Video Transcript
What is the main role of the
pulvinus in mimosa leaves and leaflets? (A) To coordinate the nervous
responses of the plant. (B) To move the leaflets in
response to a stimulus. (C) To provide essential nutrients
to the leaflets. Or (D) to release hormones in
response to an internal stimulus.
Let’s start by taking a look at the
leaves of mimosa plants. The leaves of the mimosa plant are
attached to the stem by a stalk called the petiole. The petiole branches into four
slender extensions called rachises. Each mimosa leaf is made up of
multiple smaller distinct leaflike structures called pinnules, sometimes referred to
simply as leaflets, which are attached to each rachis.
Mimosa plants have many swollen,
joint-like structures called pulvini, which in a singular form is called a
pulvinus. The specific classification of each
pulvinus depends on where in the plant it is located. The primary pulvini are found where
each petiole branches from the stem. The secondary pulvini are found
where each petiole branches into rachises. The tertiary pulvini are located
where the pinnules branch from each rachis.
Let’s take a closer look at a cross
section of a part of the rachis and two of the pinnules where they meet at tertiary
pulvini. 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 light and touch. This allows the leaflets to rapidly
open or close or the leaves to droop in response to changes in these stimuli,
providing us with the correct answer to this question.
The main role of the pulvinus in
mimosa leaves and leaflets is (B): to move the leaflets in response to a
stimulus.
