1 Practical 1
Preparing a slide of onion epidermal cells and calibrating an
eyepiece graticule
Apparatus and materials
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microscope
eyepiece graticule
stage micrometer
sharp knife or scalpel
small onion
slides and coverslips
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forceps
mounted needle
2% iodine in potassium iodide solution
dropping pipette
filter paper
eye protection
Introduction
In this practical, you will:
• make a temporary preparation of some onion epidermis cells
• stain the cells so that you can see structures within them
• set up a light microscope and use it to make observations and measurements
• make a drawing of the onion cells
• calibrate an eyepiece graticule and use it to measure the size of the cells.
Procedure
A Calibrating the eyepiece graticule
1
If it does not already have one, insert a
graticule into the eyepiece of the
microscope by unscrewing the top lens,
resting the graticule on the rim halfway
down and replacing the top lens.
2
Place a stage micrometer slide on the stage
of the microscope. Note that the smallest
division on the stage micrometer equals
100 µm.
3
Using the low-power objective, focus the
microscope on the stage micrometer.
Rotate the eyepiece and move the slide to
superimpose the scales of the eyepiece
graticule and the stage micrometer (see
diagram).
COAS Biology 1 Teacher Resources
Original material © Cambridge University Press 2008
1
4
Count the number of divisions on the eyepiece graticule equivalent to 100 µm on the stage
micrometer and hence calculate the length that one eyepiece division is equivalent to. For
example, if three divisions are equal to 100 µm, then each division is equal to 33.3 µm at low
power. Record your answer.
5
Repeat step 4 for the medium- and high-power objectives. You have now calibrated the eyepiece
graticule and you can use it to measure cells in the preparation below, or in the preparations in the
next practicals.
B
Making a temporary preparation of onion epidermal cells
1
The fleshy pieces inside an onion are leaves that store nutrients. Cut open an onion and separate
some of the leaves. Peel off one of the thin layers of epidermal tissue from the inner concave
surface of a leaf and transfer it to a drop of water on a microscope slide. Use forceps and a
mounted needle to make sure that the tissue is not folded.
2
Place two drops of iodine solution onto the tissue. Gently lower a coverslip onto the slide using a
mounted needle. Use a piece of filter paper to absorb excess stain. Place some filter paper over the
coverslip and gently press to flatten the specimen.
3
Place the slide on the stage of the microscope and use the low-power objective to locate the cells.
Now use the high-power objective to select three adjacent cells that are clearly visible in your field
of view.
4
Make a large, labelled drawing of these three epidermal cells. Use a sharp HB pencil and a ruler to
draw the label lines. Write your labels in pencil.
5
Use the eyepiece graticule to measure the length of one of the epidermal cells that you have drawn.
Now measure the same cell in your drawing.
6
Calculate the magnification of your drawing, using the formula:
magnification =
length of drawing of cell
actual length of cell
Remember: The lengths must be measured in the same units, e.g. micrometres (µm).
Write the magnification underneath your drawing.
COAS Biology 1 Teacher Resources
Original material © Cambridge University Press 2008
2