Positive phototropism of stems.
Light - growth - towards light.
Phototropism = "The way a plant grows or bends in response to light"
A. With light coming from all directions on the stem, it grows forwards and upwards.
B. The light is on one side of the stem, the plant bends in the growth towards the light source.
Auxin - Is a plant hormone. It is made at the top, & diffuses down the stem, & this stimulates growth. Auxin diffuses away from the light. So in image "B", the side of the stem in more shade, is where more auxin is found, so grows faster. This causes the stem to bend towards the light.