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Lecture Comments (2)

1 answer

Last reply by: Dr Carleen Eaton
Mon Apr 9, 2012 3:38 PM

Post by Ivy Wu on April 5, 2012

Hi, u said ferns are hemosporous, but how come they have antheridum and archegonium to produce eggs and sperms???

Seedless Plants

  • Plants are multicellular eukaryotes and photosynthetic autotrophs. Plants evolved from ancestral green algae and exhibit numerous adaptations to life on land.
  • Plants are divided into two groups: nonvascular (bryophytes) and vascular (tracheophytes). In vascular plants the tube-like xylem and phloem transport water, minerals and nutrients.
  • The vascular plants are divided into the seedless vascular plants and the seed plants. The two groups of seed plants are the gymnosperms and the angiosperms (flowering plants).
  • Bryophytes are non-vascular plants such as mosses and liverworts. They lack true roots, leaves and stems.
  • In bryophytes the gametophyte stage is dominant. The sporophyte generation is dependent on the gametophyte plant.
  • Ferns are an example of a seedless vascular plant. They inhabit moist environments, allowing the flagellated sperm to swim to the archegonia where fertilization takes place.
  • Most seedless plants are homosporous; they produce a single type of spore that develops into a gametophyte capable of producing both male and female gametes.

Seedless Plants

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Origin and Classification of Plants 0:06
    • Origin and Classification of Plants
    • Non-Vascular vs. Vascular Plants
    • Seedless Vascular & Seed Plants
    • Angiosperms & Gymnosperms
  • Alternation of Generations 3:54
    • Alternation of Generations
  • Bryophytes 7:58
    • Overview of Bryrophytes
    • Example: Moss Gametophyte
    • Example: Moss Sporophyte
  • Moss Life Cycle 10:12
    • Moss Life Cycle
  • Seedless Vascular Plants 13:23
    • Vascular Structures: Cell Walls, and Lignin
    • Homosporous
    • Heterosporous
  • Adaptations to Life on land 21:10
    • Adaptation 1: Cell Walls
    • Adaptation 2: Vascular Plants
    • Adaptation 3 : Xylem & Phloem
    • Adaptation 4: Seeds
    • Adaptation 5: Pollen
    • Adaptation 6: Stomata
    • Adaptation 7: Reduced Gametophyte Generation
  • Example 1: Bryophytes 26:39
  • Example 2: Sporangium, Lignin, Gametophyte, and Antheridium 28:34
  • Example 3: Adaptations to Life on Land 29:47
  • Example 4: Life Cycle of Plant 32:06

Transcription: Seedless Plants

Welcome to

We are going to start a series of lectures on plants beginning with the seedless plants.0002

And I'm going to start out by talking about the origin of plants as well as their classification and then, go on to talk about some types of seedless plants.0008

Plants are multicellular eukaryotic organisms, and they are also photosynthetic autotrophs.0018

Plants have cell walls that contain cellulose, and they have many similarities to green algae.0026

And in fact, plants are believed to have evolved from ancestral green algae about 500 million years ago.0032

And as you can see in this classification scheme, green algae would be right down here and from that, branched out various groups of plants.0041

As plants evolved from ancestral green algae, they adapted to live on land, and today, the vast majority of plants are terrestrial species.0058

So, plants are at the bottom of the bottom of the food chain, so they are at the base of the food chain.0069

And therefore, they are essential to the survival of other organisms such as animals that live on land.0073

In addition, plants make the soil more suitable for other life, and plants are home to organisms such as insects and birds.0080

Plants can be classified into two broad categories: vascular and nonvascular.0089

Right here, the bryophytes are nonvascular plants. These three groups up here are all vascular plants.0096

Examples of nonvascular plants are plants like moss, which we will talk about in a minute.0114

Now, what does vascular meant?0119

Well, vascular structures are the xylem and phloem, and vascular structures transport water, mineral and nutrients throughout the plant.0121

Nonvascular plants lack these structures.0134

You may also hear vascular plants referred to as tracheophytes.0138

If you go into the category of vascular plants, you can make further divisions.0149

