Chapter 32: Leaf Structure and Function

• Function – photosynthesis• Shape – – max. light absorption– Diffusion of CO2 and O2– Ordered arrangement for light– Loss of water vapor– Trade off between photosynthesis and water

conservation

External form

• Shapes – round, need, scalelike, cylindrical, heart, fan, thin, narrow

• Size – 20m to < .5 cm• Blade, petiole, stipules• Simple, compound• Axil region

Fig. 35-6a

(a) Simple leaf

Petiole

Axillary bud

Fig. 35-6b

Compound leaf

(b)Leaflet

PetioleAxillary bud

Fig. 35-6c

Doublycompoundleaf

(c)

LeafletPetioleAxillary bud

Fig. 35-6

(a) Simple leaf

Compoundleaf

(b)

Doublycompoundleaf

(c)

Petiole

Axillary bud

Leaflet

PetioleAxillary bud

LeafletPetioleAxillary bud

Leaf arrangement

• Alternate – 1 leaf each node• Opposite – 2 leaves each node• Whorled – 3+ leaves each node

Leaf Venation

• Veins = vascular tissue• Parallel• Netted– Palmately – from 1 point– Pinnately – branch from entire length of midvein

Leaf tissues

• Upper epidermis + Lower epidermis– No chloroplasts/transparent

• Cuticle – waxycutin• Trichomes – hairlike (fuzzy)– Retain moisture next to leaf, reflect light– Secrete irritants – herbivores– Texture – deter insects walk/eat– Excrete excess salts

Fig. 35-9

Very hairy pod(10 trichomes/

mm2)

Slightly hairy pod(2 trichomes/

mm2)

Bald pod(no trichomes)

Very hairy pod:10% damage

Slightly hairy pod:25% damage

Bald pod:40% damage

EXPERIMENT

RESULTS

• Subsidiary cells – epidermal; water and ions supplied to guard cells

• Stomata (opening) + guard cells– Open/close stoma– Only epidermal cells with chloroplasts– Lower epidermis (land); upper epidermis (aquatic)

Fig. 35-18b

Guardcells

Stomatalpore

Surface view of a spiderwort(Tradescantia) leaf (LM)

Epidermalcell

(b)

50 µ

m

• Mesophyll – photosynthetic ground tissue– Btw. Upper and lower epidermis– Parenchyma – chloroplasts– Air spaces – gas exchange– 2 sublayers:• Palisade mesophyll – top, columnar cells, close together–photosynthesis

• Spongy mesophyll – lower, loose and irregularly shaped–Gas exchange

• Vascular bundles – veins – through mesophyll– Xylem (top) and phloem (bottom)

• Bundle sheath– Nonvascular, around vein– Parenchyma or sclerenchyma

Fig. 35-18

Keyto labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheathcell

Xylem

Phloem

(a) Cutaway drawing of leaf tissuesGuardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

Guardcells

Stomatalpore

Surface view of a spiderwort(Tradescantia) leaf (LM)

Epidermalcell

(b)

50 µ

m10

0 µm

Vein Air spaces Guard cells

Cross section of a lilac(Syringa)) leaf (LM)

(c)

Fig. 35-18a

Keyto labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheathcell

Xylem

Phloem

(a) Cutaway drawing of leaf tissuesGuardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

Fig. 35-18c

Upperepidermis

Palisademesophyll

Keyto labels

DermalGroundVascular

Spongymesophyll

Lowerepidermis

Vein Air spaces Guard cellsCross section of a lilac(Syringa) leaf (LM)

(c)

100

µm

Functioning of Stomata• Day – open – photosynthesis– Water moves into guard cells turgid + bend pore

• Night – close – water leaves guard cells flaccid collapse close

pore• Prolonged drought – stomata close (even in day)• Drop in CO2 in leaf – stomata open, even in dark– Photosynthesis (occurs in light) reduces internal

concentration of CO2 in leaf, triggering stomata to stay open

Details of Stomatas Opening/Closing• H+ and K+ move across PM of guard cells• Blue light triggers K+ to move into guard from

subsidiary/epidermal cells– Active transport – ATP– ATP provides energy to pump H+ out of guard– Removal of H+ makes electrochemical gradient to drive

uptake of K+• Uptake of K+ in guard increases solute conc. In vacuoles water

enters guard from surrounding cells by osmosis

• Result increase in turgidity changes guard shape

• Almost opposite happens to close stomata– Evidence that increase in Ca2+ conc. In guard

triggers closure

Transpiration

• Loss of water vapor by evaporation • Responsible for water movement in plants• Factors influencing rate:– Temperature– Light– Wind + dry air

• Benefits– Cools stems and leaves– Distributes minerals

• Harmful effects– Loose more water than take in during heat loss

of turgidity wilt– Temporary wilting of plant can “come back”

Leaf Abscission

• Fall off, once/year• Many changes– Plant hormones – ethylene, abscisic acid (ABA)

• Abscission zone – near base of petiole– Weak, parenchyma and few fibers

Modified leaves

• Spines – animals• Tendrils – vine attachment• Bud scales – winter buds• Bulb – short underground stem with fleshy

leaves for storage• Succulent leaves – water storage in dryness• Insectivorous plants