Video Transcript
The diagram provided demonstrates a basic investigation into plant tropisms. What plant tropism is being displayed positively by the shoots?
To answer this question, we first must recall what a tropism is. The term tropism refers to the directional growth movement of an organism in response to a stimulus. If an organism grows toward a stimulus, the tropism is positive, and if it grows away from a stimulus, it is 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. There are multiple types of tropisms in response to different stimuli. Let’s discuss some specific examples of plant tropisms.
Thigmotropism is growth movement in response to touch. For example, some vines respond to touching immobile objects by growing in a fashion which wraps themselves around the object. This is an evolutionary adaptation which allows the plants to grow tall and access sunlight without having to build up energy expensive structural support.
Gravitropism, also known as geotropism, is the movement of the plant in response to gravity. Most plants tend to grow directly upward toward the sky. Even if it shoots on an overturned pot or on a steep slope, the plant will still grow straight up away from the pull of gravity. In addition, the roots of most plants grow downward toward the pull of gravity, which allows them to access water and minerals which are generally found deeper in the soil.
Hydrotropism is the growth movement of a plant in response to water. Water is an essential reactant in photosynthesis and a medium for transporting nutrients. Therefore, it makes sense that the roots of most plants grow toward areas of moisture. Phototropism is the movement of a plant toward or away from light. The growth toward light is apparent in many plants, and it’s helpful as it allows the photosynthesizing parts of a plant, like its leaves, to access more light for photosynthesis.
With this knowledge, let’s return to our original diagram. The plant grows toward light in each scenario, even though light is coming from different directions in each box. This allows its photosynthesizing parts to access more light. So the shoots of this plant are showing an example of positive phototropism.
