1. How a Touch on a Mimosa Leaf Turns into Movement
A mimosa leaf seems to close in an instant. However, research groups including the National Institute for Basic Biology have announced that leaf movement begins just about 0.1 seconds after calcium ion signals and electrical signals reach the swollen base of the leaf, called the pulvinus [3].
It is important to note that this 0.1 seconds is not from the moment of touch. It is the time from when the signal reaches the pulvinus. There is already time for the signal to travel to the pulvinus before the movement starts.
Observations using high-speed cameras showed that the increase in calcium happened up to 0.15 seconds before the leaf moved [4]. This confirms the sequence seen in the video: first the message, then the movement.
2. How Water Pressure in the Swollen Leaf Base Moves the Leaf
According to the Plant Physiological Society of Japan, the main cause of mimosa movement is changes in water pressure inside cells, known as turgor pressure. This change happens in a part specialized for movement called the pulvinus [1].
The pulvinus is a swelling at the base of the leaf. It bends like a joint and serves as the base that raises and lowers the leaf.
Of the cells in the pulvinus, only the cells on the lower side lose pressure [1]. They do not all shrink at the same time. Because turgor pressure drops only on one side, the pulvinus bends, causing the leaf to move.
Because only the pressure on one side drops, the pulvinus bends and the leaf moves [2].
Safety Note: If you want to try this, only do so if you are allowed to touch the plant. Touch the tip of the leaf gently with your finger. Do not touch it many times or strongly, as this can hurt the leaf.
3. How Water Leaving Cells Makes Them Shrink and the Leaf Close
A review paper published in PMC explains that potassium ions and chloride ions leave the movement cells on the lower side of the pulvinus. Then, due to osmosis, water also moves out of the cells. The turgor pressure drops suddenly, the pulvinus bends, and the leaf moves [2].
The water is not sucked up toward the leaf. It moves out of the cells. The review also states that the volume increases in the upper side of the pulvinus [2].
Think of a balloon filled with water. It is tight and swollen. When the water leaks out, the balloon becomes soft and shrinks. This is a metaphor for ions and water leaving the lower cells of the pulvinus and pressure dropping. However, cells have hard walls, so they do not shrink as much as a balloon.
4. How the Signal Travels from the Touched Spot to the Leaf Base
At the place that is touched or injured, calcium ions inside the cells increase, and an electrical signal (action potential) is created. These two travel together through the leaf. When they reach the pulvinus, the water movement described earlier begins [3].
A paper in Nature Communications found that calcium signals caused by injury traveled at about 1.3 millimeters per second in the leaf veins of leaflets, and about 3 to 6 millimeters per second in the petiolules (small stalks). With touch stimulation, the speed in the petiolules was about 5.9 millimeters per second [4].
A speed of 6 millimeters per second is about one-third of the diameter of a 100-yen coin (20 millimeters) in one second. It seems like an instant because the distance traveled on the leaf is not very long.
5. Why Mimosa Leaves Move: Grasshopper Experiments
For a long time, the reason for mimosa leaf movement was a mystery. Research by Saitama University and the National Institute for Basic Biology showed that it helps protect the plant from herbivorous insects [5].
In experiments, leaves whose movement was stopped with lanthanum ions (La³⁺) were fed to grasshoppers. The weight of the leaves decreased by 38.0%, which was about twice the 18.9% decrease in control leaves (38.0 ÷ 18.9 ≒ 2.0) [4]. When moving and non-moving leaves were placed together, grasshoppers ate more of the non-moving leaves. It is reported that when leaves moved, grasshoppers stopped eating and moved away [4].
This result shows that movement helps repel insects. However, it does not state that this is the only purpose of moving.
6. How Closed Mimosa Leaves Return to Normal
This change can be reversed. After the leaf closes, the ions and water that were outside the cells are taken back in. Turgor pressure recovers. Then the pulvinus returns to its original shape, and the leaf opens [2]. The sources used here do not give a confirmed time for the leaf to reopen.
Research from the University of Tsukuba reports that in pulvinus cells, thin parts of the cell wall are more common on faces perpendicular to the direction of stretching. The relationship with water flow is speculated, but this is not yet a confirmed fact [6].
7. How to Investigate Mimosa Leaf Movement Yourself
If you have a chance to see a mimosa plant, gently touch the tip of the leaf with your finger and watch the order of closing with your eyes. You could also measure the time from closing to opening with a clock. Do not touch it many times or strongly, as the leaf may be hurt.
For official information, the "Everyone's Plaza" Q&A from the Plant Physiological Society of Japan explains the mechanism of mimosa in Japanese. Press releases from the National Institute for Basic Biology and Saitama University explain the background of the research in Japanese.