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.
Beth was getting her vegetable garden ready for the season. She had several small plants ready to transplant into her garden. First Beth dug several holes to put the plant's in. She made sure the soil at the bottom of each hole was loose then settle the root balls into the wholes and covered them. She was also careful to select a spot where the plants would get sunlight from several different directions throughout the day.
Will the plant roots grow up or down?