Phototropism





Phototropism 2838
Photo by: Trevor Slauenwhite

Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of "tropisms" in order to ensure their survival. A tropism is the growth of a plant as a response to a stimulus, and phototropism occurs when a plant responds to light by bending in the direction of the light. Although plant physiologists (scientists who study how the processes of a plant actually work) know that this growth is caused by a plant hormone, they still do not fully understand exactly how it works.

Bending toward the light

Most of us at some time have noticed a houseplant on a windowsill that seems to have all of its thin stems leaning in the same direction, as if it were trying to press itself against the glass. Picking it up and turning the entire pot in the opposite direction so that the plant is pointing away from the window will only result, about eight hours later, in the plant having reversed itself and going about its business of pointing its leaves toward the window again. This is not because plants especially like

Plants respond to the direction and amount of light they receive. The seedlings on the left grew toward the light it received on only one side. The plant in the center received no light. The plant on the right was grown in normal, all-around light. (Reproduced by permission of Photo Researchers, Inc.)
Plants respond to the direction and amount of light they receive. The seedlings on the left grew toward the light it received on only one side. The plant in the center received no light. The plant on the right was grown in normal, all-around light. (Reproduced by permission of
Photo Researchers, Inc.
)

windows but rather because light is essential to their survival, and they have developed ways of making sure they get all they need.

We know then that it is the light coming through the window that the plants are striving to get closer to, but how is a plant, which is rooted in soil, able to "move" toward the light? Actually, the plant does not so much move toward the light source as it grows in that direction. As already noted, this growth of a plant that occurs as a response to a stimulus is called a tropism. There are several forms of tropisms, such as gravitropism or geotropism, in which a plant reacts to the force of gravity; hydrotropism, in which the presence of water causes a response; galvanotropism, in which a plant reacts to a direct electrical current; thigmotropism, in which a plant responds to being touched or some form of contact; and chemotropism, in which a plant reacts to a chemical stimulus. Since the prefix "photo" refers to light, phototropism involves a plant responding to light. In all of these tropisms, the plant's response involves some form of growth. Finally, all tropisms are either positive or negative, although these words are not always used. So when a plant's leaves grow toward the light (stimulus), it is technically called positive phototropism. When its roots normally grow away from the light, it is called negative phototropism.

Words to Know

Auxin: Any of various hormones or similar synthetic substances that regulate the growth and development of plants.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Tropism: The growth or movement of a plant toward or away from a stimulus.

How phototropism works

It is known that as long ago as 1809, Swiss botanist Augustin Pyrame de Candolle (1778–1841) observed the growth of a plant toward the light and stated that it was caused by an unequal growth on only one part of the plant. However, he could not understand how this was happening. Some seventy years later, English naturalist Charles Darwin (1809–1882) began to grow canary grass in order to feed the birds he kept, and he eventually discovered that it was the tips of the sprouting seedlings that were influenced by the direction of their light source. He and his son Francis learned this when they covered the tips of some seedlings and found that they did not move toward the light. When only the seedlings' stems were covered, however, they still moved toward the light.

It was not until the 1920s that Dutch botanist Frits W. Went (1903–1990) proved the connection between phototropism and a plant hormone called auxin. Went discovered that plants manufacture a growth stimulant (which he named auxin) in their tips, which they then send to other cells in the plant. In phototropism, however, this growth hormone is distributed unevenly when the light source comes from only one direction. Specifically, more auxin flows down the dark side, meaning that it grows faster than the exposed side of the plant. This unequal or one-sided growth (also called differential growth) brings about the curving or bending of the plant toward the light source. Went named this growth hormone after the Greek word auxein, which means "to increase." Although it was isolated and named, auxin was not understood chemically until twenty years later when it was finally identified chemically as indole-3-acetic acid.

Plants can react and adjust

Understanding what plant tropism is and, specifically, what happens during phototropism makes us realize that plants, as living things, necessarily demonstrate the several characteristics of life. Specifically, this includes growth, response to stimuli, and adaptation. It is because of its hormones that a plant's stem always grows upwards and its roots always grown downward. Since plants must make their own food to survive (by changing light energy into chemical energy—a process called photosynthesis), the ability to capture as much of this light energy as possible is crucial to its survival. Thus, plants have developed a chemical response to light or the lack of it that causes their stems to bend toward the stronger light.

