As you read earlier in this chapter, plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below , auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.
Amanda has two of the same plant in different pots sitting in front of the same light source. Amanda rotates Plant A by 180 degrees every day, but does not rotate Plant B at all.
Will the cells on the dark side of Plant B be longer or shorter than the cells on Plant A?