Video Transcript
What is the response of plants to a touch stimulus known as? (A) Hydrotropism, (B) thigmotropism/haptotropism, (C) chemotropism, (D) geotropism,
or (E) traumatotropism.
Let’s start by defining a few key terms that are important in understanding this
question. The directional growth movements of an organism like a plant in response to certain
stimuli are called tropisms. Remember, a stimulus is a detectable change in an organism’s internal or external
environment that may trigger a response in that organism.
There are many different stimuli that plants are able to respond to, and this
question concerns one particular stimulus: touch. Have you ever seen a vine wrapped around a tree, or perhaps ivy climbing up the walls
of a building? Even pea plants have small tendrils that twist around objects like sticks to keep
themselves upright. These are just a few examples of how plants may respond by growing in response to
touching another solid object.
Let’s take a closer look at this process in the pea plant example. When a part of the climbing plant, like a tendril, comes into contact with a solid
object, this triggers the release of plant hormones and proteins. This results in the cells that are directly in contact with the object to grow
slowly. In contrast, the cells on the opposite side are stimulated to grow rapidly. This causes the tendril to coil or twine closely around the object. This process is called thigmotropism or sometimes haptotropism. The prefix thigmo- means touch, so it refers to the stimulus itself. The prefix hapto- means to fasten, descriptively illustrating the action that the
plant takes in wrapping around and fastening itself to another object.
With this information, we can answer this question correctly. This question asks us to identify the response of plants to a touch stimulus. And we now know that this phenomenon is (B), thigmotropism/haptotropism.
