In any experiment, there are parameters that may change, these are known as the variables. The variables that either changes itself or is changed by the experimenter (purposely) is known as the independent variable. The change in independent variable affects other parameter/s, which are known as the dependent variable/s. Controlled variables are the ones that the experimenter 'controls'.
In an ideal experimental design, there is only one independent variable and one dependent variable, rest are controlled variables.
In this case of tomato germination comparison between normal and spaced tomato seeds, the variables can be decided as:
Independent variable: spacing between the seeds. it can be measured at the time of seed placement into the soil. It is part of your experimental design, you may use one normal and one spaced seed placement or you different sets of spacing (say 1,2,3,4,..cm, etc.).
Dependent variable: plant growth or seed germination. It can be measured visually as whether the seeds have germinated or not. A quantitative way to measure the dependent variable would be to measure the number of baby or cotyledon leaves or height of stem.
Controlled variables: Other variables would be controlled, that is, same for both the cases. These may include the amount of sun-light, type of soil, amount of moisture (or water), container, etc.
Remember it is very useful to include these variables in your experiment objective (say, how spacing of tomato seeds affect their germination) or in your hypothesis (say, spacing of 3 cm between tomato seeds helps better/earlier germination, etc.).
Hope this helps.
The independent variable is the factor that is changed or controlled on purpose within the experiment.
The dependent variable is the factor that 'gets' changed, it DEPENDS upon the independent variable or rather the change is caused by the INDEPENDENT.
Going with what you gave as an example--
The independent variable in this case would be the experiment locale-- in space or on earth?
The dependent variable would be the amount of seeds germinated while in space and compared to the amount germinated here on earth within the same time parameters of 20 days.
For seeds in space, as compared to seeds on earth, the most commonly studied parameter is the effect of gravity.
The independent variable in this case would be the presence/absence of gravity.
The dependent variable would be the same as already suggested. And the controlled variables will also be the same.
Hope this helps.
The independent variable would be the location of the seeds - Earth and space. The biggest difference between those locations is the amount of gravity and the type of atmosphere. In general, the independent variable is what was changed on purpose between your samples or trials.
The dependent variable would be the amount of germination - how many seeds started to grow in each location. It could also be the average height of the plants in each location. In general, the dependent variable is what changed due to the independent variable. The dependent variable is usually quantitative, such as the number of plants germinated or the height of the plants.