Venus Flytraps Are Even Creepier
Than We Thought
When it comes to digesting its prey, the plant is a
calculating killer.
By Ed Yong
David Loh / Reuters
JANUARY 21, 2016
If
you accidentally get transformed into a fly, and get caught in a Venus flytrap, here is some valuable advice: Don’t panic.
“If
you just sit there and wait, the next morning, the trap will open and you can
leave,” says Rainer Hedrich from the University of Würzburg. “If you panic, you
induce a deadly cycle of disintegration.”
Hedrich
and others have found that the Venus flytrap can count the number of times that
its victims touch the sensory hairs on its leaves. One touch does nothing. Two
closes the trap. Three primes the trap for digestion. And five, according to Hedrich’s latest study, triggers the production of digestive enzymes—and more
touches mean more enzymes. The plant apportions its digestive efforts according
to the struggles of its prey. And the fly, by fighting for its life, tells the
plant to start killing it, and how vigorously to do so.
The
Venus flytrap has captivated scientists for centuries, perhaps because of how
un-plant-like it is. It captures and eats animals. Its leaves look unnervingly
like fang-lined mouths. It moves quickly, with each of its traps closing shut in a tenth of a second.
It has, on occasion, a fantastic singing voice. It is, as Charles Darwin said, “one of the most wonderful
[plants] in the world.” To understand his admiration, it helps to slow things
down, and see exactly what happens when the flytrap traps.
We
start with a fly. It lands within the open halves of the flytrap’s trap, drawn
there by the red color and fruity smell. The inner surfaces of the traps are
minefields, dotted with stiff hairs. When one of these is bumped by a bumbling
fly, it triggers a spike of calcium ions that send an electrical impulse
through the lobes, much like those that travel along your neurons.
A
single impulse means nothing; it could be triggered by wind-blown debris
hitting a hair, or perhaps a falling raindrop. To avoid closing its traps for
such false alarms, the flytrap is programmed to await a second signal.
The
first impulse sets a secret timer, and what the fly does in the next 20 seconds
will determine its fate. If it avoids the hairs, it will live. If it bumps a
second one, it sets off another electrical impulse, which raises the trap’s
calcium levels above a critical threshold. The plant responds by sending water
into its leaves, which rapidly change shape from convex (bent outwards) to concave (bent inwards).
In
other words, the trap snaps shut.
__________
The fly, by fighting for its life,
tells the plant to start killing it, and how vigorously to do so.
__________
The
fly is imprisoned but not dead. It struggles, knocking the trigger hairs even
further and sending off more electrical impulses, around one per minute. The
third impulse raises the trap’s calcium levels even further, prompting it to
produce a hormone called jasmonate. In many plants, jasmonate is a touch
hormone, which is released by wounds and injuries and coordinates programs of
defense and repair. In the Venus fly trap, jasmonate doubles as a carnivory
hormone. It primes the gland cells in the trap to start making digestive
enzymes, which they finally do once they detect a fifth electrical impulse.
The
plant carefully calibrates the supply of these enzymes to meet the demand for
them. A large fly struggles more furiously, knocks more trigger hairs, and sets
off more electrical impulses. The plant responds by making proportionally more
jasmonate, and secreting proportionally more digestive enzymes.
After
six to seven hours, the trap becomes hermetically sealed, and fills with fluid.
The fly, cut off from the outside world and deprived of oxygen,
asphyxiates—which is merciful, considering what happens next. The fluid inside
the trap becomes incredibly acidic, dropping to a pH of 2. It also fills with
meat-disintegrating enzymes. It turns into what Hedrich calls a “green
stomach.”
The
stomach takes several days to digest the dead fly, and the flytrap keeps up the
assault of acid and enzymes by tasting its meal. The dead fly isn’t setting off
any electrical impulses any more, but in as-yet-unpublished work, Hedrich has
shown that the trap is also lined with chemical sensors. These can detect the
chitin in the fly’s shell and the substances in its blood. So, as long as the
plant can taste something to digest, it will keep digesting.
It
also starts absorbing, doing the job of intestines as well as a stomach. The
same five electrical impulses that trigger the production of digestive enzymes
also activate a set of transporter enzymes, which absorb the sodium liberated
from the disintegrating fly. Most plants detest salt, but the flytrap puts it
to good use. By concentrating it within the traps, it can drag the water from
the fly into its own tissues.
After
a week or more, the trap opens, revealing the fly’s empty husk, which
eventually falls out or blows away. The trap is now set for another victim. It
will capture one or two more before the plant replaces it.
Many
of these details have been known for a long time, says Andrej Pavlovic from
Comenius University in Bratislava. But Hedrich’s latest study finally shows
that the flytrap can count electrical impulses to induce the digestive process.
How it does so is still a mystery, and one that the team is actively trying to
solve.
For
now, it is clear that the plant is a paragon of efficiency. It ensures that it
wastes no energy in capturing victims. It only closes its traps when it likely
has a meal. It only starts producing fluids when it’s really sure that
it has caught something. And it only continues digesting when there’s something
to digest. And in doing so, it can thrive in nutrient-poor swamps and marshes
where other plants struggle.
Ed Yong is a former staff writer at The
Atlantic. He won the Pulitzer Prize for Explanatory Reporting for his
coverage of the COVID-19 pandemic.