Video Transcript
The diagram shows a simplified
outline of a Mimosa leaf and a leaflet that has folded upward. What letter represents the
pulvinus?
The Mimosa plant has a
fascinating response to being touched and to the onset of night. It folds up its leaflets and droops
downward.
In the diagram provided by the
question, most of the leaflets are in their horizontal, undisturbed position. One of the leaflets, however, has
folded up, as it would be when stimulated by a touch stimulus. Let’s learn the anatomy of the
Mimosa plant, which will help us to understand how the plant does that
and work out what a pulvinus is to answer this question.
Each stalk branching from the stem
of a Mimosa plant that holds a leaf is called a petiole. In a Mimosa plant, four
extensions called rachises usually branch from each petiole. We can see a zoomed-in cross
section through one rachis labeled on this diagram with the letter Y. Each rachis is covered with smaller
leaflets, or secondary leaflets, called pinnules. A single pinnule is labeled on this
diagram with the letter X. The larger primary leaflet, which
is sometimes called a pinna, composes a rachis and all its attached pinnules.
If we zoom in to where two pinnules
meet the rachis, we can see a structure called a pulvinus. Pulvini, which is the plural form
of a pulvinus, are the joint-like structures that allow the plant to fold up its
leaflets when it senses certain stimuli, such as touch. On this diagram, a pulvinus has
been labeled with the letter W. Therefore, the correct answer to
this question is W.
