Duckweed (Lemna sp.) is a small plant that floats on the surface of water. Duckweed
can be used to remove minerals from polluted water.
The plant uses water and carbon dioxide to make carbohydrates in photosynthesis.
To make other chemicals it needs a variety of mineral ions.
The plants grow mainly by vegetative reproduction. Each adult plant produces
daughter plants.
A single plant was placed in a Petri dish containing 20 cm3 of deionised water.
Another plant was placed in a Petri dish containing 20 cm3 of a solution with all
mineral ions present.
This procedure was repeated for a further seven Petri dishes containing solutions
with one mineral ion missing. These are shown in the table below.
Name the independent variable in this investigation.