Unique Tropisms
In Plants
By: MMMN
Morgan, Meghan, Miles & Noah
What is a tropism?
Definition: A tropism is the turning or bending movement of an
organism toward or away from an external stimulus such as light,
heat, or gravity.
• Tropisms are directional movement responses that occur in
response to a directional stimulus.
• The word “tropism” come from the Greek word “trope”, which
means “a turning”
• Tropisms can be positive or negative.
• One of the most commonly observed
tropic responses in plants is phototropism.
Positive and Negative Tropism
Plants are able to alter their growth so they can grow into more favorable
conditions. To do this requires a plant’s ability to detect where the conditions
are better, and then alter their growth so they can "move" in the appropriate
direction.
There are many types of tropisms, but only 2
ways that tropism can occur; positively and
negatively.
Positive Tropism
 In positive tropism, a plant will bend and grow
toward a stimulus.
 Ex. Roots of plants exhibit positive gravitropism
when growing, because they grow down,
toward the direction of gravity.
Negative Tropism
 In negative tropism, a plant will bend and grow away from a stimulus.
 Ex. Shoots of plants show negative gravitropism during growth, because they
grow upward, against the force of gravity.
Types of Tropisms
There are many types of tropisms, or ways that plants
respond to an external stimulus.
Some of these types of tropisms are:
Phototropism- movement or growth in response to light
Gravitropism (or Geotropism)-movement or growth in response to gravity
Thermotropism- movement or growth in response to heat/temperature
Hydrotropism- movement or growth in response to water
Thigmotropism- movement or growth in response to touch or contact
Chemotropism- movement or growth in response to chemicals
Heliotropism- movement or growth in response to sunlight
Host tropism or Cell tropism is a tropism process that determines which cells can
become infected by a certain pathogen
Phototropism
Phototropism is directional growth in
response to a light stimulus.
• Think: “Photo” means “light”
An example of this is “Photosynthesis”,
which means “Creation of a new
chemical (synthesis) with light”
Phototropism can be positive or negative.
• Positive phototropism is growth toward a
light source.
• Negative phototropism is growth away from a
light source.
One of the most known plant examples of
phototropism is the sunflower.
Phototropism II
• Most plant shoots exhibit positive phototropism,
and roots usually exhibit negative phototropism but
gravitropism usually plays a larger part in their
behavior and growth.
• Some vine shoot tips show negative phototropism.
This allows them to grow towards dark, solid objects,
like tree trunks, and climb them.
• The reason that phototropism is possible is
because of auxins. Auxins are hormones in plants
that cause plants and leaves to show a positive response to sunlight.
• When light shines on one side of the plant, the auxin moves to the shaded side and
causes the cells to become longer. This uneven growth makes it possible for the
plant to bend to the sunlight.
• Auxins also control the production of other hormones in plants.
• In sunflowers, a hormone inside the plant is
very important for phototropism. This hormone is
called Indole acetic acid. It regulates the entire
process. The most important job of IAA is to stimulate
cell growth and enlargement wherever it is located.
Gravitropism
Gravitropism is the turning or growth movement
of a plant or fungus in response to gravity.
 Also known as Geotropism
 Charles Darwin was one of the first scientists to
document that roots show a positive form of
gravitropism and stems show a negative form of gravitropism.
 Interesting Fact: Plants in a pot can continue to show gravitropism,
even when they are turned on their side! Based on the pull of gravity,
the roots and stem can orient their growth to the correct position.
(Stems move to face upward and roots orient themselves to grow
downward.) You can see this in the picture above and in the video.
 Auxins also play a part in gravitropism, not
just phototropism. During gravitropism, auxins
carry signals from the root cap to the zone of
elongation. This is an important part of the
process of growth response.
Thermotropism
Thermotropism is the movement of a plant
or part of a plant in response to changes in
temperature.
 An example of a thermotropic plant is the
rhododendron. It is classified as having
thermotropic or temperature-sensitive leaf
movements.
 Charles Darwin fist categorized leaf movements in the year 1880, in his book
The Power of Movement in Plants.
 Darwin said that many plant parts, particularly leaves, move in response to a
number of environmental and physiological factors. The most important
environmental factors are light intensity (phototropic), light direction
(heliotropic), water content (hydrotropic), and temperature (thermotropic).
 The Rhododendron’s thermotropic leaf movements are unusual because
these movements are in response to cold temperatures and the leaves
become pendent instead of vertical.
Hydrotropism
The prefix “Hydro” means “Water”, so, of course,
Hydrotropism is a plant growth response where
the direction of growth is determined by a
stimulus of ascent/descent in water concentration.
o A typical example of hydrotropism is when
plant roots grow in humid air. Hydrotropism
causes the roots to bend toward a place in the air with a higher
humidity level.
o The hydrotropism process is starts with the root cap sensing water and
sending a signal to the part of the root which is being made longer.
o Hydrotropism is hard to observe in roots growing underground, because
these roots aren’t very observable. Gravitropism in roots is also usually
more of an influence than hydrotropism.
o For these two reasons, most of the research pertaining to root
hydrotropism has been conducted on roots growing in humid air,
instead of in soil.
Thigmotropism
Thigmotropism is a movement or response in which
an organism moves or grows as a response to a
stimulus of touch or contact.
 The prefix “thigmo” comes from the Greek for "touch".
 Think: “Thigmotropism” and “Thumb” both start with
Th- and you touch things with your thumb.
Use this tool to help you remember what thigmotropism is.
•
Thigmotropism occurs most often when plants grow around a surface, like a
wall, pot, or trellis. Climbing plants, such as vines, develop tendrils that coil
around these supporting objects.
• Get this!: Auxin is involved in thigmotropism, too!
Touched cells produce auxin and transport it to untouched
cells. Some untouched cells will then become longer, faster,
then cell growth bends around the object.
• The plant Mimosa pudica is widely known for its rapid plant
movement. Its leaves close up and droop when touched,
but this is not a form of tropism. It is a nastic movement, which
is similar to a tropism, but not the same thing.
 Thank you for watching! 
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