Phototropism

​​Like humans, plants produce hormones that cause them to grow or that inhibit their growth. One such hormone, auxin, is formed in a plant's terminal buds and root tips. It promotes cell elongation, because it weakens connections between cellulose fibers in a plant’s cell wall. This allows plant cells to stretch and grow longer, and also promotes meristematic growth (growth in the terminal buds and roots). This chemical is also what allows plants to move towards light.

Although it may seem like plants can “see” light and point in the direction of it, this isn’t what is actually happening...

​​Phototropism, or "light-eating" in Latin, is the movement of a plant towards any light source. Heliotropism is similar to this, except it is the movement of a plant towards the light produced by the sun specifically. Both allow plants to move by the same method. When light is shone on a plant, auxin accumulates on the side of the plant opposite the source of light. This is because sunlight reduces auxin in the side of plant exposed to it. Since auxins are the hormones that elongate plant cells, this means the side opposite the light source is the one extending, causing the stem of the plant to bend towards the light. To test the abilities of plants to undergo this process, we built two obstacle courses for two sunflowers to move through in order to "find" the light source, which was sunlight coming in through the window. We wanted to see if the plants could undergo the process of phototropism effectively enough to move around the bends and corners of the maze.

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First, we had to plant sunflower seeds. We planted more than we thought we would need, since we didn't know if all the seeds would germinate. The seeds were originally planted in the biodegradable flat shown here, but once they reached a height of about 2 inches they were ready to be re-potted and put through the mazes...

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Next, we made the cardboard mazes for the sunflowers to bend through to find the light. They were fairly simple to make; all we needed were cereal boxes, duct tape, and scissors. The way we were able to make "obstacles" for the plants to bend around was by making slits in the sides of the box to slide the cardboard pieces into.

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We put one sunflower in each maze, and continued to water them over the past few weeks as they grew and moved around the obstacles to get to the light. Finally, we saw that our sunflowers had found the way out! Since we couldn't show you how they had bent without opening the boxes somehow, we (carefully!) cut the mazes in half. Although these mazes weren't very complex (as shown by the only slightly bent growth of our sunflowers), it is likely that as long as they are given enough water and at least some sunlight to grow, they could find the light in mazes with more ob​stacles.