Aerial parts of plants, usually show positive phototropism (lean toward the light). This response helps the green parts of the plant approach a source of light energy, which can be used for photosynthesis. On the other hand, roots tend to grow in the opposite direction of light.
When a stem is deprived of light in one side hormones called auxins accumulate on the dark side. Auxins are promoters of plant cell growth.
That causes the dark part to be elongated more than the illuminated part. In search of light.
