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Now this bending either towards gravity or

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away from gravity is caused by changes
on different sides of the plant.

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So here's an experiment where
we could see what happens to

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negative gravitropism in the chute.

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We see that the plant
grows up towards gravity.

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And if we look at the cells on
the side of the bottom of the chute.

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On the side on the top of the chute,

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there's actually
a difference in their size.

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The cells on the bottom are longer

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relative to the cells on
the upper part of the chute.

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And this change in the size of
the cells is what causes the bending.

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This of course, begs the question,
how is the gravity sensed?

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Where is the gravisensor?

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It might not surprise you to learn that
one of the first people to ask this

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question was Darwin in the same book,
The Power of Movement in Plants, in 1880.

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But before we go into the experiments
that Darwin carried out to identify

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the gravity sensor we need to go
a little bit more into plant anatomy.

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What we see in this cartoon is a picture
of the very tip of a root of an onion but

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it could be the tip of any root,
basically, of most plants.

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And I'm going to dissect this root,
both from the inside out, and

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from the bottom up.

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So from the inside out, we have
a tissue which is called the epidermis.

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The purpose of the epidermis is to
isolate the root from the soil of course,

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and it's the first tissue which is
involved in the absorption of water and

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minerals from the soil.

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The absorption of water and minerals
is facilitated through an increase in

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the area of the root, of the epidermis,
by these projections that come out of some

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of the epidermis cells,
which are called root hairs.

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Not every cell has a root hair,
but some of them do.

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And these just increase the area
of contact between the root and

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the soil allowing for the efficient
absorption of water and minerals.

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You sometimes see these root hairs
if you're pulling out weeds,

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you can see sort of like
a hairy substance on the roots.

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Inside of the epidermis is a tissue
which is called the cortex.

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The cortex is the main meat of the roots,
such that for

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example when you're eating a potato, or
a carrot, what you're eating is cortex.

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The cortex cells are a place where
materials such as starch accumulate.

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And they separate between
the epidermis and the vascular tissue

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which brings the water and minerals
up to the green parts of the plant.

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The endodermis encircles the vascular
tissue and is basically the gate keeper

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which determines which minerals
go into the vascular tissue and

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which are kept out.

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There's actually an active process
going on here such as the root

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regulate what's going up to
the green parts of the plant.

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And what are remaining in
the roots excluded from the green

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parts of the plant.

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In the middle of the root we
have the vascular tissue.

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In different books this has different
names, sometimes it's called the steal.

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If you cut a carrot vertically you
can see the vascular tissue in

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the middle of the root.

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And this is where we
have the xylem vessels,

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which bring water up from
the root up to the green part.

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And what are called the phloem tubes which
bring sugars from the green parts of

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the plant down to the root.

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Or actually sometimes take sugars
from the starch from the root

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up to the green part of the plant.

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So this is from the outside in.

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Now we're going to take a look at
the root from the bottom to the top.

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We can divide the root from this
part into four different sections.

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At the very tip of the root,

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we have a section which is called
the area of cell division.

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This is also called the meristem, and

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this is the only part of the root
where the cells actively divide.

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At this part of the plant
there's no difference between

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different type of cells.

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These are very similar
to human stem cells, and

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these are the cells that
are actively dividing.

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Above this we have an area which is called
the area or the zone of elongation.

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This is where the cells are increasing
in size up to 10 to 20 times difference

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in cell size, from this area of cell
division, up to the area of elongation.

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So first the cells divide,
then they elongate and

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once they've elongated,
then they can mature.

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They can assume their final role, for

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example, the epidermal cells
can then develop root hairs.

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This is the zone of maturation.

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So we have these three zones.

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I mentioned that the meristem
cells are the embryonic cells.

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These are the only part of the root that
are actively dividing to renew the roots.

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So as these are the embryonic cells,
the plant has to protect them

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because if they're damaged then there's
no way for the root to keep growing.

