Seed Plants - Online Tutor, Practice Problems & Exam Prep

Seed plants, including angiosperms and gymnosperms, have a dominant sporophyte life cycle and are primarily heterosporous. Gymnosperms produce naked seeds in cones, while angiosperms develop seeds within fruits. Angiosperms are classified into monocots and eudicots based on cotyledon number. Flowers can be complete or incomplete, perfect or imperfect, and can exhibit monoecious or dioecious traits. Pollination can occur through cross-pollination or self-pollination, with angiosperms uniquely undergoing double fertilization, forming both an embryo and endosperm for nourishment.

Seed Plants - 1

Hi. In this video, we're going to talk about seed plants. Now seed plants are made up of angiosperms and gymnosperms, and remember that they're a monophyletic group, meaning that the seed just evolved once and split off, towards angiosperms in one direction and gymnosperms in the other. Like all vascular plants, they have a sporophyte dominant life cycle. But unlike the seedless vascular plants, most of their gametophytes are actually microscopic, so they're even more diminished. And these actually can develop within the sporangia. Now, most seed plants are heterosporous and contain microsporangia that produce pollen and megasporangia that are found inside ovules, and we'll talk about what ovules are in just a second. It's also important to note that most of these plants will have the microsporangia, which is the male part, right, and megasporangia, the female structure, on the same plant. However, some will be found on different plants. You can see here, we have, 2 types of cones, these are, you know, the technical term is strobili, and here we have the male one that is going to house microsporangia, and here we have the female version that is going to house the megasporangia.

So the ovule is the structure that contains the megasporangium and the megaspore, and it's actually what's going to develop into a seed after fertilization. The outer protective layer of the ovule is known as the integument, and it's made of sporophyte tissue. You can see that gymnosperms have this singular integument, whereas angiosperms have multiple integuments. And don't worry so much about memorizing all the facts about gymnosperms and angiosperms right now; we'll be going into much more detail in other chapters. So the opening at the apex of the integument is called the micropyle; you can see them here in these ovules, and this outer wall in the angiosperms is the ovary wall. So, the angiosperms are going to actually have this ovary that is going to cover the ovule, and that's actually what develops into fruit. So, basically, when you're eating fruit, you're eating plant ovaries, and they're delicious.

Now, pollen grains are the male gametophyte covered by a pollen wall. Right? This is made of sporopollenin, and it's a tough coating. It's going to help protect the interior. These are produced by the microspores. The pollen grains are produced by the microspores, and when the pollen travels to the ovule, we have pollination. Pollination syndrome is a term that refers to flower traits that have evolved in response to pollen vectors like wind, bees, and birds. So in essence, pollination syndrome is a group of traits that have evolved, specifically in response to certain pollination vectors, and there are tons of pollination vectors, many more than I've listed here.

Now, seeds, which we can see right here, these are seeds, or this is a seed, these are ovules. Seeds develop from fertilized ovules, right, so this is going to turn into that. They contain the embryonic plant, which you can see right here, I'm going to jump out of the image, so it's easier to see. The embryonic plant, which is this structure right here, and they also have this protective seed coat, which allows seeds to stay dormant but alive for quite some time. Now, the cotyledons are these little embryonic leaves contained in the seed, and they will germinate to form a sprout. You can see they've been labeled here, but our cotyledons are actually these structures right here; I don't know what these arrows are pointing off to. So let me redraw those, the cotyledon arrow, these are the cotyledons. Now, the endosperm is basically the food for the embryonic plant, and it's all the stuff surrounding the embryo. So, if you eat a seed, mostly what you're eating is endosperm. If you've ever broken a peanut in half and noticed the little notch, on the end of it, that is the embryonic plant, and the majority of the nut is the endosperm.

Alright, with that, let's flip the page.

