Like any other multicellular living thing, a leaf is made up of layers of cells. Viewing the leaf structure under the microscope shows different types of cells that serve various functions. Using a microscope, it's possible to view and identify these cells and how they are arranged (epidermal cells, spongy cells etc). To do this a compound microscope is required given that it allows for higher magnification.
While a compound microscope is ideal for viewing the internal structure of a leaf, a stereo microscope would be the ideal tool for observing the external structure of a leaf (vein, lamina etc).
To view the external structure of a leaf, the following will be required:
Observation 1 (leaf surface)
Observation 2 (stomata)
Stoma refers to the minute pores that can be found on the epidermis of a leaf. These pores vary in size and allow for the movement of water and gases in and out of the intercellular spaces. The following is the procedure for viewing (as well as estimating stomatal frequency) stoma on the surface of a cell.
While the compound microscope would be more effective for viewing the frequency of the stoma, a stereo dissecting microscope can also be used for this purpose. For instance, by viewing a Zebrina plant leaf (older leaf) it's possible to view the stomata as green patches with a purple background.
When viewing the surface of the leaf under the stereo microscope, students will be able to clearly see hair-like structures (trichome) on the leaf surface that serve a number of functions ranging from trapping insects to trapping water/moisture. Students will also observe the intricate leaf veins (vascular bundles) running across the surface of the leaf.
With some leaves (such as the maple leaf), it's possible to isolate the vascular bundles (vein structures) for viewing under the microscope.
When viewing the cells of a leaf, this should be done using a compound microscope. The procedure used or this also allows for the stomata to be seen.
Having obtained a leaf, carefully fold it and using a pair of tweezers, peel off the lower surface of the leaf (epidermal membranous transparent layer)
Place the peel into a watch glass that contains distilled water
Remove the epidermal skin from the watch glass and transfer to another watch glass with safranin for about 30 seconds (few drops of safranin solution)
Move the epidermal skin from the safranin solution and place it in the glass with water once again to remove excess stain
Place the epidermal skin onto a clean, dry glass slide and add a few drops of glycerine
Cover the epidermal skin on the slide with a cover slip
After removing excess glycerine with a blotting paper, place the slide on the microscope for observations
When viewed under the microscope, it's possible to see the epidermal cells that tend to be irregular. In addition to the epidermal cells, one will also see the leaf spores (stomata) in between the epidermal cells. Typically, the stomata are bean shaped and will appear denser (darker) under the microscope.
Under high magnification, students can differentiate between closed and open stomata. The bean-shaped structures are referred to as guard cells and contain a nucleus and chloroplasts.
Whereas the transparent thin epidermal skin of the leaf allows the student to observe the stomata and other epidermal cells, it would be important to prepare a cross section of a leaf to observe the arrange of cells inside the leaf.
Under high magnification, students will be able to view the internal structure of the leaf. This includes the upper and lower epidermal cells (flattened cells) with the mesophyll layer in between. Here, the mesophyll section of the leaf contains two different type of cells including the palisade mesophyll (elongated cells) and the spongy mesophyll (spherical or ovoid). This part of the leaf also has air spaces.
Using the stereo and compound microscope, students can view different parts and structures of a leaf. These include both the external and internal structures. With a wide range of leafs available, students can obtain different types of leaves (thick and long leaves etc) and compare the appearance of such structures as the stomata, shape and arrangement of cells.
This can be an important lesson to help students understand the differences in the arrangement and size of the cells and stomata between different types of leaves and consequently learn the significance between these differences. For instance, students may notice larger stomata in thick leaves that allows for the leaves to release more water compared to smaller stomata in thin leaves that serve to preserve water.
Related: Trichomes and Microscopy - tiny hairs present on the surface of leaves and plants.
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