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LEAFBiology 101

Leaves

Principal appendage or lateral organ of stemPart of the shootTissue systems: dermal vascular Tissue systems: dermal, vascular and fundamentalDeterminate apical growth (vs. stem—indeterminate)

Structure-function relation

PHOTOSYNTHESISLarge external surfaceExtensive air space systemAb d f hl l t i th Abundance of chloroplasts in the ground tissueClose spatial relation bet. Vascular and ground tissue

Foliage leaf (rel. to photosynthesis)

Lacks storage tissuesDevelops no peridermPrimary tissues only

Classification of leaves

FoliageCataphyllsHypsophylls

t l dcotyledons

Foliage leaves

Principal photosynthetic organs

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Cataphylls

Cata= down; phyllon= leafLeaves inserted at low levels of shootScales on bud and underground stem)Protection or storageProtection or storage

Hypsophylls

Hypso= highLeaves inserted at high levels of the plantFloral bracts (protection)

Prophylls

Pro= beforeFirst cataphylls on lateral branchMonocots– 1 prophyllE di t 2 h llEudicots – 2 prophyll

Cotyledon

First leaf of the plant

Phyllomes

General termsInclude foliage leaves, scales, bracts, floral appendages

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FOLIAGE LEAF morphology

Blade/lamina– flattened structurePetioleLeaf sheathSimple and compound leaf (leaflets)CladodesStipulesphyllode

ANGIOSPERM LEAF

Histology of MATURE leaf

EpidermisEpidermal cellsGuard cells with subsidiary cellsT i hTrichomesSilica and cork cells (Gramineae)Bulliform cellsFiber like cells

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Epidermis

Terrestrial Living tissueNo well differentiated chloroplasts

AquaticMay show more abundant chloroplasts

Wall structure of epidermis

Presence of cutin in the outer wallsa. Thin – mesophytes and water plantsb. thick, lignified – xerophytes

Sili ifi d d llic. Silicified – grasses and allies

Mesophyll

Mesos= in the middleLiving, lacunose parenchyma with chloroplastsMesophyte Dicots– palisade and spongyP li d d l t i ff t d b li htPalisade-- development is affected by light

-- more chloroplasts(sun vs. shade plants)

Vascular system

Vascular bundles or group of it = veinsSingle vein– conifers, EquisetumDicot– largest vein occur in median position (midvein)—with rib(midvein) with rib

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Monocots– usually equal in size or may vary (larger veins alternalte with smaller ones); median bundle may be larger than others

Histologic composition

Collateral bundles– x is adaxial; p is abaxialBicollateral – adaxial phloem occurs only in large veins

Largest veins (distribution of bundles)- circular- irregular- irregular- crescent shape (if single)

Veins (dicots)

Larger veinsMay have primary and secondary tissuesVessels and sievetubes

Smaller veinsentirelyprimaryTracheary elements are tracheids; phloem part may parenchyma only at ultmateendings

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Bundle sheaths

Part of ground tissueAlso called border parenchyma (dicots)May contain chloroplasts

In monocots (Gramineae), two types exists:1. Parenchymatous– with chloroplasts2. thick-walled sheath/ mestom sheath—inner;

surrounded by parenchymatous sheath also--- procambial origin

Supporting structures

Not so developed as in the stemFlat blades– vascular systemDicots– the bundle sheaths and extensions-- collenchyma (large veins)-- sclereidsMonocots– large amounts of sclerenchyma-- fibers (assoc. with vascular bundles)

PETIOLE

Comparable to stemGround tissue =~ cortex of stem-- less chloroplasts

ti t t ll l-- supporting structures: collen or sclerenVascular bundles-- collateral (Syringa)-- bicollateral-- concentric (most dicots)

Distribution of vascular tissues

Continuous or multi-stranded arc (open toward adaxial)Form a circle (with addtl. Bundles within circle)Numberous and arranged in several superposed arcNumberous and arranged in several superposed arcscattered

Petiole

1 Collateral bundle– x is adaxial; phloem is abaxialBicollateral bundle—on both sides of the xylemIf in arcs or circles– phloem oriented peripheryIf in arcs or circles phloem oriented periphery

*rachis and pedicels of leaflets—similar to petiole but with less tissue

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Pinus leaf

GYMNOSPERM LEAF

Pinus leaf

XeromorphicLow ratio of surface to volume Epidermis heavily cuticularized/ thick-walledPresence of hypodermis—thick wall; compact yp ; p(except with stoma)Guard cells sunken (overtopped by subsidiary cells)Vascular bundles surrounded by transfusion and endodermis respectivelymesophyll not differentiated

Other features

Resin ductsVascular bundles– x adaxial side; p abaxial sideXylem is endarch

Transfusion tissue

2 kinds of cells: a] living parenchyma cells with non-lignified walls and b] thin-walled but lignified tracheids with bordered pitsParenchyma cells- deeply stainingTracheids (near xylem)Albuminous cells (near phloem) – dense cytoplasm and prominent nucleiUniversally present in gymnosFunction: water storage or auxiliary conducting system

Development of the leaf

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Origin from SAM

Periclinal division in the flank meristem

Lateral protrusion—occurs NEAR the surface

Leaf buttress formed

Leaf develops

Tunica and corpus

Participates in the formation of leaf primordiumIf single layer tunica– corpusIf three-layered tunica-- tunica

Early growth and histogenesis

After initiation cell division, enlargement and maturation

Stages of leaf developmenta Formation of foliar buttressa. Formation of foliar buttressb. Formation of leaf axisc. Formation of lamina

As the leaf axis is elevated above the buttress---procambium is differentiated

Adaxial meristemMarginal meristem

a. marginal initialsb b i lb. submarginal

initials

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Vascularization

Procambium of midvein differentiates first in the leaf axisAs the lamina is formed, the procambiumdifferentiates in the middle layersyThe development progresses BASIPETALLY

Leaf abscission

Separation of leaf from the stem without injury to the living tissuesWhile giving protection to newly exposed surface from dessication and infectionOccurs in abscission zone

Abscission zone

Occurs within the petiole or at its base

Facilitating separationa. Histologic structure of g

petioleb. Presence of

separation layer

Histologic structure

Contains minimum of strengthening tissuesParenchymatous except in vascular tissuesVascular elements (tracheids) are shortW k tiWeak portion

Separation layer

Cell walls are chemically modifiedCell walls increase in volume, swell, assume gelatinous appearance

cells separate from each other or are easily cells separate from each other or are easily broken

Calcium pectate water soluble pectin

Protection of the surface exposed

a. Formation of scar or cicatrice- deposition of suberin, lignin, or wound gum

b. Periderm formation beneath the scar