Another major division are the seedless vascular plants versus the seed plants.0153

Seedless vascular plants such as ferns are also going to be covered today, and then, in later lectures, we will talk about vascular plants that have seeds.0159

So, these are the seed plants.0170

Again, we can break these down into further categories: the gymnosperms and the angiosperms.0172

Gymnosperms are plants such as conifers. Angiosperms are the flowering plants.0178

And there is one final part of the classification scheme that you should be aware of, and that is the division of the angiosperms into monocots and dicots.0188

For now, we will cover this briefly, and the idea is that within a developing seed, there are embryonic leaves called seed leaves.0200

Some types of plants have one seed leaf. These are monocots.0209

Others have two. These are the dicots.0213

And as we talk about plant structure and this in subsequent lectures, I am going to discuss some differences between the monocots and dicots.0216

For right now, though, we are focusing on the seedless plants both nonvascular seedless plants, the bryophytes, and seedless vascular plants such as ferns.0225

We are going to start out with the review of alternation of generations since we are going to be covering life cycles of plants.0235

So recall, we talked about life cycles back during the section on the protists and discussed how some protists exhibit alternation of generations.0241

Plants exhibit this, as well.0250

When we say alternation of generations, what we mean is that there are two multicellular generations within the life cycle.0253

The gametophytes down here and in this bottom half of the figure, these are all haploid stages.0262

So, gametophytes are multicellular and haploid, so multicellular, and in the grey area, they are all haploid.0271

Up here is the second form of the plant, which is multicellular but diploid.0281

Now, if you look at animals, if you look at humans, you could say "well, we have a diploid stage, we are diploid".0289

However, egg and sperm are haploid, so that is a haploid stage, as well. What is the difference?0298

Well, the difference with alternation of generations is that there is a multicellular haploid stage and a multicellular diploid stage.0303

Gametes, sperm, egg are single-celled, whereas here, we have an actual multicellular structure.0310

And that defines alternation of generation.0317

Now, just to go through this generalized life cycle before we talk about specific life cycles of moss and ferns,0321

starting out with the sporophyte, the sporophyte is the dominant generation in most plants.0329

So, when you look around you and see trees and bushes and grass, mostly what you are seeing are the sporophytes.0336

They are the independent dominant structure. However, we are going to talk about moss today in which the gametophyte generation is actually dominant.0345

Here, we have the sporophyte, which is a multicellular diploid plant.0356

And within this structure is called the sporangia. Within the sporophyte are structures called sporangia, and these are the site of spore production.0362

So, meiosis occurs within the sporangia, and that is going to have the chromosome number; so the spores will end up being haploids.0373

Meiosis within the sporangia produces haploid spores.0382

These spores actually germinate and undergo mitosis and grow into a gametophyte, a multicellular structure that is haploid.0386

And most of the time, this is not a structure that you can just readily see.0396

It is protected. It is very small, and it is dependent on the larger sporophyte, but we are going to talk about some exceptions.0400

So, again, spores germinate. They undergo mitosis, development and form what is called a gametophyte.0406

The gametophyte contains archegonium or antheridium or sometimes both on a single gametophyte depending on the plant.0415

And the archegonium is the site of egg production, so eggs are produced within the archegonium. Sperm is produced in the antheridium.0425

So, these structures are already haploid, and mitosis occurs to form gametes.0438

Here is shown a gamete. Let's say that is the egg.0446

And then, a sperm from another gametophyte comes along and fertilizes the egg, so these are both haploid.0449

After fertilization, the result is going to be a diploid zygote.0458

Mitosis occurs, development, to get an embryo, a young sporophyte and then, eventually a mature sporophyte, and the cycle goes again.0465

So, this is a generalized plant life cycle.0475

What we are going to talk about first are the bryophytes, which are nonvascular plants, and these were the earliest plants.0479

The plants that arose from ancestral green algae were nonvascular, and green algae live in aquatic environments and moist environments.0487

