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artificial light experiment

Beverly Erlebacher bae at cs.toronto.edu
Wed Nov 25 12:07:59 EST 1998

In article <73cm79$cgf6$1 at newssvr04-int.news.prodigy.com>,
VICKI L JONES <VICTORIAJ at prodigy.net> wrote:
>  I'm doing an experiment for my biology class.  I'm using artificial
>light to grow bean plants.  After germination, I placed them under
>clear plastic bottles using cello for the different color lights.  I
>used clear, yellow, red, green and blue.   I thought that this was
>also the order in fast growing to least growth; however, my experiment is
>not working out this way.  All were about the same size after germination,
>but my blue is growing the fastest.  This should not
>be the case should it?  Could someone please tell me what order they should
>be growing in and why?  

Seedlings live mainly off the stored food in their cotyledons (seed leaves) 
while they are developing true leaves, so you may need to grow your plants
on for a while to get away from this effect, which may not be easy in a 
small space like a pop bottle.  Also, individual beans can vary enough in
weight, therefore in the amount of stored food, to make a substantial
difference between individual plants.  This is a problem if you don't have
multiple plants under each color.

You can get around some of these problems by using smaller-growing plants
with smaller seeds.  You might try cress, lettuce, radish or bluegrass.  I
realize you may not have time to repeat your experiment with different 

Another problem is that your experiment doesn't distinguish between effects
from color of light and amount of light.  If you use dark blue vs light blue
cellophane the difference between growth under blue may vary quite a lot
compared to growth under other colors.  Similarly, your artificial light
source may be putting out substantially different amounts of light at
different wavelength.  You can find out the spectrum of various kinds of
fluorescent tubes at the web sites of their manufacturers.  Note that
fluorescent bulbs emit less light near their ends than in the middle.

Another factor is, how are you measuring growth?  If it's just height, you
may be measuring the tendency of plants to stretch (etiolate) to reach the
light.  I.e. the blue light plant may be getting less light than the others,
so stretching further even though it isn't photosynthesizing as well.  Also,
different wavelengths of light have different effect on plant growth forms.
In general, plants will tend to stretch under redder light and grow more
stocky under bluer light.  One more accurate way of determining relative
growth would be to clean and totally dry the plants at the end of the 
experiment and measure the dry weight to determine how much growth was due
to photosynthesis.

>I'm confused and I don't want to fail, but my
>experiment seems to be distorted from what the results I thought should be.
>Thanks so much,     Tiffany (9th grade)

Discuss these issues with your teacher, and see what he/she says.  It looks
like you are serious about your science project.  Remember that if experiments
always turned out exactly as expected we would know a lot less science!

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