The Definition, Function, Structure & Microscopy Of Epidermal Cells In Plants

On the surface, plants can seem very simple when compared to animals, but in reality they are often much more complex.

The Definition, Function, Structure & Microscopy Of Epidermal Cells In Plants

This misconception is rooted in the fact that plants don’t move and don’t consume other living matter for energy.

As such, many people see them as one of the simplest forms of life on the planet, even though this is far from the case. 

Plants are incredible life-forms capable of taking energy directly from the sun to fuel their growth and expansion.

While this process sounds simple in theory, it is in fact the work of intricate chemical reactions and the complex biological structures that facilitate them. 

Here at WhatAreBacteria.com, we aim to help you better understand the natural world and the processes that govern it.

In this article, we will be looking at epidermal cells in plants. We will tell you what these cells are, what functions they perform, as well as how they work to help the plant grow and thrive. 

What Are Epidermal Cells?

Epidermal cells make up the epidermis of a plant, which is its outer layer, much like the skin on an animal.

These cells form a protective layer around the entire exterior of the plant to keep the other cells on the inside safe from damage or over-exposure.

The epidermis also plays an important role in regulating water loss and transpiration rates. 

Since they are the main barrier between the internal cells of a plant and the outside world, these cells are located very close together.

Epidermal cells can be found in every part of the plant, from the roots all the way up to the leaves.

There are three main types of epidermal cells in plants including, pavement cells, stomatal guard cells, and trichomes. 

When a developing seed is undergoing the process of embryogenesis, the epidermal cells will begin to differentiate into these three different types.

Each of the three serves a vital function towards helping the plant to not only survive, but thrive in its specific habitat. 

We will cover each of these cell types in turn to explore their function, structure and microscopy. 

Pavement Cells

Of the three different types of epidermal cells, pavement cells are the most common and numerous. They are responsible for providing support for the plant’s body by giving it strength and stability.

Pavement cells are usually flat and elongated in shape, with clearly defined boundaries that lock together to prevent water or other internal substances from escaping the plant.

They are not very specialized cells, since they don’t need to carry out complex functions like the stomatal guard cells do.

As such, one of their main defining features is the way that they are tightly packed together, with the cell wall of each one touching its neighbors. 

Although they are relatively simple in their structure, pavement cells can vary from plant to plant.

Arabidopsis thaliana for instance, has very strong leaves, and the reason for this is often attributed to the jigsaw-like arrangement of its pavement cells. 

Meanwhile, pavement cells in the stem of a plant will often be more elongated than those found in the leaves or roots.

This is because the stem is the part of the plant that requires the most strength for it to grow upright and be able to compete for light. 

As well as providing structural integrity, pavement cells help to maintain the internal temperature of a plant as well.

By keeping the temperature of the plant constant, they help to ensure that all the chemical reactions occurring in other cells can happen with optimal efficiency. 

They keep the inner cells locked in place, so they can’t move around too much, as well as keeping any harmful bacteria or particles out.

Finally, they also separate the stomatal guard cells so that they aren’t too close together.

You will understand how important this function is when we cover stomatal guard cells in our next section. 

Stomatal Guard Cells

The second type of epidermal cells are called stomatal guard cells. These cells are responsible for regulating what is allowed into the plant, and what is allowed back out.

The stomata are microscopic openings on the surface of the leaf which allow air to enter and leave the plant.

They are enclosed by guard cells, which are capable of changing shape to open and close the stomata as necessary. 

When open, air can pass freely both into and out of the plant. This allows it to absorb carbon dioxide from the air for photosynthesis and excrete oxygen back out.

Water vapor also escapes the plant via these pores, which is essential for transpiration (the process by which a plant turns liquid water into water vapor). 

But how do these cells allow the stomata to open and close? The answer lies in an essential organelle that can be found in all plant cells, called the vacuoles.

Vacuoles are sacks inside a plant cell that are filled with water and essential ions.

When full, these sacks allow the cell to expand, making it turgid, and when they are empty the cell can relax to become flaccid. 

When flaccid, the cell walls of both guard cells will touch each other, closing the stomata and preventing air from leaving the plant.

This is often done in dry conditions, when the plant needs to hold onto as much water as possible. 

On the other hand, when the vacuoles are full of water and the cells are turgid, the cell walls separate to open the stomata.

This allows air to leave the plant, facilitating both photosynthesis and respiration.

With the stomatal guard cells evenly spaced between pavement cells, the plant can regulate its own internal environment to hold onto water when it is scarce and let it escape in the form of vapor when it is plentiful.

Trichomes  

These are microscopic hairs that are located on the surface of the epidermis. They are actually an extension of the epidermis, rather than separate cells in their own right.