And nonvascular plants are actually very closely tied to water, as well.0498

They are much more dependent on being in a moist environment than vascular plants are.0503

And the reason is that without the tube-like structures, xylem and phloem, vascular structures, to transport the water and nutrients throughout the plant,0508

bryophytes have to rely on diffusion, so bryophytes do not have true roots, leaves or stems like the vascular plants.0519

In a vascular plant, they can absorb water through the roots and then, transport it from the xylem throughout the plant.0529

Nonvascular plants cannot do that. They have to be in a moist environment and rely on water diffusing into them.0536

The other thing to remember, and we are going to emphasize this as I talk about the life cycle of moss, is that the gametophyte stage is dominant.0546

And I just said that usually, the sporophyte stage is dominant.0554

You look around. Most of the time what you are seeing is the diploid sporophyte tree, the major structure, but it is the opposite in nonvascular plants.0558

And here is a picture of moss.0569

And right here, what you probably picture when you think of moss is just this mat of moss, low-lying green structures on a rock or on the ground or on a tree.0573

And that is actually the gametophyte part of the plant.0584

Here, what you see sticking up out of the gametophyte is the sporophyte.0591

And the sporophytes in moss and other nonvascular plants are dependent on the gametophytes, so the gametophyte is the dominant generation.0596

The sporophytes, overall, are smaller, and they are dependent on the gametophyte.0607

So, let's take a look at the life cycle of moss as an example of a nonvascular plant.0613

Moss are more primitive plants than vascular plants, and reproduction occurs in water. It requires water.0619

Starting out with the mature sporophyte, the sporophytes were those structures that I pointed out that stick up out of the gametophyte0628

and are dependent on the gametophytes, so these are diploid; and they obtain nutrients from the gametophyte, as well.0638

Within the sporophyte, meiosis occurs, and the result is the release of haploid spores.0647

So, meiosis halves the chromosome number, and haploid spores are released.0655

These spores germinate and undergo mitosis to form gametophyte plants.0660

Gametophytes start out as filamentous structures called protonema, and that is what you see here.0667

And arising out of the protonema are structures called gametophores, and the gametophores are the site of gamete production.0675

This, we have the gametophore, which is the site of gamete production. At the tips of the gametophores are gametangia.0686

I already mentioned antheridium and archegonium.0701

An antheridium is the male gametangia. The archegonium is the female gametangia.0704

Sperm are formed from cells in the antheridium. Eggs are formed from cells in the archegonium.0712

And the sperm cells in moss are actually flagellated. They have to swim to fertilize the eggs.0719

You can see why the moss is dependent on water for reproduction.0726

And when I said they are dependent on water, the moss, obviously, it does not have to be totally submerged under water in a pond or something.0730

But, it does need to be moist. Even a water droplet can be enough to allow the sperm to go ahead and fertilize the egg.0736

And fertilization occurs within the archegonium, so the archegonium is a structure that provides protection for the developing embryo.0745

Sperm swims over, fertilizes the egg within the archegonium, and that fertilization returns the plant to its diploid state.0755

So then, what you have is this diploid zygote.0763

It undergoes mitosis and develops into an embryo, a young sporophyte and then eventually, a mature sporophyte, and the cycle goes around again.0766

Key points here are the facts that the gametophyte generation is dominant and that the moss depends on water to allow fertilization to occur.0778

This was an example of a nonvascular plant, nonvascular seedless plant.0796

What we are going to talk about next are seedless vascular plants using ferns as an example.0804

Bryophytes, nonvascular plants, definitely have some adaptations that allow them to live on land like cell walls.0810

Cell walls, I mentioned, in plants, contain cellulose, and that allows the plant to be stronger.0821

But, as you can see, moss do not grow very tall, and that is because they lack the vascular structures.0828

So, vascular structures do a couple things. The first is they do transport water and nutrients throughout the plant.0835

But, their second function that is very important is support.0841

The reason that trees can grow extremely tall or even that ferns can grow taller than moss is because they contain lignin in their cell walls.0845

