Plant organisation

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.

Part of

Combined Science

Plant structures and their functions

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 stomata, 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

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

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 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.

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 calibratedeyepiecegraticule, 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

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