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 , eg with petroleum jelly

Method

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

Example results

The table below shows the decreases in mass due to transpiration when different leaf surfaces were coated with petroleum jelly.

	Neither surface coated	Upper surface coated	Lower surface coated	Both surfaces coated
Repeat 1	43%	37%	5%	2%
Repeat 2	38%	38%	3%	1%
Repeat 3	37%	35%	6%	3%
Repeat 4	42%	36%	4%	2%
Repeat 5	40%	34%	3%	2%
Mean	40%	?	4%	2%

Repeat 1
Neither surface coated	43%
Upper surface coated	37%
Lower surface coated	5%
Both surfaces coated	2%

Repeat 2
Neither surface coated	38%
Upper surface coated	38%
Lower surface coated	3%
Both surfaces coated	1%

Repeat 3
Neither surface coated	37%
Upper surface coated	35%
Lower surface coated	6%
Both surfaces coated	3%

Repeat 4
Neither surface coated	42%
Upper surface coated	36%
Lower surface coated	4%
Both surfaces coated	2%

Repeat 5
Neither surface coated	40%
Upper surface coated	34%
Lower surface coated	3%
Both surfaces coated	2%

Mean
Neither surface coated	40%
Upper surface coated	?
Lower surface coated	4%
Both surfaces coated	2%

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:

\(\text{mean percentage decrease in mass} =\) \(\frac{\text{loss in Repeat 1} + \text{Repeat 2} + \text{Repeat 3} + \text{Repeat 4} + \text{Repeat 5}}{5}\)

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

Question

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

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 – 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
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.

To find out how many stomata there are on a leaf surface, make an epidermal strip, and examine using a microscope:

Paint a thin layer of nail varnish onto a leaf surface and allow to dry.
Stick clear adhesive tape over the dry nail varnish and use this to peel the nail varnish off the leaf surface.
Stick the tape with the nail varnish imprint of the leaf surface onto a clean microscope slide.
Repeat this process several times using the same leaf surface to provide a number of different strips so that you have a representative sample for your estimate.
Compare the microscope images of the nail varnish imprints using low power magnification.

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.