The Closing Force and Pressure of a Venus Flytrap

This is a review of the research article: “Venus flytrap biomechanics: Forces in the Dionaea muscipula trap” by Volkov, et al., from the Journal of Plant Physiology, 2013.

In this research, the authors measured three different forces related to Venus flytrap trap closing.   They wanted to measure the impact closing forces, the constriction forces during digestion and the force necessary for prey to escape a closed trap.

First, they measured they measured the closing force of a trap.  To do this, the scientists used a piezoelectric film.  This looks like a small piece of paper that, when squeezed, outputs a small electrical signal.  By placing the piezoelectric film into a closing trap, the force of the closing trap was converted into an electrical signal which could be easily measured.  Now, as any Venus flytrap owner will tell you, the lobes often don’t appear to make contact when they first snap shut.  In this research, they claimed to measure the impact of the rims of the lobes striking each other.  I am still not entirely clear on how they did this.  The single picture in the article depicting this is from the side of the trap and the cilia are pointing straight upward along the piezoelectric film, not toward each other, like normal.  Nowhere in the article do they explain this.  Anyway, however they did it, they calculated the average trap closing force to be 0.149 Newtons.

To measure the force applied to the prey inside the trap during constriction/digestion, the scientists used something similar to carbon transfer paper.  They placed a tiny slip of this paper completely inside a trap.  On each side of the paper they placed a bit of gelatin to simulate the trapped prey.  The trap closed and squeezed both pieces of gelatin, which squeezed the carbon paper in the middle.  By looking at how much carbon was transferred by the pressure, they could estimate how hard the trap was constricting.  The cool part of this is that they found a system that is completely enclosed by a digesting trap.  They also used a second method to measure the constriction force.  They placed a small piece of sponge inside the trap and measured the width of the trap after it had squeezed the sponge flat.  They compared these measurements with how thin the sponge would get when different weights were set on the sponge outside of the trap.  The cool part of both these methods is that they found systems that are completely enclosed by a digesting trap.

Finally, the scientists wanted to know how much force an insect would need to use to escape a closed trap (closed is when you can still see through the teeth into the trap – different from constricted, when it is sealed shut).  To do this, they used a small metal tube, about the size of an insect.  They tied fishing line through the tube and placed the tube in a trap.  After the trap closed, they slowly lowered the entire Venus flytrap plant by lowering a jack that the pot was resting on.  The other end of the fishing line was attached to a scale.  As the plant was lowered, the fishing line would pull down on the scale until the metal tube came out of the trap.  When that happened, they would know how much force it took by looking at the reading on the scale.  They calculated that a trap will hold prey initially with an average force of only .188 Newtons but could build up to an average holding force of 3.9 Newtons within a short period of time.

Now, to me, the escaping force experiment doesn’t hold a lot of water.  By the way the article described the experiment, the ‘escaping’ of the metal tube meant that it broke free of the entire trap at once because the fishing line was tied through a hole in its center.  But, in the real world, an escaping insect would likely slip through or displace only a few cilia and not burst open the entire trap, which is what the authors seem to have been measuring here.

Nevertheless, it was an interesting experiment. maybe they shouldn’t have claimed to have been measuring ‘escape’ force and claiming that their metal tube simulated an insect. Instead, they could have done the experiment and simply claimed to be measuring the holding force of the trap.  I feel this would have been much more accurate and far less presumptuous.

This article wasn’t the best example of single-variable experiments, but, as with all research on Venus flytraps, I’ll certainly take it!

 

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