Consider the response of a sprouting plant left in a closet to horizontal light coming from a crack at the bottom of the door. What causes this?
Plants are organisms that make their own food. For this they primarily require carbon dioxide from the atmosphere, water from the soil and energy in the form of sunlight.
In addition to these, plants also require other nutrients like nitrogen, potassium, phosphorus, calcium, etc, which they obtain along with water from the soil through their roots.
To acquire sunlight for photosynthesis, plants have a tendency to grow towards the source of light. This is called phototropism and is facilitated by a plant hormone called auxin. Auxin promotes the elongation of cells in plants. This hormone moves from regions of the plants that have a lot of light falling on them towards regions on which a lesser amount of light falls. As a result of this, the cells in the darker side grow larger and cause the stem to bend towards the source of light. This results in plant movement and increases the amount of light available to it.
In the plant kept in the closet phototropism would make it move towards the crack at the bottom of the door from where light is coming in.