Auxin is a plant hormone, which is a growth substance. Auxin is made in cells near the tips of plant roots, or shoots; where the plant divides as it grows. All the time, new cells are being produced, which eventually find themselves behind other cells. These older cells have to grow, by getting longer. Auxin makes these cells get longer - it diffuses backwards towards the older cells. Auxin stimulates these cells to lengthen. Eventually, auxin is broken up by enzymes in the plant. If light is shone on one direction, the auxin tends to accumulate on the other side, the more shady side. The cells on this side therefore grow longer faster, meaning the shoot bends towards the light. This is positive phototropism. The growth of a plant is also affected by gravity. When a shoot is growing latera