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I am doing a science project with plants. Two tomato plants are grown under boxes made of four types of cellophane: red, green, blue and clear. The red one grew the most, both in height and width, then the clear followed by the green, then the blue. Why did it turn out this way?
Question Date: 2002-03-21
Answer 1:

I remember doing an experiment very similar to this one when I was in high school. My results were a little different than yours so I think that there may be factors that affect the growth of plants other than light at work in our experiments. I used a single celled plant (I don't remember what it was anymore) and found that they grew best in red light, then clear (though the red and clear were almost the same), then blue, then green. You may already know that plants get their energy from sunlight and that sunlight contains all the colors. To get turn light into energy the plant can use, plants have a special molecule called chlorophyll. It turns out that the molecule works best with red light, and works a little with blue light, but not very well with green light. That is why plants look green. The green light in the sunlight gets reflected back into your eyes. The red and blue light are absorbed by the chlorophyll and used for energy. I would expect the clear cellophane to work about as well as red cellophane since clear cellophane contains lots of red light also. I hope that helps.

Answer 2:

You let your plants all have the same kind of soil, and the same water. What did you change? The kind of light. Light is what the plant uses to make its food from carbon dioxide (that it gets from the air). Plants use a certain colors of light much more than others, mostly blue and some red. So plants absorb (take in) blue and red light. That is why they look green! It's because plants reflect green light and absorb the other colors. The cellophane you used blocks out the color of light that it appears to be, and lets the rest through. The red cellophane blocks out red light, and lets blue light through. The plant covered with red cellophane got the most blue light, so that plant had the most energy to grow. The clear cellophane-covered plant should have gotten about as much blue light, so it should have been nearly as big.

Maybe giving a plant ONLY blue light makes it easier for it to grow somehow. The green cellophane probably blocked out some of the blue light that plants use (look at a rainbow - green is very close to blue. They are similar colors) When they make green cellophane they probably add yellow dye to same mix they use for blue cellophane. So the green and blue cellophane block out the blue light and those plants have the least energy to grow.

Answer 3:

Plant leaves absorb more light of certain colors than others. Given that the color of opaque objects, like the green leaves, is determined by the light that they reflect, rather than absorb, which color(s) do you think the plant leaves absorb the most? Does that make sense with which plants grew the most?

As an interesting aside, green-leafed plants were not always dominant on Earth. The spectrum of sunlight reaching the Earth's surface is peaked around green, such that there is sun's light is more intense in the greens and yellows than it is in the reds, blues, or purples. Ancient plants evolved to make the most of the light arriving from the sun, and so absorbed green light. The leaves of these ancient plants would probably have appeared purplish in color. The green-leafed plants evolved to use the light that the majority of the plants then reflected and did not absorb. If you're curious, you can investigate to find out when the purple-leafed plants died out, and why. There are still some plants that have purple, rather than green, leaves. Can you think of some?

Answer 4:

I would hate to give you the answer, as the point of doing a science project is to look at the results and think for yourself what they mean. First of all, you must know something about how plants absorb light or you wouldn't have chosen this project. The clear cellophane is letting all colors of light pass through. The colored cellophane is letting light of only one color pass through. Plants absorb light and convert it to chemical energy, which they use for growth. Plants don't absorb all colors equally, though, and which color of light they absorb the most depends on which light-absorbing pigments the plant has. For the most part, land plants use chlorophyll-a to absorb light. Chlorophyll-a has a specific chemical structure that absorbs red and blue light more than green (which, by the way, is why plants look green: the green light is reflected rather than absorbed). Red, especially, is absorbed.

Here are some guidelines for interpreting your results. Assume that all factors affecting growth other than light (genetic differences, water, soil nutrients, temperature, pests) are the same for each plant. Then imagine several scenarios. I'm guessing the outcome of your experiment is a combination of these.

1) There is enough light coming through the cellophane so that plant growth is not limited by light, but by some other factor. Predict the order of growth.

2) All of the cellophane colors, even clear, reduce the light enough so that plant growth is limited by light. The colored cellophane is light enough that the main difference among the cellophane treatments is color of light and not the intensity of light. Knowing that plants absorb red light the most, then blue, predict the order of growth.

3) Some of the cellophane colors are darker than others and so reduce the intensity of light to very different extents. Place the cellophane against a white background and order the colors according to how much you think they might reduce incoming light. Predict the order of growth.

Plants are like animals in that even those from the same batch of seeds (the same "parents") can have very different individual growth rates. To account for this, you would need to keep some "control" plants (say four extra plants) nearby but without cellophane, and measure their growth. If the differences in growth among the four plants without cellophane are similar to the differences among the four plants under the different cellophane colors, you're not looking at the effect of cellophane color but at individual genetic differences in the plants. The control plants would also tell you if the clear cellophane was reducing light, which would help with scenario 2.

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