THE LEAF

STRUCTURE & FUNCTIONS

FUNCTIONS

• Photosynthesis

• Transpiration

• Storage

Development

• Primary growth – From the shot apical meristem

• Specifically from the leaf primordia – earliest stage of leaf development.

Parts of a Leaf

• Blade

• Petiole – stalk that supports the blade.

Petiole

Blade

• Location of where the leaf attaches to the stem is the node, the space between the two successive nodes is known as the internode.

Stem and Leaf Structures

• Axil – upper angle where the leaf joins the stem.

• Axillary bud can grow into a branch with leaves or flowers.

• Pulvinus is the swelling in the joint like structure of the petiole that allows the plant to respond to stimuli.

• Alternate leaf arrangement have one leaf per node, typically found alternating around along the stem.

• Opposite leaf arrangements have two leaves per node.

• Whorled leaf arrangements have three or more leaves (up to 25).

Leaf Arrangement

Leaf Arrangement

• Simple leaves – flat, undivided blade that is supported by the petiole.

• Compound leaves have a blade that is divided into leaflets.

Leaf Arrangement

• Compound leaf arrangement– Pinnately compound leaves – leaflets form in

pairs around a central, stalk-like rachis.– Palmately compound leaves – Leaflets attach

at the same point and radiate.– Peltate leaves are simple leaves with the stem

attached to the middle of the blade, (Fig 9.8 a; Page 209)

– Perfoliate leaves are simple, sessile leaves that surround the stem. (Fig 9.8 b; Page 209)

Leaf Arrangement

Com

poundS

imple

Leaf Arrangement

Pinnately Pinnately CompoundCompound

Palmately Palmately CompoundCompound

Other leaf types

• Leaves of ferns are called fronds, usually form in curled structures called a fiddlehead.

Venation

• Vascular tissues (xylem and phloem) form in strands which are called veins.

• Normally supported by fibers and a layer of the parenchyma cells called the bundle sheath.

• Dicots – Netted venation, where larger veins and smaller veins branch into a mesh-like network.

• Monocots – Parallel venation, or long strands of vascular tissues run parallel to each other.

Leaf Tissues

• Epidermis with cuticle– Upper epidermis– Lower epidermis

• Guard cells

• Mesophyll (parenchyma tissue)– Palisade– Spongy

Tissues

• Epidermis – Typically transparent and non-photosynthetic.– Contains numerous stomata

• Monocots – stomata form in parallel rows• Dicots – stomata form in a random,

scattered pattern.• More numerous on the underside of

horizontally oriented leaves, and equally distributed amongst vertical leaves.

Guard cells

Stoma

Chloroplasts

Regulation of Transpiration

• Opening and closing of the stomata

Organization of Mesophyll Tissues

• Photosynthetic, parenchyma cells.• Store products produced by photosynthesis.• Upper layer is the Palisade Mesophyll, which are

densely packed, and contain large numbers of chloroplast – specialized for light absorption and photosynthesis.

• Lower layer of mesophyll tissues are the Spongy mesophyll, which are irregularly shaped, loosely organized and contain chloroplast. Specialized for gas exchange and promotes movement of gases into and out of the leaf through the stomata.

X. S of a Typical Dicot Leaf

Upper epidermis

Lower epidermis

Palisade mesophyll

Spongy mesophyll

Guard cells

Water and Water Loss

• Water accounts for ~85-95% of the weight of the plant and 5-10% of the seeds weight. Water lost from transpiration is replaced by water absorbed from the roots – water can move up to leaves ate the rate of 75 cm/hr.

• Used for – Making organic molecules– Solvent for chemical reactions– Medium in which nutrients move

• 95% of the water a plant gather is lost to transpiration.

• Transpiration – loss of water from plants depends on:– Amt. of surface area available for evaporation

• Internal SA of the leaf can be more than 200X greater than the outside of the leaf

• One corn plant can transpire more than 500L of water during its four month growing season.

