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.
