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Student Introduction: Plants adjust to the conditions of their environment through growth responses. For example, plant stems generally bend toward light sources while roots grow away from light sources. This process is called phototropism. Substances called auxins play a strong role in controlling various plant responses. Auxin increases the plasticity of plant cell walls causing them to stretch more during active plant growth. During phototropism, the auxin concentration in stem cells is usually higher on the side of the stem away from light. These cells will stretch more during active growth, causing the plant to move toward the light source. In gravitropism, plants respond to gravity, generally causing shoots to grow up and roots to grow down.

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