Six Ways Plants Move
Tropisms are plant
movements directed toward
or away from a stimulus."
developmental response to water) and thermotropism (response
dependent upon temperature).
It is helpful to also designate plant responses that are triggered
by a definite stimulus, but not in a direction that is related to
the direction of the stimulus. Such movements are called nastic
movements. Examples of this are a mimosa folding its leaves
in response to a touch or a Dionaea (Venus flytrap) closing in
response to the presence of an insect.
A lot of tropism research has focused on how plant hormones
known as auxins control plant growth by stimulating cell
elongation. It is well accepted that phototropic and geotropic
bending of shoots and roots results from cells on one side of a
plant elongating faster than cells on the other side, thus causing
the plant to bend and change the direction of its growth.
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Maximum Yield USA | March 2012
Phototropism
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. Auxins are important in regulating how plant organs move and grow toward a
light source and photosynthesis is one of the most-studied topics in botany.
In the basic model of phototropism, the concentration of
auxin is elevated on the unlit side of a plant shoot exposed to
light, which causes the cell walls on that side to become less
fixed in structure. The cells on the unlit side elongate and then
re-solidify their cell walls—the effect of this deceptively simpleseeming process is that the elongating cells cause the shoot to
bend toward the light. Exactly how the concentration of auxin
gets to be higher on the dark side is not perfectly understood,
but current research suggests that auxin is transported from one
side to the other and perhaps from other places in the plant. The
biochemistry of auxin-regulated bending is still being worked
out, even after many decades of research.
Two other pieces of the phototropism puzzle lie in the
proteins phototropin and phytochrome. Although these two
molecules are almost certainly on the light-receiving end of the
phototropism system, the biochemical steps that go from there
to the action of auxin are still under investigation.