The indepedent variable is the variable you have control over - in this case, it's the amount of hormone added to the plants (you controlled how much hormone to add).
The dependent variable is the variable that depends on the independent variable. It's the variable you can't control; another hint is that it's the data is collected. In this case, it's the number of leaves on each plant.
The control is what you would compare each group to, in order to see if the hormone actually had an affect on the plants. In this case, it's the plant that received no hormone. Let's say the plants with no hormone have 5 leaves, and the plants with hormone also had 5 leaves, this helps us to make the hypothesis that the hormone had no affect on the number of leaves.
Other variables to consider are those that affect how plants grow, such as how much light the plant received, the fertilization of the soil, the temperature, or the humidity. If one plant received more light than another, this could make the results inaccurate.
