Working with the Light Microscope
Observing Onion Cells

Materials

Protocol

1. Obtain a 2 cm² section of a scale from an onion bulb.

2. Using a scalpel, remove a single layer of epithelium from the inner side of the scale. Be sure that the layer is thin enough to be transparent to light before you continue. It should look like skin peeling after a sunburn.

3. Prepare a wet mount of the section of onion tissue, being careful to not let the skin double over. (See Nelson Biology, page 38.)

4. Stain the slide with iodine. To do this, place a drop of iodine on one edge of the cover slip. Touch the opposite edge with the edge of a paper towel. This will draw the iodine stain onto the specimen.

* Try acetocarmine. Place a drop on the slide, then add the onion root tip. Mash the tips. Let stand for two minutes then heat in Bunsen burner flame for 3 or 4 seconds. Do not let stain boil. Cool the slide for one minute. Add a drop of acetocarmine and heat as before. Cool for one minute then apply cover slip. Repeat if too light.

5. Focus the cells under low power. Select a group of cells and move the slide so that the cells are in the middle of the field of view. Now view the cells under medium power. Again, move the slide so that the group of cells is in the centre of the field of view. Switch to high power.
a) Draw a group of four cells as they appear under high power.
b) Estimate the width of a single onion cell in micrometers.