Grades: K – 7

The word phototropism is a mouthful! It’s made up of two parts: photo which means light and tropism which comes from the Greek word tropos meaning to turn.

To grow tall and strong, plants use energy from light to make food in a process called photosynthesis. We think of plants as being stuck in one place, but they can move their stems to grow towards light. This movement is called phototropism: where plants turn or orient towards a source of light. This happens in plants and even some fungi.

Here’s how it works: Plants use many hormones to grow. One hormone in particular called auxin, tells individual cells to grow longer. It’s one of the ways that plants grow taller. Normally, plants growing with an unshaded light source will grow straight up towards the sun because auxin is evenly distributed all around the shoot.

But when something blocks or shades the sun like a tree or other plants, something interesting happens. Auxin starts to concentrate on the shaded side of the plant instead, and as a result, the cells on the sunny side stay the same size but the cells on the shaded side grow longer. This causes the plant to turn or bend towards the light.

Phototropism is an evolutionary adaptation that helps plants to move access light needed for photosynthesis.