Final answer:
The independent variable is the number of seeds per pot; the dependent variable is the length of the plants; constants include pot size, water amount, and plant species.
Step-by-step explanation:
In the experiment with the tomato plants, the independent variable (IV) is the number of seeds added to each pot, as this is what is being changed to test its effect on the outcome. The dependent variable (DV) is the length of the tomato plants after two weeks, because it is what is being measured and what may be affected by the changes in the IV. The constants in the experiment are the size and type of pots, the amount of water given daily (50ml), and any other conditions that are kept the same, such as the sunlight exposure, temperature, and type of tomato plants, as these could affect the growth of the plants but are not being tested in this experiment.