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Lecture Comments (6)

0 answers

Post by Muna Lakhani on May 18, 2013

Are only vessels elements cells dead and cannot divide, or both tracheids and vessel elements cannot divide?

0 answers

Post by Jonathan Aguero on February 26, 2013


1 answer

Last reply by: Dr Carleen Eaton
Sun Mar 3, 2013 5:15 PM

Post by Jonathan Aguero on February 26, 2013

I thought dicots vascular part was scattered?

1 answer

Last reply by: Dr Carleen Eaton
Tue Mar 13, 2012 11:46 PM

Post by John Wadsworth on March 13, 2012

great job

Plant Structure

  • Roots anchor the plant to the ground. They are also the site of absorption of water and minerals from the soil and a site of food storage.
  • The leaf is the primary site of photosynthesis in most plants. The outer layer of the leaf is covered with a cuticle to prevent the loss of water. The cells of the mesophyll contain numerous chloroplasts where photosynthesis takes place.
  • Stomata are pores on the surface of leaves that allow for gas exchange. Guard cells flanking a stoma change shape in order to control the opening and closing of the stoma.
  • The xylem conducts water and minerals from the roots to the rest of the plant. The xylem is composed of tracheids and vessel elements arranged in chains.
  • The phloem conducts nutrients from sites of production to the rest of the plant. Companion cells associated with the sieve-tube elements regulate the flow of nutrients.
  • Plants can grow indefinitely due to the undifferentiated, embryonic cells in their meristems. Primary growth increases the length of the plant. Non-herbaceous plants also undergo secondary growth, which results in an increase in width.

Plant Structure

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
  • Plant Tissue 0:05
    • Dermal Tissue
    • Vascular Tissue
    • Ground Tissue
  • Cell Types in Plants 2:14
    • Parenchyma Cells
    • Collenchyma Cells
    • Sclerenchyma Cells
  • Xylem 5:04
    • Xylem: Tracheids and Vessel Elements
    • Gymnosperms vs. Angiosperms
  • Phloem 8:37
    • Phloem: Structures and Function
    • Sieve-Tube Elements
    • Companion Cells & Sieve Plates
  • Roots 10:08
    • Taproots & Fibrous
    • Aerial Roots & Prop Roots
    • Structures and Functions of Root: Dicot & Monocot
    • Pericyle
  • The Nitrogen Cylce 18:05
    • The Nitrogen Cycle
  • Mycorrhizae 24:20
    • Mycorrhizae
    • Ectomycorrhiza
    • Endomycorrhiza
  • Stems 26:53
    • Stems
    • Vascular Bundles of Monocots and Dicots
  • Leaves 29:48
    • Blade & Petiole
    • Upper Epidermis, Lower Epidermis & Cuticle
    • Ground Tissue, Palisade Mesophyll, Spongy Mesophyll
    • Stomata Pores
    • Guard Cells
    • Vascular Tissues: Vascular Bundles and Bundle Sheath
  • Stomata 36:12
    • Stomata & Gas Exchange
    • Guard Cells, Flaccid, and Turgid
    • Water Potential
    • Factors for Opening Stoma
    • Factors Causing Stoma to Close
  • Overview of Plant Growth 44:23
    • Overview of Plant Growth
  • Primary Plant Growth 46:19
    • Apical Meristems
    • Root Growth: Zone of Cell Division
    • Root Growth: Zone of Cell Elongation
    • Root Growth: Zone of Cell Differentiation
    • Stem Growth: Leaf Primodia
  • Secondary Plant Growth 48:48
    • Secondary Plant Growth Overview
    • Vascular Cambium: Secondary Xylem and Phloem
    • Cork Cambium: Periderm and Lenticels
  • Example 1: Leaf Structures 53:30
  • Example 2: List Three Types of Plant Tissue and their Major Functions 55:13
  • Example 3: What are Two Factors that Stimulate the Opening or Closing of Stomata? 56:58
  • Example 4: Plant Growth 59:18

Transcription: Plant Structure

Welcome to

In this section, we are going to be focusing on plant structure.0002

And I am going to start out by talking about the different tissue types that are found in a plant.0005

There are three types: the dermal tissue, vascular tissue and ground tissue.0010

The outer covering of a plant consists of dermal tissue, so the epidermis. Epidermal cells are an example of dermal tissue.0016

The epidermis provides a protective covering on the plant, and it also secretes a waxy layer called the cuticle.0028

And that prevents water loss from the plants, so that is one type of tissue.0036

The second is the vascular tissue. The vascular tissue is composed of the xylem and phloem.0041

And we will talk in detail about the xylem and phloem in a few minutes.0049

For now, though, you should just know that the xylem transports water and0053

minerals throughout the plant from the roots up through the stem and leaves.0057

The phloem transports nutrients from where they are created via photosynthesis in the leaves.0062

It transports those nutrients from the site of their creation in the leaves to the rest of the plant, these carbohydrates.0069

And the region, the areas containing vascular tissue are...if you cut through a cross-section, you can see that they are arranged in these cylinders.0076

And these cylinders are known as vascular cylinders that contain vascular tissue.0086

The third type of tissue is the ground tissue. The majority of the plant is ground tissue.0093

Ground tissue is the site of photosynthesis. It is also important for support of the plant, and it is the site of storage of water and nutrients.0098

All three types of tissue are found throughout the plant. For example, the roots, leaves and stem all have a covering composed of dermal tissue.0110

