Video Transcript
Which of the following is correct about plant roots? (A) Plant roots are positively phototropic and negatively gravitropic. (B) Plant roots are positively hydrotropic and negatively gravitropic. (C) Plant roots are negatively phototropic and positively gravitropic. Or (D) plant roots do not display any major tropisms.
This question asks us about plant roots, which are the parts of a plant responsible
for obtaining water and minerals, usually from soil. The answer options describe various tropisms that can be displayed by plants. Let’s start by defining what a tropism is before we explore some of the different
tropisms mentioned in the answer options.
A tropism refers to the directional growth movement of an organism like a plant in
response to a particular stimulus, such as light or gravity. If a part of the organism is growing toward a particular stimulus, we would call this
a positive tropism. If a part of the organism is growing away from the stimulus, this would be referred
to as a negative tropism.
We can work out the specific stimuli that the different tropisms mentioned in the
answer options are in response to by taking a look at the prefix in each word. For example, the prefix photo- in “phototropic” means light. You might remember this from a process common in organisms like plants,
photosynthesis, which literally means synthesizing or making substances using
light.
Photosynthesis is how a plant makes its own food and is usually carried out by the
shoots and leaves of plants. So do you think these plant parts will be positively or negatively phototropic? They will be positively phototropic, growing toward a source of light to absorb as
much as possible for photosynthesis.
However, this question asks us about tropisms displayed by plant roots, which
generally do not contain any photosynthesizing cells, as they tend to be found
underground, where little light is available. Instead, the plant roots will actually usually grow away from a source of light
through negative phototropism, deeper into soil, where more water and minerals are
commonly found.
With this information, we can already eliminate option (A), which suggests that the
roots are positively phototropic, which is not usually true. We can also eliminate option (D), which suggests that the roots do not show any major
tropisms.
We now know that the roots show at least one major tropism: negative
phototropism. The prefix gravi- tells us that gravitropism is the response of a part of an organism
to grow toward or away from the pull of gravity. While the plant shoots and leaves will generally grow upward, away from the direction
of gravity, the plant roots will usually grow downward, toward the direction of
gravity, which can help them find more water and minerals deeper in the soil. Therefore, the plant roots are positively gravitropic. This means that we can eliminate answer (B), which suggests that the roots are
negatively gravitropic.
Now we have found the correct answer to this question. We are asked which of the statements is correct about plant roots. And we now know that the correct answer is (C). Plant roots are negatively phototropic and positively gravitropic.
