In a plant, there are several different auxin producing areas of a plant. They can be found in different areas of the stem (most prominently the tip of the stem), the tips of the roots, and on the buds of the plant. Auxin is a plant hormone that promotes the elongation of its stem. In this picture, the auxin producing area is found on the tip of the stem. The auxin moves to the darker side of the plant, near the shaded area, resulting in less auxin on the side that receives light. This uneven growth is what causes the plant to curve towards the lighter side and helps more light reach the plant, which is also depicted in the picture above.