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
upperepidermis
palisademesophyll
spongymesophyll
lowerepidermis
21.4 Leaves
– 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
• Guard cells surround each stoma.
21.4 Leaves
• Leaves may be simple, compound, or double compound.
Simple leaf Compound leaf Double compound leaf
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
xylem
phloem
cuticle
upperepidermis
palisademesophyll
spongymesophyll
stomata
lowerepidermis
21.4 Leaves
• Leaves have many adaptations.
– for extreme temperatures, ex: pine needles
– for water loss,ex: cactus spines
21.4 Leaves
– for aquatic environments, ex: water lily
• Leaves have many adaptations.
– for extreme temperatures, ex: pine needles
– for water loss,ex: cactus spines