You may think of a leaf as a simple green organism that falls off trees and means you have to rake your yard. Yet they are actually very interesting and complex cellular organisms with a pretty big job.
Leaves are a vital part of any vascular plant as this is the site at which photosynthesis occurs in order to feed itself.
They also play a huge role in the food chain, providing many mammals with a vital source of nutrition.
Different microscopes can be used for different levels of magnification.
Stereoscopic microscopes for example are used for low magnification observations which is why we can use them to examine the surface of a leaf rather than the internal structure.
A compound microscope has multiple lenses giving the users the highest level of magnification (around 1000x), making it perfect for viewing the cross-sectional area of a leaf.
External Leaf Structure
A common leaf consists of a broad expanded blade or lamina, which is attached to a plant stem by a stem-like structure called a petiole.
You may refer to these as a stem or stalk, but they are actually considered to be part of the leaf itself. Leaves that are attached directly to the plant stem are called sessile leaves or apetiolate leaves.
Most leaves have a central vein or midrib which directly travels the length of the leaf and contains the primary vein as well as supportive ground tissue, both vital for supporting the structure of the stem as well as storing water and sugars.
Internal Leaf Structure
Here we’re going to take a look at the complex internal structure of a typical leaf. The main parts of the leaf are the epidermis and stomata, the mesophyll cells and the vascular system.
These tissues all have various functions that contribute to the survival of the plant.
The outermost layer of a leaf is called the epidermis, with the upper epidermis located on the top side of the leaf and the lower epidermis on the bottom side of the leaf.
Plants in excessively hot or cold conditions may have several layers to protect against water loss, however, the epidermis is usually one cell layer thick.
The main function of the epidermis is to protect the leaf whilst allowing sunlight to reach the cells underneath, regulate gas exchange and also stop the leaf from losing water.
The epidermal cells are protected by a waxy layer called the cuticle. This layer is rich in lignin (helping to give the leaf rigidity) and waxes which give a leaf its waxy texture.
Not only does this serve as a protective and waterproofing barrier it also protects the leaf against drought, extreme temperatures and pest infections to name a few.
The surface of a leaf can also feature trichomes or small hairs, which are adaptations to decrease the chances of being eaten by insects.
The small hairs can reduce the insects’ ability to move, deterring them from using the leave as a food source.
Under The Microscope
In order to see the structure of the surface or epidermis of the leaf up close, you will need to find a small healthy leaf to put under a stereo microscope.
- Start with low power and gradually increase the magnification.
A singular stoma is a minute opening on the epidermis of a leaf that can vary in size, through which gases such as carbon dioxide and oxygen are exchanged.
Carbon dioxide enters the leaf through the stomata to be used for photosynthesis, whereas oxygen and water vapor exit through the stomata as a product of the process.
Stomata can also facilitate the movement of water between cells. Usually the lower epidermis or abaxial surface contains more stomata than the upper epidermis or adaxial surface as it is cooler and therefore less prone to water loss.
The opening and closing of stomata can be influenced by external factors such as climatic conditions, water availability and the time of day.
As plants photosynthesise in the daytime when sunlight is present, the guard cells absorb water causing the stoma to open. During the nighttime or when there is a lack of sunlight, these guard cells lose water and the stoma closes.
Each stoma has two guard cells surrounding it which help to regulate the opening and closing.
Under the Microscope
To see a stoma up close you’re going to need to use a compound microscope, you should be able to see a stoma when increasing the magnification 100 times.
- Flatten a leaf and apply clear nail varnish to the surface
- When dry, peel off a film of thin skin from the surface of the leaf
- Place this film onto a microscope slide and place a cover slip on top
Layers of cells below the epidermis are referred to as the mesophyll. This area houses the many chloroplasts needed for photosynthesis.
In typical leaves, the mesophyll layer is split into two: the palisade parenchyma and the spongy parenchyma.
Palisade parenchyma cells are tightly packed and arranged in column shapes over 1 to 3 layers.
