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Syllabus Theme ASyllabus Theme ASyllabus Theme ASyllabus Theme APlant Structure and FunctionPlant Structure and FunctionPlant Structure and FunctionPlant Structure and Function
A2: Structure and function of the basic plant organs
Campbell & Reece Chap. 35 Selected page numbers
ROOTS
� FunctionsAnchors the vascular plant� Absorbs minerals and water
� The absorption of water and minerals occurs near the root tips
� Vast numbers of tiny root hairs – function?
� Often stores organic nutrients
Root systems
� Taproot system� One main root formed from the radicle� Multiple lateral roots extend from the main
root
� Fibrous root system� Multiple adventitious roots of the same size
(developing from the end of the stem)� Multiple lateral roots branching off from the
adventitious roots
Taproot system
Fibrous root system
� Many plants have modified roots� What is a modified root?
Prop roots Storage roots “Strangling” aerialroots
Buttress roots Pneumatophores
Primary Growth of Roots
� The root tip is covered by a root cap� Function?
DermalGroundVascular
Key
Cortex Vascular cylinderEpidermis
Root hairZone ofmaturation
Zone ofelongation
Zone of celldivision
Apicalmeristem
Root cap
100 µm
2
� The primary growth of roots� Produces the epidermis, ground tissue, and
vascular tissue
CortexPith (monocots)Xylem
Phloem
Tissue Organization of RootsCortexVascularcylinder
Endodermis
Pericycle
Pith
Xylem
50 µm
Endodermis
Pericycle
Xylem
Phloem
Key
100 µm
VascularGroundDermal
Phloem100 µm
Epidermis
EUDICOT MONOCOT
� Lateral roots� Arise from within the pericycle, the
outermost cell layer in the vascular cylinder
Cortex
Vascularcylinder
Epidermis
Lateral root
100 µm
1 2
3 4
Emerginglateralroot
STEMS
� A stem is an organ consisting of � An alternating system of nodes, the points at which
leaves are attached� Internodes, the stem segments between nodes� An axillary bud
� Is a structure that has the potential to form a lateral shoot, or branch
� A terminal bud� Is located near the shoot tip and causes elongation of a young
shoot
� Lenticels � are sites of loosely-arranged cells allowing oxygen to diffuse into
interior of woody stem
This year’s growth(one year old)
Last year’s growth(two years old)
Growth of twoyears ago (threeyears old)
Scars left by terminalbud scales of previouswinters
Leaf scar
Leaf scar
Stem
Leaf scar
Bud scale
B
D
C
A� Many plants have modified stems
RHIZOMES
TUBERSBULBS
STOLONS
Storage leaves
Stem
Root Node
Rhizome
Root
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Primary Growth of Shoots
� A shoot apical meristem� Is a dome-shaped mass of dividing cells at the
tip of the terminal bud� Gives rise to a repetition of internodes and leaf-
bearing nodes Apical meristem Leaf primordia
Developingvascularstrand
Axillary budmeristems
0.25 mm
Tissue Organization of Stems
� In gymnosperms and most eudicots� The vascular tissue consists of vascular
bundles arranged in a ring and a distinct cortex and pith XylemPhloem
Sclerenchyma(fiber cells)
Ground tissueconnecting pith to cortex
Pith
EpidermisVascularbundle
CortexKeyDermal GroundVascular1 mm
Groundtissue
Epidermis
Vascularbundles
1 mm
� In most monocot stems� The vascular bundles are scattered
throughout the ground tissue
Tissue Organization of Stems
� Occurs in stems and roots of woody plants but rarely in leaves
� Consists of the tissues produced by the vascular cambium and cork cambium� Name them!
Secondary Growth
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Vascular Cambium and Secondary Vascular Tissue
� Vascular cambium� Cylinder of meristematic cells one cell thick� Develops from parenchyma cells� Produces
� Secondary xylem (wood) – to the inside� Secondary phloem (inner bark) – to the outside
Cork Cambium and the Production of Periderm
� The cork cambium� Gives rise to the secondary plant body’s protective
covering, the periderm
� Periderm� Consists of the cork cambium plus the layers of cork cells
it produces
� Bark� Consists of all the tissues external to the vascular
cambium, including secondary phloem and periderm
Secondary phloemVascular cambiumLate wood
Early woodSecondaryxylem
Corkcambium
CorkPeriderm
Xylem ray
Bark
0.5 mm0.5 mm
Vascular cambium
Vascular cambium
4 First cork cambium
Secondary xylem (twoyears ofproduction)
PithPrimary xylem
Vascular cambium
Primary phloem
2
1
6
Growth
Primary xylemSecondary xylem
Secondary phloem
Primary phloem Cork
Phloem ray3Xylem ray
Growth
Bark
8 Layers of periderm
7 Cork5 Most recentcork cambium
CortexEpidermis
9
Secondary phloem
� Vascular cambium� appears as a ring,
with regions of dividing cells
� fusiform initials –produce xylem and phloem tissue cells
� ray initials – produce vascular rays (move water and nutrients laterally)� Also store starch
Vascularcambium
C X CP
C
XC
XC
PP
PCX
X PC
XX
C C
� As a tree or woody shrub ages� The older layers of secondary xylem, the
heartwood, no longer transport water and minerals
� Is darker – has resin that protects against fungi/insects
� Closer to the centre of the stem
� The outer layers, known as sapwood� Still transport materials through the xylem� Is lighter� Closer to the vascular cambium
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Growth ring
Vascularray
Heartwood
Sapwood
Vascular cambium
Secondary phloem
Layers of periderm
Secondaryxylem
Bark
LEAVES
� The main photosynthetic organ of most vascular plants
� Leaves generally consist of
� In classifying angiosperms� Taxonomists may use leaf morphology as
a criterion
Petiole
Simple leaf
Compound leaf
Double (bipinnately) compound leaf
Axillary bud
Leaflet
PetioleAxillary bud
Axillary bud
LeafletPetiole
� Monocots and dicots� Differ in the arrangement of veins, the vascular
tissue of leaves
� Most monocots� Have parallel veins
� Most dicots� Have branching veins (net veined)
� Pinnately� palmately
Leaf venation
Leaf arrangement
� Leaf arrangement on a stem may be� Alternate (one leaf at each node)� Opposite (two leaves at each node)� Whorled (three or more leaves at each node)
� Many plants have modified leaves
Tendrils
Spines
Storage leaves
Bracts
Reproductive leaves
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Tissue Organization of Leaves
� The epidermal barrier� Has stomata – gaseous exchange and
transpiration� Coated by a waxy cuticle – why?
� The ground tissue - mesophyll� Is sandwiched between the adaxial and abaxial
epidermis
� The vascular tissue� Is continuous with the vascular tissue of the
stem
Keyto labels
DermalGround
Vascular
Cuticle
Sclerenchymafibers
Stoma
Adaxialepidermis
Palisademesophyll
Spongymesophyll
Abaxialepidermis
Cuticle
Vein
Guard cells
Xylem
Phloem
Guard cells
Bundle-sheathcell
Cutaway drawing of leaf tissues(a)
MONOCOTS
� Cannot distinguish between palisade parenchyma and spongy mesophyll
� Presence of bulliform cells (mainly grasses)
� Broad, flattened leaf blade� Stomata� Transparent epidermis� Air spaces in mesophyll tissue
Photosynthesis and leaf structure