Onion Epidermal cells
Introduction:
This is one of the most common plant cell microscope exercises. The epidermal layer of cells is
one-cell thick and easily handled.
Method
Cut an onion into segments about 1 cm wide (like the segments of an orange). Separate the
individual onion scales and bend then as shown until they ‘snap’. As you carefully separate the two
halves a thin film (epidermal layer) will peel away from one half of the scale.
Epidermal layer
Two halves of onion scale are
separated as indicated by arrows
When the two pieces have completely separated the thin layer of cells can be cut away with fine
scissors. Be careful not to let the layer fold on itself as it may be difficult to flatten on the slide.
Immediately transfer the layer of cells onto a microscope slide that has a drop of iodine solution on
it. Add another drop of stain on top of the cell layer and lower a cover slip. Use a small piece of
absorbent paper to mop up any excess fluid.
Exercise to illustrate the value of stains
Try mounting two pieces of onion skin
on the same slide using water and
iodine stain. Compare stained and
unstained cells under the microscope
and the details that can be seen only
when the cells are stained.
Images
Onion Epidermal cells Low Power (x40)
Unstained specimen
mounted in water
Specimen mounted
with stain
Onion Epidermal cells Medium Power (x100)
Despite the simplicity of this exercise, there are a number of interpretations and questions that are
worth considering when you record your observations in a diagram. You should remember that the
layer of cells might appear flat but is actually three-dimensional.
Are the nuclei in the centre of the cell or are they around the outside?
Are some of them inside the vacuole as they appear to be?
If you look at the diagram above you will see examples of both. Some nuclei are close to the edge
of the cells while some are central. You also need to understand that the bulk of interior of these
cells is occupied by a large storage vacuole. Does this mean that some of the nuclei are inside the
vacuole?
Look at the diagram below:
The cell layer is drawn as if from a side view. The red arrows show the path of light from below the
specimen through the nucleus and to the objective lens. In two of the four cells, it could appear as
if the nucleus is within the vacuole but it is fact above or below. When drawing these cells therefore
it is best to draw the nucleus clear of the vacuole so as not to give an incorrect impression.
Light Ray
Cytoplasm
Vacuole
Cell wall
Nucleus
How should cell walls be drawn? As a single layer or as two layers?
Before this question is answered, we need to know what the structure of the cell wall is. The
following diagram shows part of the cell walls of two neighbouring cells.
Position of cell membranes of
neighbouring cells (these will not
be visible as they are pressed up
against the cell walls)
Middle lamella
Cell walls of the 2 adjacent cells
Cytoplasm of cells
You will not normally be able to see this much detail but several facts need to be appreciated:
 each cell has its own cell wall
 the cell walls are held together by a layer called the middle lamella (literally middle layer).
This layer contains pectin and other substances that act like glue (in fact when we boil fruit
to make jam we are extracting this pectin and it is the pectin that makes that jam thicken)
 each cell has a cell membrane but you are unlikely to see this in plant cells when it is
pressed up hard against the inside of the cell wall.
Does the cell wall between the cells appear as two layers?
Look at the next 3 photographs
Look here
Onion Epidermal cells High Power (x400)
Look here
Onion Epidermal cells High Power (x400) above and below
Look here
Note in each photograph there is an area where the Cell wall does in fact appear as a double layer.
This would be easier to see in reality because you could focus up and down as you look at the
cells. The problem is often that the double cell wall is not always obvious. Why not?
Look at the next diagram for an explanation:
This diagram shows three cells side on. The red arrows show the path of light from below the
specimen through the cells and to the objective lens.
On the left of the diagram, the two cell wall layers would not be seen because the cell walls are not
parallel to the light that passes through the specimen.
On the right hand side, the cell walls would be seen as two distinct layers because the cell walls
and the middle lamella are all parallel to the light. This situation does not occur frequently so you
will need to search a large number of cells to see it.
Intercellular spaces.
This is a detail that many students miss in their diagrams. If you look back at the photographs on
the previous pages, you will note that there are no intercellular spaces in the corners between cells
in a layer of onion epidermal cells.
However, in many plant cells, these spaces exist and you need to look for them and draw them if
they exist in other plant tissues.
Drawn with intercellular spaces.
Incorrect for Onion Epidermal cells
Drawn without intercellular spaces.
Correct for Onion Epidermal cells
What does this mean when it comes to drawing cell walls?
 Each cell has a complete cell wall that surrounds the cell
 There is a middle lamella between all cells even if it is only visible under ideal conditions
 There are no intercellular spaces between onion epidermal cells
 Proportions need to be accurate i.e. ensure the correct thickness of the cell relative to the
width of the cells.
What other organelles are visible in Onion cells?
Look carefully at these two photographs
Onion Epidermal cells High Power (x400) above and below
These are the two photographs of the same cells. The difference between them is the height at
which the microscope was focused. Note that the nucleus is in focus in the top photograph but not
in the bottom photograph.
If you look at the zone indicated by the white arrows, you can see that there is a boundary or
change of texture. This is the where the membrane that surrounds the vacuole is found. This
membrane is very thin. You can also see that this boundary is not always smooth and that there
appear to be some strands of cytoplasm especially at the ends of the cells.
Because these cells have been stained with iodine the cytoplasm has a number of granules and
particles that appear darker. These are various cell organelles but they cannot be distinguished at
this magnification. Many appear to be inside the vacuole but as explained previously these
organelles will be above or below the vacuole. If you focus through the cell from top to bottom, you
will notice that the organelles above the vacuole focus at a different level from those below the
vacuole.
Onion Skin cell drawing: