We are dealing with two things here (1)
Plant Physiology (2) How you are addressing
this - and this is a question of how science is
Perhaps the best answer is for you
to consider what happened? Did you do an
experiment "wrong". Why do you think so? Turn
the question on your head . What did the PLANT
do? What is that telling you? You could grow
it in a clay pot & why do you think that would
make a difference? (IT would not - the stem
would just die because it could not make it out
of the top of the pot if the latter was solid).
But you could do that experiment and you should
then be asking what have I learned. What is
the plant telling me?
You could play with the experiment.
You could illuminate the upside down pot from
the bottom - this would be an experiment to see
if the plant is responding to light. You could
also use a clay pot WITH a hole in its botttom
(a drain hole) and a second without - and
illuminate from both below & above as two
separate experiments. (again, testing the role
Finally, for your own pleasure
(responding to (4) You should take a well-
sprouted plant that was grown rightside up &
suspend it upside down. The stem will turn. If
you grew three such plants, and took two &
turned them up side down & then took the third
& depotted it and photographed the distribution
of the roots, you could then in a few weeks
take other two & de pot them, think you will
see that the roots will be distributed towards
the "top" of the pot not the bottom.
Again, your problem is that your
hypothesis was "I expected the plant grow down,
and then curve up." I think you assumed light
would dominate. Your hypothesis was disproven.
You now need to further test light. And you need
to erect another hypothesis (also tested here),
that gravity dominates over light.
(Getting to the bottom of this gets into
primary literature, e.g. the response is to
auxin distribution, in turn probably (in my
understanding) Having to do with some sensing of
gravity by the plant - (see Gravity signal
transduction in primary roots, Perrin et al
2005. (Annals of Botany Volume 96, Issue 5 Pp.
737-743) This suggests that columella cells of
the root cap sense gravity & then influence the
redistribution of auxin, in turn determining the
growth of the root - in this case downwards.
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