The main job of these cells is to capture incoming sunlight and rotate chloroplasts to the top of the leaf, allowing them to regenerate. They can also decrease the intensity of the sunlight that enters the spongy mesophyll underneath.
The spongy parenchyma cells (or mesophyll) are loosely arranged and of irregular shape compared to palisade cells. This is so the air between the cells can facilitate gaseous exchange from the incoming stoma.
Despite these cells being located in the middle of the leaf, they still contribute to photosynthesis.
Veins (Vascular Bundles)
Contrary to what you may think, the leaf contains vein-like structures as well as the stem. These are the third types of tissue that make up the leaf structure.
Vascular bundles enter the leaf through the stem and contain xylem and phloem tubes, each with an individual purpose.
Xylem tubes usually face upwards, transporting water and minerals to the leaves. Phloem generally faces downwards and transports photosynthetic products created in the leaf to other areas of the plant.
No matter the size of a vascular bundle, it will contain both xylem and phloem tissues.
Under The Microscope
To isolate the vascular bundles or vein structure for viewing under a microscope you’ll need to simmer a Maple leaf for 1 ½ hour.
- When the leaf starts to feel slimy, place it on a plate or petri dish
- Add a small amount of water and slowly remove the soft tissue using a small brush such as a paintbrush from both sides of the leaf
- Place the vascular bundles between 2 hard surfaces such as books to prevent this curling up or twisting
- After an hour or so, view the leaf vein tissue under a stereo microscope or a compound microscope set to low power
Cross Section Of A Leaf
Under The Microscope
Studying the cross-section of a leaf enables us to look closely at how the arrangement of cells inside the leaf structure. Below we will indicate how to do this.
- Take a fresh, healthy leaf and lay it flat on a work surface
- Slice about a 1” section out of the center using a sharp knife – the cells surrounding the central vein are the main area of focus here, so make sure you manage to include a section of the vein
- Take the strip of leaf you’ve cut and tightly roll it from one end to the other
- Make several very thin slices of one end of the rolled-up leaf with a knife to finish off your cross-section
- Place the inner part of the leaf face up on a microscope slide and add a few drops of water to the leaf cross-section and cover with a coverslip
- Finally, take your slide and look at it under the microscope (10x objective) to see the structure and use higher power to see the cell in detail
As you can see, although a common leaf may not look very interesting to the naked eye but using a combination of stereoscopic and compound microscopes you can observe the complex structure.
The external and internal structures are composed of different tissues to serve different purposes. However, they all contribute to the same goal of photosynthesis and the survival of the plant.
Here we have used a typical leaf as an example to demonstrate how the structures are layered and what procedures are needed to see these up close.
These practices can be applied to compare different types of leaves and learn about the differences in structures. We would suggest recording your observations to note key characteristics.
We hope this has given you a better insight into how a leaf is structured, what they are made up of, the specific functions carried out by each group of cells and how to use a microscope to prepare and view these tiny structures.
Frequently Asked Questions
What Can You See In A Leaf Under A Microscope?
Using a stereo microscope, you can see the epidermal layer or surface layer of a leaf as well as the stomata cell structures.
These will appear in a ‘bean’ shape and are usually denser cells making them easier to identify. If you want an even more up close view of a leaf, you can view the cross-section using a compound microscope.
How Do You Prepare A Leaf For A Microscope?
How you prepare a leaf to view under a microscope depends on which structure you’re looking at.
- To look at the surface structure of a leaf you can place a fresh, healthy leaf under a stereo microscope and gradually increase the power.
- In order to see the stomata, you must use clear nail varnish to peel off a thin layer of film and place it between a microscope slide.
- When preparing a leaf for cross-sectional examination, you cut a very thin slice of a leaf with a sharp knife to get an almost transparent slice. You must add water to this when on a glass slide in order to observe using a compound microscope.
What Are The 4 Structures Of A Leaf?
The four structures in a leaf from top to bottom are the waxy cuticle and epidermis, the palisade cells where photosynthesis happens, the spongy mesophyll layer which aids in the transport of water and the final layer of the epidermis and cuticle, also containing stomata and guard cells which are vital for the exchange of gases.
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