A student wanted to study the effect of nitrogen fertilizer on plant growth, so she took two similar plants and set them on a windowsill for a two-week observation period. She watered each plant the same amount, but she gave one a small dose of fertilizer with each watering. She collected data by counting the total number of new leaves on each plant and measured the height of each plant in centimeters.
Which of the following is a significant flaw in this experimental set-up?