The vascular tissue, the xylem and phloem, runs throughout the plant from the roots0119

up through the stem and leaves to deliver nutrients and water to all areas of the plant.0123

Delving a little deeper going from tissue to the cellular level of organization,0130

there are three major cell types in plants that I am going to talk about right now.0135

And those are the parenchyma, sclerenchyma and collenchyma.0140

Parenchyma cells have a primary cell wall that is thin and flexible, so they have a thin, flexible primary cell wall.0147

However, they lack a secondary cell wall.0165

Often, when people imagine a plant cell or when we talk about a plant cell, what we are talking about is the parenchyma.0169

They are thought of as this is the typical plant cell, and they have a large central vacuole usually.0174

They are specialized depending on their location. So, a parenchyma cell in the leaf would be specialized to carry out photosynthesis.0182

Those in the roots are a site of storage for starches within plastids.0188

Parenchyma cells retain the ability to divide. Therefore, if a plant is injured, repair can occur through division of parenchyma cells.0193

The second type of cell are...let's talk about the collenchyma cells next.0202

Collenchyma cells also lack a secondary cell wall. They have primary cell walls of varying thickness.0208

One region will be a little thicker, the other a little thinner but no secondary cell walls.0219

And their function is to support the stem as the plant is growing. An example of collenchyma cells are the strings that you will find in a stalk of celery.0231

Finally, sclerenchyma cells, these cells have a thick primary cell wall, and a thick secondary cell wall, thick primary and secondary cell walls.0240

The secondary cell wall contains lignin. It provides a lot of strength, and the function of a sclerenchyma cell is to provide support to the plant.0257

You should be aware that there are two types of sclerenchyma cells: fiber cells and sclereids.0271

Fiber cells are longer and thinner than the sclereids, so fibers are longer and thinner. The sclereids are shorter and more than irregular shaped.0277

An example of fiber cells are flax. Flax fibers are used to make linen.0290

An example of sclereid know how a pear has that gritty feel? That is due to sclereid cells.0296

Next, we are going to talk about the xylem and then, the phloem.0305

So, remember, vascular tissue is composed of xylem and phloem. Right now, I am focusing on the structure.0308

We are going to talk about transport of nutrients in water throughout the plant in a later lecture. Right now, the focus is on structure.0313

General features and just talking about vascular tissues, recall from the previous lecture,0323

while nonvascular plants like moss have certainly some adaptations that allow them to live on land,0329

tracheophytes or vascular plants evolved even further to adapt to new environments and to succeed in terrestrial environments.0336

Vascular systems and their supporting structures contain lignin, and that is the polymer that strengthens the cell walls in these plants0345

and allows them to grow very tall and also provides a structure through which water and nutrients can reach all areas of the plant.0356

In addition, vascular plants have true roots, stems and leaves.0366

First, talking about the xylem, the job of the xylem is to conduct water and minerals from the roots to the rest of the plant.0374

And there are two types of cells that compose the xylem: tracheids and vessel elements.0382

And these are arranged in chains, and the result is that they form essentially tubes through which0389

water can be conducted from the roots up through to the leaves, to the stem, to all parts of the plants.0396

Tracheids are long and thin. Vessel elements, they tend to be shorter and fatter.0404

They are represented as, kind of, similar with hair, but usually the vessel elements are a little shorter and fatter than the tracheids.0411

Something else to note is that these cells are dead. They cannot divide, so they can do their job of conducting water, but they can no longer divide.0417

So, they have lost the ability to divide. Now, they have these thick secondary cell walls, but the water needs to be able to go from cell to cell.0428

And the solution in tracheids is these areas called piths, and water can flow from cell to cell via these piths.0437

These piths lack secondary cell walls, so they are regions of the tracheid that lack secondary cell walls, and they allow water to move from cell to cell.0450

The vessel elements are lined up end to end and have openings through which the water can pass. That is shown right here.0463

Also, tracheids are found in both non-flowering plants, gymnosperms and in angiosperms.0474

However, only angiosperms generally have vessel elements.0484

So, gymnosperms, which are non-flowering plants, they are vascular. They are seed plants, but they are non-flowering.0489

They only have tracheids, whereas, angiosperms have both cell types.0498

So, that is the xylem.0514

The second type of cell found in vascular tissue is the phloem, and the phloem conducts nutrients from the leaves to the rest of the plant.0515

And it is composed of what is called sieve-tube elements and companion cells.0524

So, the sieve-tube elements are the cells through which the nutrients actually move through.0534

Up in the leaves, photosynthesis occurs. Glucose and carbohydrates are made, and those move down through the phloem.0542

The sieve-tube elements are accompanied by companion cells.0552

And the companion cells are located near plates called sieve plates through which the nutrients can move.0557

And the job of the companion cells is believed to be to regulate the flow of the nutrients through the sieve-tube elements through the sieve plates.0569

So, companion cells are associated with the sieve-tube elements and provide supporting0578

function and allow for regulation of the flow of nutrients through these cells, OK?0589

So, xylem conducts water, and phloem conducts nutrients.0598

Now, we are going to go ahead and talk about the different systems in the plant.0603

OK, there are two major systems in the plant: the root system and the shoot system. The root system is largely below ground.0609

And the shoot system are the parts of the plant that are above ground such as the stem, leaves and flowers.0615

The root is a structure that anchors the plant to the ground. It is also the site of the absorption of water and minerals, and a site of food storage.0622

