Video Transcript
A tendril is a specialized part of
a climbing plant, pictured. If one side of a growing tendril
touches an object, signals are transmitted to encourage the tendril to grow around
the object. What tropism is being displayed
here? (A) Negative thigmotropism,
haptotropism; (B) positive thigmotropism, haptotropism; (C) positive hydrotropism;
(D) negative gravitropism; or (E) positive gravitropism.
To answer this question, first we
need to understand what a tropism is. A tropism is a directional growth
movement response either away from or toward a stimulus. While growth of an organism like a
plant away from a stimulus is called a negative tropism, growth toward a stimulus is
known as a positive tropism.
In the case of the tendril in the
picture, it is growing toward the object, which causes the tendril to wrap around
it. As the tendril is growing toward a
stimulus, it is displaying an example of a positive tropism. So we can already rule out answers
(A) and (D), since they both describe negative tropisms.
Let’s take a look at the tropisms
that have been described in the other answer options. Option (B) describes positive
thigmotropism, which is sometimes known as haptotropism and is the directional
growth movement of an organism like a plant in response to a touch stimulus. Positive thigmotropism describes
when a part of a plant comes into contact with an object, like another plant’s
branch, triggering the release of plant hormones and proteins, which cause the cells
on the side that is in contact with the object to grow slowly. In contrast, the cells on the
opposite side are stimulated to grow more quickly, causing that plant part to coil
or twine closely around the object. This sounds a lot like what’s
happening to the tendril that we can see in the picture. So option (B) is likely to be the
correct answer.
The prefix hydro- means water,
which helps us to understand that positive hydrotropism is the growth of a part of
an organism toward water. In the case of the climbing plant
in the question, water is not the stimulus, so option (C) is incorrect.
Gravitropism describes the growth
of an organism like a plant in response to gravity. In the case of the climbing plant,
gravity is not the stimulus, so option (E) is also incorrect.
As we know that the stimulus in
this scenario is touch, we can confirm that the correct answer is (B). The tropism being displayed by the
tendril is positive thigmotropism, haptotropism.
