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So getting back to plants,
what did they feel.

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Check out this video.

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Of course its a video
of the Venus flytrap.

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But notice, that's it's not that
the trap closes immediately,

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it's actually feeling where its
prey is in relation to its leaves.

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So what's going on here?

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How does it know where it's being touched?

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So, here we have a close
up of the Venus flytrap.

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We could see it's general structure.

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We have two symmetrical lobes
which are separated by midrib.

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At the edge of these lobes are cilia,
hair-like structures which actually

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close-like teeth making a type of
jail cage for the enclosed prey.

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As the prey come along over the leaves,
over the lobes of the Venus flytrap,

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they sometimes touch these large
black hairs that are on both sides.

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And it's these hairs which
are called the trigger hairs.

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It's the sensation of touching of these
hairs which causes the flytrap to close.

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May not surprise you that one
of the first people to study and

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report on the Venus Flytrap,
again was none other than Charles Darwin.

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And he did this in a book that came out
before the book we talked about earlier,

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before The Power of Movement in Plants.

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This was a book that was
published in the 1870's

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called Insectivorous Plants
by Charles Darwin.

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Darwin originally suggested that
an impulse, somehow or another,

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must travel rapidly to enable the very
quick closing of the fly trap.

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But with all of Darwin's talents, he
could not figure out what the signal was.

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He tested many, many, many options.

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For example,
he even put meat on the fly trap.

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He thought somehow on there it was
some chemical coming out of the meat,

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out of the animal that was
triggering it to close.

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But that didn't work.

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So he was left pretty much empty handed.

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But it was one of his colleagues
who came up with the answer.

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It was a doctor, a physician named
Burdon-Sanderson who published in 1878

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the following paper in the proceedings
of the Royal Academy of Sciences.

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And I think it's sometimes interesting
to read these early scientific papers.

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This paper was entitled A Note on

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the Electrical Phenomena Which Accompany
Irritation of the Leaf of Dionaea.

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Dionaea is the a Latin name for
the Venus flytrap.

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And here's what he writes.

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Okay, let's go through this old
English as he writes it here.

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If the leaf being so placed on the
electrodes that a normal leaf-current is

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indicated by a deflection leftwards,
the fly is allowed to creep onto it.

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It is observed that the moment
the fly reaches the interior, so

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as to touch the sensitive
hairs on the upper surface,

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the needle swings to the right, and
leaf at the same time closes on the fly.

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What I neglected to tell you
is that Burdon-Sanderson

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was one of the first people to
actually study electrophysiology.

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He was one of the first scientists
to notice action potentials,

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these change in electricity
in muscles of animals.

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And he applied the same technology to
studying plants using the Venus flytrap.

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So what he goes on to say is the same
series of phenomena, which is the actual

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potential, the electrical current,
present themselves if the sensitive hairs

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of a still expanded leaf are touched
with a camel-hair pencil.

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In other words, in simpler English, what
Burdon-Sanderson discovered in 1873 was

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that the impulse that causes the closing
of the flytrap is electrical.

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And it actually resembles the signal
of an animal nerve and muscle.

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What he and Darwin also showed was that
it was not enough to touch only one hair.

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When one air was touched there
wasn't an electric signal

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that went through the entire plant.

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It takes the touching of one hair and

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then several seconds afterwards a second
hair that causes the depolarization.

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To get the depolarization, two hairs have
to be within 20 seconds of each other.

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If one hair is touched and

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a long time passes before the second
one is touched, let's say 30 seconds or

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a minute, there's no depolarization and
the trap doesn't close.

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What this ensures is that the fly or

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the prey that is coming in the trap is
of a certain size, it's rather large.

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That makes it worthwhile for
the plant to close.

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If it's a very small bug that's
doesn't have a lot of protein,

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not very nutritious, it'll touch one hair.

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But the small bug will take a long
time til get to the second hair.

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By the time it touches the second hair,
it won't cause it to be closed.

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So the trigger hairs have to be touched,
close to each other in order

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to cause the depolarization,
in order to cause the closing.

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You may notice here that there is
a phenomenon that I don't want to go in

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to now,
maybe in one of the later lectures.

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What we're seeing here
is a type of memory.

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The plant, the Venus flytrap, remembers
that the first hair was touched, stores

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this memory and if a second one is touched
within 20 seconds, then it will close.

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If it's more than 20 seconds,
it forgets that the first one was touched.

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We'll come back to that in later lectures.

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So it's not only that
the touching causes the closing.

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It's not only that the touching
causes an electric signal.

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This electric signal is very similar, or

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the mechanisms are very similar to what's
happening in human nerves and muscles.

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Recently, scientists have shown that you
can even just add an electric impulse

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by putting electrodes in
the Venus flytrap, and

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without touching any of the hairs, this
electrode signal will close it to close.

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If you put on certain medicines, certain
drugs which stop human nerve signaling.

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For example,
there are chemicals which will stop

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ion channels from opening in humans.

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These will also stop the ion potential,
the amplitude of the action potential

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in the Venus flytrap or
inhibitors of what are called aquaporins.

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Aquaporins are the type of channels
which like water in and out of cells.

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So, inhibitors of human aquaporins also
inhibit the closing of the Venus flytrap.

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I just want to emphasize this,
the same chemicals, the same drugs that

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inhibit action potentials or that
inhibit the movement of water in humans,

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influence plants the exact same way.

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Which leads us to realize that
the basic cellular mechanisms

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are conserved between plants and humans.