And lignin is found in the vascular structures, and this additional strength allows vascular plants to grow much taller than the nonvascular plants.0859

The earliest vascular plants were seedless, and ferns are more primitive and evolved earlier than most plants in existence today.0873

So, they are a really good example of a more primitive plant.0882

And if you think about where ferns live and where they thrive, it is, again, in a moist environment.0886

As I said, they do have vascular structures, so they are a bit more adapted to land than the moss in a way.0891

And they can transport water to various areas of the plant, and they can grow taller.0902

But they are still dependent on a wet environment because they do not have seeds.0909

Without seeds, water, again, is required. Without seeds and pollen, water is required for reproduction.0914

In addition to ferns, just to give you another example, horsetails are also a seedless vascular plant.0925

But, what I am going to focus right now is on the fern life cycle.0932

One thing to note, what you see with seedless vascular plants is that the sporophyte generation is dominant.0936

The sporophyte generation is dominant, and that is in contrast to what we saw with the nonvascular plants.0943

As we move to seed plants, we are going to see the gametophyte generation reduced even further.0951

At this point, the gametophyte generation with ferns is an independent living structure.0957

Whereas, once you get to the seed plants, the gametophyte is much smaller.0965

It is reduced, and it is protected within the sporophyte.0970

Let's start out by looking at the dominant generation, which is the sporophyte, and again, these are diploid plants.0974

So, when you go outside and see a fern, what you are seeing mostly is the diploid sporophytes.0981

Certain leaves on a fern, if you looked on the underside, they have something called sori, and these are clusters of sporangia.0988

So, if you just looked on the underside of a fern, you will see these dots.0997

And these are sori that are located on the underside of a fern; and they are clusters.1003

They are composed of clusters of sporangia.1010

A sporangium would be located in these sori, and within the sporangium, spores are produced.1016

Remember that this is a diploid plant, so meiosis must occur in order for spores to be produced.1024

Most seedless plants are homosporous, so ferns like other seedless plants including the bryophytes are homosporous.1032

This means that they produce only one type of spore, so one type of spore.1045

The result is one type of gametophyte and archegonium and antheridium located on that same gametophyte.1057

This is in contrast to the seed plants.1068

When we discuss the seed plants, you will see that they are heterosporous, meaning that there are two types of spores.1071

And these produce two different types of gametophytes: a male gametophyte that contains antheridium and a female gametophyte that contains archegonium.1081

So, it has male gametophyte that has antheridium. Female gametophyte has archegonium.1111

So, a spore will germinate to produce a structure that produces either sperm or eggs, not both, in most seed plants.1115

However, in seedless plants like what we are looking at right now, these are homosporous. They create one type of spore.1124

That spore germinates into a gametophyte.1130

And the gametophyte, which is a multicellular haploid structure, contains archegonium that are the site of seed production,1134

and antheridium - excuse me - egg production are produced here and antheridium that are the site of sperm production.1146

These mature gametophytes are small independent plants.1158

So, if you go over and look at a fern, and you look on the ground, you will see some low-lying structures that are, sort of, leafy looking.1161

And these are actually the gametophytes of the fern, so they are still independent at this stage.1169

When we move onto the seed plants, you will see that the gametophytes are completely dependent on the sporophyte.1174

Within the archegonium, the eggs are produced. Within the antheridium, sperm is produced.1183

The sperm is released, and as we talked about with moss, the sperm are flagellated.1187

So, they need to swim to the archegonium to fertilize the egg, which results in a diploid zygote.1192

Mitosis occurs, embryo development of a young new sporophyte. Further development results in a mature sporophyte.1201

Now, one thing to note is that if a single gametophyte has both archegonium and antheridium on it,1213

the result could be self-fertilization, which does not result in a lot of genetic diversity.1220

To make sure that different gametophytes cross-fertilize, plants have different techniques.1225

And one of them is that the archegonium and the antheridium often mature at different times.1231

On a particular gametophyte, maybe the eggs will be ready, but the sperm are not.1237

