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Welcome back to the second week of What a
Plant Knows.

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In this lecture today we're going to talk
about what a plant sees.

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And to start the lecture I want you to
think about this.

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Plants see you.

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Now of course I'm not talking about they
see you like I see you or

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you see me on the lecture, but plants

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actually monitor their visible environment
all the time.

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They see if you come near them.
They see when you stand over them.

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They know if you've painted your house.
They see

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if you've moved their pot from one side of
the house to the other.

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They even know if you're wearing a red or
a blue shirt.

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Now of course plants don't see in pictures
like you and I do.

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So what do I mean when I say see?

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So this is the view from my office at Tel
Aviv University.

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You can see here the Mediterranean in the
background

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and the green part of the middle of the
campus.

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We all agree that this is seeing for us.
And if you were

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blind, this is what you would see,
nothing, no picture only black.

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So we all agree that there's a difference
between seeing and being blind.

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But let's redefine this.

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If we would take someone who's blind,
who's

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only seeing this black here what you see
on

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your screen, and through some type of
intervention, allow

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them to just differentiate between
different shades of gray.

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For this person, even though he doesn't

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see pictures, would this be considered a
form of rud, rudimentary sight?

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Would this be a type of seeing?

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I think we can agree that seeing, gray

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and white and black is better than seeing
nothing.

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So it must be a type of simple sight.

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And if we could now allow this person, to
differentiate between red and

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blue, you maybe see the blue sky above and
some colors below it.

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This is even a slight improvement on just
seeing gray.

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So if we can agree that this is a form

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of sight, that sight even for people
doesn't have to

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be only pictures, then I think we can
definitely see,

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or agree that plants do have a form of
vision.

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While plants don't see in pictures like we
do.

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They do see in many ways in colors that we
can only imagine.

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For example, plants see the same UV light
that gives us sunburns.

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And infrared light that, heats us up.

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These are two types of light that we are
blind to.

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Plants can tell when there's very little
light,

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like from a candle or when there's a
strong

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light in the middle of the day, or when

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the sun's just about to set in the
horizon.

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Plants know if the light is coming from
the

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left or from the right, or from above
them.

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They even know if another plant has grown
over them, blocking their light.

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And they know how long the lights have
been on.

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What is sight for humans?

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So how did I catch this baseball?

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Light signals came from the ball, bounced
off the ball

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into my eye, went from my eye through to
the

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brain, the brain, computed what was
happening, sent the signal

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out to my hand and luckily I caught the
ball.

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So what we have, we have a detection of
the light.

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A transfer of the light signal and some
type of response.

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What we see as light is actually

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part of the electromagnetic spectrum and
we actually only see

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a small part of that, what we call visible
light.

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It goes from the blue light, which is

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about 400 nano-meters which are the
shortest light

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waves, up to red light which is about

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700 nano-meters which is the longest light
waves.

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But this is only as I said a small part of
the electromagnetic spectrum.

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Slow smaller waves or what we see as
ultraviolet light, we can't see those,

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all the way down to x-rays, which go
through our

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body and even from gamma rays coming from
the universe.

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Longer light waves, or longer
electromagnetic waves could

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be infrared heat waves, again which we're
blind to.

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Microwaves which heat our food, radio
waves which can be

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even a kilometer in length, which we don't
see either.

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So we only see a small part of the
electromagnetic spectrum.

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When light is coming off the baseball,

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the light bounces off the baseball, goes
in through my eye and hits

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the retina, and from there it sends the
signal to the optic nerve.

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Within the retina there are cells which
are specifically attuned for what we

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call visible light and these are the cone
cells and the rod cells.

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The rod cells let us see black and white
and different

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shades of grey, while the cone cells let
us see color.

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There are actually three

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types of cone cells, some that see red,

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some that see green and some that see
blue.

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So, for example, red light coming off the

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stitches of the baseball are absorbed only
by the

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red cones and then they send that signal
to

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the brain which we translate as a red
color.

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If we see purple light, for example,
coming

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through, the purple light would be
absorbed both

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by the red cells and by the blue

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cone cells and our brain translates that
into purple.

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So human sight, again to repeat this.

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We have photoreceptors in the retina,
which absorb the light.

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The light is signaled to, or tran, the
light signals transfer to the brain.

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It's translated into pictures, and this
signals some type

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of response, or in this case, I caught the
ball.

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So we have perception, signal
transduction, and the response.