Thank you for the very interesting question!
Gravitropism is very important for plant
growth because it maximizes the plant’s exposure
to sunlight, which the plant uses to produce
energy through photosynthesis.
Plants have a type of cell called a
statocyte which can sense the direction of
gravity. This is possible because of a special
part of the cell called the amyloplast,
heavy particles that “roll” towards the bottom the
Signals sent by these cells direct the growth
of a plant’s roots and stems. The biologist
Charles Darwin was the first to demonstrate that
roots show positive gravitropism: they grow in
the direction of gravitational pull.
Conversely, stems show negative
gravitropism, meaning they grow in the
opposite direction of gravity. This accounts for
why roots are underground and stems are overhead.
Now for the interesting experiment you propose!
Should a plant be planted upside-down, it
turns out that the amyloplasts will adjust to
the new orientation and send signals that reverse
the direction that the roots and stems grow
typically in. This means the stems will grow
down and the roots up! You can see an example of
this in the amazing
PHOTO I have attached to this message. Notice
how the branches of this tree are growing down,
meaning the roots are growing upwards.
You can try a variant of this experiment at
home by turning a potted plant on its side. Given
what we know about plants, we could hypothesize,
or expect, that the stem will likely start to bend
upwards in response to the pull of gravity towards