Two major types of root systems are tap roots and fibrous roots.0631

With tap roots, there is a single large root from which lateral roots called branch roots arise, and tap root systems penetrate deep into the ground.0639

Carrots are tap roots for example. Carrots or turnips and other root vegetables are also an0654

example of what is called a storage root because they are a site of long-term storage of food.0660

Fibrous roots have a different structure. Fibrous roots are bound in grasses such as wheat.0665

They are thinner, and they are more spread out to form a mat, and they do not penetrate as deep.0671

They do not have a single main root. They are more spread out, so it is more of a mat or net of roots.0678

And what is very important about these roots is that they also prevent erosion of the soil.0690

Because they are thin and spread out like this, they help to hold the soil down and prevent erosion.0696

Other types of roots that are specialized, one is aerial roots.0702

So, although I said for the most part, the root system is below ground, there are some exceptions.0707

Plants such as ivy cling to the sides of buildings and trees, so aerial roots allow plants to attach to these surfaces and grow up along a vertical surface.0712

Another type of root that extends above ground are prop roots.0722

Prop roots grow from the base of the stem, and they help support a plant like corn0726

that is tall and, sort of, heavy on top and might fall over without the support.0730

If this is the ground, and then, you have a plant growing up, a tall, sort of, heavy plant, what prop roots do is they come out from the base of the stem.0735

They grow from the base of the stem and into the ground, and they hold - literally prop up - the plant.0750

This prop root is a type of adventitious root.0760

So, adventitious roots arise from the stem, branch or another area of the plant other than the primary root.0763

Prop roots are adventitious roots because they do not originate from the primary root. They grow out from the stem of the plant.0774

Next, we are going to look at the structure of a cross-section of the roots and the different structures within monocots and dicots.0782

First, starting from the outer layer, we talked about dermal tissue, and dermal tissue is, it forms the epidermis.0796

The epidermis is right here. This is an outer covering, and in roots, the epidermal cells have projections called root hairs.0807

Root hairs are projections of epidermal cells.0815

And their function is to increase the surface area of the root, which makes sense because the job of the root is absorption.0819

This increased surface allows for absorption.0826

Now, seeing that there are two different structures here, this is a monocot. Remember that angiosperms can be divided into monocots and dicots.0830

Flowering plants can be divided into monocots and dicots.0838

Slightly different structure of the vascular tissues of a monocot versus a dicot, OK, the epidermis found in both, that is the outer covering.0841

Next, what you will see is the cortex, and the cortex is composed of parenchyma cells.0852

These cells contain plastids where starch is stored, so cortex cells, which are parenchyma, and they are a site of storage.0860

The cortex is also right here on the monocot.0873

The pith, these are similar cells, but the difference between cortex and pith is that cortex lies outside the vascular cylinder.0878

Whereas, pith lies within the vascular cylinders.0888

So, it is more of the location versus the function, but pith also functions as a site of storage.0891

Now, roots do not undergo photosynthesis.0898

They are non-photosynthetic parts of the plant, and they depend on the leaves to make the carbohydrates.0901

And these are transported by the phloem. Meanwhile, the xylem is transporting the water absorbed from the roots up to the rest of the plant.0908

And these vascular tissues are found in what is called the vascular cylinder, so this entire structure is the vascular cylinder.0917

This layer right here, this black line, represents what is called the endodermis. This is a layer of cells.0933

This whole thing is the vascular cylinder, all these structures, but surrounding it is this endodermis.0943

So, this is just a layer of cells, and the purpose of the endodermis, one purpose is to regulate the entry of water and minerals into the xylem.0956

These endodermal cells, to carry out this job, are packed tightly together, and along the endodermal cells is a structure called the Casparian strip.0965

And this is a belt made of a waxy substance called suberin.0977

Some of the endodermal cells have suberin impregnated in areas of their cell wall, and the result is this strip that is impermeable to water.0983

This means that water cannot just come up to the root and then, just go straight into the xylem, be transported around. It is actually regulated.0993

And because of this Casparian strip, water cannot slip past the endodermal cells.1002

In a later lecture on plant nutrition, I am going to talk about how this regulation1007

occurs and how transport of water and nutrients occurs throughout the plant.1012

Just inside the endodermis is a layer of cells called the pericycle, so we will draw that in right inside the endodermis, the pericycle.1018

And this is the site from which the lateral roots arise.1027

Now, within the vascular cylinder are xylem and phloem.1037

In the monocots, what you see is a circle of xylem here, so the xylem is marked in brown, circle of xylem, and just outside of that, a ring of phloem.1044

In dicots, the structure is different. It is this star or X-type structure of xylem with the phloem located just outside that.1056

So, remember, the central area of parenchymal cells is called the pith. The outer area of parenchymal cells is called the cortex.1065

This is just to give you an idea of the cross-section- what the root structure is if you took a root, cut it and looked at a cross-section in a monocot and a dicot.1074

While we are talking about roots, it is a good time to introduce the nitrogen cycle.1084

And I am going to focus mainly on the plants and how they obtain nitrogen, but remember that they are part of the overall nitrogen cycle.1088

Right now, though, our focus is on plants.1099

The atmosphere is about 78% nitrogen. However, the nitrogen in the atmosphere is dinitrogen N2, and this is a gas.1101

It is not a form that plants can utilize, but nitrogen is needed by plants and other organisms.1110

Plants use it to make proteins, nucleic acids and other organic compounds, so nitrogen is a very important part of organic compounds.1116

