How on earth did the venus flytrap happen?
A venus fly trap closes its jaws in as little as 100 milliseconds. This is nuts, given that the plant has no muscle fibers or nervous system. So the plant is rightly famous, lauded both by Charles Darwin (“one of the most wonderful plants in the world”) and creationists (as proof of “evolutionary fantasy”).
So how does it work? And why?
The traps themselves are what mechanical engineers call a “bistable system” – they can exist happily in either the open or snapped condition, but not in between. Think of a snap bracelet, straight as a rod until you apply a little energy in the right place… it magically curls around your wrist.
An insect wiggling the inner hairs of a flytrap triggers a small but critical change in the plant that pushes the trap from its initial state to past its intermediate point, causing it to snap shut. Now, a bistable system isn’t that hard to design – snap bracelets are often made out of recycled tape measures, which aren’t all that complicated. But the speed is impressive.
To close, the venus flytrap has to send a signal from the hair, to a (still poorly understood) system that changes the mechanical properties of the trap, pushing it past its point of instability. And it has to do so before the insect leaves. So the flytrap has evolved the ability to send electrical signals an order of magnitude faster than most other plants. All plants move a little – think of how plants will turn its leaves to face the sun – but in terms of speed, the flytrap is a champ, closer in speed to an insect than a grass.
Optimizing both the mechanics and the electrical system seems like an awful lot of evolutionary trouble. Why bother? And why is the flytrap so unique?
Selective pressures push plants towards canivory when there aren’t micronutrients to sustain them. A flytrap can fix carbon through photosynthesis just like other plants. But the bogs it inhabits in North and South Carolina are covered with peat moss and almost completely lacking in available nitrogen and phosphorous. And plants need fertilizer to thrive. So evolution drove plants to capture from the air what they could not take from the soil.
There are other carnivorous plants in those bogs, ones that rely on passive techniques such stickiness to capture small insects, absorbing their nutrients after they die. The flytrap hit upon a system that could capture large insects like spiders and ants as well.
It’s not a great system – a trap can only close two or three times before it becomes jammed with the undigestible exoskeletons of its meals. Flossing may be an evolutionary bridge too far. But as snap traps evolved around 65 million years ago, it seems to be plenty good enough for survival.