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 laterally, auxin accumulates on the lower surface of the shoot, causing the cells on the lower side of the shoot to grow faster, so the shoot grows up. Besides the shoot, the root is also affected by auxin. The roots are more sensitive to auxin, and when the auxin accumulates (builds up) on the lower side of the root, the high concentration of auxin inhibits (stops/slows down) the growth of the root on the lower surface, therefore the root will grow downwards. This is geotropism.