Here, we have in the atmosphere, N2, dinitrogen gas. That ends up in the soil.1128

Bacteria play an extremely important role in the nitrogen cycle because what they are capable of doing1135

- nitrogen-fixing bacteria - is converting this gaseous nitrogen in the atmosphere to ammonia, NH3.1141

So, nitrogen-fixing bacteria can take the dinitrogen and convert it to ammonia.1149

I am going to talk here about legumes in a second. Right now, I am talking about nitrogen-fixing bacteria that are free-living.1155

These are in the soil, and these are free-living nitrogen-fixing bacteria.1162

Within the soil are free hydrogen ions, H+, and these combine with the ammonia to create ammonium ions, NH4+.1167

Nitrifying bacteria can take that ammonia and oxidize it to form nitrite.1181

So, nitrifying bacteria take the ammonia and convert it to nitrite, NO2- and then, further convert, oxidize that to nitrate, NO3-.1190

Alright, so, what has happened so far is nitrogen-fixing bacteria converted the N2 to ammonia.1204

The ammonia combined with hydrogen ions in the soil to make ammonium, NH4+ ions.1210

Then, nitrogen-fixing bacteria oxidized that to nitrite and then, further to nitrate.1221

And this is a form that the plants can absorb and then, convert back into ammonium and use.1233

Now, I do want to note that plants can absorb some ammonium from the soil, but most of the nitrogen they absorb is in this form of nitrate.1242

And in fact, if the ammonia level of the soil gets too high, it is actually toxic to plants.1253

The plants really depend on bacteria to fix nitrogen and then, convert it to nitrate.1260

Denitrifying bacteria actually take some of the nitrate that is created by the nitrifying1269

bacteria and end up releasing nitrogen gas that just goes back up into the atmosphere.1275

Second thing I want to talk about are nitrogen-fixing bacteria that exist on the root nodules of legumes.1286

A group of plants called legumes have a mutualistic relationship with bacteria, and the bacteria are from the genus Rhizobium.1293

These Rhizobium live in close association with legumes like pea plants.1306

Legumes have root nodules, and within the root nodules, within the cells of the root nodules, are bacteria and vesicles, and the bacteria fix nitrogens.1311

They perform this function of changing it from N2 to NH3 for the plant.1324

What the Rhizobium get from this association is protection and nutrition from the plant. The plant gets a supply of nitrogen.1331

Now, while we are focusing on plants right now, just to complete the discussion, which we will take up in ecology section about the nitrogen cycle,1339

so the plants get their nitrogen one way or another, and make it into organic compounds and survive and live.1349

Well, animals eat the plants, and the animals release waste products; and the animals also die and decay.1357

In addition, plants die, and their organic matter is returned to the soil.1368

And within the ground, decomposers break down the waste products and parts from dead plants1373

and animals and return that nitrogen to the soil where it is acted upon by nitrifying bacteria.1382

So, in that way, the nitrogen has been incorporated into plants,1387

gets incorporated into the animals and makes its way back into the soil to complete the cycle.1391

One other thing is just to look at the overall reaction of nitrogen fixation, so this is nitrogen from the atmosphere.1396

Nitrogen gas combines with 16 molecules of ATP plus 8 electrons plus 8 hydrogen ions to yield 2 molecules1403

of NH3, ammonia, plus 16 molecules of ADP plus 16 inorganic phosphates plus hydrogen.1415

You do not need to memorize these, but just to look at the overall reaction to see what is happening,1427

that fixation of nitrogen is an energy-requiring process.1432

And the result is that nitrogen from the atmosphere is converted to ammonia that the nitrifying bacteria can act upon.1436

So, that is the nitrogen cycle, and you see this relationship between bacteria and plants that allows the plants to grow and thrive.1445

A second relationship between plants and another organism is a relationship between plants and fungi.1454

We talked about this in the lecture on fungus.1460

And a mycorrhiza refers to the symbiotic relationship between certain fungi and the roots of a plant.1464

Plural is mycorrhizae, and mycorrhizae provide the plant with an increased surface area.1473

The advantage to a plant that has mycorrhizae associated with it is that it has a1481

bigger surface area that can be used to absorb water and minerals from the soil.1485

Certain fungi have a special type of hyphae that allows them to extend over the root surface and actually partly into the root.1492

I said the advantage for the plant is that this increases the surface area of the root.1502

And the fungus, then, helps the plant root to absorb water and minerals.1506

The added benefit to the fungus is that they take carbohydrate from the root cells, and that allows them to live.1510

In addition, some of these fungi secrete growth factors that help the plant to grow.1517

Mycorrhizae are very commonly associated with plants, and in fact, over 90% of plants have mycorrhizae associated with them.1523

And experiments have been done growing certain types of plants with and without mycorrhizae.1530

And those plants with mycorrhizae grow significantly better than those without them.1536

There are two types of mycorrhizae: ectomycorrhiza and endomycorrhiza.1541

Ectomycorrhiza are a type of mycorrhiza in which the hyphae remain outside the root cell, so hyphae outside the root cell.1563

In this case, the fungus are confined to the extracellular space of the cells in the cortex of the root.1578

In endomycorrhiza, the hyphae actually enter the cell walls. Hyphae enter the plant cell walls.1586

But they do not actually penetrate the cell membrane or enter the cytoplasm of the cell so just two subtypes of mycorrhizae.1597

OK, so we talked about root structure. We also talked about associations between roots in bacteria and roots in fungus.1605