Seed Plants - 2

Gymnosperms produce naked seeds. Basically, that just means that they're not enclosed by an ovary, like angiosperms. Seeds of gymnosperms are usually packed into cones. And as we were talking about earlier, generally, most plants will have the male and female sporangia. Right? The male cone and the female cone on the same plant. We call these monoecious plants. Some plants, however, are dioecious, and you'll actually find the male and female sporangia on different plants. So those male and female cones will be found on different plants. They won't be found on the same individual. Conifers are probably the gymnosperms you're most familiar with. These are like pine trees. And they're basically the group of gymnosperms that have those distinctive needle-like leaves. Right? What, you know, we think of as just pine needles. I mean, there are many different types, but that's a feature that defines the conifers. And also their sporangia are contained in those cones which are technically called strobilae. Now the cones are actually modified leaves, right? They're sporophyll. And the pine needles represent an evolutionary modification of the leaf that has a very thick cuticle. And this helps these plants prevent water loss because, you know, conifers, I mean, there are conifers all over the place but they can live in areas that have restricted access to water. So these pine needles allow, and especially in the wintertime, they live in cold places where it can be very hard for the roots to obtain water due to frost and the freezing temperatures. So these pine needles are an evolutionary trait that allows them to really prevent water loss, very effectively.

Now, let's take a look at the life cycle of a gymnosperm. When the pollen grain reaches the ovule, it's actually going to germinate and form what's called a pollen tube that digests its way through the megasporangium. So here we have our pollen grain. It's gonna get to our ovule and then here, you can see the pollen tube digesting its way through the ovule to the megasporangium. Now the megasporocyte is going to undergo meiosis and produce 4 haploid cells. One of which will become the megaspore. And the megaspore will actually develop into a female gametophyte containing multiple eggs. When the pollen tube reaches the eggs, it's going to eject sperm that will fertilize the egg and then the ovule will become a seed. And after fertilization, of course, we're gonna have the formation of the embryo. And that is our little seed right here. It's resting on one of the scales of the cone. So if you look at a pine cone, it has all these little scales, these little, I mean they're like little diamond-shaped things in the figure but, you know, they're little cone scales. Well, if you were to peel that back and look at it, that's where the seed develops, right? That's where your seed is going to be. Now, the seed, of course, will get carried away and sprout into a sporophyte and that is what grows into these big hulking trees that of course will develop their male and female cones and repeat this cycle over again. Don't worry too much about memorizing all the aspects of the life cycle of gymnosperms. We're gonna cover that in much more detail when we take a deeper look at plants. This is just meant to be sort of a broad strokes overview.

With that, let's flip the page and talk about angiosperms.

Seed Plants - 3

Angiosperms are perhaps best known for their beautiful flowers and their delicious fruit, which surrounds their seeds. They're actually also the most diverse group of seed-producing land plants. Angiosperms are generally classified based on how many cotyledons their seeds have. Monocots have one cotyledon, and eudicots have two cotyledons. You can see a monocot over here and notice how it just has one leafy bit to it, and over here, we have a eudicot. Notice how it has two leafy bits on its sprout. And yes, "leafy bit" is the technical term there. Now, you might be wondering why it's "eudicot," "eu" meaning true, and not just "dicot." The reason is the group actually used to be called dicots and included other species that have since been rearranged into other lineages. Those other major lineages are basal angiosperms and magnoliids. Now, let's take a look at the flower, the defining feature of the angiosperm.

The flower is the reproductive organ of angiosperms and has many parts to its anatomy. You've probably taken note of these if you've looked at flowers before, but you may not have known all the terms for each of the parts. To begin, there's the sepal, which, if you've ever seen a flower bud, is those green leaves that encase the little flower bud, and when the flower bud matures, they look like those green leafy bits underneath the petals. So right here, this part is a sepal. Now, we also have the petals; these are modified leaves that surround the reproductive organs of the flower. Their purpose is often to attract pollinators, which is why they have these beautiful colors and sometimes interesting patterns inside of them.

The stamen is the male part of the flower, the pollen-producing part, and is made up of the filament and the anther. You can see a stamen right here; this is the filament or the stalk. There's also one marked over here; this is our filament. And here we have the anther, that nub on the end. If you've ever had fresh flowers hang around your house for a little too long, all the stamens fall off, the anthers knock pollen everywhere, and it's like a huge mess. The trick is you got to pull them out before the flowers die.

