Depending on the environment a plant is in, more or less gas may be produced. What is a method for measuring the rate of gas production from the aquatic plant in?
In principle, the simplest method would be to collect the gas in an inverted graduated cylinder.
(From IB Biology Syllabus Home - WikiFoundry)
You would place the aquatic plant under the mouth of the cylinder and collect the gas by the displacement of water.
Then you would measure the volume of gas collected at various times.
You could study the effect of temperature, the amount of light, and the availability of nutrients as factors that might affect the rate of gas production.