The Q&A Archives: Will A Plant Grow Down

Question: My son is doing a science project, his question for his project is will a plant grow downward if the only light source comes from underneath the plant, he is using a grow light from below and has the project in a dark closet, can you give us your thoughts on the project and what might happen?

Answer: It might be helpful to first explain how plants move in some situations. For example, when you put a tomato seedling in a sunny window, the stem will begin to bend toward the light. This reaching or bending toward the light is called phototropism.

A group of substances within the plant cells called plant growth regulators are responsible for this change in growth. (Plant growth regulators are sometimes called hormones, as in rooting hormone.) On type, called auxin, stimulates cells to increase in length. When the sun shines on one side of our tomato seedling, the auxin collects on the dark side, causing those cells to elongate. Because these cells enlarge faster than the ones on the sunny side, the stem bends toward the light. Just why and how the auxin actually moves is still a mystery to scientists.

How your project will turn out depends upon how strong the seedling is. My guess is that the stem will bend down toward the light.

You (or your children's teachers) might be interested in looking into National Gardening Association's new online botany/horticulture courses. These month-long courses will explain all sorts of interesting phenomena like that. Check out the web site at We love to get people interested in botany!

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