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Stoma density and pollutionFeatured science projectScience project video


This science fair project was conducted to determine the effect of pollution on the stomata density of coniferous trees. The science fair project was done using samples of pine tree leaves from 3 locations with different levels of traffic flow.


The stomata density in pine tree leaves taken from locations with a higher traffic flow is lower.

Scientific Terms

Transpiration, stomata, stoma, photosynthesis, hydrostatic pressure


Stomata in plants

Stomata are tiny plant structures found on the epidermis of plants. They allow water vapor, carbon dioxide and oxygen to move in and out of a leaf. Most plants have stomata on the bottom side of the leaf. Each stomata consists of a tiny hole called a stoma, and 2 guard cells at the sides. The stoma opening allows for the diffusion of carbon dioxide and oxygen gases while the guard cells at the side serve to open or close the stoma. The stoma opens for carbon dioxide molecules to enter in order to enable photosynthesis to take place. However, when the weather gets too hot, the stoma closes to prevent loss of water.

The stomata provides a channel or opening for the plant to interact directly with its surrounding atmosphere. Diffusion of gasses in and out of the plants takes place through the holes in the stomata. Water molecules are also released into the air through the opening in the stoma. When water molecules escape from the surface of the leaf, hydrostatic pressure reduces. This causes the water and nutrients collected in the roots to be pulled upwards. This transpiration and flow of water also helps in cooling down plants. The amount of water lost by a plant will depend on the humidity, temperature, wind conditions and the amount of water in the soil.

Stomatal density is the number of stomata per square millimeter. It can range from 100 to 1000 stomata per square millimeter, depending on the type of plant and the environmental conditions during the plant’s development.  Stomatal density will be greater if the plant develops in environments with greater amounts of light, lower atmospheric concentrations of carbon dioxide and in moist environments.

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Complexity level:
Project cost ($):
Time required:
1 hour to prepare, 3 days for observation
Material availability:
Easily obtained. Access to a microscope is required
Safety concerns:

Basic safety requirements