The next part of the plant we are going to focus on are the stems.1612

The stems contain nodes, and these nodes are a point of attachment for the leaves; so these are points of attachment for the leaves.1617

Like the roots, the stem is covered with a layer of dermis.1639

So, if we took a root and cut it or a stem and cut it and got a cross-section of the stem, we would see this dermal tissue.1644

Or epidermis is the outer layer, so the epidermis is the outer layer, and this is continuous.1654

The root is covered with epidermis that continues up on the stem and then, out to the leaves.1661

In addition, the vascular system is continuous, as well.1667

So, we talked about the roots absorbing water and that water being carried up through the stem to the leaves.1670

Meanwhile, the phloem is absorbing nutrients that were made in the leaves and carrying those down through the stem to the roots.1674

Now, again, there is a difference between the arrangement in the vascular bundles of monocots and dicots.1684

And we talked about the roots of monocots and the roots of dicots and how those are structurally different.1691

Here in monocots, the vascular bundles are scattered throughout the stem.1699

So, if this is a monocot, then, you would find these vascular bundles throughout.1711

Contrast that with a dicot of the stem. Take the stem of a dicot.1717

You cut it into a cross-section, and then, what you are going to see is that the vascular structures, the vascular bundles, are arranged in a ring.1722

Remembering that in a stem in dicots, the vascular bundles are arranged in a ring.1735

Whereas in monocots, the vascular bundles are just scattered throughout.1743

The cortex is unspecialized tissue that lies between this epidermis and the vascular or conducting tissues.1750

So, we talked about the cortex earlier on. It can be a site of storage.1760

It could be a site of photosynthesis depending on the area of the plant.1765

The cortex, the pith is a central area of parenchymal cells.1770

So, the main point about the stems that you should know is that there is a difference in the vascular bundles between dicots and monocots1775

and also that the nodes are points of attaching for the leaves, and leaves are the structure that we are going to talk about next.1782

The leaf is a primary site of photosynthesis in most plants.1791

Now, there are exceptions. For example, in some cacti, a lot of the photosynthesis takes place in the stem.1795

And in fact, in cacti, leaves may be modified to form spines, and those spines are great protection for the cactus and minimize water loss.1801

The flat part of the leaf is called the blade, and the petiole is this little stock that attaches.1813

So, if you have a leaf, if you pull the leaf off of a tree, and this part will be the blade.1826

And then, the petiole is the part - the little stock - that attaches the leaf to the stem.1832

Like the root, the outer layer of the leaf and like the stem, as well, the outer layer is the epidermis.1839

So, here is a cross-section of a leaf, a close-up view, a cross-section of the leaf.1846

And here, this outer layer is the upper epidermis, so the top of the leaf, the upper epidermis.1852

And down here, protecting the leaf on the underside, is the lower epidermis1862

The epidermis produces a cuticle. The cuticle is a waxy...if you feel leaves, they have, sort of, a waxy feel.1873

And that waxy layer prevents the water loss from the plant.1879

The epidermal cells secrete a substance that comprises the cuticle, a waxy layer that prevents water loss.1885

The middle of the leaf is composed of ground tissue and vascular tissue.1896

We are going to talk about the ground tissue first. The ground tissue, these are the mesophyll cells.1902

And as you can see, there are two different types of mesophyll cells.1913

And these right here, these more columnar cells, are known as the palisade mesophyll or palisade parenchyma.1918

Here, we have a second layer of mesophyll cells, and these are known as the spongy mesophyll cells/layer.1934

The mesophyll cells contain many chloroplasts because these are the site of photosynthesis.1946

And starting out by contrasting the shape and the arrangement of the palisade versus the spongy mesophyll,1960

in the palisade mesophyll, the cells are columnar, and they are much more tightly packed than the spongy layer.1966

The irregularly shaped spongy mesophylls are purposely less tightly packed to allow for these air spaces.1972

And what the air spaces do is that they allow for gas to diffuse up and reach the cells throughout the leaf.1984

The circulation of gases for photosynthesis to occur, CO2 needs to go into the cell, and oxygen needs to be released.1991

So, there needs to be circulation of gases throughout these air spaces.1998

The underside of the leaf, in particular, contains pores called stomates or stoma singular, stomata plural or sometimes stomate.2004

The upper area can contain these, as well, but it is shown here in the lower.2016

And the purpose of a stoma is to allow for gas exchange, so CO2 needs to enter the plant, go into the cell.2020

Photosynthesis occurs. Oxygen needs to be released.2030

But because the epidermis is covered with this cuticle layer, which prevents water from leaving, it also prevents gas exchange from occurring.2034

So, that is why there are these stoma, which are pores.2046

I am going to talk about stoma in detail in a few minutes, but right now, what you should know is that they are flanked by guard cells.2053

These stoma are flanked by guard cells, and the guard cells control the opening and closing of a stoma.2061

Most of the water loss in a plant occurs through these pores, occurs through the stomata.2072

And so, the guard cells need to minimize water loss while at the same time, allowing the stomata to be open enough for gas exchange to occur.2078

Alright, we have talked, now, about the dermal tissue and the ground tissue, now, the vascular tissue.2086

So, the last part of the leaf here is what is shown, a vascular bundle or a vein, and they contain the xylem and the phloem.2093

We already talked about xylem and phloem being present in the root and in the stem, and it goes up into the leaf, the vascular bundle.2105