Today, we know that a certain minimal amount of light (whether natural or artificial) has to be present for the plant to react chemically. This is called its threshold value. Despite our understanding of the basic stages and phases of phototropism, we are only now beginning to obtain the most basic knowledge of what goes on at the genetic and molecular level. We do realize however that plants are living, sensitive things that can adjust to their environment and actually seek out the light they need if they are not getting enough.

[ See also Plant ]



User Contributions:

hiju
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Aug 7, 2006 @ 6:18 pm
This was a good site! I only looked at the pictures!
Stephanie
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Apr 17, 2008 @ 5:17 pm
Lots of good information!! For my phototropism assignment!! ahahahah
isabella
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Aug 8, 2008 @ 10:22 pm
really good! very easy to understand! definately helped
toby
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Aug 13, 2008 @ 10:22 pm
good website, helped alot for my assignment, very nice
candee
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Sep 3, 2008 @ 8:08 am
has alot of awsome pix and good info... esspesially for ascience project.
Ian
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Sep 18, 2008 @ 7:19 pm
Great website, helped me alot with phototropism assignment, wickapidias is WAY to long and complicated.

I will reccomend this site to others at my school.

propz
Betta
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Oct 19, 2008 @ 9:21 pm
THIS WEBSITE IS A LIFE SAVER ESPECIAALY WHEN IT COMES TO PROJECTS AND ASSIGNMENTS.....

THANX GUYS FOR THE GREAT HELP....
bob
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Oct 20, 2008 @ 5:17 pm
interesting

has alot of awsome pix and good info... esspesially for ascience project.
fluffy
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Dec 12, 2008 @ 8:08 am
got me an a on my tropism resaerch paper. : )also i learned alot about phototropism
shelby
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Dec 18, 2008 @ 12:12 pm
this website was a lot of help for my science fair project
HARRY POTTER LOVER!
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Mar 11, 2009 @ 7:19 pm
This information is also very interesting. I had no idea that phototropism had anything to do with a plant hormone.

Again,
My Thanks,
HPL ♥
Summer
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Apr 8, 2009 @ 6:06 am
Great information and unlike other websites, i actually understand what i'm reading but it would be great if you could describe the other types of tropism a bit more. Thank you for all the help and great work!
autumn
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Apr 20, 2009 @ 4:16 pm
Wow! Thanks so much for such good information that was very easy to understand. I appreciate it very much!
BubbleBees
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Apr 23, 2009 @ 12:12 pm
This was the most helpful website i absoluetly loved iT! The pictures and information were the bomb!! Wikepedia is way to complicated and this was understandable I will recomend this website to anyone for a science project becuase it really helped!!! Thanks a million
hue g but
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Apr 29, 2009 @ 9:21 pm
this saved me on my bio project
very very very good my friend
Gagan
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May 6, 2009 @ 6:18 pm
this is a very good website and has got lots of information regarding phototropism
Louu
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May 9, 2009 @ 11:23 pm
I can use this on my phototropism assignment and experiment! Thankyou!
v-girl
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May 10, 2009 @ 8:20 pm
this website rocks!huge help with my project. this is great!
Mi
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May 15, 2009 @ 10:22 pm
Thanks so much this website was the only one that had what i was looking for, literally a life saver
lollii
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Jul 28, 2009 @ 2:14 pm
this sit is gr8. Really help me understand it better.
kg
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Jul 29, 2009 @ 8:08 am
thanks for the info! it helped for my prac assignment.
Science
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Sep 19, 2009 @ 2:14 pm
Good website. I needed this information for my science research papers.
Stephanie
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Oct 7, 2009 @ 6:06 am
This has loads of info for my great assingment on phototropism!! woo lets hope my plants grow like the one on the left did
jon
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Oct 12, 2009 @ 9:09 am
good website it really helped me on my ag project. thanks guys :):):):)
Nareta
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Oct 13, 2009 @ 9:21 pm
This site gave me alot of help with my phototropism assignment learnt alot of stuff that i didnt know before. CLASSIC site
Nareta
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Oct 13, 2009 @ 9:21 pm
Thanks mark alot of hard work and a lot
of over night studies ; this is the site i basically
got my information off . really good site.
Joe mamma
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Oct 18, 2009 @ 11:11 am
really good...could use more info though but all in all it was good for science fair
Kaylee
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Oct 20, 2009 @ 10:10 am
Very good site!
I actually understood!
:) And that says alot!
club man rules
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Oct 21, 2009 @ 3:15 pm
i love this site really cool stuff saved my behind
Candice Johnson
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Nov 17, 2009 @ 4:16 pm
hi this is candice and i would like for you to send me some info on phototrpism asap


