The Q&A Archives: The effect of light on beans

Question: My daughter is doing a science project. She planted beans in three types of environments, In the dark (In a cooler), In a window with partial light and under a grow light (24 hours a day). The beans that were totally in the dark are twice as long as the beans in partial light and the beans in total artificial light have grown the least. Those in the dark are very skinny and not very green. The partial light beans have been put under a plastic bag. We were surprised by the growth in the dark and believe germination is the cause. Can you help explain this in easy terms? Mark Hoyere Farmington Hills, MI

Answer: A bean seed contains a certain amount of reserve carbohydrates to sustain the new seedling. After this nutrient reserve is used up the seedling must be able to sustain itself through photosynthesis (the process by which green plants use sunlight to manufacture their own food in the form of simple sugars.) Those tall, spindly, pale plants in the dark were stretching and reaching to find the light they need for photosynthesis. In time, as their initial food reserve is depleted, they will die from lack of nutrients. Healthy bean plants will appear stocky, lush, and bright green. So, as you can see, plant height is not necessarily an indicator of plant health. Note too that sunlight is generally much brighter than artificial lights. So plants grownon a windowsill may actually be receiving more light than those grown under artificial lights 24 hours a day.

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