21.4 Leaves
KEY CONCEPT
Leaves absorb light and carry out photosynthesis.
21.4 Leaves
Most leaves share some similar structures.
• The blade is usually broad and
flat.
– collects sunlight for
photosynthesis
– connects to the stem by a
petiole
blade
petiole
21.4 Leaves
• Mesophyll is between the leaf’s dermal tissue layers.
cuticle
upper
epidermis
palisade
mesophyll
spongy
mesophyll
lower
epidermis
21.4 Leaves
• Guard cells surround each stoma.
– Stomata open and close when guard cells change
shape.
– When stomata are open, water evaporates and gas
exchanges.
– Stomata close at night and when plant loses too much
water.
guard cells
stoma
21.4 Leaves
• Leaves may be simple, compound, or double compound.
Simple leaf
Compound leaf
Double compound leaf
21.4 Leaves
• Leaf veins may be parallel or pinnate.
Parallel veins
Pinnate veins
21.4 Leaves
• Leaf margins may be toothed, entire, or lobed.
Toothed margin
Entire margin
Lobed margin
21.4 Leaves
Most leaves are specialized systems for photosynthesis.
• There are two types of mesophyll cells.
– both types contain chloroplasts
– palisade mesophyll absorbs sunlight
– spongy mesophyll connects to stomata
cuticle
upper
epidermis
palisade
mesophyll
xylem
spongy
mesophyll
phloem
lower
epidermis
stomata
21.4 Leaves
• Leaves have many adaptations.
– for extreme temperatures,
ex: pine needles
21.4 Leaves
• Leaves have many adaptations.
– for extreme temperatures,
ex: pine needles
– for water loss,
ex: cactus spines
21.4 Leaves
• Leaves have many adaptations.
– for extreme temperatures,
ex: pine needles
– for water loss,
ex: cactus spines
– for aquatic environments,
ex: water lily
21.4 Leaves
• Leaves have many adaptations.
– for extreme temperatures,
ex: pine needles
– for water loss,
ex: cactus spines
– for aquatic environments,
ex: water lily
– for getting food,
ex: Venus’ flytrap