In article ,
Phred wrote:
In article ,
"Don H" wrote:
Tropism ("turning") in plants is their orientation when growing: upwards for
stems, and downwards for roots. The upwards trend can be attributed to an
attraction to light (phototropism), but downwards - due to gravity?

Plant shoots also respond to gravity. They grow "up" in the dark when
seeds germinate buried in soil. I also have a vague recollection of
experiments done decades ago where plants growing in rotating
chambers lost the effect of geotropism and grew in "random"
directions.

Maybe someone in sci.bio.botany will put me straight on this.

Wow, what a blast from the past!

In 1968, I was at Argonne National Labs near Chicago as an undergrad
summer student. On a tour, we were shown the place where some of these
experiments were in progress. It was a concrete blockhouse, and you
had to go through a series of doors to make sure no light got in.
There was an extremely faint green safelight to turn on, and once your
eyes adapted to the very near total darkness, you could see the three
"gravitational compensators" going through a slow synchronized dance
controlled by IIRC a PDP-6 minicomputer.

The seeds were placed in containers of vermiculite which were moved in
three dimensions in a pattern that ensured that the gravitational
vector was never constant long enough for the plants to "perceive" it.
They did, indeed, send their shoots and roots in random directions. I
remember that there was a problem in that seeds which rooted up into
the air portion of the chambers tended to dry out and die, skewing the
results, and they'd had to go to some trouble to deal with this.

IIRC, the mechanism of geotropism as understood at the time had to do
with starch granules settling towards the bottom of the cells. I don't
know what the current belief is. It's an old problem - Darwin wrote
about it.

The research at Argonne was funded by NASA. In those days before the
Vietnam War clobbered so much research funding, it looked like the
growth of plants in microgravity might become of considerable practical
interest in the not-too-distant future. So it goes.