Within the vascular bundle, the xylem and phloem.2112

And since the nutrients in the plant, the carbohydrates, are produced here in the leaf, the phloem is going to carry those to the rest of the plant.2116

The vascular bundle is surrounded by a layer of cells called the bundle sheath, so a sheath,2123

so bundle sheath or bundle sheath cells that are just a layer surrounding the vascular bundle, and these are specialized mesophyll cells.2132

Monocots and dicots have different vascular bundle patterns or different vein patterns.2142

So, if you look at the leaves of a monocot, it is going to have a parallel structure of the veins.2147

Whereas, a dicot, it is going to be more of a net-type pattern, so parallel in a monocot versus a net-type pattern in a dicot.2157

We are going to talk more in detail, now, about stomata, so as I introduced, stomata are pores on the surface of leaves that allow for gas exchange.2173

More than 90% of the water loss from a plant is lost through the stomata.2183

To prevent this, the opening of stomata is tightly controlled by the guard cells.2188

Guard cells are modified epithelial cells, and they open and close the stoma by changing shape.2197

This picture here shows two guard cells, and it is showing this stoma as being open, so this is an open stoma.2204

And I said that the stoma or the guard cells control the opening and closing of the stoma by changing shape.2213

And what it has to do with is that when water enters a guard cell, the guard cell becomes turgid, so it is full of water.2220

And guard cells have microfibrals that are radially oriented, so they radiate out.2229

And this arrangement causes the cell when filled with water to bow outward.2240

When these cells are flaccid, they are not filled with water. They come together.2245

When they fill with water, they bow outward like this. They bend away from each other, and the result is that the stoma is open.2252

So, when the cell is flaccid, it does not have water, or it has minimal water in it. The stoma is closed.2263

When the guard cells fill with water, they become turgid. They block the opening to the stoma, and the stoma is open.2274

To understand how guard cells work, you need to understand the concept of water potential. The water potential of pure water is zero.2285

If a solution is more concentrated than pure water, it has more solutes in it, it will have a more negative water potential.2302

The most important point is that water moves from areas of higher water potential to areas of lower water potential.2309

So, water is going to go from an area of high water potential to an area of low. It is going to move from high to low water potential.2318

If pure water is zero for example, then, if a cell has a lower water potential, it is going to be more negative, then, the water is going to move into that cell.2332

Solutes are not the only factor that affects water potential. For example, pressure can affect water potential.2347

In a plant, the cell wall exerts pressure, and that could decrease the water potential from the inside of the cell as that pressure is pushing outward.2354

So, it is not just about solutes. It is about other factors, as well.2363

Now, potassium is very important in the activity of guard cells, and this ties in to water potential.2368

Because what happens is a guard cell, and then, there are dermal cells nearby; and what happens is potassium is taken up by the guard cells.2374

As the potassium is taken up by the guard cells, the water potential inside the guard cell will decrease.2389

So, potassium goes into the guard cell. It is going to decrease the water potential inside the guard cell.2396

The water potential is going to become more negative, and this water will be stored within vacuoles inside the guard cell.2401

And as the water potential inside the guard cell becomes more negative, water is going to enter the guard cell.2409

The guard cells will bow outward like this, and the stoma will be open.2415

When potassium moves out of the guard cell, water is going to follow it out through osmosis, and the cell is going to become flaccid.2421

These two guard cells will be flushed together, and they will block the opening of the stoma.2430

You should know conditions that stimulate the stoma to open or to close.2436

So, factors or conditions for the opening or that stimulate the opening of the stoma, one is decreased CO2 levels within the air space of the leaf.2442

We talked about the airspace of the leaf where the gases can circulate around, and CO2 is used in photosynthesis.2463

Therefore, when the CO2 levels drop, the stoma needs to open so that more CO2 can come in.2468

Oxygen can leave, and photosynthesis can continue. Therefore, decreased carbon dioxide leads to the opening of the stoma.2475

The second factor that will cause a stoma to open is the presence of light.2484

In addition to CO2, light is needed for photosynthesis, and in fact, stoma are typically open during the day.2487

The plasma membrane of guard cells contain blue light receptors.2497

These blue light receptors, when they are stimulated due to the presence of light, will cause potassium to be absorbed from surrounding cells.2503

Water will enter the cell, and again, stoma will open.2516

The third factor is Circadian rhythm, which refers to 24 hour cycles that organisms have.2520

And plants have a Circadian rhythm, a 24 hour cycle, for the opening and closing of stomata.2529

And in fact, even if it is dark, if you put a plant in a dark area, the stomata will still open during the day and then, close at night.2536

Now, these other factors can affect it if there is light, and it is daytime, then, more stomata will open.2548

But, there is already this underlying 24 hour cycle of opening during the day, closing at night.2554

Now, these are some factors that cause the stomata to open, factors causing a stoma to close, a couple factors for that, a few factors.2560

One: decreased water. If there is not enough water available, water cannot enter the guard cells.2576

And so, even if potassium enters the guard cells, if there is not water available, the water cannot enter, and these guard cells will not bow out.2585

So, in this case, the cells will remain flaccid, and the stoma will close,2591

which is actually good because if there are conditions of low water, the stoma needs to be close to prevent further water loss.2595

Another factor is temperature. If there is increased temperature, then, the plant needs to conserve water, so if it is hot out, the stomata will close.2602

Finally, there is a hormone called abscisic acid, and abscisic acid is secreted by plants.2613

