- [Instructor] When
plants appear on the land
for the very first time, these plants
were extremely tiny like moss and algae.
And for a few million years,
there were no plants on our planet,
which could grow taller than few inches.
But why?
Well, turns out growing
tall has some challenges.
However, with time, plants
slowly started evolving
and eventually they were able
to overcome those challenges.
And as you know, today
we can have tall plants,
plants which can grow into trees,
trees which can grow
hundreds of meters tall.
And so the question we wanna
try and answer in this video
is what are the challenges
that the early plants faced
because of which they couldn't grow tall?
And how did they overcome them?
So, the biggest challenge
that these early plants faced
was a lack of a transport system.
What's a transport system
and why do they need them?
Well, let's take an example
and understand this.
For example, consider my house plant.
In order for this plant to stay alive,
all the parts of it require
food and water and minerals
so that they can grow.
But where are they available?
Now you might know that most of the water
and the minerals are
available in the soil.
So the soil contains water,
and it has minerals in it.
Minerals.
And the plants have roots
which can absorb these water
and minerals, so great.
But what about the rest
of the part of the plants?
Like the leaves, how
do they get that water?
Well, and you see, the
plants need to transport
that water from the roots
all the way to the leaves
and all the other parts.
So can you see that a
transport system is needed
to transport this water and
its dissolved minerals up?
Right?
Similarly, think about food.
How do plants get their food?
You might know that they photosynthesize.
They use the energy of the
sun to create their own food.
And most of these photosynthesis
happens in the leaves.
So let's say some of the food
is synthesized in these leaves.
Consider these are food.
These are basically sugars.
Let's just call them as food as of now.
Now, what about the parts of the plant
that cannot photosynthesize?
For example, roots can't photosynthesize
because they're under the ground,
they're not exposed to sunlight.
There maybe certain
growing parts of the plant
which might require this food
because they cannot photosynthesize.
Maybe there are some leaves somewhere
which are in absolute
shade and because of it,
they're not able to photosynthesize.
So again, what to do?
We need a transport system.
The food needs to get transported
to the different parts.
So again, the food need
to get transported down
maybe to the roots sometimes.
It might have to get transported
up to some other parts.
So can you see, a transport
system is necessary.
Without which, these
plants cannot survive.
And the earlier plants didn't
have a transport system.
And therefore, they are very tiny.
The thing is, if you are very tiny,
then, this transportation
happens just by diffusion.
Diffusion is a process
in which stuff moves
from a region of high concentration
to lower concentration.
So, if it's very tiny,
these food particles,
these sugars can easily move into the
different parts of the plant.
They can easily diffuse into it.
Similarly, the water can
easily diffuse to all
the different parts of the plant.
But as the plants get taller,
then, diffusion becomes super
slow and it almost becomes
impossible to transport
it just by diffusion.
This is where specialized
transport system is needed.
And so, eventually, plants
evolved this transport system.
Today, they are called
as vascular tissues.
So the transport system evolved
are called vascular tissues.
And the word vascular,
vascular means vessels.
I like to think of them as pipes,
so you can think of them as pipes.
So piping system through
which they can move the food
and water all around their body.
And what do these vascular
tissues look like?
Well, just like the name
suggests, they look like pipes.
So pipes which start from the roots.
So let me show you the roots.
And so, pipes that start from the roots
and go all the way to
each and every leaves.
I'm not showing it for all the leaves,
but it goes to each and every leaves.
But wait, you maybe thinking,
how can we use one pipe to
transport both food and water?
Wouldn't they get mixed up?
Well, to make sure they
don't get mixed up,
we have two separate piping systems.
So one for water and one for food.
And we give them names.
So the piping system that we
use for water is called xylem.
So xylem transports water.
And the dissolved minerals.
When we say water, it already
has dissolved minerals.
And for food, we have
another tissue called phloem.
And this one transports
food, mostly sugars,
but it can also transport
other things like hormones,
amino acids and other
stuff, but mostly food.
And the way I like to remember this,
I remember watching this
in some YouTube video,
I like to remember this as fo for food
and pho for phloem, so
food, phloem, fo pho.
