What Is Phototropism? - Definition, Experiments & Examples

  • 0:01 What Is Phototropism?
  • 0:41 Early Experiments on…
  • 1:44 Examples of Phototropism
  • 2:32 Lesson Summary
Create An Account
To Start This Course Today
Used by over 10 million students worldwide
Create An Account
Try it free for 5 days
Lesson Transcript
Instructor: Sarah Friedl

Sarah has a Master's degree in Zoology and a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Why do plants bend toward sunlight? Phototropism is one way a plant can maximize its exposure to sunlight, and because plants need sunlight to make food, this is an important survival tactic.

What is Phototropism?

If you put a plant in a window, you may notice that, after a few days, it has bent toward that window. This is because the plant needs sunlight to make energy, so it grows toward the light. The growth of a plant toward any stimulus is called tropism, and the growth of a plant toward a light stimulus is called phototropism - photo means light.

Seedlings of plants grow straight upward in dark environments in order to reach the sunlight above ground. Once they break through the surface, they start bending toward the light because the growth of cells on the dark side is faster than the cells on the light side. However, if the amount of light is the same on all sides of the plant then it will continue to grow straight upward instead of bending.

Early Experiments on Phototropism

Early experiments on phototropism were based on the idea that plants were bending toward the warmth of the light, not necessarily the light itself. Charles Darwin performed many experiments in the late 1800s, and his results showed that photoreceptors, which are cells that detect light in the tip of the plant, communicated to the lower part of the plant that curves. From this, he concluded that there must be some substance produced in the tip of the plant that is sent to the lower part of the plant, signaling it to bend.

Darwin's work eventually led to the discovery of a plant hormone called auxin. Auxin is the chemical signal that causes plants to elongate and grow cells faster on the side of the plant farthest from the light. When auxin collects in the cells on the side of the stem that is away from the light source, the cells on this side are stimulated to grow. This is what causes the curvature in the stem towards the light source.

To unlock this lesson you must be a Member.
Create your account

Unlock Your Education

See for yourself why 10 million people use

Become a member and start learning now.
Become a Member

Already a member? Log In

Earning College Credit

Did you know… We have over 100 college courses that prepare you to earn credit by exam that is accepted by over 2,900 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

You just finished your first lesson. has thousands of lessons to help you meet your educational goals.
You're making great progress. Keep it up!
Congrats on viewing 10 lessons! You're doing great.
Keep clicking that 'next lesson' button whenever you finish a lesson and its quiz. Got It
You now have full access to our lessons and courses. Watch the lesson now or keep exploring. Got It
You're 25% of the way through this course! Keep going at this rate and you'll be done before you know it.
Two days in a row, nice! Keep your streak going to get the most of your learning and reach your goal faster.