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

Method

  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.
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
Mean40?42

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

Question

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

36.

Calculation:

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} = 360

Water loss through the stomata

Water is lost through open stomata. Scientists sometimes count all the stomata on a leaf surface, but usually they take a sample. This must be a representative sample.

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 calibrated eyepiece graticule, you could estimate the number of stomata per millimetre cubed.

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