The carpal is the female part of the plant, and some plants have multiple carpals. If a plant has multiple carpals, they are referred to collectively as the pistil. Now, the carpal is also made up of different parts, and you can see, we have the carpal right here. The top of the carpal, let me use a different color, the top of the carpal is referred to as the stigma. The portion that goes from the stigma down to the ovary is called the style. You can see the style is this portion here. The ovary is this lower portion that contains the ovules, which will become the seeds in the fruit. Now over here, we have another ovary, or I guess they're showing the stigma as well. So, technically, this is a full carpal, though the top of this image is a little cut off. So that's why it looks a little funny. But anyway, within the ovule, here is our embryo sac. It's basically the female gametophyte inside the ovule. And you can see here, a pollen tube is extending to it. And, of course, the fruit is formed from the ovary, the ovary of the carpal. Alright. With that, let's turn the page.

Seed Plants - 4

Hello, everyone. In our last lesson, we talked about the anatomical structures of flowers. Now we're going to talk about the different types of flowers that you can find on different types of angiosperms.

Okay, we previously discussed things like sepals, petals, anthers, filaments, and things like that. The structures that you find within a flower. But these structures can vary depending on which type of flower you have. There are complete flowers, incomplete flowers, perfect flowers, monoecious flowers, and dioecious flowers. What we learned about previously were complete flowers. They contain sepals, petals, stamen, and pistils. So, remember that stamen includes the anther and the filament, which are the male structures that create pollen and sperm. And then, pistils include the stigma, style, and ovaries, which are the female components of the flower, creating the egg and eventually the seed or fruit. Now, if a flower is missing any of those four structures, it is considered an incomplete flower. Otherwise, it is a complete flower. Some examples of complete flowers are hibiscus flowers, roses, pea plants, and tulips. There are many more examples, but those are just a couple.

Okay, now let’s talk about the perfect flower. You may think that the perfect flower and the complete flower are probably the same thing. It’s actually not true. They have confusing names. You would think a complete flower would be a perfect flower, but in fact, it's not. A perfect flower is going to be a flower that is bisexual, meaning it has both male and female structures in the same flower, which means it has a stamen and pistils. Now, you may think, oh, well then it’s probably a complete flower. No, because it doesn't have to have sepals and it doesn't have to have petals and it can still be qualified as a perfect flower as long as it's got a stamen and a pistil. So, perfect flowers can be complete flowers, but complete flowers are not always perfect flowers.

Monoecious and dioecious are going to be two other types of flowers. So, monoecious and dioecious, you’ve heard these terms used before, generally when talking about animals and the sexes of animals. Monoecious plants, or monoecious angiosperms are going to have unisexual flowers, meaning that the entire flower is either male or female. But, these male and female flowers are found on the same plant. So what that’s going to look like is if we have a monoecious species of angiosperm, we’re going to have male flowers, and we’re going to have female flowers on the same plant. This is monoecious. And, generally, when we talk about monoecious organisms, we're talking about hermaphroditic organisms, organisms that possess both qualities of both sexes, and these angiosperms do fit that category.

Now, dioecious flowers, or dioecious angiosperms also have unisexual flowers, meaning that they have either male-only flowers or female-only flowers. However, these are found on completely different plants. So, if this species is dioecious, we're going to have one male plant and one female plant. That is a dioecious species of angiosperm. And that’s because dioecious generally refers to two different sexes. Human beings are dioecious. There are individuals that are solely male, and there are individuals that are solely female. The same thing is happening with these plants.

Examples of plants include squash and corn, which have both male and female flowers on the same plant. Some examples of dioecious plants, or dioecious angiosperms, would be willow trees, holly, sassafras, and also poison ivy. These are just some examples of the two different categories.

Now, since we talked about flowers, let's talk about the different forms of pollination that can occur with these flowers. First, we'll discuss cross-pollination. Cross-pollination is where pollen is transferred from one plant in the species to a completely different plant in the species. This is preferred in the reproduction of angiosperm plants because it allows for genetic diversity. It is sexual reproduction. This is how two separate individuals mix their genetics to create a zygote and eventually a seed or a fruit, which will create the new plant.

