Video Transcript
What type of response is the
𝑀𝑖𝑚𝑜𝑠𝑎 plant demonstrating in the picture provided? (A) Geotropism, (B) gravitropism,
(C) phototropism, (D) thigmonasty, or (E) hydrotropism.
While many people might think of
plants as fairly immobile organisms, they are actually able to move and grow in
response to many different stimuli in the world around them. Remember, a stimulus is a
detectable change in an organism’s internal or external environment that can trigger
an effect in that organism. And there are different ways in
which a plant can move in response to them. One type of plant movement, called
tropisms, are fairly slow as they involve the plant growing in response to a
stimulus. Specifically, tropisms are
directional growth responses toward or away from a stimulus.
A house plant growing toward light
shining through a window is an example of a tropism. If parts of the plant grow toward a
light source, the stimulus that triggers the growth response is light and we
specifically call the response phototropism. The prefix photo- is referring to
the stimulus, light. The growth response of an organism
toward or away from water is called hydrotropism, as the prefix hydro- refers to
water. This is often observed in a plant’s
roots, which grow toward areas of moisture in soil to absorb more water into the
plant.
The growth response of an organism
to grow toward or away from touch is called thigmotropism, as the prefix thigmo-
means touch. An example of where thigmotropism
might be observed in plants is ivy wrapping around solid objects. The growth response of an organism
to grow toward or away from the pull of gravity is called geotropism or
gravitropism. While the shoots of plants usually
grow away from the pull of gravity and so are described as negatively gravitropic,
the roots of most plants are positively gravitropic, growing toward the direction of
gravity downward into deeper soil to absorb more water and minerals.
Some plants like the 𝑀𝑖𝑚𝑜𝑠𝑎
plant can move quite fast through responses that, unlike tropisms, do not require
growth. Upon being touched, 𝑀𝑖𝑚𝑜𝑠𝑎
can close its leaflets in a matter of seconds. The response to touch of a
𝑀𝑖𝑚𝑜𝑠𝑎 plant is nondirectional, as it does not matter where the plant is
touched. It always responds in the same
way. A response of plant parts that is
independent of the direction of the external stimulus is called a nastic
movement.
We have learned previously that a
response to touch can be indicated by using the prefix thigmo-. The suffix -nasty refers to
nondirectional nastic movement responses. The folding of the leaflets of a
𝑀𝑖𝑚𝑜𝑠𝑎 plant is therefore called thigmonasty, as the direction in which the
plant moves is independent of the direction of the stimulus. No matter in which direction the
𝑀𝑖𝑚𝑜𝑠𝑎 plant is touched or shaken, its leaflets will always fold up in the
same way and droop downward. So the type of response being
demonstrated by the 𝑀𝑖𝑚𝑜𝑠𝑎 plant in the picture provided by the question is
(D) thigmonasty.
