AP Biology Gymnosperms and Angiosperms

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We are continuing our discussion of plants with the seed plants.0002

In previous lectures, I talked about seedless plants.0006

These include the bryophytes such as moss, which are nonvascular, as well as seedless vascular plants such as ferns.0010

Today, we are going to go on and talk about the two groups of seed plants.0018

Seed plants have adaptations that further allow them to adjust to life on land.0024

Remember when we talked about the seedless plants,0031

I discussed adaptations that were made from the ancestral green algae to the early plants such as the bryophytes.0034

As evolution proceeded, more adaptations occurred, which allowed plants to thrive on land.0044

For this reason, the seed plants are the most common type of plants on earth today.0052

Starting out with seeds and pollen, seeds and pollen are resistant to drying because they contain sporopollenin.0059

Both seeds and pollen have sporopollenin in their walls, which makes them resistant to desiccation.0068

In addition, pollen allows the dispersal of sperm without water.0074

Recall that when we talked about moss, I said that sperm are flagellated, and therefore, they have to swim to fertilize the egg within the archegonium.0079

And that tightly ties the seedless plants to having to live in a moist environment.0088

Recall that moss and ferns are found in very moist environments.0094

In addition, the seed is a very complex structure. We will talk about the structure later on today.0100

And it provides not only protection for the developing embryo but also nutrition for the embryo.0105

And there are two groups of seed plants: the gymnosperms, and the gymnosperms are primarily the conifers.0112

There are some groups of gymnosperms that are not conifers. However, most of them are conifers, and they are also the angiosperms.0122

The angiosperms are the flowering plants.0130

Seed plants are heterosporous. Recall that when we talked about ferns, I mentioned that ferns are homosporous as well as other seedless plants.0136

Homosporous means that the plant produces one type of spore.0148

And that type of spore develops into a single type of gametophyte, which produces both male and female gametes.0151

In contrast, the seed plants are heterosporous meaning that they produce two different types of spores: megaspores and microspores.0159

So, within the megasporangia are produced megaspores.0169

From these megaspores develop the female gametophyte, and within the female gametophyte are the eggs.0184

Microsporangia are the other type of spores, so microsporangia are the site of production via miosis of microspores.0200

These develop into pollen grains, and within the pollen grain is the male gametophyte and the sperm nuclei, which fertilize eggs.0214

Another adaptation that seed plants have to terrestrial environments is the reduction of the gametophyte generation.0232

Recall that in moss, the gametophyte generation is dominant.0238

If you go and see a bunch of moss growing on the ground, you will see this big mat of moss, and that is actually the gametophyte.0242

That is the dominant generation in moss, and in fact, the sporophytes are just small structures that project out from the gametophyte.0249

And those sporophytes are completely dependent on the gametophyte in moss.0258

In ferns, the gametophyte generation is reduced, and the sporophyte generation is dominant; but the gametophytes are still small independent plants.0262

Here, now, we see the gametophyte is just extremely reduced. The sporophyte is completely dominant.0271

And the gametophyte is just very small microscopic structures within the larger sporophyte and dependent on the sporophyte for nutrition and protection.0277

So, when you go outside and see the fir trees and bushes and vegetables and the vast majority of the plants you see,0286

which are seed plants, what you are seeing is the sporophyte.0297

And the gametophyte, you would have to really open the plant up, look for - get on a microscope - very, very small structure.0301

This is another way in which seed plants have adapted to life on land because it allows the sporophyte to protect the developing gametes.0308

We are going to start out by talking about gymnosperms. Gymnosperms were the earliest seed plants.0319

Flower plants developed more recently in evolution, and gymnosperms are primarily composed of the conifers such as firs, pine trees, spruces, red woods.0326

However, not all gymnosperms are conifers.0339

In fact, there is a couple of few other small phyla such as the Ginkgos and the cycads, which are also gymnosperms.0343

Cycads look somewhat like palms, and they were actually very common in the age of the dinosaur. Though, there is not many species of cycads left today.0351

Conifers often live in dry environments, and the needles of conifers are actually, they are leaves.0360

They are modified to minimize water loss since conifers live in dry environments.0367

And they do this because needles have a relatively low surface area compared with a large, flat leaf.0371

Gymnosperm seeds are described as naked seed. These are seeds that are not enclosed within fruits, so these are described as naked seeds.0379

And this is in contrast with the flowering plants, and the flowering plant, what happens is the ovary, which surrounds the ovule,0390

when the ovule is fertilized, when the egg is fertilized, the ovule becomes the fruit - excuse me - the ovary.0399

The ovary becomes the fruit, and the seed in enclosed within the fruit in flowering plants.0408

In gymnosperms, there is no fruit, so the seeds are just exposed.0413

The seeds are, instead, in the conifers located on cones. Cones are also modified leaves.0419

And if you look at most types of conifers, you will actually two types of cones, and here, within this photo is an example.0425

The larger cones are the ovulate cones.0431

And the smaller cones are called the pollen cones where pollen is produced versus where the female gametophyte and eggs are produced.0436

