Ricardo wanted to conduct an investigation to determine what color light is best for plant growth. He predicted that orange light will cause tomato plants to grow the tallest. He developed the following steps to conduct his investigation: Five identical tomato plants were placed in the same type of container with equal amounts of soil and water. He shined one color of light on each of the five tomato plants for the same amount of time each day, using the colors red, orange, blue, green and purple. The height of each plant was measured and recorded at the beginning of the experiment and daily for 30 days. What essential part of his investigation is missing?
