Lessig: When plants attack

Most of us have had an opportunity to see a Venus Flytrap at work. We use a toothpick or other small item to touch the inside of the open “mouth” and it rapidly closes.

But how often have we ever asked the question “How does this work?”  

A Venus Flytrap starts as a bulb with about seven leaves emerging from the base of the plant. The petiole is somewhat flattened and capable of photosynthesis. The leaf itself consists of a pair of terminal lobes hinged at the midrib, forming the trap. The inside of the trap contains red anthocyanin pigments which attracts insects.  

Teri Lessig

The trapping mechanism is tripped when the prey contacts one of the three hair-like trichomes that are found on the inside surface of these lobes. This trapping mechanism is so specialized that it can tell the difference between something physical touching the surface and something non-living such as raindrops or wind.  

Two of the three hairs must be touched within 20 seconds of each other for the trap to close. Once these hairs are touched, the trap will close in 1/3 of a second. The edges of the lobes are fringed by cilia (look like eyelashes) that mesh together and prevent large prey from escaping until the trap is completely closed tight. If the prey is small enough, it will be able to escape through the cilia hairs.  

This is actually a benefit to the plant because it will not waste the time and energy digesting a meal that is too small to replenish the energy it needs to digest it.  

The speed of the trap’s closure can depend on several factors including overall plant health, temperature, light conditions, size of prey and humidity.

The actual mechanism that causes the sudden movement of the trap closing is still disputed. It is believed to be a complex interaction between elasticity, turgor, and growth. Before the trap is tripped, the leaf halves are convex, or bent outwards. When it is closed, they are concave, forming a cavity. It is this rapid flipping that closes the trap.  

When the trigger hairs are stimulated, an action potential is generated. This action potential is what is currently being debated. Generally, it is believed that water is rapidly moved from the cells of the inner wall to the cells of the outer wall. Therefore, the inner cells shrink and contract while the outer cells enlarge and swell. It could be charged ions that create this rapid water movement or another chemical reaction.

Once the prey is trapped and unable to escape, it will continue to stimulate the inner surface of the lobes causing a further response that forces the edges of the lobes together even tighter. It eventually seals the trap hermetically and forms a “stomach” in which digestion occurs.  

Digestion is catalyzed by enzymes secreted by glands in the lobes. Digestion takes about 10 days, after which the prey is reduced to a husk of chitin.

The trap gradually reopens and is ready for its next victim.  

Venus flytraps rarely feed on flies at all. As a matter of fact, their diet is 33 percent ants, 30 percent spiders, 10 percent beetles, 10 percent grasshoppers and less than 5 percent flying insects.  

They are native to a couple of areas in the United States – mainly in North and South Carolina. We tend to think of them as tropical plants, but they actually need a mild winter to go through a period of dormancy for their survival. They thrive on nutrient poor soils, which is the reason that they developed their taste for insects, so they could extract the nitrogen for protein formation that they require when the soil could not provide it.  

Ahh, the wonderful world of plants.