The apical meristem is the quickly growing part of the roots and shoots of plants. It is an ideal place to observe many dividing cells. The zoom image shows two onion root tips. Try to find the different stages of mitosis, in which the chromosomes are then clearly visible. Compare the cell division stages of the onion with those in the lily: Mitosis stages of the Lily

Make a cut beneath the red layer and firmly press the red part of the onion against the edge of the knife, without cutting yourself...

Carefully tear off the layer of red cells. Remove the thick part of the onion (where the cut was made) and only observe the thin layer. Many cells will probably break open during this process and be useless, we only need a few intact cells.

The top image shows the cells before plasmolysis. The cells are filled with a red pigment and appear pink. The bottom image shows the same cells after the addition of saturated salt water. Intact cells will lose much of the water due to osmosis. The concentration of the pigment rises resulting in a darker color. The shape of the cell wall remains unaffected.

Materials: kitchen knife, red onions, salt, tap water, microscopic slides, cover slips

Method – Obtaining a single layer of red onion cells.
For this experiment, we can not use the onion skin which is found between the layers of the onion. We need a single layer of pigmented cells. These cells, however, do not separate easily.

1. We need a thin layer of cells of the red part of the onion. It is not possible to directly cut a single cell layer, so we need to use the “peeling method” to obtain a single layer of cells. Obtain a small piece of onion about (1cm x 1cm). The onion layer is about 2mm thick.
2. With the red side of the onion facing you, cut beneath the red layer, about half way into the onion. This cut does not have to be very thin. There will be about 1mm of onion between the knife and the red pigmented layer.
3. Press the onion firmly against the knife with your thumb.
4. Now tear off or peel away the red part of the onion. The red layer will become thin. Some red pigment may be released from broken cells.
5. Cut away and discard the thick part of the onion (the place where the initial cut was placed).
6. Observe the remaining cells (the thin, peeled part) under the microscope (using a glass slide, water and cover slip, of course.
7. Only consider those cells that are filled with the red pigment. White cells are broken and have lost the red pigment.

Method – Plasmolysis.

1. Make a saturated solution of salt-water
2. Using a pipette, add one drop of this solution to the specimen. The salt water should flow beneath the cover slip. There should be no need to remove the cover slip to add the salt water
3. If there is too much water beneath the cover slip, then the salt water will not flow between the cover slip and the slide. In this case use tissue paper to withdraw water from one side of the cover slip while adding the salt solution at the other side.
4. Observe what happens to the red pigment inside the cells.

Explanation: Water from the cells moves to the surrounding salt water. The shape of the cells does not change, the cell wall maintains the cell shape. The cell content (the red part of the cell) starts to shrivel up. At the same time it is possible to see that the intensity of the red pigment increases because it becomes more concentrated as water is removed (the red pigment is not able to move out of the cell). The process can be reversed when the salt water is removed and when distilled water is added.

Materials: Slide Projector (an overhead projector will not work!), slide frames with glass, onion, ruler, marker.

Method:

1. Cut out a piece of onion skin of about 1 cm² size. The onion skin is the membrane which can be found between the layers of the onion.
2. Using a ruler, draw a 1 cm long line on the inside glass of the projector slide. This is our internal reference.
3. Place the onion skin flat into the slide.
4. Project the skin on the wall and measure the length of 10 cells using a ruler. The students then calculate an average. Also measure the length of the 1cm reference line.
5. We now can calculate the magnification by dividing the length of the projected line by the original size: Magnification = length of projected line / length of original line E.g. if the projected line is 30 cm, then the magnification is 30 cm / 1 cm = 30x.
6. Now divide the size of the projected cell by the magnification to obtain the real cell size.

The nuclei of onion cells stain blue.

Materials: Onion, tap water, alcohol, fountain pen ink, several small beakers or film containers.

Method:

1. Using a sharp knife, cut out about 1 cm² of onion material. This may be done by the teacher to reduce the risk of injury.
2. The individual layers of the onion are separated by a thin skin. Peel this skin off using your fingernails or tweezers. The skin can be found on the inside part of each layer.
3. Place the skin into pure alcohol for about 30 seconds. This procedure removes water from the cells.
4. Place the skin into the ink. The ink, which is water-based, will enter the cells and strain the onion skin deep blue.
5. Using tweezers, transfer the skin into pure water and rinse it for about 30 seconds or until no more ink is given off. The skin will still have retained its blue color.
6. We now start the washing steps. We have to remove excess ink from the cells. Transfer the skin into pure alcohol for about 30 seconds. Some of the ink will be removed staining the alcohol slightly blue.
7. The skin is transferred into pure water for about 30 seconds.
8. The skin is carefully spread on a microscope slide and a cover slip is placed on top. Excess water is removed with filter paper
9. If the cytoplasm of the cells is still too dark, then it is necessary to repeat the washing steps 6 and 7 for a second time.

Troubleshooting:

Problem: The cells are too blue and too dark.
Solution: The cells were insufficiently washed. Prolong the washing times or introduce another washing cycle.

Problem: The nuclei are not stained, or not stained enough.
Solution: There could be several reasons for this.

• The onion was not placed into the alcohol. Therefore the water inside the cell was not removed and the ink could not enter the cell.
• The onion was not submerged long enough in the ink or the onion was not fully covered by ink on all sides
• The onion was washed too intensively. This is the most probable cause. If the onion was washed twice, then the washing step should be conducted only once.
• It could also be that the washing times were too long. If the nuclei are stained lightly blue, then this indicates that the staining procedure is in principle working, but that too much of the ink was removed.