Phototropism is a growth movement induced by a light stimulus. Growth towards a source of light is called positive phototropism, that away from the source is termed negative phototropism. The tips of shoots are usually positively, that of roots negatively phototropic. Charles Darwin and his son Francis discovered (in 1880) that the phototropic stimulus is detected at the tip of the plant. The Darwins used grass seedlings for some of their experiments. When grass seeds germinate, the primary leaf pierces the seed coverings and the soil while protected by the coleoptile, a hollow, cylindrical sheath that surrounds it. Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it. The Darwins found that the tip of the coleoptile was necessary for phototropism but that the bending takes place in the region below the tip. If they placed an opaque cover over the tip, phototropism failed to occur even though the rest of the coleoptile was illuminated from one side. However, when they buried the plant in fine black sand so that only its tip was exposed, there was no interference with the tropism – the buried coleoptile bent in the direction of the light. From these experiments, it seemed clear that The stimulus (light) was detected at one location (the tip) The response (bending) was carried out at another (the region of elongation). This implied that the tip was, in some way, communicating with the cells of the region of elongation.