thanks,

Candice
chipmunk
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Nov 24, 2009 @ 12:00 am
this website is so good..
with all this info I can do my holiday assignment..
thanks.. ^-^
nikka
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Dec 7, 2009 @ 3:03 am
this really help us in our projects...
this is really an educational website.
biobiobio
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Dec 13, 2009 @ 7:19 pm
helpful :) i get it now thanks to this helpful little website dealyy
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Feb 3, 2010 @ 4:16 pm
Omg this really helped me and my science project!!! Good information! I'm doing a project on how plants respond to light by being put at different points in a room, but my farthest plant away looks healthy, but I was expecring it to be the one to most likely lean the most towards the window. Any tips on the subject?
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Mar 23, 2010 @ 6:06 am
I found this really helpful for my biology EEI. Anyone know what the chemical compitisition of indole-3-acetic acid?
John
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May 18, 2010 @ 3:03 am
Really Good!! Thx!!!

I used it for science project! Will acknowledge in science assignment bibliography.
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May 22, 2010 @ 2:14 pm
It was the best website i found talking about my experiment
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May 26, 2010 @ 11:23 pm
is this true?
is it backed up by facts?
anyways this information helped me with my report for biology thanks;)
mibz man
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Sep 11, 2010 @ 9:09 am
thanks 4 the help! ill remember to use this cite more often!
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Sep 22, 2010 @ 3:15 pm
Thanks for the help i have a science project to do
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Jan 2, 2011 @ 8:20 pm
love the information however,how do I cite this in APA
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Jan 5, 2011 @ 8:20 pm
i was looking for a answer like what chemical helps plants move and i am onley 9 years old
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Jan 28, 2011 @ 1:13 pm
that is so cool because the plant grows toward the light
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Feb 16, 2011 @ 11:11 am
very good ... helped alot with my science homework helped me so much agood website for research
DeDeSweet
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Apr 27, 2011 @ 5:17 pm
THIS INFO IS GREAT IT REALLY HELPED ME WITH MY PHOTOTROPISM PROJECT
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Aug 28, 2011 @ 5:17 pm
Great website for my biology research project. :)

Thanx again,
MBL
amber
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Oct 29, 2011 @ 7:19 pm
omg
totaly help on my science fair project
last minute reseach helped at lot
got good info :)
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Jan 12, 2012 @ 5:05 am
i realy like the arrangement and love the information given it has been realy helpful. Thank you so much.
Victoria
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Feb 7, 2012 @ 9:21 pm
Thanks! This helped a lot with my Report!!! I got an A on it!
Brandon
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Mar 2, 2012 @ 11:11 am
Thanks for the info, this is going to help my group with our experiment
Charette
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Mar 30, 2012 @ 9:09 am
Awsome info about phototrophism.It will be pretty helpful to my research project on Homeostasis.
Elise
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May 12, 2012 @ 11:11 am
wow thank you for having this website it gives me really good information on my science project i have to do and it came so fast thank you again, Elise luv
bob
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Jun 17, 2012 @ 4:16 pm
wow that was great i love the it help a lot for biology research!!!
bonnie
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Nov 18, 2012 @ 12:12 pm
real good site it helped me on science project i love phototropism!!!
ronald john
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Jan 22, 2013 @ 8:08 am
what would be a good discussion about phototropism
Kera
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Feb 10, 2013 @ 3:15 pm
This is a really great site. It helped me with my science fair report. However, maybe you could define a few more words that people may not know.

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