Scientific calculations

Investigating transpiration

A simple method for investigating water loss from plant leaves is to measure their change in mass over a period of time.

Various factors that affect water loss from the leaf can be investigated using this method, for instance:

  • air movement – direct a fan on the leaves
  • temperature
  • obstructing the stomata, eg with petroleum jelly


  1. Remove a number of leaves from a bush or tree.
  2. Find the mass of each leaf.
  3. Suspend each leaf from a piece of wire or string.
  4. After a set period of time, re-measure the mass.

Example results

Experiment number1234
Surface coated with petroleum jellyNeitherUpperLowerBoth
% decrease in mass in Leaf 1433752
% decrease in mass in Leaf 2383831
% decrease in mass in Leaf 3373563
% decrease in mass in Leaf 4423642
% decrease in mass in Leaf 5403432

Analysis of results

There may be variation in the decrease in mass of different leaves.

It is important to repeat the experiment and calculate a mean for each set of data.

For experiment 1:

mean percentage decrease in mass = \frac{loss~in~Leaf~1 + Leaf~2 + Leaf~3 + Leaf~4 + Leaf~5}{5}

 = \frac {43 + 38 + 37 + 42 + 40}{5} = \frac {200}{5} = 40


What is the mean percentage loss in mass in experiment 2?



mean percentage decrease in mass = \frac{loss~in~Leaf~1 + Leaf~2 + Leaf~3 + Leaf~4 + Leaf~5}{5}

 = \frac {37 + 38 + 35 + 36 + 34}{5} = \frac {180}{5} = 36

Water loss through the stomata

Water is lost through open stomata. Scientists sometimes count all the stomata on a leaf surface, but usually, there are too many to count. In these instances, they take a sample. This must be a representative sample - it must give a true picture of the numbers of stomata on the leaf.

To be representative of the whole leaf, the representative sample must:

  • include a sufficient number of counts - not just one or two - of stomata over different parts of the slide
  • must be random, and not select areas where there are many or few stomata

A number of random counts of stomata should be made with a microscope. Count the number of stomata in the field of view. Then move the slide slightly and count the number of stomata in a different field of view.

Make at least five random counts, then calculate a mean.

A diagram of 12 stomata - both open and closed

In this field of view, there are 12 stomata - nine open and three closed.

Using this method, and a calibratedeyepiecegraticule, you could estimate the number of stomata per millimetre squared.

The images show the fields of view of a plant leaf viewed with a microscope.

Count the number of stomata in each sample.

A second sample of stomata

Sample 1

For these counts, the mean is:

mean\; =\; \frac{sample\: 1\: +\: sample\: 2\: +\: sample\: 3\: +\: sample\: 4\: +\: sample\: 5}{numbers\; of\; samples}

=\; \frac{14\: +\: 12+\: 11+\: 12+\: 11}{5}\; =\; \frac{60}{5}\; =\; 12\; stomata\; in\; the\; field\; of\; view