Trichomes are highly specialized, and their function can vary greatly from plant to plant. 

On some plants, they are simply barriers that protect the inner cells of the plant.

However, in other species, these hairs may be filled with poison to deter predators.

Stinging nettles are a great example of a plant that uses its trichomes as a defensive mechanism to avoid being eaten by animals. 

Some species, like sage, mint and even cannabis, use their trichomes as a store of oil and resin. Other plants will have hardened trichomes that form their spines or thorns.

These obviously serve a more defensive nature, like the poison filled trichomes we mentioned above. 

Cuticle

The final part of a plants’ epidermis is the waxy cuticle. This is a layer made from cutin (an ester formed from fatty acids) often located on the upper epidermis of a plant.

Here it acts as a water-repellent to prevent the plant absorbing too much water in heavy rainfall. 

The cuticle is normally much thinner, or not present at all, on the underside of the leaves.

This is because it would interfere with the stomatal guard cells and their ability to let air in and out of the cell. 

The thickness of the cuticle varies greatly from plant to plant, and some species that live in arid conditions will have a much thicker cuticle.

This is to lock water into the plant so that it can’t easily escape, which is essential for desert dwelling species like cacti. 

Since the cuticle is shiny, it also helps to reflect excess sunlight. 

How To Observe The Epidermal Cells Of A Plant Under A Microscope

How To Observe The Epidermal Cells Of A Plant Under A Microscope

The classic example taught to most students when looking at epidermal cells under a microscope is those of an onion.

Since the epidermis of an onion is one cell thick, it is very easy to take a layer of onion skin and view the pavement cells under a microscope. 

When doing this, you will be able to clearly see the arrangement of rectangular pavement cells, distinguished by their thick cell wall.

Inside each cell you will be able to see a small circle which is the nucleus. You may also be able to spot larger, irregular shapes which are the vacuoles of each cell. 

If the cells are blurring together, and you are struggling to see them properly, then you can use a drop of iodine or methylene blue on your slide to make them clearer.

Onion skin contains lots of starch, which will bind with the iodine to stain blue. 

However, the epidermis of an onion is relatively simple and if you want to have a chance of seeing all the different epidermal plant cells, you will be better off with a common leaf. 

The Microscopy Of A Leaf

To see all the different types of epidermal cells, you will need to gather a few items. First, you will need to find a smooth leaf.

Look for one with a slight sheen to it, as this may be the easier type to collect an epidermal sample from.

If your leaf is too rough, crinkled or hairy, you may struggle to remove a layer of epidermis in one piece.

You will also need a compound microscope, glass slides for placing your sample on and coverslips for holding it in place.

To collect your sample, it will help to have some forceps or tweezers. Finally, you will need a small amount of tap water and a syringe. 

Break your leaf in half with both hands to create an uneven edge. Look for a thin, transparent layer jutting out from the underside of the edges of your torn leaf.

Use forceps to carefully remove this layer and place a small amount of it on your slide.

Try to get your sample to lay flat without any folds or wrinkles, as this will make it much easier to examine. 

Once you have your sample placed on your slide, cover it with a drop of water from your syringe and place a coverslip over the top.

You are now ready to start looking at your carefully prepared sample under the microscope.

Start at the lowest magnification setting and work your way up, taking the time to focus your image each time. 

As you zoom in, you will be able to see the pavement cells, which will look like interlocking rectangles arranged side by side.

In between these, you will be able to spot larger pairs of cells that are the stomatal guard cells.

Since the leaf has been removed from the plant, they are likely to be closed, with their cell walls meeting in the middle. 

You may also see the tiny hairs that are the trichomes of the leaf. Depending on the species of your sample, you may see lots of trichomes or only a few.

Some other cool things you may notice while observing your leaf is the large vacuoles in the stomatal guard cells when compared to the pavement cells.

You might also be able to spot several tiny circles, which are the cell’s nuclei. 

If you are having trouble seeing your sample clearly, then there are few things you can do to improve your image.

Slightly closing the iris diaphragm on your microscope may increase the contrast, showing the different cells much more clearly without them blurring together.

Using a methylene blue stain will also help to show up the different cells much more clearly. 

Conclusion

Hopefully, after reading this guide, you now know a lot more about the epidermal cells of plants and what they do.

It may also have helped you to realize just how complex plants are, and how incredible their biology is.

If you have a compound microscope, then preparing a sample of epidermal cells from a smooth leaf is a great activity to use it for.

You can even get your children involved to teach them more about biology and plants, in a way that is very fun and engaging.

Should you wish, you can experiment by taking samples from different types of leaf, to see the differences and similarities between varying species. 

Jennifer Dawkins