So, that forces fertilization to have to occur from a sperm from another gametophyte.1241

Alright, again, major points here is that although seedless vascular plants like ferns have vascular tissue, they are still very dependent1249

on a moist environment because they have to reproduce on a moist environment since they lack pollen, they lack seeds.1258

And they have to rely on the flagellated sperm swimming over to the egg in an archegonium.1265

Now, I discussed some adaptations of more primitive plants to life on land.1272

As we go on to talk about the seed plants and both flowering and non-flowering seed plants, you are going to see more adaptations.1276

However, right now, I just want to take a moment and summarize adaptations to life on land that plants have from the time when they1284

evolved from their ancestral green algae to the present, when there is a huge array of plants in all kinds of terrestrial environments.1292

The first adaptation is that plants have cell walls with cellulose in them, and this provides them with support.1298

In the water, an organism does not need as much support. They are buoyed up by the water.1310

But, if you take an organism out of the water, it is going to need some, sort of, supporting structure.1314

Additional supporting structure is produced by, in some plants, in vascular plants, lignin in the cell walls.1320

So, in vascular plants, lignin adds further strength to the cell walls.1331

So this is a polymer that is in the cell walls of vascular plants, of vascular structures.1345

Also, in addition to containing lignin, xylem and phloem, the vascular structures,1352

transport nutrients and water to the distant parts of the plant throughout the plant.1362

So, the plant does not have to rely on diffusion to get water.1378

It can get water just from the ground, take it up through the roots and disperse it throughout the plant.1381

When we talk about seed plants, I am going to talk about the structure of seeds and pollen.1388

Seeds and pollen represent additional adaptation to terrestrial environments.1393

Seeds are resistant, so they contain the developing embryo; and they are resistant to drying.1399

The hard covering on the outside of the seed is resistant to drying or desiccation, resistant to drying out.1408

The same thing with pollen, pollen is resistant to drying out, and it can be dispersed without water, so dispersed, for example, by wind.1416

The other way that pollen can be dispersed or one major way is by animals by, for example, attaching to an animal.1428

And they are also resistant to drying out, whereas, in the moss and ferns, the sperm actually had to swim over to the egg.1436

Pollen can be dispersed by wind or by animals. Water is not required for fertilization to occur.1445

There is a material called sporopollenin, and this is a polymer; and it protects the pollen and spores from drying out actually.1453

Let's put here "sporopollenin protects certain structures from drying out".1474

Now, land plants have a lot of means to prevent water loss, drying out, desiccation.1486

And the result, though, is that some of these structures prevent gas exchange from occurring.1494

So, the answer to that is the development of stomata.1500

These are pores on leaves that open to allow gas exchange, but they do it in a way that minimizes water loss.1504

They close up when the plant really does not need to get additional CO2 in or let oxygen out.1518

And they open when gas exchange needs to occur that way water loss is minimized, but gas exchange can still occur.1526

Finally, the reduced gametophyte generation: as we move from nonvascular to vascular seedless to seed plant,1533

you will see a reduction in the gametophyte generation.1552

And the result is that in seed plants, what you see is that the larger sporophyte provides protection and nutrition for the developing eggs.1554

And this, therefore, allows the plant to live where there is not necessarily a lot of moisture around because that gametophyte is protected and is covered.1569

A reduced gametophyte generation is the gametophyte is protected and nurtured by the sporophyte.1581

Again, this summarizes some adaptations to land in both seedless and seed plants.1590

But it gives you an overview for what we are going to talk about later on, so let's go ahead and review what we discussed today.1595

Example one: why are bryophytes small in height?1602

Moss just clings to rocks and the ground. It does not grow very tall.1606

Why is that? Well, this is because bryophytes, nonvascular plants, lack vascular tissues.1612

This means that these plants do not have the support that is provided by lignin-containing cell walls.1622

In addition, bryophytes cannot transport water and nutrients to the distant parts of plants.1644