So, let's go ahead and take a look at the life cycle of a typical gymnosperm.0447

There are differences in specific species, but this is just going to be a generalized life cycle of a typical conifer.0452

We are going to start out with the mature sporophyte, and that is what this is.0460

So, you go out. You see a fir tree.0464

You see a pine tree. What you are looking at is the mature sporophyte generation, and all the areas with the white background are diploid.0466

Remember that the sporophyte is diploid.0473

And as I mentioned, there are two types of cones. There is the larger ovulate cone and the smaller pollen cones, and cones have many scales on them.0476

So, if you take one of these scales and look at it and see what all the parts are, and we are able to see and name the parts,0487

what you would see is that one of the structures is an ovule; and we will look at, kind of, a close-up of the ovule structure.0495

But for right now, what you should know is that the ovule is covered by what is called an integument.0504

And that is going to develop into the seed coat after the fertilization of the egg, so it becomes the seed coat.0517

On the scale of the ovulate cone are ovules. Surrounding the ovule is a covering called an integument.0527

And within the ovule is a megasporangium - plural is sporangia - and the megasporangium contains megasporocytes, and these are diploid.0533

And the megasporocytes undergo meiosis, so that is what occurs right here at this step- meiosis.0552

It halves the chromosome number, as you know, which is going to result in megaspores.0564

And from the megaspores develop the egg, so this is what is going on with the ovulate cone.0572

Meanwhile, over here with the pollen cone, we have microsporangium, and within the microsporangium are the microsporocytes.0583

Again, meiosis needs to occur to produce haploid microspores, so this should be down here in the haploid area.0601

The microspores are haploid, and those develop into pollen grains. Pollen grains contain the male gametophytes.0612

Pollen grains are covered, and they have a protective covering around them, as I talked about, impregnated with sporopollenin.0622

And within the pollen grains are the male gametophytes.0629

Now, the pollen grains are released, and then, they are carried via wind to the ovule.0638

When the pollen grains reach the ovulate cone, they can germinate and form a pollen tube.0645

And the pollen tube is the means by which the sperm enters the female gametophyte.0652

However, things are a little bit complicated in terms of the timing.0658

And in fact, this entire life cycle can take two or three years because the pollen does not0662

just land on the ovulate cone where the egg is ready and then, undergo fertilization.0667

In fact, what happens is that pollination occurs back here before formation of megaspore, formation of the eggs.0672

What is going on the ovule is that the cells are still diploid.0682

So, what happens is the pollen pollinates the ovulate cone, or it lands on the ovulate cone; and the pollen tube begins to grow.0686

We are still at the stage where there is megasporocytes, which are diploid.0697

As the pollen tube grows into the ovule, as it reaches the ovule, then,0701

the diploid megasporocyte, at that point, will undergo meiosis to produce four haploid megaspores.0707

So, I gave you the overview.0719

But as far as the timing, the meiosis does not occur within the ovule until after the pollen tube has grown and pollination has occurred.0720

And it can take a year for the pollen tube to grow, so this is not a fast process.0731

Meiosis, then, occurs, so now, we have four haploid megaspores in a megasporangium.0736

Usually, only one megaspore survives, and that megaspore, within that develops the female gametophyte.0742

And notice that this gametophyte is contained within the ovule, which is within this large sporophyte plant and completely dependent on it.0755

Now, within the female gametophyte are several archegonia, and within each of these archegonium develops an egg.0767

So, we have gone from the diploid megasporocytes. Meiosis has occurred to produce four haploid megaspores.0784

One of those will survive and develops into a female gametophyte. Within that are archegonium, and within each of those is an egg.0794

The formation of the pollen tube has occurred, so now, we have finally got the egg ready.0804

The pollen tube has grown into the ovule near this egg so that the sperm can reach the egg.0809

At this point, sperm nuclei are released through the pollen tube, and they fertilize the egg.0815

So, with fertilization, so now, I am going to have the sperm reaching the egg via the pollen tube.0821

With fertilization, the plant is returned to the diploid part of its life cycle because the zygote is, of course, diploid.0830

Sperm nuclei can fertilize multiple...one sperm can fertilize one egg in an archegonium, and another can fertilize another egg.0839

But usually, only one of those will survive, and then, this zygote becomes an embryo.0848

Just to talk a little bit about structure, remember that the integument becomes the seed coat, so now, the zygote is protected within.0858

Here is the seed, and the outer layer is this, so this whole thing is the seed. The outer layer is the seed coat.0867

And within that is the embryo. In addition, there are nutrients within the seed to nourish the developing embryo.0874

Then, what happens is, so now, these seeds are located on this ovulate cone.0884

And eventually, the scales separate, and they are dispersed by the wind.0888

If the seeds land in a hospitable environment, then, they will germinate, and they will begin to grow.0893

They will form a young sporophyte, continue to grow and then, eventually, a mature sporophyte.0900

