Experiment: Observation of onion epidermial cells

Material

• Raw onion
• Slides and cover glass
• Scalpel and forceps
• Water and dropper
• Filter paper
• Methylene blue stain solution
• Microscope, Digital microscopy camera

Procedure

Place a couple of drops of water in the center of the slide.

 Using the scalpel, slice the raw onion. Cut one of the onion rings into 6mm sections.

By using the forceps, gently remove the skin from the inner side of the onion.

Place immediately the skin on the drop of water to avoid it curls up. Gently place the cover glass. If necessary remove the water excess with filter paper.

Observe the slide under the microscope by using both the low-power and high-power magnifications.

Using the digital camera take a picture of the sample, at the different magnifications used.

Prepare a new sample by placing a couple of drops of diluted methylene blue solution in the center of a slide. Place a new onion skin on this solution and cover with the cover glass, as before. If necessary remove the staining solution excess with filter paper.

Observe the slide again under the microscope using the same low-power and high-power magnifications.

Take new pictures at the same magnifications used, as before.

Observe the pictures and identify the shape and pattern of the onion cells.

Compare the pictures and find the differences on the sample before and after staining.

Observation

The onion epidermis, because of its simple structure and transparency, it is often used to introduce students to plant and cell anatomy.

Students should be able to distingue the rectangular shape of the individual cells, which are perfectly lying side by side, forming the typical epidermis pattern.

After the staining students should be able to identify cellular structures (mainly the cell walls and nuclei) which are almost indistinguishable on the live samples.  Staining enhances the optical contrast of these structures making them visible.