the egg. So essentially, this means that those organisms can only reproduce when it's wet out and this kind of restricts them to environments where there's enough moisture. The adaptation of pollen, which is the male gametophyte surrounded by a spore and pollen encoding, basically it's just like a tough coating. Allows for the male gametophytes, and ultimately the sperm to exist without water for a much longer period of time. They can be exposed to the air for a long time and furthermore, they can actually take advantage of the air as a medium through which to travel to the egg. So pollen was a huge, important adaptation and it essentially allows the sperm to travel through the air to the egg and not require water to actually fertilize the egg, and we'll talk about the specifics of that later when we talk about seed plants. Now the seed itself, as I said, was a big evolutionary adaptation. And the seed is basically just the embryonic plant, right? The embryo of the plant and a food supply. And this is all surrounded by a tough coating that allows the seed to resist environmental damage and degradation from being digested by an organism, for example. And seeds form from fertilization by pollen. You can see pollen here. All these little yellow dots are bits of pollen. Here we actually have an electron microscope image of pollen. You can see that there's a lot of diversity in form. Pollen can take a lot of different shapes. And here we actually are looking at the inside of seeds. This is a seed cut in half, and let me jump out of the image here. This is our embryonic plant. It's a dicot. You'll learn why later. Don't worry about it now. But if you know why it's a dicot, thumbs up. It's because it has these two things. Again, we'll talk about this later. So don't worry if you don't know what I'm talking about right just yet. So this is our embryonic plant, and this is what's called endosperm. And basically, it's the food supply for the embryo. The embryo is going to use this endosperm for energy to grow and turn into a sprout. And you can't really see it, but there you can just make out this coating around the outside of the seed here. And that's going to be the tough protective shell that will allow the seed to lay dormant for a long time and resist digestion, for example, until the conditions are right for the seed to sprout. So essentially, in a way, the adaptation of pollen, the adaptation of seeds is kind of a similar theme. They allow these facets of reproduction to persist in the environment much longer. Rather than requiring, for example, the presence of water. Now the last major evolutionary adaptation I want to talk about is flowers, and these are the reproductive structures of angiosperms. We're going to talk about them in much more detail when we talk about angiosperms, but the flower is one of the last major evolutionary adaptations of land plants, and it's in part due to the flower that angiosperms saw this explosion of diversification. They, in fact, saw what they saw was an adaptive radiation. So angiosperms actually are one of the most diverse groups of land plants around today, and it's in part because of the evolution of the flower. Alright, that's all I have for this video. I'll see you guys next time.

Here’s what students ask on this topic:

What are the main adaptations that allowed plants to transition from aquatic to terrestrial environments?

The main adaptations that allowed plants to transition from aquatic to terrestrial environments include the development of a cuticle, stomata, and vascular tissue. The cuticle is a waxy layer that covers the epidermis of plants, helping to retain water and prevent desiccation. Stomata are pores that facilitate gas exchange and regulate water loss, controlled by guard cells that open and close based on water pressure. Vascular tissue, comprising xylem and phloem, enables the transport of water, minerals, and nutrients throughout the plant, allowing it to grow larger and survive in diverse environments. These adaptations collectively enabled plants to thrive on land by addressing the challenges of water retention, nutrient transport, and gas exchange.

How do nonvascular plants differ from seedless vascular plants?

Nonvascular plants, such as mosses, liverworts, and hornworts, lack specialized vascular tissue (xylem and phloem) and are generally small and low-growing. They are gametophyte dominant, meaning the haploid stage is the most prominent in their life cycle. In contrast, seedless vascular plants, like ferns and horsetails, possess vascular tissue, allowing them to grow taller and transport water and nutrients more efficiently. These plants are sporophyte dominant, with the diploid stage being the most prominent. Additionally, nonvascular plants require water for reproduction, while seedless vascular plants have adaptations that allow them to reproduce in a wider range of environments.

What is the significance of seeds and pollen in the evolution of land plants?

Seeds and pollen are significant evolutionary adaptations that allowed land plants to reproduce without the need for water. Pollen, which contains the male gametophyte, can be transported by wind or animals, facilitating fertilization over long distances. Seeds, which contain the embryonic plant and a food supply, are encased in a protective coating that allows them to withstand harsh environmental conditions and remain dormant until conditions are favorable for germination. These adaptations enabled plants to colonize a variety of terrestrial environments, leading to greater diversity and ecological success.

What are the differences between gymnosperms and angiosperms?

Gymnosperms and angiosperms are both seed plants but differ in several key aspects. Gymnosperms, such as conifers, produce naked seeds that are not enclosed in a fruit. They typically have needle-like or scale-like leaves and rely on wind for pollination. Angiosperms, or flowering plants, produce seeds enclosed within a fruit, which develops from the ovary of a flower. They have a wide variety of leaf shapes and structures and often rely on animals for pollination. Angiosperms are the most diverse and widespread group of land plants, largely due to their efficient reproductive strategies involving flowers and fruits.

How do xylem and phloem function in vascular plants?

Xylem and phloem are the two main types of vascular tissue in plants, each serving a distinct function. Xylem transports water and minerals from the roots to the rest of the plant. It consists of tracheids and vessel elements, which are specialized cells with thick, lignin-reinforced walls that provide structural support. Phloem, on the other hand, transports sugars, amino acids, and other nutrients produced during photosynthesis from the leaves to other parts of the plant. Phloem is composed of sieve tube elements and companion cells, which work together to distribute nutrients. Together, xylem and phloem enable vascular plants to grow larger and thrive in various environments.