- Photo tropism refers to growth responses as a result of light.

- When uniformed light coming from all directions hits the plant, it causes the stem to grow upwards.

- Positive photo tropism however is when light comes from the side of the stem (a lateral light source), which causes the plant to have a bend in the growth, and tilt towards the direction of the light. The reason why this happens is because of a plant hormone called Auxin.

- Auxin is a plant hormone which is produced in the plant tip. As a plant grows, it needs more water and minerals. Because of this Auxin will build up in the cells on the shady side of the plant. The Auxin will loosen up the cell walls which will allow the cell to elongate. This causes the shady side to grow more than the sunny side so the plant bends towards the light.