And as I said, the cycle can take two or three years, and it could take a whole year just for the pollen tube to form.0906

So, this is a typical gymnosperm life cycle.0912

Now that we have talked about the gymnosperms, we are going to go ahead and focus on flower structure and angiosperms.0916

Most plants on earth today are flowering plants, and the angiosperms are all members of one phylum; and this is the phylum Anthophyta.0923

And we are going to start talking about the angiosperms focusing on one of the structures that makes them unique which is flowers.0937

And this is the reproductive structure of flowering plants.0944

Starting out with structure you are already familiar with, petals, these are brightly colored in order to attract insects, birds and other pollinators.0950

And the thing that differentiates pollination by an insect or a bird or another animal is that it is more precise than wind pollination.0960

With the wind pollination we talked about with the gymnosperms, huge quantities, masses of pollen, need to be produced.0968

And then, they are blown around because it is imprecise.0975

It is just sheer numbers, and then, hopefully, some of that pollen will land on an ovulate cone where pollination and then, fertilization can take place.0978

Whereas, if there is an insect pollinator, that insect will go and land on a flower, take the pollen and then,0987

take it over, hopefully, to another flower, carry it over, so it is much more efficient.0994

Now, there are some angiosperms that use wind pollination.1000

They do not all use an animal pollinator, but this is in advance that came with the angiosperms.1004

So, petals help to attract pollinators.1011

The second structure are the sepals. The sepals are green modified leaves that wrap around the flower.1014

So, before the flower buds, they can provide protection for it.1022

Next, we get to the male and female reproductive organs.1027

The stamen is the male reproductive organ, and the pistil or carpal is the female reproductive organ.1030

And we are going to start out with the male reproductive organ- the stamen.1045

There are two structures here. One is the long filament, and on top of the filament, at the end of the filament, is the anther.1051

The anther contains the microsporangia. I just talked about microsporangia when we talked about the gymnosperm life cycle.1058

The microsporangia are within the anther. These are also called pollen sacs, and they are the site of production of pollen.1068

Now, the female reproductive organ, the pistil, you might see in some sources called a carpal. I am going to use them interchangeably.1076

Many sources use them interchangeably. Some sources differentiate and say that a pistil is one structure, and that a pistil refers to a set of fused carpals.1090

So, sometimes, they are differentiated in that a pistil is a set of fused carpals. I am just going to use them interchangeably as is common.1103

There are three parts to the pistil. The first part is the stigma, and the stigma is sticky.1111

It is the top-most structure, so if pollen lands here, it will stick to the stigma.1117

Then, when the pollen germinates, the pollen tube will grow down this long, thin style, which leads to the final structure- the ovary.1122

The ovary, here, contains the ovules. Once the ovule is fertilized, it develops into a seed.1134

And with fertilization, the wall of the ovary thickens, and it becomes a fruit.1144

Fruit like peaches or grapes or apples, those are actually the fertilized ovules of a flowering plant.1151

Now, a couple advantages to a fruit for the plant, one is that it provides protection for the seeds.1157

But, another is the advantage it provides in dispersal. It is a way to disperse the seeds.1164

Instead of just dispersal by wind, what can happen is an animal can eat the fruit, and the fleshy part, the fruit itself, will be digested by the animal.1170

But, the seeds will pass through the animal's GI tract undigested.1180

And then, when the animal defecates, that seed will be eliminated into the ground along with natural fertilizer from the animal's waste.1184

And then, the seed can germinate.1196

So, this is a method of dispersal that the fruit also helps with dispersal as well as protection.1197

These structures that I talked about - the petal, sepal, stamen and pistil - are all modified leaves.1205

And they are sometimes called/known together as the floral organs.1211

Complete flowers have all four of these structures, so they have all four structures meaning sepals, petals, stamens and pistils.1215

Incomplete flowers are missing one or more structures.1231

And we will talk about situations where a flower may have pistils but not stamens or just stamen and not a pistil, and that is an incomplete flower.1234

Alright, now, I am going to focus on gametophyte formation in angiosperms.1245

Although, I do have a picture here, as well, of the gymnosperm ovule for comparison.1251

First, starting out with the male reproductive organ, the anthers of the flower contain four microsporangia, so these are pollen sacs.1258

First, starting with the anther, let's start with the anther, and this contains the microsporangia or pollen sacs.1275

These are diploid, so these are 2n; and they contain microsporocytes, so the microsporocytes within the sporangia.1291

These structures are also diploid, so they are 2n. They undergo meiosis, and they, therefore, produce four haploid microspores.1304

From these four haploid microspores come four grains of pollen, so each of the microspores develops into pollen grains.1334

The pollen consists of the male gametophyte enclosed within a pollen wall, so within that is the male gametophyte.1347

But, what you should know is that the male gametophyte consists of two cells: a tube cell and a generative cell.1364

The pollen tube forms from the tube cell. Two sperm are produced from the generative cell.1377

Now, that is the male gametophyte development and the production of sperm and the pollen tube.1388