– Equal us drinking 40L of water a day

Water and Water Loss

• Other environmental factors include:Humidity – inc. in dry air Internal conc. of CO2 – lower CO2 concentrations

cause stomata to open, inc. transpiration.Wind – thin moist layer around the leaf called the

boundary layer, wind removes this layer replacing it with dry air.

Air temperature – inc. temperature results in faster transpiration rates, temp. inside the leaf can exceed the outside temp. by as much as 10°c.

Soil - the amount of soil moisture affects the rate of transpiration

Light intensity – Light stimulates the stomata to open and therefore increase transpiration.

Water and Water Loss

Adaptations to prevent water loss

• Leaf position – plants re-orient leaves to avoid the midday sun, decreasing temperatures.

• Abscic Acid – prevents desiccation by causing stomata to close preventing transpiration.

• Cuticle – Waxy substance that prevents water loss from the leaf, thicker in dry, arid regions.

• Sunken stomata – increase the thickness of the boundary layer.

• Reduced leaf areas – reduce the size of the leaf, which in turn reduces the amt. of water lost to transpiration.

• Leaf abscission – plants drop their leaves when temperatures or water becoming limiting.

• Circadian rhythm – Daily, 24 hour cycle where plants open stomata during the day and close them at night, regulated by light and an internal biological clock.

Water Transport

• Plants rely on vascular tissues for transportation of water to leaves for photosynthesis.– Evolution of xylem and phloem is largely due size and

distances that water must travel in multicellular plants.– Diffusion of water is slow and inadequate in

multicellular organisms.• Molecule that my diffuse in a single celled organism in

seconds can take up wards of 8 years to diffuse in a multicellular organism.

• Transport of molecules between cells is speeded up by cytoplasmic streaming – continuous movement of cytoplasm in the cell.

Water Transport

• Guttation or the loss of water from the leaves of the plant, is a common phenomenon. Caused by root pressure, the influx of minerals into the root at night causes the water pressure in the xylem to increase, eventually pushing water out of marginal openings in the leaves.

Guttation is Caused by Root Pressure

Deciduous and Evergreens

• Deciduous – plants that drop all of their leaves in the fall. (Ex. oak, pecan)– Larger leaves can produce more sugars from

photosynthesis, but lose more water from transpiration.

• Evergreen – leaves can live 3-5 years and can be shed anytime of the year.– Smaller leaves lose less water from

transpiration, smaller leaves produce less sugars.

Deciduous and Evergreens

Specialized Leaf Structures

• Stipules – small, leaf-like projections at the base of the petioles; these are photosynthetic and can form protective spines.

• Tendrils – modified leaves for support, coiling around objects and holding plants off of the ground, allow certain plants to climb up structures.

• Spines – modified leaves that protect plants from herbivores, and reduce water loss.

• Bud Scales – tough, overlapping leaves that protect buds from low temperatures, drying out and pathogens.

• Storage leaves – fleshy leaves that concentrate and store food.

• Succulent leaves – common asexual means of reproduction, produce tiny plantlets when the fall off of the parent plant.

Carnivorous Plants

• Live in nutrient poor soils, evolved to use other organisms as a source of nitrogen and other minerals they require.

• Many species have evolved to where they have no roots and lack chlorophyll.

• Examples are;– Venus Fly Trap– Genlisea– Butterwort– Pitcher plant

Poisons

• Many plants have evolved to concentrate toxins in their leaves to protect against herbivores.

• Many of these are very toxic• Examples are;

– Hemlock– Milkweed (some sp. Of grasshoppers only eat weed,

they then concentrate these poisons in their body which they then spray at attacking predators)

Economically Important Plant Leaves

• Spices (thyme, oregano, peppermint, spearmint and sage)

• Fibers for producing textiles• Foxglove – drugs produce from foxglove are used in

digoxin and digtalis which are used to treat low blood pressure and strengthen the heart.

• Coffee and tea – caffeine• Cannabis (marijuana) and coca (cocaine) – used to

produce illegal narcotics, oils produced by Cannabis sativa produce tetrahydrocannabinols (THC).

• Carnuba wax is produced from the carnuba palms.