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

(a) Simple leaf Petiole Axillary bud Fig. 35-6a Figure 35.6 Simple versus compound leaves Axillary bud

Leaflet (b) Compound leaf Petiole Axillary bud Fig. 35-6b Figure 35.6 Simple versus compound leaves Axillary bud

(c) Doubly compound leaf Leaflet Petiole Axillary bud Fig. 35-6c Figure 35.6 Simple versus compound leaves Axillary bud

(a) Simple leaf Petiole Axillary bud Leaflet (b) Compound leaf Petiole Fig. 35-6 (a) Simple leaf Petiole Axillary bud Leaflet (b) Compound leaf Petiole Axillary bud Figure 35.6 Simple versus compound leaves (c) Doubly compound leaf Leaflet Petiole Axillary 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 – waxycutin Trichomes – hairlike (fuzzy) Retain moisture next to leaf, reflect light Secrete irritants – herbivores Texture – deter insects walk/eat Excrete excess salts

EXPERIMENT Very hairy pod (10 trichomes/ mm2) Slightly hairy pod Fig. 35-9 EXPERIMENT Very hairy pod (10 trichomes/ mm2) Slightly hairy pod (2 trichomes/ mm2) Bald pod (no trichomes) RESULTS Very hairy pod: 10% damage Slightly hairy pod: 25% damage Bald pod: 40% damage Figure 35.9 Do soybean pod trichomes deter herbivores?

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)

Surface view of a spiderwort (Tradescantia) leaf (LM) Fig. 35-18b Guard cells Stomatal pore 50 µm Epidermal cell Figure 35.18 Leaf anatomy (b) Surface view of a spiderwort (Tradescantia) leaf (LM)

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 Bundle sheath Xylem (top) and phloem (bottom) Bundle sheath Nonvascular, around vein Parenchyma or sclerenchyma

Fig. 35-18 Figure 35.18 Leaf anatomy Guard cells Key to labels Stomatal pore 50 µm Dermal Epidermal cell Ground Cuticle Sclerenchyma fibers Vascular Stoma (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll Bundle- sheath cell Spongy mesophyll Figure 35.18 Leaf anatomy Lower epidermis 100 µm Cuticle Xylem Phloem Vein Guard cells Vein Air spaces Guard cells (a) Cutaway drawing of leaf tissues (c) Cross section of a lilac (Syringa)) leaf (LM)

(a) Cutaway drawing of leaf tissues Fig. 35-18a Key to labels Dermal Ground Cuticle Sclerenchyma fibers Vascular Stoma Upper epidermis Palisade mesophyll Figure 35.18 Leaf anatomy Bundle- sheath cell Spongy mesophyll Lower epidermis Cuticle Xylem Phloem Vein Guard cells (a) Cutaway drawing of leaf tissues

Cross section of a lilac (Syringa) leaf (LM) Fig. 35-18c Upper epidermis Key to labels Palisade mesophyll Dermal Ground Vascular Spongy mesophyll Lower epidermis 100 µm Figure 35.18 Leaf anatomy Vein Air spaces Guard cells (c) Cross section of a lilac (Syringa) leaf (LM)

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 Harmful effects Cools stems and leaves Distributes minerals 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