Now, let's look at what is going on with the female reproductive structure.1396

Within a flowering plant, here, we have the style and the ovary. Within the ovary is the ovule.1400

Recall that as I mentioned in gymnosperm, here is a gymnosperm ovule. It is covered with an integument.1410

With the gymnosperm, there is usually one layer of integument, whereas, there are two layers around the angiosperm ovule.1416

This gap right here in the integument is called a micropile.1426

And the pollen tube can grow down and then, reach the egg by entering that opening called the micropile.1433

The ovule contains the megasporangium, which is diploid, and within the megasporangium are diploid megasporocytes.1444

Within the ovule are the megasporangium, then, within that are diploid megasporocytes.1455

What happens is meiosis produces the megaspores, and again, as we talked about with gymnosperm, usually, only one megaspore survives.1462

So, we started out with diploid megasporocytes. They undergo meiosis to produce the haploid megaspore.1474

The megaspore, then, undergoes mitosis to develop into a female gametophyte. This is also called an embryo sac.1482

One of the cells within this gametophyte is an egg, so there is going to be one egg cell, and there are also two polar nuclei.1507

These are the major cells you should be familiar with as far as the female gametophyte, so one egg and two polar nuclei.1518

These polar nuclei share a cytoplasm.1524

Now, we have gotten to the point where the egg has been formed. Pollen has been formed, and we talked about formation of the sperm and the egg.1532

So, that takes us to the next step, which is fertilization.1541

In an angiosperm, fertilization is actually double fertilization, and I will explain now why it is called double fertilization.1544

Starting with pollination, pollen is released, and it travels via wind or via animal and lands on another flower.1554

When it lands on another flower, self-fertilization can occur.1565

But most fertilization is cross fertilization, which means that a pollen from one flower will pollinate another flower.1570

And we will talk about how self-fertilization is prevented, but for right now, let's just focus on the process of fertilization, itself.1579

So, the pollen is released. It somehow gets to another flower, and it attaches, then, to the stigma.1587

Remember that the stigma is sticky. The pollen will land on the stigma and attach there.1593

Then, the pollen grain may germinate. If it germinates, what will happen is from that tube cell will be produced a pollen tube.1598

Remember in a male gametophyte, there is a tube cell and a generative cell.1607

The pollen tube is produced from the tube cell, and then, the pollen tube is going to grow down the style towards the ovary.1611

It will reach the ovule by passing through that opening in the integument, which is that opening called the micropile and reach the ovule.1624

At this point, the nucleus of the generative cell within the pollen grain divides.1633

So, within the pollen grain, there is a tube cell and a generative cell.1644

The generative cell divides and produces two sperm nuclei, and these sperm nuclei are released into the ovule via the pollen tube.1646

Now, there are two sperm nuclei, and here, within the ovule, is the egg as well as the two polar nuclei that share a cytoplasm.1663

And here is why it is called double fertilization.1676

Two fertilizations occur. Two sperm are released into the ovule.1679

One of those sperm fertilizes the egg to form a zygote. The other sperm fuses with the two polar nuclei to form an endosperm, which is triploid or 3n.1683

So, what we have is sperm nuclei. Each of these is haploid.1697

They are n.1702

When you unite the sperm, which is haploid, with the egg, which is also haploid, the result is a zygote, which is diploid.1703

The other sperm nuclei is haploid, and that is united with the two polar nuclei.1721

Each of those is n, n + n, to give a total of 3n or triploid endosperms, so this structure is called an endosperm.1730

And the endosperm is a source of nutrients for the developing embryo, so once fertilization has occurred, the ovule, then, becomes the seed.1743

The endosperm is the nourishment for the embryo within the seed, and then, the wall of the ovary thickens to become the fruit.1758

So, within the fruit is the seed, and then, within the seed is the plant zygote, which is going to develop into an embryo.1771

Now, I focused on gametophyte formation and fertilization, the most complex steps of this life cycle.1777

And now, I am going to put that into the context of the overall angiosperm life cycle.1784

Again, we will start out with the mature sporophyte plant.1788

So, you go around. You see some flowers on a plant.1792

What you are seeing is the diploid sporophyte structure and then, just looking specifically at the reproductive organ, which is the flower.1795

Recall that the pistil of the flower includes the ovary, and the ovules are contained within the ovary.1805

And inside the ovules are the megasporangia, so inside the ovules are the megasporangia.1816

And the megasporocytes are created within the megasporangia.1826

Meiosis produces the megaspores, so within the ovule, meiosis occurs to produce haploid megaspores.1834

One of the megaspores, as I said, will survive and undergo mitosis and develop into the female gametophyte or embryo sac.1845

One of the cells within the gametophyte is the egg, and there are also the two polar nuclei, so just a review of gametophyte formation.1854

Meanwhile, in the anther are the microsporangia, and within the microsporangia are the microsporocytes that undergo meiosis to produce microspores.1864

The microspores develop into pollen, and within the pollen is the sperm nuclei. The male gametophyte and the sperm nuclei form.1879

