
How Do Plants Know Which Way Is Up And Which Way Is Down? - shawndumas
http://www.npr.org/blogs/krulwich/2012/06/21/155508849/how-do-plants-know-which-way-is-up-and-which-way-is-down
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StavrosK
There was a bit of a non sequitur at the end, where they mention that the
statolith theory has support from plants growing in space. That's no
confirmation for the theory, we already knew they sensed gravity, the question
is _how_ they sense it.

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Tloewald
Right. The experiment does nothing to confirm the conjecture. A neat trick
would be, say, to figure out how to line up the statholiths (sp?) in zero g
and see what happens relative to a plant whose statholiths are randomly
arranged.

Note that ultimately we would like to come up with ways to convince human
bodies that they're in a gravitational field, so this is not entirely
academic.

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Avitas
I seem to remember learning the pecise reason why plants both respond to
sunlight and to gravity during my Freshman year Biology class in high school.
I remember that it is due to a single plant hormone that stunts the elongation
of cells. I think it begins with an 'a'.

Aren't the mechanics of this well-known?

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cezary
Auxin, I did a science project on this in 5th or 6th grade. My father helped
me rig up a centrifuge-like apparatus that would spin seeds at what was
supposed to simulate greater than earth gravity. I remember doing research
(reading) about auxins and coming to the conclusion that higher gravity would
stunt plant height.

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alberich
And what happened to the seeds on your science project? :)

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Jun8
"...where the pull of gravity is nearly zero": No it's not! This is sloppy
science journalism physics where there's a gravity shield around Earth. The
pull is there but objects are in free fall.

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jmanamj
"would the plants respond to the centrifugal pull of gravity"

Sometimes I have to reread sentences like this to get what the author is
trying to convey. I don't understand why the author didn't leave it at
"centrifugal pull. " It's already an adequate layman's term. A sentence like
this only serves to confuse and make physics sound harder than it is. If a
good teacher can open kids eyes to the wonders of the world, a bad teacher can
easily fog the world in obscurity and inconsistencies.

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praptak
Some bacteria know up and down by knowing magnetic north and south. They take
advantage of the fact that magnetic north usually points slightly down or up.

By knowing up and down the bacteria can move to the area with optimal
parameters like temperature/oxygen saturation - these usually change with
depth.

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strictfp
I'm surprised that the plant was robust in space. It could just as well have
stopped growing at all. Makes you wonder if there are any low gravity growth
places on earth. If not, where has the plant learned this skill?

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nmb
I'm not sure if you're joking, but you should know that I had a good laugh at
this comment. :) In case you are indeed wondering, gravity has no effect on
cell growth, or indeed on any of the cell's vital functions. More than
anything, cellular processes rely on chemical diffusion, which at that scale
is more strongly affected by temperature than by gravity.

Also, gravity is almost constant everywhere on the earth's surface.

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strictfp
Wow. Complete cognitive dissonance between us :) I was reasoning about the
plant as a biological device which has adapted to life according to Darwin's
theory of evolution. Since gravity, as you point out, is quite constant on
earth, one could expect that a plant has come to rely on it for its existence.
Unless trained otherwise. So I was contemplating that the ability to survive
without gravity would have had to be a skill learned off the surface of the
planet. And if it has never experienced such an environment before in its
evolutionary development, one could consider it extreme luck that it kept on
living without the previously nearly constant gravity parameter. If the plant
manages to survive in spite of fundamental environmental changes without
previous training, it must have a very robust design. It's like a computer
program which would keep on running if you removed main memory. Not
impossible, but very unlikely that it wouldn't be dependent on main memory by
now. If it still works, must it not be designed in a very robust manner?

Are plants this robust? One could be lead to believe that this is indeed a
useful property in a evolutionary sense. But it must surely also have high
costs. So it is nonetheless a very interesting question in my opinion.

Addendum: Regarding cells not being dependent on gravity for growth: In the
article is was mentioned that plant in fact _are_ dependent on gravity for
their survival, in fact very much so. Without gravity the plant would not be
able to physically locate its root system correctly or get its leaves above
ground. It was, however, also mentioned that the plant survive in space just
fine.

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maxerickson
There's no reason to leap from the presence of an environmental factor to
reliance on that factor for survival.

For instance, air is thick with nitrogen, something that most organisms need
but are unable to extract from that air.

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ww520
I think it's just a function of how the liquid in the plant's cells settled by
gravity. The denser liquid is pulled to the bottom by gravity while thinner
liquid floats to the top. The plant must grow toward the thinner liquid, away
from gravity.

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biot
While this sounds plausible, it doesn't explain how roots grow downwards.

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teekarja
did not click cause of one obvious word: gravity

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prawn
You would've been a bundle of excitement in school science when the teacher
discussed humans knowing which way was up and which way down. Yelled "gravity"
and then blocked your ears while the teacher explained the utricle and so on?

