Ask Pippa

How do Certain Plants Move When they Sense Something Touching Them?

The plants around your home may not be picking up their roots and running races, but plants do move.

At the very least, plants track the sun throughout the day. Sunflowers face the rising sun in the east, and by late afternoon they are facing west as the sunlight disappears. Many flowers open during the day and close at night -- more motion.

But some plants move more dramatically than others -- like the Venus Fly trap, and the Sensitive Plant (also called a mimosa). The Venus Fly trap has hollow, bean-shaped leaves that are open along one edge. The edge is covered with sharp spines.

These odd shaped leaves is what the plant uses to trap bugs -- which it likes to eat.

When a bug crawls inside the leaf it snaps shut, trapping it inside. The more the bug moves, the tighter the trap closes.

The plant then dissolves amnd digests the bug. Yum!

The leaf closes because the bug stepped on sensitive "trigger" hairs. When the trigger hairs are moved, it causes an electrical signal to be sent to other parts of the leaf.

These signals cause changes in water pressure in the leaf's cells (tiny parts of the leaf), causing it to close.

Here’s how it works.

Water from cells on the inside of the leaf moves up to cells on the outside of the leaf.

Normally, when the inside cells have lots of water the leaf keeps its shape and stays open. But when water is moved to the outside cells, this causes the first cells to go limp, and the outside cells (which are fat with water) help push the leaf close. The leaf snaps shut.

The Sensitive Plant also has tiny trigger hairs. When touched, they too cause a change in water pressure in the leaves.


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