Alright, so, we are at the point where we have talked about formation of the male gametophyte and the female gametophyte, the egg and the sperm.1895

So then, pollination occurs. The pollen is released from the flower.1903

It travels via wind, water, animals, some method to reach another flower, and it attaches to the stigma there.1908

And so, pollination has occurred, and the pollen grain will germinate.1917

And these have certain conditions such as water to trigger germination, and when that occurs, the tube cell will form the pollen tube.1922

The pollen tube is going to grow down through the style towards the ovary, pass through the micropile.1929

And the two sperm nuclei are released within the ovule.1936

One of these will fertilize the egg to create a zygote. The other will fuse with the two polar nuclei to form an endosperm.1941

So, this is actually the double fertilization that is occurring.1950

Here, we are talking about haploid, sperm and egg, and then, fertilization occurs; and we end up with the diploid zygote.1954

The ovule is going to develop into a seed, so this is the seed. The developing embryo is going to be within the seed.1961

Also, the endosperm is going to be within the seed to provide nourishment for the developing embryo while the ovary will thicken.1970

And then, this seed will be contained within a fruit. The fruit, then, will drop or go off the tree or somehow be dispersed.1979

And then, the seed will eventually end up on the ground, hopefully, where it can germinate and then,1991

grow into a young sporophyte plant and then, eventually a mature sporophyte plant, and then, this cycle continues on.1999

So, this is the entire angiosperm life cycle just generalized.2006

Now, we are going to look at the structure that makes the seed plants, seed plants in a little bit more detail.2012

We are going to look at seed structure as well as development of the embryo within the seed.2018

Remember that after fertilization, the ovule develops into a seed.2024

And in an angiosperm, the ovary develops into a fruit that encloses and protects the seed.2028

As I have already mentioned, the outer covering of the seed is called a seed coat, and it contains sporopollenin.2034

It is very resistant to drying, to other environmental stressors, so it can protect the developing embryo.2044

And it can remain dormant and protect that embryo until conditions are favorable for germination.2050

The endosperm, which is triploid, contains nutrients like starches that nourish the embryo.2056

Seed dispersal, I mentioned, are by animals. Some seeds, however, are light.2065

They are aerodynamic, and they are actually just dispersed by the wind like maple seeds.2071

Maple seeds have structures that are wing-shaped, so they travel really well on the wind.2075

Others, aquatic plants, have seeds that are dispersed by water.2080

With fruits, again, one of the advantages of fruits is that animals can disperse the seeds after they eat the fruit and then, eliminate the seeds.2086

Other seeds actually are distributed by animals because the seeds actually have spines on them, and they are carried away.2095

Also, seeds can be buried by animals, which is a big help to the seed as long as the animal does not come back and eat it.2103

So, a squirrel might bury some seeds to eat later on and then, never actually end up getting to those, and then, those can germinate.2111

After the seed is developed, has been dispersed, the zygote will develop. It will undergo mitosis and form an embryo.2119

And that is what we are looking at now, is the structure of the embryo within the seed.2129

And this structure consists of the embryonic root and the seed leaves. The seed leaves are called cotyledons.2133

So, what is being shown here is a seed that has been sectioned opened. It has been split open, and there are actually two seed leaves.2145

There are two cotyledons, so if there are two cotyledons, the plant is called a dicot.2153

Angiosperms have been broadly divided into two major groups: the monocots and the dicots.2161

Those plants that have only one seed leaf, one cotyledon, are called monocots.2167

In dicots, food storage is transferred from the endosperm to the cotyledons, so the cotyledons are a site of food storage.2180

In monocots, food storage remains in the endosperm.2189

Other structures within the embryo are first, the epicotyl. The epicotyl eventually becomes the shoot system.2194

So, this develops into the shoot system meaning the stem and leaves.2201

In some plants, just below the epicotyl is the hypocotyl, is the structure that forms the roots.2209

However, in some plants, there is actually another structure lower down called the radical that forms the roots.2216

If there is a radical that forms the roots, then, the hypocotyl usually becomes the lower stem.2222

We have the shoot system, epicotyl, the hypocotyl, which is the lower stem or the roots.2229

And then, there may be a radical that forms the roots instead of the hypocotyl.2234

We are going to talk in a little while using a table to talk about differences between monocots and dicots.2239

But, just to start thinking about it, some examples of monocots are grasses. Many grasses like wheat and corn are actually examples of monocots.2245

Dicots are many wood plants such as oaks, willows.2257

Other plants that are herbaceous plants like marigolds, a lot of garden vegetables like tomatoes and pea plants, those are also dicots.2263

When we talked about the life cycle of angiosperms, I did touch upon pollen dispersal, and I would talk about that now in more detail.2278

To maintain genetic diversity, pollen from one flower needs to travel to the pistil of another flower where pollination and fertilization can occur.2285

So, there are various methods of dispersing the pollen, and these are broadly categorized as abiotic or biotic.2295

