The objective here is to establish a figure for the size of a plant cell.
The problem is there is no such thing as a typical plant cell.
The cells we will be observing are from the leaf of Elodea the Canadian pond weed. These leaves are so thin they are transparent and easy to prepare for the microscope.

MATERIALS

PREPARING YOUR CELLS

• Using forceps, pick a healthy green leaf of Elodea and mount it in a drop of water.
   

• Observe the leaf under medium and high power. The chloroplasts should be visible as small green spheres.

Organs Tissues and Cells

When you observe the leaf of Elodea you are observing an organ whose functions are photosynthesis and gas exchange. Organs are made of tissues with more precise functions and tissues are groups of cells with similar structure. These cells are specialised (adapted) for a function.

So when you observe the leaf it will appear to be composed of different areas of cells (tissues) and within these areas the cells will all look very similar.

Selecting your sample

• Observe the leaf under medium power. Move the slide around you will notice the cells are not all the same. For example those in the middle of the leaf (the midrib) are different from the cells either side of it.
   

• Select a tissue in the cell and draw a representative cell.
  What do you think “representative” means?
  What is the difference between a drawing and a diagram?
   

• Identify the features on it and annotate your drawing.
  Note Annotation does not just mean labelling the parts, it includes adding observations especially of features that cannot be shown by the drawing itself (e.g. colours – your drawing should be in pencil – or movements). (See sheet on biological drawings)

Sampling

You cannot measure all the cells in a tissue, there are too many. So you must take a sample; but how many and which ones?
A sample must be representative of the population which is being studied.

Samples sizes

Samples of less than 5 are considered too small to be representative.

Picking a random sample

If you let yourself choose the cells to be measured you may go for the biggest or the smallest. In other words you may be biased. We need to avoid bias by measuring a random sample of cells from the tissue.

• Place the eyepiece micrometer in the microscope and calibrate it, if necessary, on high power. (see sheet on Using the micrometer eyepiece )

• Turn to medium power.

• Use you micrometer scale (0 to 100) and the random number tables to select your cells.
  Count along the graduations to the first random number on your list. The cell under this graduation is your first sample. Centre this cell in your field of view and turn to high power. Refocus, measure this cell and record your result. Which way are you going to measure it?

• Return to medium power and count to the second random number on your list. Measure this cell on high power. Continue until you have measured at least 15 cells.

Analysing your data

Statistics are useful mathematical tools which are used to analyse data. Perhaps the best known statistic is the average. This is a single figure which is used to represent a set of data.

Averages

There are three types of average:
The median which is the middle value of a range of results.
The mode which is the value that appears the greatest number of times.
The mean which is the sum of all the results divided by the number of results.

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© Paul Billiet 2007