So, they also cannot transport nutrients throughout the plant.1652

Why are bryophytes restricted to living in a moist environment?1666

So, these are somewhat related. They rely on absorption to attain water, to get water.1672

And the second reason is that the flagellated sperm must swim to fertilize the eggs.1684

They lack pollen, so the flagellated sperm needs to swim over to fertilize the egg; and that is going to require water.1693

And so, this explains why plants like moss are low-lying, why they are small.1705

They cannot grow very tall, and they are restricted to living in very moist environments.1710

Example two: this is a matching question. You need to match the following terms to their descriptions, and the first term is sporangium.1715

So, is this a dominant generation in bryophytes? No.1724

Structure in which haploid spores are formed- well, we see the word "spore" here, so that is a clue; so this one is looking pretty good.1729

A polymer that is found in the cell walls of vascular plants and provides additional strength - that is not correct - or the site of sperm production in plants.1736

The correct answer for no. 1 is B: the structure in which haploid spores are formed is called a sporangium.1747

Two: lignin- the dominant generation in bryophytes- that is not correct.1756

In fact, lignin is a polymer, and it is found in the cell walls of vascular plants and provides additional strength to those cells- C.1760

Gametophytes: the dominant generation in bryophytes- that is correct.1770

The gametophyte generation is dominant in nonvascular plants, whereas, the sporophyte generation is dominant in vascular plants.1773

Finally, antheridium is the site of sperm production in plants.1781

List four adaptations that allow plants to survive on land.1789

Well, there are certainly more than four, but you are only asked to list four so starting with cell walls containing cellulose.1793

The second is vascular structures meaning xylem and phloem.1809

Xylem and phloem contain lignin, and the lignin strengthens the vascular structures and allows the plant to grow much taller.1824

In addition, the vascular structures can transport nutrients throughout the plant.1838

A third development is the reduced gametophyte generation.1843

This reduced gametophyte generation, well, in the vascular plants especially in the seed plants means that the egg is protected.1854

The developing egg is protected within the gametophyte that is within the largest sporophyte that it depends on for nutrition.1865

Seeds: seeds have a coat around them that prevents desiccation, so the development of seeds to protect the developing embryo.1877

Pollen: pollen is protected by sporopollenin, which is also found in spores and prevents it from drying out.1886

The other thing with pollen is that it allows for dispersal by wind or by animals1896

so that the plant is not so dependent on the flagellated sperm having to swim over and fertilize the egg.1903

So, these are some of the developments. I listed five.1914

You only had to list four that allowed plants to survive and actually thrive on land.1918

Finally, example four: describe the general life cycle of the plant.1927

Include the following terms in your description: gametophyte, sporophyte, gamete, spore and zygote.1931

Remember that in just a general life cycle of a plant, there is alternation of generations.1938

There is a haploid gametophyte multicellular generation, and there is a diploid sporophyte multicellular generation.1942

So, let's go ahead and start here with the gametophyte, which is haploid and multicellular, so the haploid or (n) gametophyte.1952

And the gametophyte is going to produce gametes.1962

The gametophyte is going to contain archegonia and antheridia, which they do not ask us to use those words.1966

A male gametophyte would contain antheridium. A female would contain archegonia, and mitosis will occur; and gametes are produced.1973

Sperm and egg are the gametes, and these could be produced by the same gametophyte or by separate gametophytes.1984

Fertilization will occur, so the union of sperm and egg, which is going to produce a zygote.1994

At this point, now, the plant is diploid, so it is diploid. It is 2n.2007

Mitosis will occur. The zygote will develop into an embryo.2017

The embryo, then, will further develop into a young sporophyte and eventually, a mature sporophyte plant.2026

There are structures within the sporophyte called sporangia that are the site of spore production.2034

Within the sporangia, meiosis will occur, which is going to have the chromosome number, and haploid spores will be released.2042

These spores will germinate and eventually grow into a mature gametophyte that can produce gametes continuing the cycle on.2051

This is the generalized life cycle of a plant.2061

That concludes this lecture on seedless plants at Educator.com2066