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How Plants Grow
Auxin's Not a Toxin

What makes plants lean toward the light? Even though plants are firmly rooted in the ground, they can still move as they respond to changes in their environment. Light is so important for plant growth that they actually "bend over backwards" to get their share!

Think about it this way. The plant cells that are on the "shady" side of the plant actually grow longer than the cells on the "sunny" side. These longer cells grow this way because of a chemical in them called an auxin. An auxin, is a plant growth hormone that helps regulate shoot growth. Under normal light conditions auxins are spread out in the plant. But when sunlight varies, auxin is broken down on the sunnier side of the stem. The higher concentration of auxin on the shady side causes the plant cells on that side to grow more so it bends toward the light.

Because light is the energy source for plant growth, plants have evolved highly sensitive mechanisms for perceiving light. That information regulates their development so they maximize the light they can take in and use for photosynthesis. The process by which plant development is controlled by light is called photomorphogenesis.

This bending toward light is called phototropism. Phototrophism is a response that causes house plants to lean towards the window and trees to branch over the road. Take a walk in the woods and look for fallen trees. Auxins cause fallen trees to turn at their tips and grow upright again.

Real-life Plants in Motion
Watch an amazing video clip showing fast-motion seedling growth in the light and in the dark:

National Science Education Standards

  • Organisms have basic needs. For example, animals need air, water and food; plants require air, water, nutrients, and light.
  • Each plant or animal has different structures that serve different functions in growth, survival, reproduction.
  • All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions in a constantly changing external environment.

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