New SAT Reading Practice Test 13

This passage is adapted from Daniel Chamovitz, What a Plant Knows: A Field Guide to the Senses. ©2012 by Daniel Chamovitz.

The Venus flytrap [Dionaea muscipula] needs to
know when an ideal meal is crawling across its leaves.
Closing its trap requires a huge expense of energy,
and reopening the trap can take several hours, so
5 Dionaea only wants to spring closed when it’s sure
that the dawdling insect visiting its surface is large
enough to be worth its time. The large black hairs on
their lobes allow the Venus flytraps to literally feel
their prey, and they act as triggers that spring the
10 trap closed when the proper prey makes its way
across the trap. If the insect touches just one hair, the
trap will not spring shut; but a large enough bug will
likely touch two hairs within about twenty seconds,
and that signal springs the Venus flytrap into action.
15 We can look at this system as analogous to
short-term memory. First, the flytrap encodes the
information (forms the memory) that something (it
doesn’t know what) has touched one of its hairs.
Then it stores this information for a number of
20 seconds (retains the memory) and finally retrieves
this information (recalls the memory) once a second
hair is touched. If a small ant takes a while to get
from one hair to the next, the trap will have forgotten
the first touch by the time the ant brushes up against
25 the next hair. In other words, it loses the storage of
the information, doesn’t close, and the ant
happily meanders on. How does the plant encode
and store the information from the unassuming
bug’s encounter with the first hair? How does it
30 remember the first touch in order to react upon the
second?
Scientists have been puzzled by these questions
ever since John Burdon-Sanderson’s early report on
the physiology of the Venus flytrap in 1882. A
35 century later, Dieter Hodick and Andreas Sievers at
the University of Bonn in Germany proposed that
the flytrap stored information regarding how many
hairs have been touched in the electric charge of its
leaf. Their model is quite elegant in its simplicity.
40 In their studies, they discovered that touching a
trigger hair on the Venus flytrap causes an electric
action potential [a temporary reversal in the
electrical polarity of a cell membrane] that
induces calcium channels to open in the trap (this
45 coupling of action potentials and the opening of
calcium channels is similar to the processes that
occur during communication between human
neurons), thus causing a rapid increase in the
concentration of calcium ions.
50 They proposed that the trap requires a relatively
high concentration of calcium in order to close
and that a single action potential from just one
trigger hair being touched does not reach this level.
Therefore, a second hair needs to be stimulated to
55 push the calcium concentration over this threshold
and spring the trap. The encoding of the information
requires maintaining a high enough level of calcium
so that a second increase (triggered by touching the
second hair) pushes the total concentration of
60 calcium over the threshold. As the calcium ion
concentrations dissipate over time, if the second
touch and potential don’t happen quickly, the final
concentration after the second trigger won’t be high
enough to close the trap, and the memory is lost.
65 Subsequent research supports this model.
Alexander Volkov and his colleagues at Oakwood
University in Alabama first demonstrated that it is
indeed electricity that causes the Venus flytrap to
close. To test the model they rigged up very fine
70 electrodes and applied an electrical current to the
open lobes of the trap. This made the trap close
without any direct touch to its trigger hairs (while
they didn’t measure calcium levels, the current
likely led to increases). When they modified this
75 experiment by altering the amount of electrical
current, Volkov could determine the exact electrical
charge needed for the trap to close. As long as
fourteen microcoulombs—a tiny bit more than the
static electricity generated by rubbing two balloons
80 together—flowed between the two electrodes, the
trap closed. This could come as one large burst or as
a series of smaller charges within twenty seconds. If it
took longer than twenty seconds to accumulate the
total charge, the trap would remain open.