Question Video: Identifying the Tropism That Encourages Root Growth Towards Water Biology • Third Year of Secondary School

Video Transcript

What tropism is illustrated in the diagram? (A) Thigmotropism/haptotropism, (B) geotropism, (C) hydrotropism, (D) chemotropism, or (E) phototropism/heliotropism.

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 to.

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.