# Calculating a mean and principles of sampling

## 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

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
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 =

$\frac{loss~in~Leaf~1 + Leaf~2 + Leaf~3 + Leaf~4 + Leaf~5}{5}$

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

## 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 - 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.

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

Using this method, and a , you could estimate the number of stomata per unit area (mm2).

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

Count the number of stomata in each sample.

Sample 1

For these counts, the mean is:

mean =$$\frac{sample~1 + sample~2 + sample~3 + sample~4 + sample~5}{number~of~samples}$$

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