Which of the following best defines tropism? (A) Tropism is the response of a plant to external stimuli via its nervous
system. (B) Tropism is the response of a plant to grow toward or away from a stimulus. (C) Tropism is the movement of a plant toward a more hospitable environment. Or (D) tropism is the mechanism by which a plant increases its cell division and
This question asks us to find the best definition for the term “tropism,” and the
answer choices provide us with definitions specific to tropisms in plants. Let’s explore some tropisms that can be exhibited by plants to find the best
One tropism carried out by plants that you’re probably most familiar with is called
phototropism. Have you ever noticed how many plants when placed near a light source like the Sun
shining through a window will move in response to light? The plant’s shoots and leaves are showing a growth movement response to a particular
Remember, a stimulus is any detectable change in an organism’s internal or external
environment that can trigger a response in that organism. The prefix photo- itself means light. So the stimulus that phototropism is in response to is fairly easy to remember. These photosynthesizing parts of the plant, like the leaves, benefit from growing
toward areas where there is more light, as it means they can photosynthesize at a
higher rate to make their own food more efficiently.
Some parts of most plants, such as their roots, generally do not grow toward light
but instead grow away from this stimulus. This is helpful as the roots are not usually photosynthetic. Instead, their aim is to grow downward, which is usually away from the light of the
Sun, to absorb water and minerals, generally found deeper in the soil.
Another stimulus that certain plant parts can grow in response to is water itself,
through a process called hydrotropism. This is usually observed in a plant’s roots, which grow toward areas rich in moisture
in soil so more water can be absorbed. This is also easy to remember, as the prefix hydro- means water.
Each of these tropisms involve a plant detecting a stimulus and responding by either
growing toward or away from it. This means that we have found the correct answer to our question. The best definition of a tropism in plants is answer choice (B). Tropism is the response of a plant to grow toward or away from a stimulus.