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Question Video: Understanding Tropism in the Roots of Legumes Biology

If, when growing, the roots of a bean shoot touch an object (like an underground rock), signals are transmitted to encourage the root to grow away from that object. What tropism is being displayed here? [A] Positive gravitropism. [B] Negative thigmotropism/haptotropism. [C] Negative hydrotropism. [D] Positive chemotropism. [E] Negative phototropism.

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Video Transcript

If, when growing, the roots of a bean shoot touch an object, like an underground rock, signals are transmitted to encourage the root to grow away from that object. What tropism is being displayed here? Positive gravitropism, negative thigmotropism or haptotropism, negative hydrotropism, positive chemotropism, or negative phototropism.

The directional growth movement of a plant in response to a stimulus is called a tropism. Tropisms can be negative, when something is growing away from a stimulus, or positive, when growing towards a stimulus.

Let’s take a closer look at the question and the options provided. The question describes the roots of a bean shoot being stimulated to grow away from an underground object it comes into contact with. The growth or movement away from a stimulus is called a negative tropism. If we look at the different answers, two of them describe positive tropisms, so these options can be ruled out straight away.

Let’s break down the three remaining tropisms into their word parts so we can work out which one is correct. The prefixes thigmo- and hapto- both mean touch, which explains how the roots of some plants, like legumes, rely on their sense of touch to help them grow into soil without encountering much resistance. When the roots of such plants come into contact with solid objects underground like rocks or stones, signals are transmitted to encourage the root to grow away from them. This helps the roots to find areas of soil that are free to expand in and might increase their ability to take up minerals and water.

This process seems to explain the example in our question pretty perfectly, but let’s have a look at the other tropisms to make sure that this option is correct. The prefix hydro- means water, so this option is not describing a plant’s response to touch. And the prefix photo- means light, so this option is also not referring to a touch stimulus. Therefore, we can deduce that the example in this question is in fact describing negative thigmotropism or haptotropism.

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