One seed was planted in each of 5 pots found around the house that contained 500g of "Pete`s potting Soil." The pots were given the following amounts of distilled water each day for each day of 40 days: pot 1, 50mL; Pot 2, 100mL; Pot 3, 150mL; Pot 4, 200mL; Pot 5, 250mL. Because Pot 3 received the recommended amount of water it was used as a control. The height of each plant was measured at the end of the experiment. What is the independent variable?