And another reason why
they have to be separate
is because if you look at water,
the water transport is
only one direction, up.
Can you see why?
Because all the water and
minerals are present in the roots.
So, we just have to
transport up to all leaves.
But the food transport has
to be in both directions.
Sometimes, you would
want the food to go up,
maybe to the growing areas of the plants,
sometimes, you want the food
to go down in the roots,
that's where the storing organs are.
So, you see food needs to be
transported in both directions
and therefore, xylem is
only one way transport up
and phloem, well that
is a two-way transport.
So it makes perfect sense
to have two separate
vascular tissues altogether.
And so this one line I have drawn,
don't think of this as single
pipe but imagine it's a bundle
of a lot of xylem pipes and a
lot of phloem pipes together.
In fact, let me show you a little better.
If I were to zoom in onto this,
and if we could see
inside, these are the xylem
and the phloem bundles.
Again, each one has a lot of xylem
and phloem tissues inside.
And if you could see from the top,
if you could have a
microscopic view of this,
then, it would look somewhat like this.
This is what you would
see in a microscope.
And what you're seeing over here,
these are the vascular tissues.
Again, let me zoom in even further.
So if we zoom in one of
these parts, you can see
a lot of round things over here.
These are the xylem,
let me just mark them.
These round things that
you're seeing over here,
these are the xylem tissues.
They are the pipes that are
coming out of the screen.
Think of it that way.
They're the one that transport water.
These over here.
And then, you can see
pipes over here as well.
You can see small, small round
things over here as well,
they are much thinner than
xylem, these are the phloem.
In between, you have other cells.
But these are the phloem.
And so, you can see a lot
of xylem and a lot of phloem
are bundled together,
along with some other
supporting cells as well.
And that bundle is what
I've drawn over here.
So the single thing that
I've drawn over here
is both lots of xylems and
lots of phloem together.
And these pipes, they start from the roots
and they end all the way into the leaves.
So, let me show you a zoomed
in version of the leaf.
You may have seen a leaf like this.
The veins that you see in
the leaves are actually
the xylem and the phloem.
These are the vascular tissues.
And then you can see they branch off,
into becoming smaller and smaller,
let me zoom in even further.
And you can see them branching
off like a network of roads
connecting different
different parts of the city.
This way, they make sure
a maximum number of cells
are able to get the food and water.
That's how the transport
system in plants work.
Isn't that amazing?
And now that they have the
developed the vascular tissues,
nothing can stop them from growing tall,
growing into trees, to grow
hundreds of meters tall.
Because of which, we have forests
and jungles and everything.
But finally, you might
have another question.
How do these transport systems even work?
I mean in tall trees, how are
they able to get that water
from the ground all the way to the top?
Is there some kind of a pump?
And I guess that's where things
get even more interesting.
No, they don't have a pump.
Yet, they use some clever
mechanism to get their job done.
And we look at these
mechanisms in great detail
in the future videos, okay?
But one of the things
about not having a pump
is that this whole
system becomes very slow.
So plants, because they don't have a pump,
they have slow transport system.
But you know what?
That's fine for them.
The plants are saying, it's
okay, we don't have a rush.
We don't have to go anywhere.
They have pretty low energy demands
so they can easily survive
with a very slow transport system.
They don't need a pump.
Unlike animals who have
very high energy demands
and animals do need a pump,
that's why we have a heart,
heart is our pump.
But plants and trees,
they don't need that.
And as I said, we will look
at how these things work
without a pump in the future videos.
Anyways, that's it for this video.
So let's quickly summarize.
What did we learn in the video?
We saw that plants have
developed a piping system
to transport food and water
which we call the vascular tissues.
We have two different ones.
One to transport water,
unidirectional transport called xylem.
And one to transport
food in both directions,
bidirectional of called phloem.
And the way I like to remember this is
fo for food, pho for phloem.
And since they don't have a
heart, their transport system
is pretty slow, but that's fine
because their energy
demands are pretty low.
As they don't have to go anywhere,
they're pretty much still.
And I would like to end by asking you
to wonder about one thing.
Can you imagine about how
different our world would be
without vascular tissues?
