Terrestrial plants, anchored, by necessity, to their substrate, have limited
mobility and few ways in which they can respond to environmental stimuli. The
primary way in which plants do respond is through changes in growth. As
we have seen in Essential Processes, Plant Hormones
, hormones are
often
responsible for changing the growth patterns of plants. This section will
explore tropisms and turgor movements, two forms of plant movement that
allow the plant to react to stimuli.
Tropisms
Tropisms are responses to stimuli that result in the long-term growth of the
plant toward or away from the stimulus. This growth results from cell
elongation occurring at different rates on different sides of the plant, so that
the plant bends in one direction. Phototropism, a reaction to light, causes
the plant to bend toward the light source (see Essential Processes,
Auxins).
Thigmotropism, a reaction to touch, causes parts of the plant to thicken or
coil as they touch or are touched by environmental entities. Tree trunks, for
instance, grow thicker when exposed to strong winds and vines tend to grow
straight until they encounter a substrate to wrap around. Gravitotropism, a
response to gravity, causes parts of a plant to grow either upward or downward.
If a plant is placed on its side, its shoot will begin to grow upward (against
gravity) and roots will follow the pull of gravity to grow downward.
Turgor Movements
Turgor movements occur more rapidly than tropisms and are easily reversible.
They rely on changes in turgor pressure (exerted by water on cell walls)
within certain plant cells instead of on differential cell growth. Turgor
movements are responsible for many plant responses, such as when leaves or
flowers droop and fold up at certain times of the day or night or in response to
an external touch. A Venus flytrap, for instance, depends on changes in turgor
pressure to close its "jaws" around insects when they land on the plant.
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