artificial light please help?

Janice M. Glime jmglime at MTU.EDU
Tue Nov 24 14:17:41 EST 1998

Dear Tiffany,
  There are several things that you need to consider.  First, blue and red
light are the portion of the spectrum in which photosynthesis is most
active.  That may explain why your plants grew "better" in these colors
than in other colors.
  However, the clear glass gets white light, which has all the colors, so
you need to consider why your beans may have grown taller in just one
color.  I assume you had the same light source for all your treatments, so
those plants in jars with colored cellophane on them received less light
than the plants in the clear jar.  Now you must ask why they would grow
taller with less light.  Think about the way grass grows if it is under a
board or rug or something that blocks part or all of the light.  The grass
and other plants get tall and thin.  Also, non-grass plants in a field
with tall grass will grow tall and thin.  This is a response to low light.
I will not tell you the whole story because your teacher probably wants
you to try to find out what is happening, but I will tell you that this is
a response to a hormone called IAA.  If you can find a plant physiology
book or a general botany book, they usually have an explanation of this.
Even a college general biology book usually will explain how IAA works in
a plant.  (The book may call it auxin instead of IAA; IAA is one of the
auxins and is the most common one in plants.)  Once you understand this
hormone, you need to think about your own experiment to see if your
conditions would cause this kind of response.
  When I was in 8th grade, I did a science fair project that looked at the
effects of IAA.  The project happened by accident.  I bought two plants
and intended to add fertilizer to one and keep one with no fertilizer. 
However, on the way home from buying the plants, the top got broken on one
of them.  I was lucky because I had read an article about IAA in a
gardening magazine and knew that IAA was produced in the tip of the plant. 
If the tip was removed, the buds along the stem were supposed to grow and
make branches, so I compared the branch growth of the two plants.  That is
exactly what happened, and I won the 8th grade science fair with my
"experiment."  Now I know I should have had many plants in my experiment,
but it was exciting to win.
  I have given you some information to think about.  Good luck with
interpreting your experiment.  You have interesting results and can learn
a lot from them.
 Janice M. Glime, Professor  
 Department of Biological Sciences
 Michigan Technological University
 Houghton, MI 49931-1295
 jmglime at
 FAX 906-487-3167 

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