Re: Blue Sky Snack

From: On-line Snack discussion (
Date: Mon Feb 05 2001 - 10:58:51 PST

Dear Jim,
I asked one of our resident biologists to answer your question and here is
her answer:

Jim asks an excellent question! He also has some very important facts at
his fingertips, and has made astute observations of the results of his

He's absolutely right that white light contains all the colors of the
spectrum, so the plants grow the best. When filters are used over the
light used for growing plants, the color (that is, wavelength or light) of
the filter is what the plant receives. In Jim's experiment, the plant
grown with red filters received only red light.

Plants respond to wavelengths of light from 300 - 800 nm, pretty much the
entire visible spectrum, violet through red. Different pigments in the
plant absorb blue, green, yellow, or red light, but mainly plants use the
pigment chlorophyll to absorb light energy that it will then use to
food. Chlorophyll absorbs primarily red and blue light.

The plants covered with yellow and green filters were not receiving enough
light energy to produce lots of leaves. Light inhibits "internode
elongation" of a plant --- that means that adequate light prevents a plant
from becoming spindly with large distances between the places where leaves
or side branches develop. It sounds like the lack of red and blue light
contributed to this phenomenon, but there's also a second aspect to this
question, one that helps to explain the results of the plants coverd with
red plastic.

As the colored plastics cut out certain wavelengths of light, they are
cutting down the intensity of the light. The plants covered with the
colored plastic are actually receiving less total light than the ones
covered in clear plastic. The plants covered in red are probably not
getting adequate light to form leaves properly, and the ones under the
yellow and green are able to use even less of the light, so they are even
spindlier and have fewer leaves.

I hope that you also remembered to grow some plants without cellophane
covering. It's what scientists call a "control" and helps you to make
that your seeds, soil, and lighting conditions under normal circumstances
are all optimal before you start your experiments. Without a control, it
is difficult to draw conclusions that are valid from your experiment.

Good luck!

Nina Thayer
Snacktalk Moderator
3601 Lyon Street
San Francisco, CA 94123-1099

On Sat, 6 Jan 2001, Jim Mormile wrote:

> Hi, I'm in 6th grade and am doing an experiment on how colored light affects plant growth. I was reading your Blue Sky snack because I am trying to understand how the colors do affect growth. I know that different colors of lights project different wavelenghts. My plant covered with clear plastic is the fullest. I think that's because white light contains all the colors of the spectrum and it's getting all the energy. My plant covered with red is the tallest, but has very few leaves. I know that red light has the longest wavelenght and longer wavelengths have less energy than shorter ones. The plants covered with yellow and green are about the same size, not as tall as red but have long, skinny stalks and most of the leaves are at the top. From what I've read, I think the plants covered with colored cellophane are being deprived of some energy, but I can't find any information to help me understand exactly what's happening. Can you help? Thank you very much.

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