Biotic mechanisms require a living organism like biotic biology- life. Abiotic would be without a living organism.2303

So, starting with some methods of abiotic- pollination. The first one is wind.2312

This is the method that is used by gymnosperms, but as I mentioned, some angiosperms use this, as well.2319

Most flowering plants actually rely on animals for pollination, but some angiosperms use wind pollination.2325

As mentioned, wind pollination is not as exact as the other methods, so very large quantities of pollen need to be produced.2334

They blow around on the wind, and you can sometimes some days see the pollen in the air. That can cause allergies for some people.2341

So, wind pollination, large quantities of pollen are produced. Many grasses and trees utilize this method pollination.2349

The second abiotic method is water. Aquatic plants may rely on water to disperse their pollen.2357

These methods - abiotic - account for about 20% of angiosperms. The majority of angiosperms, 80%, rely on biotic methods.2365

These are living organisms: pollinators such as these shown here, animal pollinators such as bees, birds, butterflies, various other insects.2377

So, what happens is the flowers on the plant are often very showy for those that rely on biotic pollination.2397

And what the flower is trying to do is attract a pollinator.2407

There may be nectar produced by these flowers, which is the payoff for these insect pollinators. It is what draws the insect pollinators.2415

And nectar is very rich in carbohydrates, so as a bird or an insect feeds on the nectar, the pollen will stick to their body.2422

Then, they go to the next flower to feed on nectar, and some of that pollen will drop off and attach to the stigma of the next flower.2430

So, about 80% of angiosperms rely on biotic methods of pollination.2437

Now, as I mentioned, what is ideal is to maintain genetic diversity, so plants have many means of preventing self-pollination.2443

Some plants do self-pollinate. However, in most plants, cross-pollination is favored.2452

Cross-pollination, again, means that the pollen from one flower fertilizes, pollinates, another flower, not the same flower, to maximize genetic diversity.2458

Mechanisms to prevent self-pollination: 1. The pistils and stamen mature at different times.2469

Even though the pollen may be ready - the stamen matures, the pollen is ready - the egg is not mature, so therefore, self-fertilization cannot occur.2485

Another method is that some plants produce flowers that contain either stamens or pistils but not both.2498

These are called dioecious plants, so dioecious plants have flowers with stamens - staminate flowers or pistilate flowers - or pistils.2506

Monoecious plants have flowers that produce both, flowers with both male and female reproductive structures,2527

so preventing self-pollination by having a flower that has either a stamen or a pistil but not both on the same flower.2545

The third mechanism is the structure of the flower. The flower may be structured such that it makes it difficult for self-pollination to occur.2553

For example, if there is a flower, and the stamen is short; and then, the pistil is very tall, it makes it much less likely that self-pollination will occur2562

because if pollen drops, it is not going to end up here in the pistil, so various structural adaptations to prevent self-pollination.2578

The most important mechanism, however, is a biochemical mechanism, and this is called self-incompatibility right here.2589

And self-incompatibility refers to biochemical methods of blocking fertilization in a plant when it is the2604

same plant or preventing fertilization that is attempted by a plant that is very genetically similar.2617

Plants can recognize self as their own genetic makeup or very similar genetic makeup, and they recognize non-self a different genetic makeup.2626

And they will allow fertilization with non-self but not with self, so how did they do this?2636

There are a set of genes called S genes on plants that allow the plant to recognize its own pollen.2642

So, a flower will have S genes, and it will be able to recognize if pollen comes from a plant with the same or various similar S genes.2651

If these S genes are the same, growth of the pollen tube is blocked, so block pollen tube from pollen with same or similar S genes.2659

There are two mechanisms through which this occurs. One is called gametophytic self-incompatibility or GSI.2679

In this case, if the S-allele on the pollen matches one of the S-alleles on the flower it is trying to fertilize or pollinate, the fertilization is blocked.2698

The pollen tube stops growing, so if the style recognize the pollen tube is self, it will destroy the RNA from that pollen tube.2710

If it recognizes that it is non-self, it will not, and this is also called haploid incompatibility.2717

And you think of it this way: the gamete, the pollen is haploid, and it is the haploid genotype that is being looked at.2723

Just to illustrate this, let's say there is pollen, and the parent plant from the pollen are the two S genes- S_a, S_b.2729

And this particular pollen grain, of course, only gets one of these alleles, and let's say it gets S S_a.2749

So, here is the parent plant from the pollen. Here is the pollen, itself.2756

Up here is the parent, and it goes to fertilize a flower; and this flower - not the egg, itself but diploid structures on the pistil - will have two alleles.2760

So, it will be diploid, and let's say it is S_a, S_c. This flower will recognize this S_a allele as self, and fertilization will be blocked.2774

However, if this S_a pollen went and tried to fertilize an S_c, S_d, plant or flower, fertilization can occur.2786

See, the S_b, pollen grains could actually pollinate this flower, so this is haploid incompatibility or gametophytic self- incompatibility, GSI.2800

