Six Ways Plants Move
Photosynthesis is a popular
way to make a living on Earth
and plants—as well as some
microscopic organisms—have
finely tuned, light-controlled
positioning systems."
Geotropism
Geotropism is a form of gravitropism,
which is growth in a direction parallel
to a gravitational field. Earth’s gravity
results from its mass, is directed toward
the center of the planet and diminishes
in strength as one gets further from the
Earth’s surface. Plants are able to detect
the presence of gravity and align themselves with it accordingly. Primary stems
are negatively geotropic in most plants and
have a strong tendency to grow upward
away from the Earth, while primary roots
are positively geotropic and grow toward
the earth. Secondary stems and roots are
plagiogeotropic, meaning they grow at an
oblique angle—not exactly straight down
and not perfectly horizontally, either. And
then there are the diageotropic rhizomes,
which snake along perpendicular to the
pull of gravity.
It is clear why plants benefit from having a gravitational sensor system—after
all, roots need to get into the ground
where the water and minerals are and
shoots need to get into the air where the
light, carbon dioxide, pollinators and the
rest of the exciting world exists—but
how exactly do they do it?
Early researchers suspected that auxins
played a role in gravitropism and they
were correct. Roots curve into the Earth
because of differential cell elongation
in the root. This is pretty much exactly
the same mechanism that guides phototropism, but the sensing and signaling
system is different.
As is the case with phototropism, the
detection phase of the gravitational
response is well known. Starchy grains
called amyloplasts in root caps settle, under the influence of gravity, to the lower
side of cells—this transmits a biomechanical signal of some kind that indicates which way is up (or down, as the
case may be) and the auxin transport and
regulation machinery takes it from there.
Hydrotropism
Plants can detect the presence of gravity and align themselves with it accordingly; primary stems in
most plants grow upward away from the earth and primary roots grow toward the earth.
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Maximum Yield USA | March 2012
Plants cannot detect water at a distance
and do not have the ability to direct their
growth toward it. However, if they do
detect water in their environment, plants
are able to direct growth in the direction
of greater water concentration. Plants
also respond to water by rapidly growing
when it is present and slowing growth
when it is not. Roots can be sparse in
a region of low moisture but suddenly
explode in a riot of highly branched
growth in a spot where water is plentiful.
Roots grow in all directions exploring the local substrate and when a good
source of water is found it makes sense
for a plant to take advantage of it by