And so, it is a hormone that is secreted by plants when they are dehydrated.2627

When the abscisic acid is secreted, it stimulates the guard cells to close, so this is a sign of dehydration.2631

So, decreased water, increased temperature or increased abscisic acid, will all stimulate the stomata to close.2639

Decreased CO2, the presence of light and a Circadian rhythm, all are factors involved in opening a stoma.2648

The next area that we are going to talk about today and the final area is plant growth.2658

And I am going to start out with an overview of plant growth.2664

Plants are capable of indefinite growth. Animals, humans, grow to a certain height and then, we stop.2668

By contrast, plants can grow indefinitely, and this type of indefinite growth is called indeterminant growth.2679

Indeterminant growth is possible because of undifferentiated cells present in areas of the plant called the meristems.2687

These are embryonic cells that are actively dividing. They are undifferentiated, and because of this, plants can grow on and on.2695

Some plants have a life cycle of a year. Their lifespan is a year.2702

They grow during that year. These are annuals.2706

Whereas, perennials have a lifespan of many years and may continue to grow throughout this long lifespan.2709

With some plants like the ancient red woods, the lifespan can be over a thousand years, and they continue growing.2714

There are two types of growth that occur in plants: primary growth that increases the length of the plant.2721

The plant grows up into the air and down into the ground, and all plants undergo primary growth.2729

However, non-herbaceous plants, woody plants like trees, also undergo secondary growth.2735

With secondary growth, the plant increases in girth or width.2742

To understand the growth of plants, we need to look more closely at the meristem- the structure that contains embryonic cells.2747

And there are two types of meristems. One is an apical meristem, and these are present at the tips of roots and shoots.2754

These would be found at the tips of the roots and the tips of the shoots. These are responsible for increase in the length of the plant.2764

In contrast, lateral meristems are responsible for the secondary growth of a plant.2772

Let's start out talking about primary plant growth of roots and of stems.2779

As I said, the apical meristems are responsible for the primary growth of a2787

plant for the plant growing up towards the sky and down into the ground.2791

These are found at the tips of roots, shoots and on axillary buds, which are regions on the stem where a branch can grow from.2795

We are going to first look at the zones of growth within a root.2804

So, here is a root, and the apical meristem is located at the tip of the root.2809

And it is covered by a root cap, a structure called a root cap that provides protection.2813

And if you look at the tip of the root, you can divide it into three regions of growth or zones.2820

The first zone is the zone of cell division, and that is roughly this region right here.2825

The apical meristem is in the zone of cell division, and this is an area where mitosis is actively occurring.2835

Cell division is ongoing, and the result is new root cells will be created; and also replacement meristem cells will be created.2847

Just above that is another zone called the zone of elongation.2856

In the zone of elongation, these newly created cell elongate, just as the name suggests.2863

And as they elongate, they push the root more deeply into the soil, so it grows downward.2870

Finally, there is the zone of differentiation. Different cells have different functions.2876

Some are epidermal cells. Some are parenchyma cells.2884

And within the zone of differentiation, cells differentiate into a specialized cell type where their structure and function is determined.2887

This shows you the root structure. Here is a stem.2896

So, the shoot also undergoes increase in length, primary growth, but the apical meristem is a bit different structure.2903

And one area you should note are what is called the leaf primordia.2911

And the leaf primordia are the site from which new leaves originate from the apical meristem of the shoot system.2916

That is primary plant growth, but remember that woody plants undergo secondary plant growth, as well.2927

And secondary plant growth is an increase in the girth or width of a plant.2934

Parts of the plant that are the result of primary growth constitute what we call the primary plant body.2939

For those plants that undergo secondary growth, they have additional areas that are called the secondary plant body.2946

Secondary growth occurs at the same time as primary growth but not in the exact same spots.2954

While the plant is growing taller at the tip and adding cells at the bottom of the root,2960

meanwhile, in older areas of the plant, secondary growth can be occurring to thicken that up and increase the width.2968

They are both occurring at the same time but not in the exact same spots.2975

The site of secondary plant growth is the lateral meristems.2978

And there are two types of lateral meristems: the vascular cambium and the cork cambium.2983

So, we are going start by talking about the vascular cambium.2990

As the name suggests, the secondary xylem and secondary phloem originate from the vascular cambium.2993

The vascular cambium consists of a cylinder of parenchymal cells.3004

So if you took a cross-section of the stem, you would see a cylinder of parenchymal cells running all up and down the stem.3008

And this is the vascular cambium.3016

Secondary xylem, which is created from this, developed from this, is wood, so secondary xylem is wood; and secondary xylem has a lot of lignin in it.3020

The cell walls are very strong and provide excellent support for the plant, which is why trees can grow so tall.3035

The xylem functions, of course, in water transport and as a mean of support.3042

However, older secondary xylem eventually stops transporting water, and its only function is to support the plant.3048

So, that is the vascular cambium, xylem and phloem or secondary xylem and secondary phloem, developed from the vascular cambium.3056

The second type of lateral meristem is the cork cambium.3071

Now, primary growth of plant produces the epidermis, so that is outer layer covering the plant.3078

However, in woody plants, that epidermal layer eventually falls off and is replaced by a thicker covering like bark.3083

And this is produced largely by the cork cambium.3093

Like the vascular cambium, the cork cambium is a cylinder of cells that runs along the length of the stem.3097

So, within the cork cambium are produced cork cells, and cork cells produce a substance called suberin.3104