The other method is called sporophytic self-incompatibility, SSI.2811

Now, sporophytes are diploid, and in this mechanism, it is the diploid genotype. It is the parent genotype, the parent of the pollen that matters.2822

So now, let's look back at this plant again. We have the parent of the pollen-producing plant is S_a, S_b.2831

The pollen is S_a. Then, this pollen goes and tries to fertilize an S_a, S_c flower.2839

What the flower is going to be looking at is the parent genotype from the pollen. Well, how does it know the parent genotype?2849

We have just got the pollen here. We do not have the whole parent plant.2856

Well, some cells from the parent plant stick to the outside of the pollen.2859

And the style, the pistil, is able to recognize, to look at, those cells that have stuck to the pollen and to check out what their genotype is.2864

Now, what this flower will do is say2874

"OK, I have S_a, S_c. You have S_a, That is self. I am going to block fertilization. I am going to block the growth of pollen tube".2878

This is haploid in compatibility, gametophytic, just the haploid genotype of the pollen is being looked at.2888

Here is the diploid genotype. It is the parent of the pollen that has the genotype that matters.2895

The major thing to understand here is that this is a biochemical mechanism of2901

preventing fertilization based on a plant recognizing self versus non-self pollen, and it maintains genetic diversity.2905

We have been talking about sexual reproduction, which is mostly what occurs in plants. However, plants can reproduce asexually.2915

This occurs through cloning, and in cloning, the result is going to be offspring that are genetically identical to the parent plant.2921

And this is called vegetative reproduction.2928

A part of the plant like the root or the stem can contain cells that are undifferentiated.2932

And those cells can produce the other specialized tissues of the plant, so from just one part of the plant like the root can come an entire plant.2938

For example, you could go to someone's house, and you see a plant that you like; and you might say "oh, can I take a cutting?".2948

So, you take a part of the plant. In certain plants, it might be the root, or it could be a stem or even the leaf in certain plants.2953

Then, you go home, and you put that cutting in water; and you hope that it sprouts roots, it forms roots.2960

And then, you would go plant it in the ground, and you will have a whole plant from that cutting,2965

which is genetically identical to the plant that you got it from; so this is a form of asexual reproduction- vegetative reproduction.2969

Tulips can reproduce through bulbs, and bulbs are actually part of the stem, so this is a type of vegetative reproduction.2977

The eyes of potatoes are called tubers, and they are also a modified part of a stem.2989

And again, this is reproduction that is asexual, so it is vegetative reproduction.2999

Bulbs and tubers are both structures that can initiate a sexual reproduction.3005

And this vegetative reproduction can occur in nature without human intervention. However, humans have also used this to our benefit.3011

In grafting, the stem of one plant is grafted or fused onto the root of another plant.3020

And this allows us to take the qualities that we want from the roots and from the other parts of the plant.3039

Now, the plant that provides the root is called the stalk or root stalk. This is the stalk, and the plant that provides the stem is called the scion.3044

Qualities that we want in the fruit, which is often what we are growing it for, are maybe the flowers for beauty.3058

The fruit or the flowers are determined by the stem.3064

However, maybe there is a plant that produces really large tasty fruit, but it is getting killed by some disease.3067

And there may be another plant that has fruit that does not taste that great, but it has roots that are resistant to disease.3075

So, what could be done is to take the stem from the plant with the great fruit and graft that onto the disease-resistant root.3082

That is how we have used that to our advantage in agriculture.3090

We talked about monocots being plants that have one embryonic seed leaf/cotyledon and dicots being plants that have two cotyledons or seed leaves.3095

This table summarizes differences between the two groups, and I do want to note that dicots have been further divided up into several groups.3110

Most dicots are now in a group called the eudicots, and then, the rest of the dicots were put in several other groups.3120

And this is based on DNA evidence of evolutionary and genetic relationships.3128

And as we have talked about with the protists and the fungi and throughout this course,3133

divisions that were once made based on morphology and biochemistry and life cycles are being overturned because of molecular evidence.3138

But, I am going to stick with these traditional divisions, and they do have some useful differences that we can focus on.3147

As you will see in the pictures down here, some monocot leaves, they tend to have leaves that are longer and narrower than dicots.3158

Also, looking at - let's start out with the leaves - the vein pattern in the leaf, a monocot like this has a vein pattern with parallel leaves.3166

Whereas, if you look at a dicot leaf like this one, it is a net-type pattern.3174

So, that is one difference between monocots and dicots- monocots with one cotyledon, dicots with two.3179

As I mentioned, in monocots, the nutrient storage remains in the endosperm.3184

In dicots, nutrient storage is transferred from the endosperm to the cotyledons.3190

Floral parts: floral parts in monocots are usually in multiples of three, so here we see 1, 2, 3, 4, 5, 6 leaves.3197

This is likely a monocot, whereas, with dicots, usually, the floral parts are in multiples of four or five.3209

Finally, or two more, vascular bundles in stems in monocots, and we talked about this when we talked about plant structure.3220

