Leaves - Overview
The leaf is the main photosynthetic
organ of most vascular plants
Leaves generally
have a flattened
blade and a stalk
called the petiole,
which joins the leaf
Blade
Leaf
to a node of the
Petiole
stem
Shoot
system
Leaves - Comparisons
Monocots and Dicots differ in the arrangement of veins,
the vascular tissue of leaves
Most Dicots have
branch-like veins and
palmate leaf shape
Monocots have parallel
leaf veins and longer,
slender blades
LEAF PARTS
1. Stoma (stomata) – small pores that act as a
doorway for gases involved in photosynthesis
(found mainly on the underside of leaves).
2. Guard Cells – Cells that regulate the opening
of the stomata.
3. Lower and upper epidermis – outer layer of a
plant that serves for protection, like skin.
LEAF PARTS CONTINUED…
4. Palisade layer – layer where majority of
chloroplasts are found (site of photosynthesis).
5. Spongy layer – cells surrounded by air spaces;
allow water, CO2 and O2 to diffuse during
photosynthesis.
6. Waxy layer/cuticle – protects leaf from water
loss and from feeding insects.
Leaves – Structure and Development
• Most dicots have 2
types of mesophyll
– Palisade
mesophyll
high photosynthesis
– Spongy
mesophyll
air spaces for gas
& water exchange
• Monocot leaves
have 1 type of
mesophyll
CROSS SECTION OF A LEAF
Leaves – Structure and Development
• Leaves are
several layers
thick – each
with different
cell types
Epidermis Leaves
• Leaf epidermis contains stomata - allow CO2 exchange
• Stomata flanked by two guard cells, control open vs. closed
• The ground tissue in a leaf, called mesophyll, fills the middle
Guard
cells
Key
to labels
Vascular
Cuticle
Sclerenchyma
fibers
50 µm
Dermal
Ground
Stomatal
pore
Epidermal
cell
Stoma
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Upper
epidermis
Palisade
mesophyll
100 µm
Spongy
mesophyll
Bundlesheath
cell
Lower
epidermis
Cuticle
Xylem
Vein
Phloem
(a) Cutaway drawing of leaf tissues
Guard
cells
Vein
Air spaces
(c) Cross section of a lilac
(Syringa)) leaf (LM)
Guard cells
VASCULAR TISSUE
Xylem – cells that carry water and dissolved
minerals from the roots to the leaves.
 Phloem – cells that carry food (sugar) that is
made in the leaves to all parts of the plant.
Use: Collectively the vascular tissue, xylem and
phloem create a vein or vascular bundle to
transport material in plants.

STOMA OPEN VS. CLOSED
Open – with light and when guard cells are
filled with water.
 Closed – without light and when guard cells are
lacking water (dehydrated).
 Why is it important to have stomata open?

Closed vs. Open
Question
time????????