Suberin has a waxy consistency. I have mentioned this before, and these cork cells along with the tissues, they produce what are called the periderm.3115

This periderm covering replaces the initial epidermal covering. Periderm is cork cells plus the tissues produced by them.3127

The periderm plus the secondary phloem constitute bark, so the secondary phloem is also a part of the bark.3143

This periderm provides protection from water, so it is impermeable. The periderm is impermeable to water and to gases.3151

There are structures in bark. There are little slits or pores called lenticels, and they allow gas exchange across the periderm.3164

Secondary plant growth occurs at the lateral meristems. There are two types of vascular cambium and the cork cambium.3182

The vascular cambium is the site of production of the secondary xylem and phloem.3188

The cork cambium produces cork cells, which come along with their tissues to constitute the periderm,3192

which is an outer covering that is impermeable to water and gases.3198

Alright, so today, we covered quite a bit. We talked about plant structure as well as growth.3205

So, we are going to go ahead and do some review questions.3210

Label the following structures on the diagram of the leaf below.3213

OK, first, we have bundle sheath. The bundle sheath is a layer of cells surrounding the vascular cylinder, so the bundle sheath is right here.3217

And within that is the vascular bundle.3229

Stoma: well, a stoma is an opening or pore in the leaf that allows for gas exchange, so the stoma is right here.3233

Upper epidermis: the epidermis is the outer covering of the leaf, so this is the epidermal layer; and this is actually the upper epidermis.3246

Next, vascular bundle: so, the bundle sheath is this outer covering. This entire structure, though, is the vascular bundle.3258

Next, the spongy mesophyll: well, this is all the mesophyll layer. The ground tissue is all right here.3270

The vascular bundles are within that.3277

And these spongy mesophyll are the irregularly shaped mesophyll cells that leave room for air spaces so gases can circulate.3279

So, right here is the layer of spongy mesophyll.3288

Palisade mesophyll cells are columnar, and they are more tightly packed. That are these cells right here, so we have the palisade mesophyll layer.3295

Those are all the sections of a leaf that we are asked to label.3308

List the three types of plant tissue and their functions.3316

At the beginning of the class, we talked about three types of plant tissue. The first one is the dermal tissue.3321

The second type of tissue was the vascular tissue, and finally, we talked about the ground tissue and functions.3332

Well, a major function of the dermal tissue is to produce the outer covering of a plant.3345

The dermal tissue is the epidermis. That is one type of dermal tissue.3354

So, the outer covering or epidermis consists of dermal tissue, and these epidermal cells secrete a waxy layer, a cuticle that protects the plant.3361

The second type of tissue is the vascular tissue, and this includes the xylem and phloem.3373

And the function of vascular tissue is to transport nutrients and water.3379

Finally, the ground tissue: most of the plant tissue is ground tissue, and ground tissue is a site of storage.3390

It is a site of photosynthesis. It is also where other metabolic functions of the plant are carried out.3400

So, these are the three types of tissue and their major functions.3413

What are two factors that stimulate the opening or closing of the stomata?3420

We talked about some factors that stimulate the stoma to open and then, other factors that would stimulate it to close.3426

They only asked you for two total, but I am going to do more; so we are going to start out with factors that would cause the stoma to open.3434

What would stimulate the stoma to open? Well, decreased CO2 within the air spaces because when CO2 level is dropped,3444

it means photosynthesis has been going on and using up CO2, the plant needs more CO2 to continue, and it needs to let the oxygen out.3454

The second factor that stimulates that stoma to open is the presence of light.3462

Remember that there are blue light receptors within the guard cells that can detect3468

light and signal the guard cells to allow the stoma to open because light is present, and this is the time when photosynthesis can occur.3476

Finally, stomata open according to a 24 hour cycle, so simply Circadian rhythm or Circadian cycle that stomata open according to a 24 hour cycle.3484

So, those are factors that would stimulate stomata to open. Factors that would stimulate stomata to close would be a lack of water.3508

The plant needs to conserve water when there is not much around, and if there is a lack of water, water cannot enter the guard cells.3518

The guard cells will not bow outward and become turgid and allow the stoma to be revealed, so lack of water.3525

The second would be increase in temperature. In high temperature, the stoma will close to minimize water loss.3532

And finally, the presence of abscisic acid, abscisic acid is a hormone-made when plants3541

are dehydrated and signals the guard cells that they should close and conserve water.3551

Finally, what is the difference between primary and secondary growth in a plant?3560

What structure is responsible for each type of growth, and what are the zones of primary growth in a root?3566

Well, primary growth is an increase in the length, and this occurs in both herbaceous and non-herbaceous or woody plants.3574

Secondary growth only occurs in non-herbaceous plants, and secondary growth is an increase in girth or width.3589

What structure is responsible for each type of growth?3600

Well, primary growth occurs in the apical meristems, and these are present at the tips of roots and shoots.3603

Secondary growth occurs in the lateral meristems, and there are a couple different types that we talked about- vascular cambium and cork cambium.3615

Finally, what are the zones of primary growth in a root?3634

We talked about several different zones in the root. The bottom zone is the zone of cell division.3639

This includes the apical meristem, and it is a region of active mitosis.3647

Just above that and overlapping it lies the zone of elongation. The newly created cells elongate and push the root further into the ground.3653

And finally, just above that, the zone of differentiation where cells become specialized in structure and function.3664

So, that was our last question and concludes this section on plant structure at

Thank you for visiting.3679