Plant leaves are adapted for photosynthesis and gas exchange. Roots absorb water and mineral ions through root hair cells and are transported up the plant by the xylem.

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

- 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

Experiment number | 1 | 2 | 3 | 4 |
---|---|---|---|---|

Surface coated with petroleum jelly | Neither | Upper | Lower | Both |

% decrease in mass in Leaf 1 | 43 | 37 | 5 | 2 |

% decrease in mass in Leaf 2 | 38 | 38 | 3 | 1 |

% decrease in mass in Leaf 3 | 37 | 35 | 6 | 3 |

% decrease in mass in Leaf 4 | 42 | 36 | 4 | 2 |

% decrease in mass in Leaf 5 | 40 | 34 | 3 | 2 |

Mean | 40 | ? | 4 | 2 |

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 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.

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 unit area (mm^{2}).

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:

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