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.

Scientists use sampling and counting techniques to investigate the distribution of stomata on leaves. They count stomata to investigate:

- their numbers, density and distribution on upper and lower surfaces
- numbers that are open and closed at any time
- adaptations of plants to environmental conditions, eg desert and water plants
- effects of changing conditions such as increased carbon dioxide concentrations from climate change

Below are two methods with which stomata can be counted.

- put a small drop of water on a microscope slide
- hold the leaf with the surface you want to examine uppermost
- tear the leaf at an angle so as to reveal part of the epidermis
- place the leaf on the microscope slide and examine

- paint the surface of the leaf with clear nail varnish
- allow to dry
- peel off the nail varnish with forceps
- place on a dry microscope slide and examine

The density of stomata on a leaf is recorded per unit area, usually the number per sq mm.

A microscope is calibrated so that its field of view is known.

In the illustration, the diameter of the field of view of the microscope is 0.40 mm.

Its area can be calculated using formula \( \pi {r}^{2} \)

Where \( \pi = 3.14\)

\(r\) = radius of the field of view

If the diameter of the field of view is 0.40 mm, the radius is 0.20 mm.

\[ Area = \pi {r}^{2} = 3.14 \times 0.20 \times 0.20 = 0.13~mm^{2}\]

The number of stomata in the field of view is 12.

The area of the field of view is 0.13 mm^{2}.

Therefore, based on this single count, the density of stomata over 1 mm^{2} is:

\[12 \times \frac{1.00}{0.13} = 92~stomata\]

The density of stomata is therefore 92 stomata per mm^{2}.

- Question
Using the same microscope, what is the density of stomata per mm2 based on the following counts?

24 23 22 27 28 25 26 24 26 25

192 stomata/mm

^{2}.The average count = \( \frac{24+23+22+27+28+25+26+24+26+25}{10} = \frac{250}{10} = 25\)

The mean count = 25.

The field of view is 0.13 mm

^{2}.Therefore, the number of stomata over 1 mm

^{2}= \( (25 \times \frac{1}{0.13} = 192)\)The density of stomata is 192 per mm

^{2}.