Question Video: Defining the Process by Which Plants Grow in Response to a Light Source Biology

Video Transcript

Complete the following statement. The response of a plant to grow toward a light source is known as blank.

Plants are not as static and immobile as they are often perceived to be. In fact, they can move and grow in response to the world around them. To exemplify this, think of a house plant growing toward a window or even roots growing toward water underground. These directional growth responses of an organism toward or away from stimuli are called tropisms. A negative tropism is when an organism grows away from the stimulus, and a positive tropism is growth toward a stimulus. There are lots of potential tropisms in plants. Let’s take a look at some of them.

Phototropism describes plant growth in response to a light stimulus. Phototropism is very useful to many plants as they require light for photosynthesis to make their own food. Therefore, many plant shoots and leaves are usually observed to grow toward a light source, like the sunlight streaming through the window in our house plant example.

Thigmotropism refers to plant growth in response to touch. An example of positive thigmotropism is a vine latching onto a pole and climbing up it. As the vine touches the pole, it grows toward it and as a result wraps around it.

Hydrotropism describes directional plant growth in response to water or moisture. Plant roots often grow toward water sources underground as they need to extract water from the soil, partly because water is a key reactant in photosynthesis. The growth of plant roots toward water in soil is an example of positive hydrotropism.

Geotropism, which is sometimes called gravitropism, refers to directional plant growth in response to gravity. Plant shoots usually grow upward against the pull of gravity. This is an example of negative geotropism, as the plant shoot is growing away from the stimulus, which is the downward force of gravity. The roots, on the other hand, can be seen as positively geotropic as they grow downward toward the pull of gravity.

Let’s look back at the question. We have now learned that the response of a plant to grow toward a light source is known as positive phototropism.