The Venus flytrap (Dionaea muscipula) is a carnivorous plant typically found in North and South Carolina. Because of its scary looks and unconventional feeding methods, it’s often portrayed in movies as a work of a mad scientist or project gone wrong. These plants, while still alarming if you were to find yourself next to a larger sample, become much less blood-curdling once some light is shone on the mechanics behind their prey catching ways.
Deciding whether or not the prey is worth the required effort
Closing its trap requires a huge expense of energy, and reopening can take up to several hours, so the Venus flytrap only wants to spring closed when its sure that the insect strolling around its surface is large enough to be worth the effort and time. This is where the large black hairs on its surface come into play; they act as triggers that spring the trap closed once a suitable prey makes its way across the trap. If the insect touches just one hair, nothing will happen; but a large enough bug will likely make contact with two hairs within a certain time window, in this case about 20 seconds, which will send a signal to the Venus flytrap to spring into action.
We can look at this system analogous to short-term memory
Once the first hair has been touched, the flytrap forms the memory, so to say, that something (not knowing what) is strolling around its surface. Then it stores this information for a very specific time period of about 20 seconds after which that memory is gone. If inside that time window there is contact with a second hair, the flytrap recalls that memory and sends the signal needed for the trap to close.
So, a small insect such as an ant, for example, would have a very small stride which would translate into it taking longer than 20 seconds to get from the first to the second hair. By the time that the ant brushes against the second hair, the flytrap will have forgotten the first touch and so the ant would happily meander on.
How does it store this information?
German scientists Dieter Hodick and Andreas Sievers, were the first to propose that the flytrap stored information regarding the number of hairs touched in the electric charge of its leaf. The beauty of their model lies in its simplicity. They discovered that contact with the trigger hair results in an electric action potential (a temporary reversal in the electric polarity of a cell membrane) which causes calcium channels in the trap to open, causing a rapid increase in the concentration of calcium ions. This coupling of action potentials and the opening of calcium channels is similar to the processes that occur in humans during communication between neurons.
However, they proposed that for the trap to close there needs to be a relatively high concentration of calcium ions, a concentration unattainable by a single action potential resulting from the contact with a single trigger hair. Therefore, a second hair needs to be touched to reach the amount of calcium ions required for the trap to close, and it needs to happen before the calcium ions from the first trigger hair are able to dissipate (so inside the previously mentioned time window of about 20 seconds). Otherwise, the total concentration of the calcium ions won’t be enough to go over the threshold.
Alexander Volkov and his colleagues at Oakwood University in Alabama were the first to demonstrate that it is indeed electricity that causes the Venus flytrap to close. They accomplished this by attaching very fine electrodes to the open lobes of the trap and applying an electrical current. This made the trap close without any direct contact to the trigger hairs. The only flaw of the experiment is that the design omitted, rather than corroborated, a central element of the Hodick and Sievers’s model-namely, the physical stimulation of the hairs.
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