How do some plants move? There are three ways that plants move. Plants can move to face the sun or to move into sunlight. Plants can move when touched or stimulated in some way. And plants can move to find a structure to wrap around. Each type of movement has a different mechanism. Let’s look at them in order.
Almost all plants make energy by photosynthesizing. They make this in the form of sugar and they do it by using the energy from the sun, carbon dioxide, and water. Plants need a certain amount of sunlight to produce enough energy and different species require different amounts. The sun moves through the sky because the Earth is revolving, which means the amount of sunlight that falls on the plant changes throughout the day. To cope with this, many plants have evolved the ability to grow towards the place with the most sunlight and some have evolved the ability to turn with the sun.
If you place a plant on a dark shelf and have a window with sun coming through it, the plant will grow towards that sunlight. There are hormones in the plant’s tissue called auxins. All of the cells in the plant have auxin in them, but it is spread out. Auxin is the chemical that makes the cells grow. Plants grow towards the sun because the auxin on the side of the plant with more sunlight is broken up and the auxin on the side of the plant in the shade isn’t broken up. The broken up auxin cells grow less than the cells with normal auxin, which makes the plant curve.
A few plants are able to track the sun throughout the day. This type of motion is called heliotropism. To move with the sun, these plants have an area called the pulvinus just below the flower. The pulvinus can move potassium ions into the nearby tissue, which causes water to move into or out of the cells. This changes the pressure of cells on different sides of the plant and pushes the flower around.
Some plants move when touched or stimulated. A venus fly trap is a good example of this. When a fly lands inside the head of the flower, it stimulates the flower to close. The mimosa pudica is another example. It is also called the sensitive plant because its leaves close when it is touched. The mimosa pudica works in the same way as flowers that track the sun. It has a pulvinus that stretches the length of the leaf. When it is touched, the pulvinus alters potassium ions in the cells on either side of it, changing their water content and altering their pressure. When the pressure is lowered, the leaves collapse, closing. When the water is returned and the pressure increases, the leaves stand up again. The leaves sense the touch through very sensitive mechanoreceptors in the surface of the leaf that conduct calcium ions when they are touched. When the ions move, they make an electric signal.
The last way that some plants move is to find a structure to latch on to. Some types of vines can move when they are trying to find a new place to latch on to. Vines don’t have a hard trunk or stem and they are small plants, so they are very close to the ground. They grow in areas with a lot of trees and they want to get as close to the sunlight as possible, so they have evolved the ability to climb up trees or any structure that might be around. To do this, the plant grows out tendrils that move around in the air until they touch the limb of a tree. When they sense they have touched it, they start to grow around the tree. How do they do this? It is the same process that plants use to grow towards the sun or to close and open leaves. The tendrils have sense receptors on their surface, just like the mimosa pudica. When they come into contact with a branch, they detect it and an electrical signal is sent. This signal stops hormones from letting the cells grown in the side of the tendril that has touched the branch. The cells on the opposite side grow as normal, and the tendril curls around the branch or whatever it has gripped. The vine grows up on this tendril and then repeats the whole process as it claws its way towards the sunlight. And this is what I learned today.
Photo by Pixabay: https://www.pexels.com/photo/green-plant-sprout-66319/
Sources
https://extension.psu.edu/how-plants-move
https://en.wikipedia.org/wiki/Mimosa_pudica
https://en.wikipedia.org/wiki/Thigmonasty
https://theconversation.com/why-do-plants-grow-straight-169337
https://en.wikipedia.org/wiki/Heliotropism
https://en.wikipedia.org/wiki/Turgor_pressure
https://journeynorth.org/tm/tulips/AuxinsGrowth.html
https://www.nps.gov/articles/000/vines-on-trees-at-forest-edges.htm