This is in contrast to self-pollination. If an angiosperm self-pollinates, it is able to take pollen from its own stamen and fertilize its own egg in the ovary. This is not particularly preferred because it does not introduce new genetic diversity; it is essentially cloning. While self-pollination is not preferred, some plants also can perform it if necessary, but cross-pollination is favored for its contribution to genetic diversity.

Now, let's talk about double fertilization. Angiosperms perform a unique process called double fertilization. Usually, you think about fertilization involving sperm fertilizing the egg to create a zygote; that's single fertilization, and that's how a lot of reproduction occurs. Angiosperms, however, do double fertilization. This is where one sperm fertilizes the egg to create a zygote, and another sperm is used to fertilize what's called the polar nuclei, which will become the endosperm. The endosperm serves as the nutritional substance inside the seed that allows the baby plant to grow. You can think of it like the yolk of an egg, providing sustenance to the developing embryo. This unique characteristic of double fertilization is very important for angiosperms.

Okay, everyone. Let's move on to our next topic.

Here’s what students ask on this topic:

What are the main differences between angiosperms and gymnosperms?

Angiosperms and gymnosperms are both seed plants but have distinct differences. Angiosperms, also known as flowering plants, produce seeds enclosed within fruits, which develop from the ovary. They are the most diverse group of seed plants and are classified into monocots and eudicots based on the number of cotyledons. Gymnosperms, on the other hand, produce naked seeds that are not enclosed by an ovary, typically found in cones. Examples include conifers like pine trees. Angiosperms undergo double fertilization, where one sperm fertilizes the egg, forming a zygote, and another sperm fertilizes the polar nuclei, forming the endosperm. Gymnosperms do not exhibit this process.

What is double fertilization in angiosperms?

Double fertilization is a unique process in angiosperms where two sperm cells are involved in fertilization. One sperm cell fertilizes the egg cell, forming a zygote that will develop into the embryo. The other sperm cell fertilizes the polar nuclei in the ovule, forming the endosperm, which provides nutrition to the developing embryo. This process ensures that the embryo has a food source as it grows. Double fertilization is significant because it leads to the formation of both the seed and the endosperm, which is crucial for the seed's development and the plant's reproductive success.

How do monocots and eudicots differ in their seed structure?

Monocots and eudicots are two major groups of angiosperms distinguished by the number of cotyledons in their seeds. Monocots have one cotyledon, while eudicots have two. This difference affects various aspects of their growth and development. Monocots typically have parallel leaf venation, fibrous root systems, and flower parts in multiples of three. Eudicots, on the other hand, usually exhibit net-like leaf venation, a taproot system, and flower parts in multiples of four or five. These structural differences are crucial for identifying and classifying angiosperms in botanical studies.

What are the roles of sepals, petals, stamens, and carpels in a flower?

In a flower, sepals, petals, stamens, and carpels each play specific roles. Sepals are the outermost parts that protect the flower bud before it opens. Petals are often colorful and attract pollinators. Stamens are the male reproductive parts, consisting of the anther and filament; the anther produces pollen. Carpels, also known as pistils, are the female reproductive parts and include the stigma, style, and ovary. The stigma captures pollen, the style is the tube that leads to the ovary, and the ovary contains ovules that develop into seeds after fertilization. Together, these structures facilitate the reproductive process in angiosperms.

What is the significance of pollination syndromes in plants?

Pollination syndromes refer to the set of flower traits that have evolved in response to specific pollinators, such as wind, bees, birds, and other animals. These traits include flower shape, color, scent, and nectar production, which attract and facilitate the transfer of pollen by the pollinator. For example, brightly colored flowers with sweet scents often attract bees, while tubular flowers with abundant nectar attract hummingbirds. Understanding pollination syndromes is significant because it helps explain the co-evolution of plants and their pollinators, ensuring successful reproduction and genetic diversity in plant populations.