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The way that the plant
protects it's embryonic cells,

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it's meristerm cells is what
the structure called the root cap.

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The root cap is a group of cells that
surround the meristem at the bottom and

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from it's side.

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They are stronger cells.

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They're more resistant to friction.

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Thus, protecting the meristem.

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Now as the root cap grows,

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as the root grows pushing
the root cap through the ground,

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the root cap secretes a type of mucus,
a lubricant, that's called mucilage.

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And you also see this sometimes that the
bottoms of the roots are sort of sticky,

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it's this lubricant which allows
the roots to actually go into the soil.

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So just to review this,
from the bottom up we have the root cap,

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we have the meristem which
are the dividing cells.

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We have an area of cells that elongate and
then the area where the cells mature.

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Now let's go back to Darwin's experiment.

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Darwin was interested in understanding
where is the gravity sensor of roots.

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To do this,
he did the following experiment.

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He placed the root on its side and
saw that the roots bend down with gravity.

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Then he started cutting off different
parts of the root and so he saw, for

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example, that if he cut off the tip of
the root before he placed it on its side,

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you could see that the root doesn't bend,
it remains straight.

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On the other hand, if you place
the root on its side for 90 minutes and

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then cut off the tip,
the root still would continue bending.

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So what's the conclusion
of this experiment,

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that it's the tip of the root
that has the gravity sensor.

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Now you might say well that makes
sense because that's where the cells

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are dividing.

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But actually it was only if he
damaged the very tip of the root

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that he got this response.

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So not only did he chop off the tip, he
then just damaged it by burning the tip.

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And this is actually a picture from his
own book, what he has here is seeds

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that are place through up a needle on
their side and you can see times where

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their root tip continues growing down and
times where it continue straight.

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You could see that this root tip has not
been cut for example here on the bottom,

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but what he did is he burned
just the cells right at the end.

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And when he burned the cells just at
the end, what we call the root cap,

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then the roots lost their
ability to sense gravity.

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This research has been carried
on now in more modern studies

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that have further shown that it's the root
cap itself which is sensing gravity, and

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we're just going to talk
about two such studies.

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In the first one, what they've done is,
genetically, they've gotten rid,

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the scientists who did these experiments,
caused the root cap to stop developing.

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What you can see here is a vertical
section through two roots.

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This one here on top is a normal root and
I think where you could

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hopefully see four levels of
the root cap and behind it,

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00:08:23.510 --> 00:08:26.860
small cells, which are the meristem,
these are the cells that are dividing.

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Below it is our root that's
been genetically modified,

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such that the root cap no longer develops.

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You could see that there's only about
one level of root cap cells and

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beyond that, the meristem.

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In the control in the upper root,
of course, there's normal gravitropism.

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Whereas in the bottom one,
that's missing the root cap,

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these roots have lost the ability
to respond to gravity.

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Now I want you to look once
second now at this cartoon.

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In Arabidopsis, there are four
levels of cells in the root cap.

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From this dark green, light green,
light gray and dark gray.

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What was done in the following experiment,

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00:09:11.620 --> 00:09:16.970
is different cells were killed
using a very fine laser.

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This is called laser ablation.

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If the first level, the most bottom level
of cells was killed with the laser,

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it had no effect on gravitropism.

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If the second level was killed, it also
had very little effect on gravitropism.

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However, if only the cells
in the third and

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00:09:38.100 --> 00:09:44.150
the fourth level were killed
by the laser then the root,

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00:09:44.150 --> 00:09:48.970
while it could still grow,
lost its ability to respond to gravity.

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What these experiments show,
first starting with Darwin,

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and then through the genetic experiment
and the laser ablation experiment

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00:09:57.880 --> 00:10:02.510
is that the gravity is sensed
in the root cap and primarily,

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00:10:02.510 --> 00:10:07.180
at least in Arabidopsis, in specific
cells in levels three and four.