The vascular bundles, if you cross-section the stem and looked at it, you would see that the vascular bundles,3229

the xylem and phloem, are scattered throughout that cross-section.3235

So, the vascular bundles would just be scattered around in a monocot.3240

Whereas, if you looked at a dicot, what you would see are the xylem and phloem arranged in a ring.3244

Finally, root systems: most monocots have a fibrous root system.3253

Remember that that is a root system that is shallower, but it is very spread out; so it helps prevent erosion.3257

Grasses are effective in preventing soil erosion, whereas dicots usually have a tap root system.3262

A tap root system has one main root, a central root, that grows very deeply and runs much deeper into the ground,3269

so difference between monocot and dicot roots.3276

So, you should be familiar with these two groups and their similarities and differences.3279

Example one: why is the process of fertilization in angiosperms called double fertilization?3284

Recall that two sperm nuclei are released into the ovule, and one of those - one sperm nucleus - goes ahead and fertilizes the egg.3290

This is typical, and the result is a zygote.3311

Recall that the sperm is haploid. The egg is haploid.3314

Therefore, the zygote is diploid. However, the second sperm nucleus is released, and it fertilizes the two polar nuclei.3317

These two polar nuclei are separate nuclei, but they usually share a cytoplasm.3332

The sperm is haploid. Each of the polar nucleus is haploid, so n + n to give triploid or 3n; and this structure is called an endosperm.3339

And it provides nutrients to the developing plant embryo, so double fertilization because there are two fertilizations that occur in angiosperms.3352

Second example: list three mechanisms by which a plant prevents self-fertilization from occurring.3363

There are more than three, but you only need three to answer this.3370

The first is that in some plants, the pistils and the stamen mature at different times.3373

So, the pollen may be ready, but the female gametophyte is not yet developed.3385

A second mechanism is that there are some plants that are called dioecious, and they produce flowers that contain either stamens or pistils but not both.3393

So, I will put "Flowers have either stamens or pistils, not both. Therefore, self-fertilization cannot occur".3404

Next: there are flower structure that discourages, prevents, self-fertilization.3419

The example I gave before is that the stamen may be shorter than the pistil.3435

So, it is less likely that pollen grains will just settle or land on the stigma of that same flower.3441

Finally and the most important one is self-incompatibility. This involves a set of genes called the S genes- self-incompatibility.3448

And with this mechanism, the plant recognizes self and non-self, and it blocks fertilization. Usually, it is pollen tube formation.3464

It blocks fertilization by self-pollen.3482

So, I gave you four. You only needed three of these to complete this question.3488

Next: complete the following chart, which compares monocots and dicots.3493

Monocots, well, number of cotyledons: mono tells you it has one cotyledon in the seed, whereas dicots, di means two, so two cotyledons.3500

Nutrient storage in seeds: remember that in monocots, nutrient storage remains in the endosperm.3511

Whereas in dicots, the nutrient storage is transferred to the cotyledons.3517

Next: this says parallel and net-type. Well, recall that the monocot leaves tend to be longer and narrower with this parallel vein pattern.3525

Whereas, in a dicot, it is usually more of a net-type spread out pattern of veins, so this is the vein pattern in leaves.3535

Next, what do we find in monocots that is in threes, whereas, in dicots in fours and fives?3548

Well, that is the floral parts like the sepals in the petals are found in threes in monocots and, generally, in fours and fives in dicots.3555

The vascular bundles in stems: if I cut a stem, I cross-sectioned it, and I looked at it,3565

what I would see in monocots are that the vascular bundles are just scattered.3570

They are scattered throughout the stem, so it is a monocot.3577

If I sectioned a dicot, I would see these vascular bundles - xylem and phloem - arranged in a ring.3582

Finally, roots: most monocots have fibrous roots. These go less deep, and they are more spread out; so they are very good at preventing soil erosion.3592

Whereas, dicots usually have a tap root system. In a tap root system, there is a central root that usually goes deep into the ground.3602

Example four: label the fine structures on the drawing of the flower below.3613

OK, stigma, so stigma is the part of the pistil that is sticky, and pollen lands on it; so it is right here at the top. That is the stigma.3617

Pistil will hold onto that, so we have done the other structures on the pistil such as the style, the long, thin structure that the pollen tube travels through.3627

Next, ovary: the ovary is the structure within which are the ovules.3640

And we were not asked to label ovule, but I will go ahead and just label the ovule right there.3647

All these put together comprise the pistil, the female reproductive structure.3652

Sepals: the sepals are the green leaf-type structures that surround the flower when it is closed. They provide protection for it.3662

Petals: petals are the brightly colored structures that help to attract animal pollinators.3676

Next, filaments: so, the male reproductive structures, I will put in blue.3685

The filament is the long, thin structure that is topped off by the anther, which is the site of pollen formation.3689

And these two, if you put these combined, they are called the stamen, which is the male reproductive structure.3697

So, that concludes this discussion on seed plants here at Educator.com.3705

Thank you for visiting.3710