How do seed cells know the difference between top and bottom?

Nanda Somarajan somarank at SLU.EDU
Mon Apr 21 12:03:24 EST 1997


	Actaully not.  I just misspelled.  It should have been 
gravitropism, instead of gravitrophism(I had an extra "h").  Gravitropism 
is the tendency of plant roots to grow towards gravity and the stems to 
grow away.  Tropism is just tendency of any organism towards something.  
There are other tropism, including towards light and certain chemicals.  
Many single celled animals that produce their own energy by 
photosynthesis does both light and chemical tropisms.

Nanda Somarajan
somarank at slu.edu
somarank at hotmail.com
http://www.geocities.com/CollegePark/4116/
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http://www.slu.edu/people/somarank-01.html
http://www.angelfire.com/ok/nanda
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http://www.slu.edu/people/somarank-00.html


On Mon, 21 Apr 1997, Barry Meatyard wrote:

> Nanda,  I trust the term 'gravitrophism' was a typo error for tropism (but
> it is repeated).
> Apologies, but I guess I'm just a pedantic dictionary writer and someone
> who has to correct student essays.
> Yours,  Barry
> 
> >Matthew,
> >        When you place a seed on its side, it will automatically adjust
> >its growth so that the shoot(stem) bends upward and the roots downward.  This
> >is a response to gravity called "Gravitrophism".  It used to be called
> >"Geotrophism."  Scientists are still unsure how this works.  ( I got the
> >following from my Biology Text book titled "Biology"  By Neil Campbell.)
> >
> >        One theory is that plants may tell up and down by the settling of
> >statoliths, specialized plastids containing sense starch grains,  to low
> >points of cells.  In roots, staloliths are located in certain cells of
> >the root cap.  According to one hypothesis, the aggregation of statoliths at
> >the low points of these cells triggers the redistribution of calcium, which
> >in turn, causes lateral transport of auxin within the root.  The calcium and
> >Auxin(A plant hormone) accumulate on the lower side of the root's zone of
> >elongation.  At high concentrations, Auxin inhibits cell elongation.  So
> >the cells on the upper side of the root elongates more rapidly than the
> >lower side with the statoliths and this causes the root to curve as it
> >grows and continues untill the root is growing straight down.
> >
> >        But, some scientists are challenging this hypothesis.  According
> >to Randy Wayne of Cornell University, the impact of the starch  grains on
> >the bottom of a cell does not release enough energy to be the mechanism
> >for gravitational detection.  He points out also that there are many
> >plants that do not have the starch grains and they still grow ok.  He
> >studies gravitrophism in Chara,which are green algae and closely related
> >to plants.  Proteins attach the protoplast of each cell to the inside of
> >the cell wall.  According to Wayne's hypothesis, the downward settling of
> >the entire protoplast, which streches the protein tethers at the top of
> >the cell and compresses those at the bottom, gives the cell its sense of
> >up and down.
> >
> >        To test this, he placed Chara in a solution more dense than the
> >laga's cytoplasm.  The protoplastes floated upward, instead of falling
> >downward and the growth pattern of the alga was also upside eown.  He is now
> >trying to study to see if this mechanism also works in vascular plants.
> >
> >        This is all I know.  Does anybody else know how plants know which
> >is up and which way is down??
> >
> >****************************************************************************
> >                                Nanda Somarajan
> >                                somarank at slu.edu
> >Ebola Information Headquarters: http://www.angelfire.com/ok/nanda/
> >My Home Page:                   http://www.geocities.com/CollegePark/4116/
> >
> >****************************************************************************
> 
> Dr.Barry Meatyard                       Tel: 01203 524228
> Science and Plants for Schools          Fax: 01203 523237
> Institute of Education                  Email: barry.meatyard at warwick.ac.uk
> University of Warwick
> Coventry
> CV4 7AL
> UK
> 
> 
> 



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