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erling.floistad at ihb.nlh.no erling.floistad at ihb.nlh.no
Mon Aug 30 02:46:25 EST 1993

In  <4gUK3Q_00iV3A6L19d at andrew.cmu.edu>  David Michael Adler <da21+ at andrew.cmu.edu> writes:
| I am a college student who just bought a few plants to lighten and
| brighten my room. I have noticed that they grow towards the light, and
| while this makes sense for the plant, it makes very little sense to me.
| Assume that a plant grows better when it's given the most light. Then
| the side of the stalk that faces the light would grow the fastest,
| causing the plant to dip away from the light. Why (or more accurately,
| how) does this happen?
|         -Frump
Your hypthesis of plant growth assumes that plants grow in a bacterial 
fashion, just simply multiplying cells at a light and nutrient- dependent
rate;  the part of the plant with the most light and nutrients grows faster.

However,  when you look closer ( as you definitely have started doing), 
you will see that plants are capable of responding to a range of different 
stimuli by modifying their crowth.   In general this is possible because
cells in different parts of the plant communicate by the action of chemical
messengers, hormones.   Auxin is the hormone involved in the response you 

Generally, when a cell recieves auxin, it will elongate, or stretch out.  
It has been shown that the tip of a plant is illuminated from one side, 
auxin will be transported away from the light and acumulate on the dark 
side of the stem.  When the auxin is transported downwards (it usually is) 
the shade-side of the stem recieves more auxin, and will grow/stretch 
than the light-exposed side.  This response is known as phototropism,  
and was first investigated by Charles Darwin (published in 
The Power of Movement in Plants, 1881).  Some of his experiments were 
quite (ingeniusly) simple, and you might enjoy trying to repeat them.
They are quoted in any textbook of Plant physiology.

For more info on this, try:

Salisbury and Ross (eds): Plant Physiology              (or even bigger)
Taiz and Zeiger    (eds): Plant Physiology

Whether or not you chose to do a closer investigation on plant physiology:

Enjoy your new plants        

(btw: they will stay prettier if you rotate them a little 2 - 3 times 
      a week, keeps them from getting too bent in one direction.)

Erling Floistad   			         e-mail: hagef at nlh.no
Ph.D. Stud.,   Dept. of Horticulture,            s-mail: Po.Box 5022
              div. of Vegetable Growing                  N-1432 AAS
          Agricultural University of Norway              NORWAY
Phone:  +47 6494 7826                               Fax: +47 6494 7802

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