Phototropism

Why are these plants turning sideways?

Plants respond to their environment in how they grow. In this picture, the light source is probably off to the left side. As a result, the plants grow in this direction to get more light.

Tropisms

Plants may not be able to move, but they are able to change how they grow in response to their environment. Growth toward or away from a stimulus is known as a tropism ( table below). Auxins, a class of plant hormones, allow plants to curve in specific directions as they grow. The auxin moves to one side of the stem, where it starts a chain of events that cause rapid cell growth on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.

Phototropism

You might have noticed that plants bend toward the light. This is an example of a tropism where light is the stimulus, known as phototropism (figure below). To obtain more light for photosynthesis, leaves and stems grow toward the light. On the other hand, roots grow away from light. This is beneficial for the roots, because they need to obtain water and nutrients from deep within the ground.

These seedlings bending toward the sun are displaying phototropism: 

http://www.ck12.org/life-science/Tropisms-in-Life-Science/lesson/Tropisms/r23/