How do plants stretch towards the sunlight?
As plants grow, they will very often turn towards the brightest light source they can find – which is almost always sunlight. The Ancient Greeks named the phenomenon phototropism, which means ‘light turning’.
Scientists have discovered that phototropism is caused by auxin, a hormone that exists in the stem, roots and buds of plants. Auxin concentrates in the dark side of a plant, and since the hormone makes plant cells more flexible, the cells stretch to grow larger on the dark side. Hence plants will stretch in the direction of the brightest light, so that plants in window boxes will lean outwards if you don’t remember to rotate them occasionally.