|Photo of Elodea plant by Kristian Peters (http://www.flickr.com/photos/fabelfroh/)|
A commonly-used plant for such experiments is the Elodea, an aquatic (underwater) plant also referred to as pondweed. You can buy this at aquarium stores.
The set up depends on which question you would like to study.
In general, the plant is placed in a test tube filled with diluted sodium bicarbonate solution (1%). Sodium bicarbonate (baking soda) gives the plant a source of carbon dioxide so that they photosynthesize more quickly.
You need to provide the plant with a fixed light source (a light bulb, for example). You could place a large beaker (around 2 liters) filled with water between the light source and the test tube so that the light heats up only the beaker water and not the plant!
For your experiment, you should change only ONE variable, never more than that (if you can help it!).
Take the case of investigating the effects of light intensity on photosynthesis rates. To change this, you can either use different wattage light bulbs, add screens between the light bulb and the plant, or change the distance of the light bulb from the plant. If you want to be really accurate, get a light meter and measure the light intensities that the plant is receiving with the different settings.
|from The Biology Web (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/)|
To change carbon dioxide levels, just change the concentration of sodium bicarbonate in the solution (e.g. 0.5%, 1%, 1.5%, 2%).
To change the temperature, you could warm the plant/test tube gently using a water bath method, i.e. place the test tube containing the plant in another large beaker which is filled with cool or warm water of known temperature. Place a thermometer in the test tube so that you know when the set temperature has been reached, and to make sure that it doesn't change too much during the experiment.
Read more about what variables you can measure, and what you should expect and look out for.