Video Transcript
All of the following parts of the
mimosa plant would undergo some sort of movement as a result of the plant’s unique
response to touch except blank. (A) The stem, (B) the leaflets, (C)
the rachis, or (D) the pulvini.
The question asks us to fill in the
blank by identifying the part of a mimosa plant that does not undergo movement as a
result of the plant’s touch response. Let’s start by taking a look at the
structure of Mimosa pudica, the mimosa plant.
Like other vascular plants, the
mimosa plant has roots, a stem, and leaves. The leaves of mimosa plants are
compound leaves, which means that each leaf blade is divided into several distinct
leaflets, all joined to a single stalk, or petiole. The compound mimosa leaf shown in
the diagram has four leaflets. The petiole branches into multiple
extensions called rachises, one for each leaflet. Attached to each rachis are
multiple smaller leaflike structures called pinnules. Each rachis, together with its
pinnules, composes a single leaflet, which can also be referred to as a pinna.
The leaves of mimosa plants have
joint-like structures located where the petiole branches into each rachis. These are the pulvini, or pulvinus
in singular. A pulvinus is a swollen, joint-like
structure found at the base of a leaf, leaflet, or petiole that is responsible for
the movement of leaves in response to stimuli. You may remember that a stimulus is
a change in an organism’s environment that can influence its behavior. For example, being touched is a
stimulus. When a mimosa leaflet is touched or
shaken, the pulvini react by converting the mechanical stimulus into an electrical
signal, with the result being that some of the cells in each pulvinus lose water,
while others swell up with water.
This difference in turgor pressure
causes the leaflets to close or fold up and the rachises and petioles to droop
downward. The unique reaction of the mimosa
plant to touch helps protect it from a possible threat, for example, an insect or
other herbivore, and is reflected in its species name. Pudica comes from the Latin word
meaning “shy” or “shrinking.”
Now let’s take a look at our answer
options. We just learned that the pulvini
originate the movement, causing the rachises and leaflets to move as well. The plant stem does not move
because it is anchored in the soil by the roots of the mimosa plant. Therefore, the correct answer
option is (A), the stem. And the completed sentence reads
“All of the following parts of the mimosa plant would undergo some sort of movement
as a result of the plant’s unique response to touch except the stem.”
