What tropism is illustrated in the
diagram? (A) Thigmotropism/haptotropism, (B)
geotropism, (C) hydrotropism, (D) chemotropism, or (E)
To answer this question, first we
need to understand what a tropism is. A tropism is the directional growth
or movement response of an organism like a plant either toward or away from a
stimulus. Remember, a stimulus is any
detectable change in an organism’s internal or external environment.
Each tropism given by the answer
choices hints at the specific stimulus that the tropism is in response to. The prefix thigmo- means touch, and
thigmotropism, which is sometimes known as haptotropism, is the movement of an
organism in response to touch.
Geotropism, which is sometimes
known as gravitropism, is the movement of an organism in response to gravity. The prefix geo- means earth,
showing how the roots of some plants grow downward in the direction of the earth,
which is also in the direction of gravity.
The prefix hydro- mains water, so
the stimulus in this case will be water molecules that an organism grows in response
The prefix chemo- means chemical,
so the stimulus in this case is chemicals.
While the prefix photo- means
light, so phototropism is movement of an organism in response to light, the prefix
helio- means Sun. So heliotropism is often observed
in organisms like sunflowers that move to follow the Sun as it travels across the
sky over the course of a day.
If we compare the two images in the
diagram provided by the question, we can see that the only difference can be
observed in the plants’ roots found within the soil. In the image on the left, the roots
are growing downward, while in the image on the right, the roots are growing
sideways. In both cases, we can see that
there’s water, or moisture, in the area that the roots are growing toward. As the stimulus that the roots are
growing toward in this example is water, we can work out the correct answer to the
question. The tropism being illustrated in
the diagram is (C), hydrotropism.