The growth of plants toward light is particularly important at the beginning of their life cycle. Many seeds germinate in the soil and get their nutrition in the dark from their limited reserves of starch and lipids even mature plants bend toward the strongest light. They do this by elongating the cells of the stem on the side that is farthest from the light. This phenomenon is called phototropism. The substance is responsible for cell elongation as auxin. This hormone is found in cells at the tip of the shoot and is then passed from cell to cell. As such the hormone is shuttled through many cells of the plant before it reaches its final destination. Export and import proteins push the auxin out of one cell into the intercellular space and then into the next cell and so on until the auxin eventually reaches its target site.