Video Transcript
Complete the following
sentence. The joint-like structure in mimosa
leaves that moves the leaflets is the blank.
The mimosa plant has a fascinating
response to being touched and even to a reduction in light intensity at the onset of
night. It folds up its specialized leaves
and droops downward. Let’s learn some more information
about the anatomy of the mimosa plant, which can help us to understand how the plant
carries out this response.
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 branch from each petiole. Each rachis is covered with small
leaves called leaflets, more specifically referred to as secondary leaflets, or
pinnules. 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 structures called pulvini.
Pulvini, which is the plural form
of a pulvinus, are swollen, joint-like structures that allow the plant to fold up
its leaflets, for example, when it senses a touch stimulus. Primary pulvini are found at the
base of each petiole, where they branch from the stem, which allow the petioles to
droop when touched. Secondary pulvini are located where
each rachis branches from the petiole, allowing the rachises to droop. Tertiary pulvini, one of which we
can see more closely here, are found where each pinnule branches from the rachis,
which allows the leaflets to close up.
Each pulvinus is divided into two
halves. The region on the upper half of the
pulvinus consists of extensor cells, shown here in pink. The region on the lower half of the
pulvinus consists of flexor cells, shown here in blue. The flexor and extensor cells are
able to change in size as a result of changes in turgidity in response to certain
stimuli, such as touch. The swelling of the flexor cells as
their turgidity increases causes the leaflet to fold up and close.
Now we know what the joint-like
structure in mimosa leaves that moves the leaflets is called, and so the correct
answer to this question. It is called a pulvinus.
