Video Transcript
The diagram provided demonstrates a basic investigation into plant tropisms. What tropism is being displayed positively by the roots?
To answer this question, we must first recall what a tropism is. The term tropism refers to the directional growth movement of an organism in response to an external stimulus. If an organism grows toward a stimulus, the tropism is positive, and if it grows away from a stimulus, it is called a negative tropism. Remember that a stimulus is the term describing a change in an organism’s internal or external environment that causes an effect in that organism.
Let’s discuss an example to make things clearer. The photosynthesizing parts of a plant, such as its shoot and leaves, usually grow toward the Sun in order to maximize the amount of sunlight they receive for photosynthesis. This movement of a plant is called positive phototropism. The prefix photo- describes the stimulus, light, and tropism describes the growth movement toward or away from the light. It is positive because the shoot and leaves grow toward, not away from, the light source.
Let’s return to the diagram of our question. The diagram we are presented with shows how the roots of a plant will move and grow in response to water. In this case, the stimulus toward which the roots grow is water. This process is known as hydrotropism, which is easy to remember as the prefix hydro- means water. Both plants are showing positive hydrotropism as their roots are growing toward water and not away from it. Therefore, the tropism being displayed positively by the roots is hydrotropism.
