A leaf consists of the following layers from top to bottom: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, and pores called stomata. Stomata are pore-like openings found on the underside of leaves that allow gases like carbon dioxide, oxygen, and water vapor to diffuse in and out. They are surrounded by guard cells that open and close to regulate gas exchange and water loss from the leaf. Keeping stomata open all the time would cause excessive water loss, threatening plant survival.

1. Internal structure of a leaf
• A leaf consists of
following layers.
• Cuticle
• Upper epidermis
• Palisade mesophyll
• Spongy mesophyll
• Lower epidermis
• Pores also known
as stoma /stomata

2. Cuticle
The outer thick waxy
covering of the
plants and leaves.
Cuticle protects
plant from drying
out by reducing
water loss.
Does not let oxygen
or carbon dioxide
pass through it.

3. Upper epidermis
• Upper epidermis
consists of a
single layer of
cells covered by
cuticle.
Sometime
contains pores
which allow gas
or water
molecule to pass

4. Palisade mesophyll
• Just beneath
the epidermis
there are column
shaped cells
containing
chloroplast are
known as
Palisade
mesophyll .
• Most

5. Spongy mesophyll
• The second type of
mesophyll tissue is the
spongy mesophyll
which is composed of
closely packed
irregular shape cells
surronded by air
spaces. There are
fewer chloroplast
because they are
under side of the leaf.

6. Lower epidermis
• Below the spongy
mesophyll is the
lower epidermis.
We will see
vascular bundle
here and stomata.

7. What are stomata or
stoma?
• The pore like openings
in the underside of
the leaf leaf is called
stoma (singular) and
stomata (plural). Each
stoma consists of two
guard cells which
control the opening
and closing of the
stomata

8. What is the function of
stoma /stomata ?
• Stomata allow
carbon dioxide
and oxygen and
water vapor
diffuse into and
out of the leaf
by opening and
closing .

9. Structure of stomata
• Stomata consists
of two guard cells.
When there is high
water pressure
guard cells open.
Low water pressure
in the guard cells
cause stomata to
close.

10. Do you think stomata are
open all the time?
• Plants keep their
stomata open
just enough to
allow
photosynthesis
to take place but
not so much that
they lose
excessive
amount of water.

11. What would happen if the
stomata were kept open all
the time?
• If the stomata
were kept open all
the time, water
loss due to
transpiration would
be so great that
few plants would
be able to take
enough water to
survive.

12. What is the time frame of
opening and closing
stomata?
• In general, • However, stomata
stomata are may be closed even
open during the in the bright
sunlight under hot,
day time when dry conditions in
photosynthesis which water
are active, and conservation is a
closed at night. matter of life and
death.