Plant organisation - AQAInvestigating 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.

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:

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.

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.

For these counts, the mean is: