Leaves: Form and structure

Chapter 6

The Plant Body: Leaves

• FUNCTION OF LEAVES – Leaves are the solar

energy and CO2 collectors of plants.

– In some plants, leaves have become adapted for specialized functions.

And so, on to leaves• Leaves are the principle

structure, produced on stems, where photosynthesis takes place. 

• Cacti are an exception.  The leaves are reduced to spines, and the thick green, fleshy stems are where photosynthesis takes place.

General leaf form• Leaves are the main

photosynthetic organs of most plants– but green stems are also

photosynthetic.

– While leaves vary extensively in form, they generally consist of a flattened blade and a stalk, the petiole, which joins the leaf to a stem node.

– In the absence of petioles in grasses and many other monocots, the base of the leaf forms a sheath that envelops the stem.

• Most monocots have parallel major veins that run the length of the blade, while dicot leaves have a multi branched network of major veins.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

BladeBlade

PetiolPetiolee

Leaf Arrangement on the Stem

Opposite: 2 leaves at a node, on opposite sides of the stem

Spiral: 1 leaf per node, with the second leaf being above the first but attached on the opposite side of the stem

Whorled: 3 or more leaves at a node

• Plant taxonomists use leaf shape, spatial arrangement of leaves, and the pattern of veins to help identify and classify plants.

– A Simple leaves have a single, undivided blade, while compound leaves have several leaflets attached to the petiole.

– A Compound leaf has a bud where its petiole attaches to the stem, not at the base of the leaflets.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Leaf Arrangement on the Stem

Venation = arrangement of veins

in a leaf• Netted-venation = one or a few prominent midveins from which smaller minor veins branch into a meshed network;

• common to dicots and some nonflowering plants. – Pinnately-veined leaves =

main vein called midrib with secondary veins branching from it (e.g., elm).

– Palmately-veined leaves = veins radiate out of base of blade (e.g., maple).

Venation = arrangement of veins in a leaf

• Parallel venation = characteristics of many monocots (e.g., grasses, cereal grains); veins are parallel to one another.

• Dichotomous venation = no midrib or large veins; rather individual veins have a tendency to fork evenly from the base of the blade to the opposite margin, creating a fan-shaped leaf

Most dicots have branch-like veins and palmate leaf shape

Monocots have parallel leaf veins and longer, slender blades

Leaves - Comparisons

Monocots and dicots differ in the arrangement of veins, the vascular tissue of leaves

INTERNAL STRUCTURE OF LEAVES

chloroplasts

• Each part of the leaf has an important job.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Structures of the Leaf

Photosynthesis

Conserves water

Transports water and sugar to stem

and rootsCopyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Structures of the Leaf

Cuticle – the outermost layer of both the upper and lower surfaces of the leaf. It is clear and waxy to prevent against water loss.

Epidermis – a layer of cells one cell thick that provides protection for the inner tissues. These cells are clear to allow light to reach the photosynthetic tissues.

Mesophyll – between the epidermal layers. It contains palisade cells that are tall, tightly packed, and filled with chloroplasts for photosynthesis.

It also has spongy cells which are irregularly shaped, have large air spaces between them, and fewer chloroplasts.

Structures of the Leaf

Stomates – openings in the surface of the leaf and stems for gas exchange. The lower surface of a leaf usually has more. Water vapor also passes out through these holes.

Guard cells – two of these special cells surround each stomate and regulate the opening and closing of the stomate.

Veins – contain the vascular tissue that is continuous with that in the stem. Xylem carries water and minerals upward. Phloem carries dissolved food throughout the plant.

Typical Dicot Leaf Cross-Section

Palisade Parenchyma

Spongy Parenchyma

Vascular

bundles

Epidermis

Cuticle

Stoma

Guard Cells

Typical Monocot Leaf Cross-Section

Xylem

Phloem

Bulliform Cells Stom

a

Epidermis

Midvein

Vein

Bundle sheath cell

Function of the Leaf

• Photosynthesis• Gaseous exchange

– take in O2 and release CO2 during respiration

– take in CO2 and release O2 during photosynthesis

Water Vapour can be lost from the surface of the leaf in a process known as Transpiration.

TRANSPIRATION• Plants must supply water to all

their tissues. It moves from the roots up the stem to the leaves by capillary action.

• Most of the water plants take up is lost to the atmosphere by evaporation.

• The evaporation of water vapor from plant surfaces is called transpiration.

• Most takes place through stomates.

• The rate of transpiration is regulated by the size of the opening of the stomates.

• They are usually closed when there is too little water available, temperature is low, or there is little light.

• Most plants open their stomates during the day and close them at night.

• This is controlled bythe guard cells.

Stomatal control• Almost all leaf transpiration

results from diffusion of water vapor through the stomatal pore – waxy cuticle

• Provide a low resistance pathway for diffusion of gasses across the epidermis and cuticle

• Regulates water loss in plants and the rate of CO2 uptake

– Needed for sustained CO2 fixation during photosynthesis

Stomatal control• When water is abundant:• Temporal regulation of

stomata is used:– OPEN during the day– CLOSED at night

• At night there is no photosynthesis, so no demand for CO2 inside the leaf

• Stomata closed to prevent water loss

• Sunny day - demand for CO2 in leaf is high – stomata wide open

• As there is plenty of water, plant trades water loss for photosynthesis products

Stomatal control• When water is limited:

– Stomata will open less or even remain closed even on a sunny morning• Plant can avoid

dehydration

• Stomatal resistance can be controlled by opening and closing the stomatal pores.

• Specialized cells – The Guard cells

GUARD CELLS AND PLANT HOMEOSTASIS

• Guard cells are kidney-shaped with thick inner walls and thin outer walls.

• When they become full of water (turgid) the unevenness of the walls causes them to bow outward and the stomate opens.

• When they lose water they become less turgid and the stomate closes.

• Guard cells gainand lose water byosmosis.

Stomatal guard cells• Guard cells act as hydraulic valves• Environmental factors are sensed by guard

cells– Light intensity, temperature, relative

humidity, intercellular CO2 concentration

• Integrated into well defined responses– Ion uptake in guard cell – Biosynthesis of organic molecules in guard cells

• This alters the water potential in the guard cells • Water enders them • Swell up 40-100%

Relationship between water loss and CO2 gain

• Effectiveness of controlling water loss and allowing CO2 uptake for photosynthesis is called the transpiration ratio.

• There is a large ratio of water efflux and CO2 influx

– Concentration ratio driving water loss is 50 larger than that driving CO2 influx

– CO2 diffuses 1.6 times slower than water

•Due to CO2 being a larger molecule than water

– CO2 uptake must cross the plasma membrane, cytoplasm, and chloroplast membrane. All add resistance

water status of plants• Cell division slows down• Reduction of synthesis of:

– Cell wall– Proteins

• Closure of stomata

• Due to accumulation of the plant hormone Abscisic acid– This hormone induces

closure of stomata during water stress

• Naturally more of this hormone in desert plants

Plants and water• Water is the essential medium of life.

• Land plants faced with dehydration by water loss to the atmosphere

• There is a conflict between the need for water conservation and the need for CO2 assimilation

– This determines much of the structure of land plants

– 1: extensive root system – to get water from soil– 2: low resistance path way to get water to leaves –

xylem– 3: leaf cuticle – reduces evaporation

– 4: stomata – controls water loss and CO2 uptake

– 5: guard cells – control stomata.

Photosynthesis• One of the most important biochemical

process in plants.– Let’s not forget cell wall biosynthesis

and adaptation during plant development, growth, interaction with the environment, and disease defense.

• Among the most expensive biochemical processes in plant in terms of investment

• The biochemical process that has driven plant form and function

General overall reaction

6 CO2 + 6 H2O C6H12O6 + 6 O2 Carbon dioxide Water Carbohydrate Oxygen

Photosynthetic organisms use solar energy to synthesize carbon compounds that cannot be formed without the input of energy.

More specifically, light energy drives the synthesis of carbohydrates from carbon dioxide and water with the generation of oxygen.

C3 and C4 Leaf structure

The C4 carbon Leaf• This is a biochemical pathway

that prevents photorespiration • C4 leaves have TWO chloroplast

containing cells– Mesophyll cells– Bundle sheath (deep in the leaf

so atmospheric oxygen cannot diffuse easily to them)

• C3 plants only have Mesophyll cells

• Operation of the C4 cycle requires the coordinated effort of both cell types

– No mesophyll cells is more than three cells away from a bundle sheath cells

• Many plasmodesmata for communication

Specialized or Modified Leaves

• Drought-resistant leaves = thick, sunken stomata, often reduced in size

• In American cacti and African euphorbs, leaves are often reduced such that they serve as spine to discourage herbivory and reduce water loss

• The stems serve as the primary organ of photosynthesis.

Specialized or Modified Leaves

• In pine trees, the leaves are adapted to living in a dry environment too.

• Water is locked up as ice during significant portions of the year and therefore not available to the plant; pine leaves possess – sunken stomata, – thick cuticles – needle-like leaves– hypodermis, which is an

extra cells just underneath the epidermis –

Cotyledons or “seed leaves”

First leaves produced by a germinating seed

Often contain a store of food (obtained from the endosperm) to help the seedling become established.

Tendrils

Garden Pea

Tendrils - blade of leaves or leaflets are reduced in size, allows plant to cling to other objects (e.g., sweet pea and garden peas.

Figure 11.8 (1)Specialized Leaves

• Some plants obtain nitrogen from digesting animals (mostly insects).

• The Pitcher plant has digestive enzymes at the bottom of the trap

• This is a “passive trap” Insects fall in and can not get out

• Pitcher plants have specialized vascular network to tame the amino acids from the digested insects to the rest of the plant

Figure 11.12 (2)Specialized Leaves

• The Venus fly trap has an “active trap”

• Good control over turgor pressure in each plant cell.

• When the trap is sprung, ion channels open and water moves rapidly out of the cells.

• Turgor drops and the leaves slam shut

• Digestive enzymes take over

The End.

Any Questions?